--- title: "Cyclotron Shielding Design: 80 Sourced Rules · Cyclotron Info" canonical: https://cyclotroninfo.com/design-guide/shielding/ description: "80 sourced shielding design rules for small and tabletop cyclotrons, each with formula, verbatim quote, and page citation. Rules for attenuating what leaves the machine: neutron and gamma yields by energy and target, shield material and thickness, geometry and streaming, and measured dose data behind the design numbers." generated: 2026-11-12 note: "Markdown rendering of the canonical HTML page, generated at build for AI assistants and text tools. Figures and interactive elements are described, not reproduced. On Design Guide rules the source quote, this site's paraphrase and this site's editorial note are labelled separately and must not be conflated." licence: "Datasets CC BY 4.0, census snapshot CC BY-NC 4.0 (https://cyclotroninfo.com/data/); cite as Cyclotron Info, cyclotroninfo.com" --- [Design Guide](https://cyclotroninfo.com/design-guide/) › Shielding # Cyclotron Shielding Design: 80 Sourced Rules 80 of the guide’s 2199 rules carry the shielding tag. Rules for attenuating what leaves the machine: neutron and gamma yields by energy and target, shield material and thickness, geometry and streaming, and measured dose data behind the design numbers. Each rule keeps its formula where the source gives one, a verbatim quote, a page-level citation, and a stable identifier (`dg-NNNN`) that resolves here and on the [all-in-one guide](https://cyclotroninfo.com/design-guide/). Where an editorial note says “the reference machine”, its parameters are on the [guide’s front page](https://cyclotroninfo.com/design-guide/#reference-machine). By applicability level: [level 1](https://cyclotroninfo.com/design-guide/?domain=shielding&level=1) (1) · [level 2](https://cyclotroninfo.com/design-guide/?domain=shielding&level=2) (37) · [level 3](https://cyclotroninfo.com/design-guide/?domain=shielding&level=3) (26) · [level 4](https://cyclotroninfo.com/design-guide/?domain=shielding&level=4) (14) · [level 5](https://cyclotroninfo.com/design-guide/?domain=shielding&level=5) (2) — levels rank breadth, never license to skip ([method](https://cyclotroninfo.com/about/methodology/#rule-levels)). Related domains, by shared rules: [Safety](https://cyclotroninfo.com/design-guide/safety/) (40), [Materials](https://cyclotroninfo.com/design-guide/materials/) (8), [Detectors](https://cyclotroninfo.com/design-guide/detectors/) (6), [Fabrication](https://cyclotroninfo.com/design-guide/fabrication/) (5), [Beam measurement](https://cyclotroninfo.com/design-guide/beam-measurement/) (3). To combine tags or levels, open this domain in the [filterable view](https://cyclotroninfo.com/design-guide/?domain=shielding). **Verify before use.** Every rule here is a *source extract* in the vocabulary of the [editorial methodology](https://cyclotroninfo.com/about/methodology/) — faithful to its cited page, not an independently validated engineering requirement. Re-read any rule that drives a real design decision at the cited page before committing metal, money, or high voltage to it. The **editorial note** under each quote is this site’s extrapolation to a tabletop machine, not something the source said: an editor’s judgement, audited for overreach, never a citation. **dg-868** · applicability level 3 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-868 - **Rule (this site's paraphrase of the source):** Size gamma shielding from the measured line energies, not worst case: for the ~510-810 keV residual-activity lines, lead half-thickness is 0.6 cm (2 cm buys 10x) and concrete 4 cm; small portable and permanent shadow shields then give safe access to key service points (valves, ion source, rf). - **Formula:** `HVL(Pb, 0.5-0.8 MeV gamma) = 0.6 cm; 2 cm Pb = 10x attenuation; 6 cm Pb shadow shield: 100 r/hr -> 100 mr/hr; HVL(concrete) = 4 cm` - **Source quote (verbatim):** “To reduce the radiation by an order of magnitude one needs only 2 cm of lead - an amount that can readily be made into a portable shield. Shadow shields of 6 cm of lead would reduce even the 100 r/hr radiation field to a quite tolerable 100 mr/hr. The same radiation has a half-thickness of 4 cm for concrete.” - **Source:** [Boom, Toth & Zucker, Residual Radiation of the LRL 184-inch Cyclotron — ORNL-3158 (1961)](https://cyclotroninfo.com/library/#lib-220) — p. 18 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** ENERGY SCOPE: a sub-MeV machine on ordinary structural materials produces no comparable residual gamma fields (light-element targets, thresholdless capture and deuteron operation are the exceptions - see the safety pages). The transferable part is the sizing discipline: identify the actual photon energy first, then buy attenuation in half-thickness units - the same arithmetic sizes the lead around a NaI detector against room background. Note the report's own arithmetic: even 6 cm of lead leaves 100 mr/hr from a 100 r/hr field - reduced is not zero. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-912** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-912 - **Rule (this site's paraphrase of the source):** Assume the shielding estimate will prove low and the experiment space too small - the quoted history: shielding initially provided 'has later proved to be inadequate' and experiment areas are 'now too small in almost every installation'. Design margin and expansion room in from the start. - **Source quote (verbatim):** “Historically, the shielding initially provided for high-energy accelerators has later proved to be inadequate ... The experiment areas are now too small in almost every installation.” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 184 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scale-free planning doctrine, and this collection's first design-stage statement of it: leave physical room (and structural capacity) to add shielding around a next machine before the first neutron is made. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-914** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-914 - **Rule (this site's paraphrase of the source):** Build the shield estimate as an explicit chain - dose limit, source term, attenuation, secondary buildup - recording at each approximation which direction the error runs; the source's own example neglected secondary production and target attenuation together because the net stayed conservative, and only to within the precision of the other data. - **Source quote (verbatim):** “we have neglected both the secondary production and target attenuation; this results in a conservative estimate still within the precision of other data.” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 189 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scale-free methodology (their 810-MeV cascade physics is not): carry the same per-step bookkeeping on a next machine's estimate. The quote's specific lesson: omissions can run in opposite directions and partially cancel, so direction is tracked per step, never assumed - and the net conservatism is only as good as the input data's precision. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-915** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-915 - **Rule (this site's paraphrase of the source):** Separate the radiation components by the question each answers: the penetrating high-energy component sets shield thickness, while the soft/evaporation component sets activation and the dose at surfaces — do not size one problem with the other's source term. - **Source quote (verbatim):** “The thickness of shielding required for a high energy accelerator is established chiefly by the cascade nucleons ... The evaporation particles must be taken into account, however, in determining the activation of materials” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 186-188 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The sort-by-question habit is the transferable part: identify which radiation component sets shield thickness and which sets activation and surface dose for YOUR source term. For a D-D-capable machine that is fast-neutron moderation for thickness, with capture gammas and nuclide-specific activation as separate questions carrying their own data - not a clean analogue of the source's cascade/evaporation split, which is high-energy physics. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-916** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-916 - **Rule (this site's paraphrase of the source):** When your field has no literature on a subproblem, adapt the quantitative methods of the nearest mature field and say so — here, accelerator maze design taken wholesale from nuclear-reactor duct shielding. - **Source quote (verbatim):** “References to maze design for high energy accelerator shields are almost completely absent from the literature. We have based our design on the methods used for nuclear reactor shielding.” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 199 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scale-free research method as practiced there: when the accelerator literature lacked the subproblem, they adapted reactor-shielding methods and said so. The pointer stands with its age showing: reactor duct/labyrinth texts remain a usable starting point for amateur questions this corpus lacks - checked against modern references (NCRP 144-class) wherever safety rides on the answer. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-917** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-917 - **Rule (this site's paraphrase of the source):** Design mazes and penetrations by multiplying per-element transmissions: straight-leg duct attenuation grows with length/radius, each bend attenuates by roughly (1/3)csc(theta) in their data (~0.1 per 90-deg bend with an extended entering leg), legs must never sight intense sources, and parallel ducts sit several diameters apart. - **Formula:** `T_total = product(T_leg_i) * product(T_bend_j); T_bend ~ (1/3)csc(theta); extended entering leg adds ~3x` - **Source quote (verbatim):** “the attenuation at a bend is approximately 1/3 csc(theta) ... An additional factor of 3 attenuation at bends may be gained by extending the entering leg beyond the bend” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 200-204 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The product-of-elements method transfers to a next machine's cable penetrations and entry labyrinth; the factors do not transfer blind - they are low-energy-neutron empirics from that facility's geometry, and spectrum, wall material, duct size and coupled legs move them. Use the method with factors from a current reference (NCRP 144-class data), never sight a source down a straight leg, and verify the result by survey. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-918** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-918 - **Rule (this site's paraphrase of the source):** Leave a designed-in recovery path in shielding layouts: if a maze or penetration proves inadequate, there should be a pre-planned location (an extended leg, a spare recess) where a plug or door can be added later. - **Source quote (verbatim):** “Should the maze design shown prove inadequate ... the attenuation can be greatly improved by the addition of plugs at the bends. The extension of the leg beyond the corner offers a convenient location for a plug door” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 204 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scale-free insurance: reserving a plug location costs little at design time - some space and a formed recess - and buys a recovery path if the survey finds the maze wanting. Initial shielding estimates often need adjustment, which is why the survey decides (dg-914's bookkeeping); this is the cheap way to be wrong. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-919** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-919 - **Rule (this site's paraphrase of the source):** Use stepped (labyrinth) joints on shield doors and plugs so ordinary construction tolerances are acceptable - with 12-in steps the report tolerated 1/2-in cracks - and account for the shielding thickness lost to mechanisms, which the report notes the wheel spaces inevitably cost. - **Source quote (verbatim):** “The steps provided at the top and sides minimize the dimensional accuracy required. With 12 in. steps, 1/2 in. wide cracks between the plug and the wall are easily tolerable. ... Inevitably the effective thickness of the shielding is reduced somewhat by the space for the wheels.” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 206 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The stepped-joint principle scales to block-wall doorways and removable concrete/poly plugs around a benchtop target station - but the tolerable crack size was specific to their 12-in steps and their radiation field, so a scaled-down plug's steps and gaps are checked against its own field (survey), not copied. Where a mechanism eats thickness, make it up locally - added length, or denser material in that spot. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-920** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-920 - **Rule (this site's paraphrase of the source):** Trade shielding construction methods on delivered cost: their study found solid concrete walls placeable for about 2/3 the cost of walls cored with compacted rock fill - the quote; the roof-method comparison is the report's neighboring analysis (scan re-read queued). - **Source quote (verbatim):** “solid concrete walls can be placed for about 2/3 the cost of walls cored with compacted rock fill.” - **Source:** [Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963)](https://cyclotroninfo.com/library/#lib-225) — p. 242 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Their specific answer is 1963 Oak Ridge civil engineering; the transferable habit is costing shielding alternatives (block vs poured vs water vs borated poly) per unit attenuation before building any enclosure for a next machine. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-962** · applicability level 2 · safety, shielding, project-management · https://cyclotroninfo.com/design-guide/shielding/#dg-962 - **Rule (this site's paraphrase of the source):** Site an accelerator below grade and the earth is your shield: the 48-inch room was planned "mostly below ground level" explicitly because it "will be easy to shield", at basement floor level for heavy-equipment transfer, adjacent to the existing building so utilities barely extend and the existing control station works without moving. - **Source quote (verbatim):** “Being mostly below ground level, the room will be easy to shield. Placing the room at the basement floor level will make it convenient to transfer heavy equipment.” - **Source:** [Howard (ed.), Electronuclear Research Division Semiannual, period ending 20 March 1955 — ORNL-1884 (1955)](https://cyclotroninfo.com/library/#lib-232) — p. 21 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Directly relevant to the facility question for any MeV-class educational machine: below-grade siting was the study's shielding strategy, and siting beside existing utilities and controls was a cost line they weighed as seriously as the magnet. Earth shields in the directions it actually covers, by its actual thickness, density and moisture - it does not blanket-replace engineered shielding, and the uncovered directions, the roof, and every penetration still get the full design treatment (see the shielding deep dive). - **Cited in:** [Choosing Your Machine](https://cyclotroninfo.com/learn/choosing-your-machine/) · [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-966** · applicability level 2 · shielding, beam-dynamics, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-966 - **Rule (this site's paraphrase of the source):** Put the beam-defining slit inside the shield wall, because the fraction of beam intercepted by the slit system is itself a strong radiation source; put the condenser as close to the beam exit port as fringe fields allow (minimizes horizontal spread), and give the analyzer a long image distance to reduce angular spread at the image. - **Source quote (verbatim):** “A considerable amount of undesirable radiation will be produced by that part of the beam intercepted by the slit system. ... it is also desirable that the analyzer image distance be large, in order to reduce the angular spread of the beam at the image point. ... [placing the condenser farther from] the cyclotron port ... required larger condenser pole pieces in order to accommodate the horizontally spreading beam.” - **Source:** [Bromley & Bruner, The Design of a Focusing and Analyzing System for the 27-inch Cyclotron Beam — NYO-3823 (1954)](https://cyclotroninfo.com/library/#lib-233) — p. 5 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** DIRECT and cheap to honor at layout time, nearly impossible later: treat every defining aperture as a place where beam power - and therefore radiation - concentrates. At 150-170 keV the intercepted beam makes mostly heat plus thick-target bremsstrahlung whose X-ray yield climbs steeply with voltage, so the slit belongs with the shielded, surveyed components, wherever the survey ranks it that day. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-994** · applicability level 3 · shielding, detectors · https://cyclotroninfo.com/design-guide/shielding/#dg-994 - **Rule (this site's paraphrase of the source):** Set flight-path length against the room, not just the resolution equation: most experiments kept paths under 1.2 m to avoid difficulty with neutrons scattered from the solid concrete floor - the quoted choice; the timing-window mechanism and the detector-shield history are the report's account (scan re-read queued). - **Source quote (verbatim):** “Most experiments have been made with flight paths less than 1.2 meters long in order to avoid difficulty with neutrons scattered from the solid concrete floor” - **Source:** [Fulbright et al., A Fast Neutron Time of Flight System for Use with Cyclotrons — NYO-9360 (1962)](https://cyclotroninfo.com/library/#lib-235) — p. 12 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** DIRECT pair of lessons: (1) geometry (short path, floor clearance, timing window) is often cheaper background suppression than shielding mass; (2) never bolt on a detector shield whose effect on efficiency you haven't calibrated - it converts a known instrument into an unknown one. Both transfer to any next-machine counting station. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-1033** · applicability level 1 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1033 - **Rule (this site's paraphrase of the source):** Size shielding around the SECONDARY radiation: the beam's interaction with the target, the accelerator structure, or the shielding itself 'most often' determines the type and magnitude of shielding required - and primary-beam containment is still assessed wherever extraction, a thin window, or an abnormal loss could make ions accessible. - **Formula:** `shield for secondaries (X-rays, neutrons) produced where the beam is lost, not for the primary ions` - **Source quote (verbatim):** “Secondary radiations produced as a result of the interaction of the primary beam with a target, portion of the accelerator, or the shielding most often determine the type and magnitude of the shielding.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 22 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** For the reference machine and a next machine the primary protons stay inside the chamber in normal operation, so the external radiation field is dominated by secondaries - dee-gap electron bremsstrahlung today; reaction products (the 11B(p,alpha) alphas) and any (p,n)-capable contaminants joining the inventory at a next machine's energies. **dg-1034** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1034 - **Rule (this site's paraphrase of the source):** Direct bremsstrahlung from a heavy projectile scales as ~1/M^2 of its mass and is usually insignificant; the X-ray sources that matter on a positive-ion machine are instead - the manual's list - characteristic X-rays from inner-shell vacancies, nuclear deexcitation, and bremsstrahlung from stray electrons. - **Formula:** `bremsstrahlung ~ 1/M^2 -> proton bremsstrahlung negligible; hazard = characteristic X-rays + stray-electron bremsstrahlung` - **Source quote (verbatim):** “The bremsstrahlung is approximately inversally proportional to the M2 where M is the mass of the incident particle. It is therefore usually insignificant for heavy particles.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 30 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Supports the program's standing model that dee-voltage electrons, not the proton beam's own bremsstrahlung, dominate the X-ray hazard on a sub-MeV proton cyclotron. Dominant is not sole: characteristic X-rays and any nuclear gammas from targets keep their own lines in the survey plan. **dg-1036** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1036 - **Rule (this site's paraphrase of the source):** Lacking design detail, the Army manual estimates the stray-electron X-ray source term of a positive-ion accelerator by assuming a reverse-directed electron current of 0.2\*I (I = ion current) accelerated through 1/3 of the terminal voltage - an assumption the authors themselves label unreliable, offered to show that even a rough guess predicts "very considerable" X-ray production, not as a bounding figure. [Corrected 2026-08-23: an earlier version and its note presented the 0.2\*I / V/3 pair as a bounding recipe. The source presents it as the opposite - an unreliable assumption that nonetheless gives a large number - and using it as a ceiling is under-conservative.] - **Formula:** `I_e(back-streaming) ~ 0.2 * I_ion at E ~ V_terminal/3 - a rough historical source-term ASSUMPTION, not a bound` - **Source quote (verbatim):** “If we assume that the ion current "I" results in a reverse directed electron current of magnitude 0.2\*I that is accelerated through 1/3 the terminal voltage we would usually get a very considerable x-ray production.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 31 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Use this only as the lesson that stray-electron X-rays exist wherever there is RF voltage and vacuum, never as a ceiling. For a next machine's hazard analysis, plan around at least the peak-to-peak dee voltage as the electron impact energy - a planning floor, not a physical ceiling, since multi-transit RF processes can exceed single-gap figures - and let the measured X-ray endpoint from the survey be the authority the analysis answers to. **dg-1037** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1037 - **Rule (this site's paraphrase of the source):** As a rough shielding estimate for a heavy-ion accelerator's stray-electron X-rays, provide the shielding that would be required at 90 degrees from the beam axis of an ELECTRON accelerator of the same beam current and energy; ion-machine shielding "may not be so much less" than the electron case. - **Formula:** `shield(ion machine) ~ shield(electron machine, 90 degrees, same I and E)` - **Source quote (verbatim):** “As a rough estimate we offer that shielding which is required at 90 deg from the beam axis of an electron accelerator, with the same beam current and energy.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 31 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The conservative sizing pattern for a product-machine enclosure: bound the ion machine by an equivalent electron accelerator at the same current and at the maximum electron energy credible in the machine (at least peak dee-to-ground; dg-1036), then read the required thickness from electron-accelerator shielding data at that energy and verify by survey. The reference machine's chamber walls stopping its soft X-rays is a measured fact about ~10 kV operation, not a rule to inherit. **dg-1038** · applicability level 3 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1038 - **Rule (this site's paraphrase of the source):** Thick-target X-ray conversion efficiency at 0.5 MeV: stopping electrons convert 0.265% of beam power to X-rays in water, 0.59% in Al, 1.34% in Fe, 4.77% in W, 6.21% in U — efficiency rises with Z and with energy (at 1 MeV, W gives 7.63%). - **Formula:** `f(X-ray) at 0.5 MeV: H2O 0.265%, Al 0.59%, Fe 1.34%, W 4.77%, U 6.21% of electron beam power (Table II-1)` - **Source quote (verbatim):** “The % of the electron energy that is converted to X-rays upon complete stopping of the electrons ... 0.5 ... 0.265 ... 0.59 ... 1.34 ... 4.77 ... 6.21” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 23 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Sets the scaling logic - conversion efficiency rises with Z and with energy - even though the table starts at 0.5 MeV. At dee-voltage energies the Z-trend persists in direction, but the table's factors do not extrapolate cleanly (characteristic lines and backscatter enter), so the design instinct is what transfers: land stray electrons on LOW-Z surfaces (aluminum, graphite) rather than tungsten or steel, and let the survey measure the actual benefit. **dg-1039** · applicability level 3 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1039 - **Rule (this site's paraphrase of the source):** At very low electron energy (few keV), bremsstrahlung is emitted with the intrinsic angular distribution of a radio antenna — intensity GREATEST PERPENDICULAR to the electron direction — the opposite of the MeV-range forward peaking. - **Formula:** `few-keV electrons -> dipole pattern, max at 90 degrees to electron path; MeV electrons -> forward-peaked` - **Source quote (verbatim):** “At very low electron energy (few keV), the intrinsic angular distribution is the same as from a radio-antenna, i.e., the intensity is greatest perpendicular to the direction of the electron beam.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 23 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** For dee-gap electrons, the INTRINSIC few-keV emission peaks sideways to the electron path - a reason to survey all around the chamber midplane and its windows rather than along any assumed axis. What actually leaks where folds in scattering, multiple electron directions, self-absorption and wall attenuation - so the survey pattern, not the dipole formula, is the finding. **dg-1040** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1040 - **Rule (this site's paraphrase of the source):** The chapter's quantitative machinery - forward intensity I(0) = 723\*tau\*(T+0.511)^2\*T\*i/d^2 \* ln(3250t/ln(183 Z^-1/3)), dose R(0) = 2.604e11\*(mu_k/rho)av\*(same), and the concrete dose-rate table scaled by W/R^2 - is tabulated for its 5.5-40.5 MeV electron range; below that range the chapter's tables simply do not reach, and the source-term assumptions are the part that generalizes. - **Formula:** `D(behind x cm concrete) = TableII-3(T,x) * W(kW)/R(m)^2, tabulated 5.5-40.5 MeV; below the table's range use X-ray-tube output data` - **Source quote (verbatim):** “The dose rate D in rads per hour is obtained by multiplying the values in the Table by W/R2, where W is the electron beam power in kwatt and R is the distance in m to the detector from the X-ray target.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 26 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scope honestly: this is the collection's only full X-ray shielding workflow, and its tables start at 5.5 MeV. For the 5-13 kV dee upgrade none of it applies numerically - take source terms and barriers from X-ray-tube shielding data (NCRP-49-class R/mA-min at 1 m vs kVp), keep the chapter for its structure (source term, then barrier, then verify), and treat the 0.2\*I stray-electron assumption as a lesson, not an input (dg-1036). **dg-1041** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1041 - **Rule (this site's paraphrase of the source):** Proton cross sections for nuclear interaction fall steeply below about 0.1 MeV because of the Coulomb barrier — but the light-nuclei exceptions the source waves off are exactly the targets amateurs use: 7Li(p,alpha) and 11B(p,alpha) run at measurable rates well below 100 keV. Evaluate the actual target isotopes before making radiation assumptions. [Corrected 2026-08-20: an earlier version endorsed the source's "nuclear-reaction-free" conclusion; nuclear data contradict it for light targets.] - **Formula:** `sigma(p,nuclear) ~ 0 below ~0.1 MeV; barrier penetration grows sharply with E thereafter` - **Source quote (verbatim):** “Because of the Coulomb barrier, proton cross sections for nuclear interaction are negligible below about 0.1 MeV. In light nuclei there are some exceptions which are of little interest here.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 48 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Closes the neutron question for the reference machine at ~150 keV-class energies EXCEPT via the light-nuclei exceptions the chapter waves off - and the exceptions differ in kind: the deliberate 11B(p,alpha) target yields charged alphas and gammas, not neutrons directly (the indirect path to check is secondary (alpha,n) on nearby low-Z materials); deuterium contamination is the direct neutron path, D(d,n) being thresholdless (see Ch. IV rule). **dg-1042** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1042 - **Rule (this site's paraphrase of the source):** (p,n) reactions are threshold-gated: the n-p mass difference (0.78 MeV) sets a floor, thresholds are of the order of an MeV for light and low-intermediate nuclei, and neutron emission becomes the dominant channel about 1 MeV above threshold - the manual's rough generalization; resonances and channel competition make real cases isotope-specific. - **Formula:** `E_thr(p,n) > 0.78 MeV (stable targets), ~MeV for light nuclei; n-channel dominant at E > E_thr + ~1 MeV` - **Source quote (verbatim):** “For light and low-intermediate nuclei, (p,n) thresholds are of the order of an MeV. Neutron emission becomes the dominant reaction when the incident particle energy exceeds the threshold by about 1 MeV.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 48 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The threshold-audit pattern for every machine energy bump: list materials the beam can strike, look up (p,n) thresholds, and confirm E_beam sits below them. At 170 keV (a next machine) every (p,n) channel on stable nuclei is closed, but the margin is not uniform: the lowest known stable-target threshold, 115In(p,n)115Sn at ~287 keV, leaves only ~120 keV, while the common structural metals and light targets sit >600 keV away (first structural channel 55Mn(p,n)55Fe at 1.03 MeV; 7Li(p,n) at 1.88 MeV). The audit must be redone on any energy bump, against the materials actually present, not a generic list. [Corrected 2026-09-13: previously claimed every stable-nucleus channel closed 'by >600 keV of margin'; that figure held for the structural metals and light targets but not for mid-Z nuclides like 115In (NNDC QCalc; see the safety page's activation section). Raised by an external site review alongside the dg-581 correction.] **dg-1043** · applicability level 4 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1043 - **Rule (this site's paraphrase of the source):** Photoneutron thresholds run 6-19 MeV for nearly all nuclei with ONE trap: deuterium at 2.23 MeV — hydrogenous (water-containing) materials with natural deuterium are the exception to "low-Z is safe around photon flux," so audit D-bearing materials wherever multi-MeV photons exist. - **Formula:** `E_thr(gamma,n): H-2 2.23 MeV; C-12 18.7; O-16 16.3; Cu-63 10.9; Pb-208 7.44 (Table III-2)` - **Source quote (verbatim):** “H2(gamma,n)H1 ... 2.23 ... C12(gamma,n)C11 ... 18.7 ... O16(gamma,n)O15 ... 16.3” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 47 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** At the reference machine's photon energies (keV-class bremsstrahlung) every channel in the table is closed. Two honesty caveats for reuse in hazard analyses: 9Be sits below the deuterium trap (S_n = 1.665 MeV, IAEA NDS/AME, retrieved 2026-08-25), so beryllium joins heavy water on the audit list wherever multi-MeV photons exist; and reaction gammas can exceed the projectile energy - 11B(p,gamma) capture emits ~16 MeV photons at small cross-section, so a p-B11 machine's analysis must bound that two-step channel rather than declare photonuclear reactions impossible. **dg-1044** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1044 - **Rule (this site's paraphrase of the source):** Neutron shielding is slow-down-then-capture in the manual's account - light nuclei (hydrogen) dominate energy loss, so hydrogenous concrete outperforms lead for neutrons - and the quoted rule of thumb: a facility shielded in concrete for X-rays 'generally contains adequate neutron shielding in the process', with the serious problem arising where the neutron hazard exceeds the photon hazard: proton and deuteron machines, the quote's own caveat. - **Formula:** `concrete X-ray shield ~ adequate neutron shield (rule of thumb, <30 MeV); capture gammas must be shielded in turn` - **Source quote (verbatim):** “it is a fairly accurate rule of thumb that for energies of interest here, the facility generally contains adequate neutron shielding in the process. The more serious neutron shielding problem occurs when the X- and gamma ray hazard is exceeded by the neutron hazard. Proton and deuteron accelerators are cases in point.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 57 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** For any future neutron-capable operation (a deuterium species test, or a >1.9 MeV machine) the caveat is the operative part: an X-ray shield does not automatically cover the neutron hazard, and the source names proton and deuteron machines as exactly the case where neutrons dominate. The source's physics points to hydrogenous material (concrete, HDPE) rather than lead for the neutron component, but sizing it is a separate design problem - worked from the actual source term and verified by survey - not settled in advance by this rule of thumb. **dg-1045** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1045 - **Rule (this site's paraphrase of the source):** For first-pass neutron shield sizing the chapter uses the reactor-derived removal-cross-section method: penetration as exp(-Sigma_r\*x), with removal cross-sections roughly three-quarters of the total at 8 MeV (somewhat larger for hydrogen), and its concrete coefficient Sigma_r ~ 0.094 cm^-1. - **Formula:** `phi(x) = phi_0 exp(-Sigma_r x); sigma_removal ~ 0.75*sigma_total @8 MeV; Sigma_r(concrete) ~ 0.0942-0.0945 /cm` - **Source quote (verbatim):** “Experimental removal cross sections are roughly three-quarters of the total cross section for 8 MeV neutrons. For hydrogen this fraction is somewhat larger. [Table III-7B] Ordinary Concrete 0.0942 ... Barytes Concrete 0.0945” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. PDF 58 (printed 49) for the quote; PDF 57 (printed 48) for the exponential/reactor framing; PDF 60 (printed 51) for the concrete coefficient in Table III-7B - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The one-line neutron shield ESTIMATOR for contingency planning - a D-D source term attenuates ~10x per 24 cm of concrete at the chapter's coefficient - used with its conditions: the coefficient is energy-derived (8 MeV; 2.45 MeV D-D neutrons remove differently), hydrogen content matters, and the chapter's own safety factors ride along. An estimate to verify by survey, never a design allowable. **dg-1046** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1046 - **Rule (this site's paraphrase of the source):** Worked pattern for a neutron shield, as the chapter runs it: its example yield (20-MeV protons on an optimized Cu target, ~6.5e10 n/s per uA), the flux at the shield face, a demanded six orders of magnitude of attenuation, and inverting exp(-Sigma_r\*x) - giving the quoted 146 cm of concrete. - **Formula:** `Y(20 MeV p on Cu) ~ 6.5e10 n/s/uA; x = ln(attenuation)/Sigma_r -> 146 cm for 1e6` - **Source quote (verbatim):** “about 6.5 x 10^10 neutrons per second are produced for each microampere of proton current ... the shield must reduce the fast neutron flux by six orders of magnitude. Therefore e-Sigma_r X = 10-6. For barytes concrete (i.e., Sigma_r = 0.0945 cm-1 ...): X = 146 cm” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. PDF 60 (printed 51) — the whole worked example, yield line included, is on the cited page - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The template to copy for any neutron-capable scenario: source yield -> flux at the shield (1/4pi r^2) -> required attenuation from the dose criterion -> x = ln(A)/Sigma_r. Also the scale anchor for why amateur neutron machines are enclosure-limited: the chapter's 20-MeV, mA-class case needs five feet of concrete. **dg-1047** · applicability level 4 · safety, shielding, ion-source · https://cyclotroninfo.com/design-guide/shielding/#dg-1047 - **Rule (this site's paraphrase of the source):** D(d,n)He3 and T(d,n)He4 are EXOENERGETIC - they run at very low bombarding energy (Cockcroft-Walton scale) - so any deuterium in source gas or beam-loaded surfaces makes neutrons with no threshold protection. Other deuteron channels are exoenergetic too (9Be(d,n) Q ~ +4.4 MeV, 7Li(d,n) Q ~ +15 MeV): thresholdlessness is a property of deuteron beams on several light targets, while the common PROTON channels, (p,n), are threshold-protected. - **Formula:** `D(d,n)He3 Q = +3.27 MeV; T(d,n)He4 Q = +17.6 MeV; also exoenergetic: 9Be(d,n), 7Li(d,n); common (p,n) and (gamma,n) channels are threshold-protected` - **Source quote (verbatim):** “Two of these reactions, the D(d,n)He3 reaction and the T(d,n)He4 reaction are exoenergetic and can be initiated at very low energies. Thus these two reactions can be produced in small Cockcroft-Walton accelerators.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 78 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** THE loophole in the 'sub-MeV machines make no neutrons' argument: natural hydrogen is ~150 ppm deuterium and D accumulates in beam-loaded surfaces, so a D-on-D source term exists in principle on any hydrogen machine - at yields the Coulomb barrier suppresses steeply at low energy, which is why the honest posture is a survey requirement, not alarm. **dg-1048** · applicability level 3 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1048 - **Rule (this site's paraphrase of the source):** Induced activity around an accelerator is a two-step process - beam makes neutrons/photons at the target; those activate surroundings - and because capture probability goes as 1/v, the chapter directs using the THERMAL cross section for estimating capture activation, with slowing-down activation negligible by comparison. - **Formula:** `activation A0 = M*phi*sigma_thermal*(1-exp(-lambda*t_irr)); slowing-down activation negligible by comparison` - **Source quote (verbatim):** “In the slowing down process ... an insignificant amount of induced activity is produced as compared with the activity produced by thermal neutrons. Therefore the thermal cross section should be used for purposes of calculating the activity produced.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 78 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The correct FIRST bookkeeping if a neutron-capable operation is ever run: inventory surrounding materials against thermal flux (Cu 3.9 b, W 34 b, Au 96 b thermal, per Table IV-2). Capture is the floor of the inventory, not its ceiling - epithermal resonances and fast threshold reactions ((n,p), (n,alpha), (n,2n)) can dominate for some materials and spectra. On today's neutron-free machines there is nothing to activate either way. **dg-1049** · applicability level 4 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1049 - **Rule (this site's paraphrase of the source):** In the report's assessment of ordinary concrete, only Na-24 (15 h) and perhaps K-42 (12.4 h) presented any hazard - a shutdown of three to five days lets them decay to very low levels; barytes concrete adds Ba-139 (83 m), which builds up during a day's running but decays away overnight. - **Formula:** `concrete activation governed by Na-24 (15 h) / K-42 (12.4 h); 3-5 day cooldown -> negligible` - **Source quote (verbatim):** “Only Na24 and perhaps K42 could present any kind of hazard. Because of the half-lives of these two isotopes, a shut down of three to five days will allow decay to very low levels.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 84 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Cooldown-scheduling logic for any future neutron-producing work, as the report's era assessed its concrete: modern assessments add impurity-driven products (Mn-56, Co-60, Eu-152/154, tritium and others) whose relevance depends on the actual aggregate and spectrum, so a real facility characterizes its own concrete rather than inheriting this list. For the current machines the practical point stands: with no neutron source term there is nothing to activate the basement structure - deuteron operation being the standing exception. **dg-1050** · applicability level 4 · shielding, fabrication · https://cyclotroninfo.com/design-guide/shielding/#dg-1050 - **Rule (this site's paraphrase of the source):** Because thermal neutrons attenuate to ~1/3 of initial flux in the first 10 cm of ordinary concrete, about 2/3 of neutron activation lives in the shield's inner skin — so design shields with a removable row of concrete blocks on the inside that can be disposed of and replaced if they grow too active. - **Formula:** `thermal flux ~1/3 per 10 cm concrete -> ~2/3 of activation in first 10 cm -> sacrificial inner block row` - **Source quote (verbatim):** “Thermal neutrons are attenuated to about one-third of their initial flux by the first 10 cm of ordinary concrete. Therefore 2/3 of the activity produced by the neutrons would occur in this region. This makes it possible to design shielding with a row of concrete blocks on the inside.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 84 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The modular-block enclosure pattern already favored for product machines gets a second justification: the inner course doubles as the sacrificial activation layer, replaceable without demolishing the shield. Two conditions travel with it: the 2/3-in-10-cm figure is for thermal neutrons in ordinary concrete, not every spectrum; and replaced blocks are surveyed and characterized before anything is 'disposed of' - activated material is a regulated waste stream (see /legal/). **dg-1051** · applicability level 5 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1051 - **Rule (this site's paraphrase of the source):** Long-lived photoproduced isotopes in shielding cannot be waited out: 'if large quantities of this isotope build up, it will be necessary to physically remove the activated shielding, so plans for this contingency should be made in the design of the walls' - the isotope's identity and production threshold are the chapter's context (Na-22-class, multi-MeV photons; cite current nuclear data when used). - **Formula:** `above-threshold gamma flux + years of operation -> Na-22 inventory -> removable-wall contingency in design` - **Source quote (verbatim):** “If large quantities of this isotope build up, it will be necessary to physically remove the activated shielding, so plans for this contingency should be made in the design of the walls.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 84 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scope closed for the machine's photon energies - photoproduction needs multi-MeV photons far beyond any dee. The design principle transfers as prudence rather than prohibition: prefer enclosure designs that could be dismantled selectively (block walls, the sacrificial inner course of dg-1050) over monoliths - cheap to choose now, expensive to regret. **dg-1055** · applicability level 3 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1055 - **Rule (this site's paraphrase of the source):** Flux-density-to-dose conversion for neutrons in the manual's table (100 mrem per 40-h week): thermal 680 n/cm2-s, 10 keV 700, 100 keV 115, 500 keV 27, 1 MeV 19, 10 MeV 17 - the table's fast-neutron minimum near 0.5-1 MeV makes those neutrons ~35x more restrictive per unit flux than thermal. - **Formula:** `100 mrem/40h flux limits: 680 (thermal), 19 (1 MeV), 17 (10 MeV) n/cm2-s` - **Source quote (verbatim):** “2.5 x 10-8 (thermal) 2 680 ... 5 x 10-1 11 27 ... 1 11 19” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 112 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The conversion pattern for any future neutron survey, used with its conditions: a reading converts only when the energy is known or the instrument already folds the spectrum in (a rem-meter does), and these are 1972 occupational numbers - modern public limits sit far lower. The design fact survives: fast neutrons near 0.5-1 MeV are the most restrictive per unit flux, which is why a D-D contamination field matters at even a few n/cm2-s. **dg-1066** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1066 - **Rule (this site's paraphrase of the source):** Estimate X-ray streaming through a maze by successive 90-degree scatters: assume conservatively that 0.05 of the incident energy scatters into one steradian per bounce - Moyer's estimate, the quote; the chained product I_p = (I_1/r_n^2)\*prod[0.05\*S_i\*cos45/r_i^2] is the chapter's application of it (validation data: scan re-read queued). - **Formula:** `I_p = I_1/r_n^2 * prod_i [0.05 * S_i * cos45 / r_i^2] per 90-deg scatter leg` - **Source quote (verbatim):** “Moyer estimated that for a 90 deg scattering of X-rays it is conservative to assume that 0.05 of the incident energy would be scattered into one steradian in the new direction.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 146 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The same hand calculation sizes a cable or vacuum-line dogleg or an instrument-port baffle in a product-machine enclosure - where a straight-through hole would dominate the leakage - with the estimate verified by survey once built (dg-917's discipline). **dg-1067** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1067 - **Rule (this site's paraphrase of the source):** Permit NO line-of-sight path for radiation through any access route or penetration, and then still evaluate the scatter path through the maze - the chapter's paired requirements. - **Formula:** `no line-of-sight through any penetration; scatter path evaluated per the 0.05/sr rule` - **Source quote (verbatim):** “Naturally no "line of sight" path for radiation would be permitted yet it is also necessary that the scatter path through the maze be considered.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. 145 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The audit rule for every feedthrough, window and joint in an enclosure: check sight-lines from the X-ray source point (the dee gap) outward, then bound the one-bounce leakage with the albedo rules (dg-1068). Geometry creates the streaming problem; material still sets what each bounce and wall costs, so both enter the estimate. **dg-1068** · applicability level 4 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1068 - **Rule (this site's paraphrase of the source):** For neutron streaming through mazes the manual uses the albedo chain phi_p = (phi_s/(4pi r_n^2)) \* prod[beta\*Omega_i]: its albedo values run from 0.66 for thermal neutrons down to ~0.05 for fast, with 0.4 offered as a conservative fast-neutron choice, and boron loading on the shield-wall surface absorbs thermal neutrons instead of reflecting them. - **Formula:** `albedo beta = 0.66 (thermal) -> ~0.05 (fast); 0.4 conservative; boron loading cuts thermal reflection. [2026-09-06 re-read: the printed eq. VII-2 divides by a bare 2*r_n^2, but the manual's own worked example uses 4*pi*r_n^2 and only that reproduces its printed 3.2e-3 answer - the 2*r_n^2 is an original typo; the card's 4*pi form follows the manual's practice.]` - **Source quote (verbatim):** “albedo for neutrons (from 0.66 for thermal neutrons to approx. 0.05 for fast neutrons) ... using an albedo of 0.4 which is relatively conservative for fast neutron ... Boron loaded concrete on the surface of the shield wall will increase the probability of absorption, decreasing the probability of scatter in reflection” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. PDF 148 (printed 139) for the albedo range and the 0.4 choice; PDF 147 (printed 138) for eq. VII-2 and the boron sentence - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Contingency reference only at current energies. If a future neutron source term streams through an enclosure penetration, the boron-surface idea (borated HDPE lining a duct) reduces the thermal reflection specifically - it does nothing for the fast component - so it is one element of a duct fix, sized with the chain using energy-appropriate albedos and checked by survey, not a standalone cheap cure. **dg-1078** · applicability level 4 · safety, shielding, fabrication · https://cyclotroninfo.com/design-guide/shielding/#dg-1078 - **Rule (this site's paraphrase of the source):** Massive shielding doors carry their own hazards in the manual's treatment: slow travel with great momentum (engineer the stopping so the door cannot trap personnel or crack walls), shielding at least equal to the adjoining wall, and - the quoted requirement - every door manually openable from BOTH inside and outside after a loss of power. - **Formula:** `door shielding >= wall; manual egress inside+outside under power loss; engineered deceleration` - **Source quote (verbatim):** “Doors should be designed to provide shielding at least equivalent to the adjoining walls ... Travel of these large doors is necessarily slow but the momentum is great ... one must be able to open these doors even after a loss of power. Some manual method of opening the door from inside and outside must be included in the design.” - **Source:** [Martin (ed.), Accelerator Radiation Protection — AD-755510 / USA-NLABS-TR-73-7, US Army Natick Laboratories (1972)](https://cyclotroninfo.com/library/#lib-239) — p. PDF 145 (printed 136) - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Scale-invariant egress principle: even an interlocked benchtop lid or a walk-in enclosure door must never imprison anyone on power loss. The criterion is manual operability from both sides without power, verified by actually trying it - whatever the mechanism - and the door's own motion is a machinery hazard (pinch points, momentum) to engineer alongside its radiological job. **dg-1097** · applicability level 2 · rf, fabrication, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1097 - **Rule (this site's paraphrase of the source):** Choose the resonator mode and geometry so tuning elements sit outside the main vacuum chamber: the half-wave resonator 'permits the rotating capacitors to be located outside... for good shielding from both the magnetic field and radiation' - the quote; the iron tuner housings are the report's detail (scan re-read queued). - **Formula:** `half-wave resonator puts voltage node / tuner outside chamber; 2-in. Fe housing shields rotors` - **Source quote (verbatim):** “a half-wave resonator permits the rotating capacitors to be located outside the main vacuum chamber for good shielding from both the magnetic field and radiation” - **Source:** [Rainwater et al., The Columbia University Nevis Synchrocyclotron Major Modification — NEVIS-189 / R-774 / CU-295 (1971)](https://cyclotroninfo.com/library/#lib-240) — p. 10 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The placement principle transfers: keep variable capacitors, trimmers and drive mechanisms of a next machine's tank outside the pole gap and chamber, where field, beam spray and pumpdown cannot reach them - where the geometry allows it. **dg-1102** · applicability level 4 · safety, materials, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1102 - **Rule (this site's paraphrase of the source):** Line surfaces struck by lost beam to REDUCE activation of the structure behind them: Nevis expected marble pole liners 'to reduce sector iron, etc., activation' - stray beam deposits in the stone instead of iron and copper. - **Formula:** `marble (CaCO3) liners over pole/sector iron in beam-loss regions - reduction, not elimination (the stone itself activates at Nevis energies)` - **Source quote (verbatim):** “We expect to use marble pole liners where possible, as in the past, to reduce sector iron, etc., activation” - **Source:** [Rainwater et al., The Columbia University Nevis Synchrocyclotron Major Modification — NEVIS-189 / R-774 / CU-295 (1971)](https://cyclotroninfo.com/library/#lib-240) — p. 6 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A higher-energy note, with a scale-free idea inside: CHOOSE what lost beam hits. At any scale that choice already governs sputter contamination and outgassing; it becomes activation-relevant the moment a machine crosses into neutron or few-MeV territory - with thresholdless capture the standing exception to 'negligible below a few MeV'. **dg-1107** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1107 - **Rule (this site's paraphrase of the source):** Buy shielding with geometry before mass, as the Nevis layout does: the underground beam stop aims away from occupied areas - which the report says greatly eases shielding and background - with secondary beams taken off at large angles and bends between production targets and experimenters. - **Formula:** `beam stop aimed away from people; large-angle takeoff; bends between target and experimenters (the report's layout choices)` - **Source quote (verbatim):** “Since the underground beam stop is aimed away from the experimental areas, this greatly eases shielding, and subsequent background problems” - **Source:** [Rainwater et al., The Columbia University Nevis Synchrocyclotron Major Modification — NEVIS-189 / R-774 / CU-295 (1971)](https://cyclotroninfo.com/library/#lib-240) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Direction-dependence of secondary radiation is universal even though the 550-MeV numbers are not: orient any future target station and Faraday-cup dump so the forward cone points at mass, not people. The specific takeoff angles and bend counts are per-facility physics rather than constants - lay out first, then let the survey confirm the geometry did what was expected. **dg-1108** · applicability level 5 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1108 - **Rule (this site's paraphrase of the source):** The Nevis synchrocyclotron modification report's shielding datum (Rainwater et al., 1971): about 6 inches of iron 'or the equivalent' per factor-of-two attenuation for the forward high-energy neutron cone (>100 MeV, charge-exchange), while at 90 degrees or more the neutrons are mainly below 100 MeV and the same 6 inches buys closer to a factor of 10 - shield thickness is budgeted per direction. [2026-09-06 erratum, scan re-read: previously attributed to 'Moyer', a name that appears nowhere in the 81-page report; and the report says 'or the equivalent', never naming concrete.] - **Formula:** `~6 in Fe (or equivalent) per x2, forward cone >100 MeV; same 6 in ~ x10 at >=90 deg (mainly <100 MeV) - per-direction budgeting` - **Source quote (verbatim):** “Forward cone > 100 MeV neutrons ... require ~ 6 in. Fe (or the equivalent) for each factor of 2 attenuation. At 90 deg or more ... closer to a factor 10 attenuation.” - **Source:** [Rainwater et al., The Columbia University Nevis Synchrocyclotron Major Modification — NEVIS-189 / R-774 / CU-295 (1971)](https://cyclotroninfo.com/library/#lib-240) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Pure high-energy datum — no tabletop relevance except as a worked example of directional shielding budgets, but it anchors the energy scaling. **dg-1112** · applicability level 2 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1112 - **Rule (this site's paraphrase of the source):** Take the fast-neutron half-value thickness of ordinary concrete for cyclotron-target neutrons as approximately 10 cm (ORNL measurement over thick C, Al, Cu, Ta targets under proton, deuteron, alpha, and carbon beams). - **Formula:** `HVT(ordinary concrete, cyclotron-target fast neutrons) ~ 10 cm` - **Source quote (verbatim):** “fixes the half-value thicknesses of ordinary concrete for neutrons from cyclotron targets at approximately 10 cm” - **Source:** [Ohnesorge & Butler, Recent Trends in Particle Accelerator Radiation Safety — CONF-741040-6, Oak Ridge National Laboratory (1974)](https://cyclotroninfo.com/library/#lib-241) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The corpus's first literal shielding number for MeV-class cyclotron neutrons. The reference machine's proton operation sits below its (p,n) thresholds and makes none - deuteron operation is the standing exception (D-D is thresholdless) - and this ~10 cm HVT is the sizing constant the moment any machine or D-beam work crosses into neutron production; it was measured for cyclotron-target spectra, so re-check it for a materially different spectrum. **dg-1115** · applicability level 3 · shielding, beam-measurement · https://cyclotroninfo.com/design-guide/shielding/#dg-1115 - **Rule (this site's paraphrase of the source):** Measure shield attenuation with the machine itself as the source - the quoted apparatus: a slab of the candidate material (3 ft x 3 ft x thickness), a detector recessed in a small cavity in a concrete igloo, and a beam MONITOR. Its evident role - normalizing detector readings to source intensity - is the method's point [editorial reading of the figure; the 2026-09-06 re-read confirmed the paper contains no analysis text stating the monitor's role - the apparatus legend is verified verbatim, the interpretation is ours and is labeled as such]. - **Formula:** `attenuation = (detector/monitor) vs slab thickness; slab 3'x3', detector in 1.5-inch cubical cavity` - **Source quote (verbatim):** “A - Slab under test. Dimensions 3' x 3' x thickness. C - Concrete "Igloo". D - Detector, in cubical cavity 1-1/2" edge. M - Beam moniter [sic]” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A shielding survey needs no separate neutron source — run the machine at a reference beam current and take detector-to-monitor ratios; the monitor normalization is what makes readings taken hours apart comparable. **dg-1116** · applicability level 3 · shielding, detectors · https://cyclotroninfo.com/design-guide/shielding/#dg-1116 - **Rule (this site's paraphrase of the source):** Keep survey electronics out of the magnet fringe field: use passive detectors (ionization chambers) at the measurement point with DC amplification, and put the indicating meters where the field cannot bias their movements. - **Source quote (verbatim):** “The monitor and detector employed were aluminum-walled ionization chambers, with DC Amplification, indicating on microammeters placed outside the magnetic field of the cyclotron.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 2 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Analog meter movements and photomultipliers misread in modest stray fields (PMTs at well under a millitesla); GM tubes themselves are largely field-insensitive, though their electronics may not be. The transferable practice is the source's separation: passive sensing volume at the measurement point, readout where the field is negligible - verified by moving the readout and watching for a change. **dg-1117** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1117 - **Rule (this site's paraphrase of the source):** Expect transition (buildup) effects at the front face of a shield in fields like Moyer's: attenuation becomes exponential only after the radiation reaches equilibrium with the secondaries it generates, so fit half-value thicknesses to the displaced linear portion of the curve - never to the first layers. - **Formula:** `fit exponential slope only beyond the equilibrium (buildup) depth; extrapolation of the linear portion back to zero is displaced from the no-absorber reading` - **Source quote (verbatim):** “The transition effects occur as the neutron beam approaches equilibrium with the secondary and scattered particles produced in the absorbing medium.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A dosimeter just behind the first inches of shielding can be measuring the buildup region rather than the attenuation slope, so thin-shield tests can misestimate a thick shield in either direction. How pronounced the transition is depends on the field and geometry; the transferable part is the fitting discipline - use the asymptotic slope. **dg-1118** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1118 - **Rule (this site's paraphrase of the source):** Layer order matters in composite shields: hydrogenous material following a high-Z layer can RAISE the ionization reading behind it — Moyer measured a paraffin transition increase of 60% following iron and 100% following lead — because the hydrogenous layer converts neutron flux to ionizing protons. - **Formula:** `Moyer's ionization readings behind paraffin: +60% following Fe, +100% following Pb, in his geometry and chambers - a measured transition effect, not a general dose identity` - **Source quote (verbatim):** “Paraffin yields a transition increase of 60% following Fe, and of 100% following Pb with similar geometry.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** When adding polyethylene or paraffin outside a metal chamber wall, a survey reading between the layers or behind too thin a hydrogenous layer can exceed the bare-wall reading - recoil protons from the hydrogen. Make the hydrogenous layer thick enough to absorb the recoils it creates, and take the dose reading OUTSIDE the complete stack: interlayer readings are diagnostics, not the answer. - **Cited in:** [Shielding a Small Cyclotron](https://cyclotroninfo.com/learn/shielding/) **dg-1119** · applicability level 4 · shielding, detectors · https://cyclotroninfo.com/design-guide/shielding/#dg-1119 - **Rule (this site's paraphrase of the source):** Threshold-activation detectors sandwiched between absorber slabs gave attenuation half-values 'generally... with better precision than those with ionization chambers' - the quoted comparison for the carbon-disc experiments; their design virtues (threshold blindness to low-energy scatter, passive in-field operation) are the method's logic rather than the quote's claims. - **Formula:** `activation of a threshold-reaction foil vs absorber depth -> half-value thickness; Moyer used C12(n,2n)C11, threshold ~20 MeV` - **Source quote (verbatim):** “Experiments using carbon disc detectors sandwiched between slabs of absorber gave exponential attenuation with half-value determination which were generally made with better precision than those with ionization chambers.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The C12(n,2n) reaction itself is blind below ~20 MeV and useless at sub-MeV neutron energies; the transferable idea is the energy-thresholded activation foil as a passive, field-immune detector that answers one question cleanly. **dg-1120** · applicability level 3 · shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1120 - **Rule (this site's paraphrase of the source):** State the geometry with any published attenuation number: with detectors close behind slabs, the measured cross section is neither pure absorption nor pure scattering removal, and a half-value thickness from one geometry does not transfer to another. - **Source quote (verbatim):** “Because of the geometry employed, these measurements are neither a true determination of pure scattering nor pure absorption.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 5 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Handbook removal cross-sections assume corrected geometry; a home measurement's bias depends on the arrangement - a narrow-beam, small-acceptance setup excludes scatter and reads MORE attenuating than a broad shield really performs, while a detector bathed close behind a slab collects scatter and reads pessimistic. State the geometry with the number, as the rule says, and compare only like with like. **dg-1121** · applicability level 2 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1121 - **Rule (this site's paraphrase of the source):** Choose fast-neutron shielding for high density combined with LOW atomic number; the attenuation cross section per nucleon falls as Z rises (nucleons shadow each other inside a large nucleus), which is why ordinary concrete outperforms lead per unit weight against neutrons. - **Formula:** `sigma per nucleon decreases with Z (shadow effect); merit ~ density x (hydrogen + light-element fraction)` - **Source quote (verbatim):** “one should seek substances which combine high density with low atomic number. Among convenient and practical materials none would seem better than concrete.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 5 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The shadow-effect argument is a >100 MeV argument. At low energy the conclusion usually still favors hydrogenous materials - elastic scattering on hydrogen dominates moderation - but merit depends on the objective: moderation, capture, dose, or secondary-gamma control (hydrogenous shields buy moderation with 2.2 MeV capture photons, dg-1329). Concrete, water and polyethylene win per dollar for neutron MODERATION, with the gamma bill accounted separately. **dg-1122** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1122 - **Rule (this site's paraphrase of the source):** Shield for machine-generated loss points, not just the target: besides the forward cone from the probe, Moyer found 'a general spray of neutrons due to the deuteron beam grazing the interior of the dee' - his report characterizes its intensity and azimuthal extent (scan re-read queued for those figures). - **Source quote (verbatim):** “Besides the neutron beam cone from the probe there was found to be a general spray of neutrons due to the deuteron beam grazing the interior of the dee.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 2 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Wherever beam is lost - dee edges, septum, probe stalk, chamber wall - is a candidate source, and a survey plan that only looks downstream of the target can miss most of the emission solid angle. Moyer's spray was found by surveying: that is the lesson. **dg-1123** · applicability level 3 · shielding, safety, detectors · https://cyclotroninfo.com/design-guide/shielding/#dg-1123 - **Rule (this site's paraphrase of the source):** Survey slow-neutron leakage through access openings separately with a BF3 (or equivalent thermal) counter: apertures and penetrations, not the bulk shield, set the slow-neutron field outside an enclosure. - **Source quote (verbatim):** “Measurements with a BF3 proportional counter have indicated diffusion of slow neutrons through various access openings from the enclosure.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 2 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Cable ways, viewport lines-of-sight and door gaps are where slow-neutron leakage concentrates ONCE the bulk shield is adequate - penetrations dominate when the walls no longer do, which is the regime a designed enclosure should be in. Thermal-neutron instruments answer a different question than fast-neutron ones; both belong in a survey. **dg-1124** · applicability level 3 · shielding, safety, beam-measurement · https://cyclotroninfo.com/design-guide/shielding/#dg-1124 - **Rule (this site's paraphrase of the source):** Publish shield performance as a normalized dose map tied to beam current: Moyer quotes 24 r/hr at 1 ft outside the tank wall falling to 10 mr/hr outside 5.5 ft of concrete and 0.5-1.5 mr/hr in the building at large, all explicitly at 0.2 uA of deuterons — so any later reader can rescale. - **Formula:** `report dose rate AND beam current together; at fixed geometry, energy, species and loss pattern, dose rescales with current - any of those changing breaks the rescale` - **Source quote (verbatim):** “With an ionization reading of 24 r/hr in the center of the neutron beam cone 1 foot outside the tank wall (9 3/4 feet from the probe), the ionization just outside the shielding in the center of the beam is 10 mr/hr, while the general building areas are 0.5 to 1.5 mr/hr. These quoted measurements are made with Al-walled ionization chambers, and correspond to a deuteron beam of about 0.2 x 10-6 amp.” - **Source:** [Moyer, Hildebrand, Knable, Parmley & York, Character of the Radiation Field and Shielding at the 184-Inch Cyclotron — AECD-2149, UC Radiation Laboratory (1947)](https://cyclotroninfo.com/library/#lib-242) — p. 2 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A survey number without the simultaneous beam current is unusable later: log dose rate, location, instrument, and Faraday-cup current as one record so the map rescales when beam current grows - and re-survey when anything besides current changes (energy, species, tune, loss pattern), because those break the linear rescale. **dg-1329** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1329 - **Rule (this site's paraphrase of the source):** Use the site as shielding: the UW building was placed to exploit a natural ravine, and the machine sits in a 40-ft-diameter circular room with 10 ft of earth on the perimeter and 24 in of water above the ceiling — earth and water doing what concrete would otherwise cost. - **Source quote (verbatim):** “It is designed so as to take maximum advantage of naturally occurring shielding of a small ravine.” - **Source:** [The University of Washington 60-Inch Cyclotron: Progress and Status Report of Design and Construction — AECU-1951, University of Washington (1951)](https://cyclotroninfo.com/library/#lib-253) — p. 5 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The siting lesson transfers even if the scale does not: cheap mass - earth berms, water, basement corners - is legitimate shielding MATERIAL for a D-D-capable machine, once treated as engineering rather than slogan: effectiveness depends on composition, thickness, geometry and the capture gammas that moderation produces (hydrogenous media slow neutrons well, then emit 2.2 MeV capture photons), so earth and water get designed and surveyed like any shield (see the shielding deep dive). Spec detail: PDF p.128. **dg-1496** · applicability level 2 · safety, shielding, controls · https://cyclotroninfo.com/design-guide/shielding/#dg-1496 - **Rule (this site's paraphrase of the source):** Personnel protection as built: the accelerator sits in a concrete brick room with an interlock control system preventing the machine from being turned on while a person is in the room. - **Source quote (verbatim):** “in a concrete brick room with an interlock control system to prevent the accelerator from being turned on when a person is in the room” - **Source:** [Cressman, The Design and Construction of a Small Cyclotron — Houghton College thesis (2006)](https://cyclotroninfo.com/library/#lib-256) — p. 29 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The documented access-control arrangement of the source machine - occupancy interlock plus (per its electronics chapter) remote operation - is a COMPONENT of protection, not a certified minimum: shielding calculations, surveys, monitors, fail-safe interlock design and applicable regulatory requirements decide sufficiency for any neutron-capable machine, and the thesis presents no dose analysis. **dg-1777** · applicability level 4 · detectors, shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1777 - **Rule (this site's paraphrase of the source):** (draft report) The Rutgers/UMD neutron detector for the diffusion measurement was a roughly two-foot-long 3He tube nested within a stack of pure polyethylene blocks, with the two sides and back stacked with neutron absorbing borated polyethylene blocks to set the boundary condition. - **Source quote (verbatim):** “The neutron detector consisted of a ~2-foot-long 3He tube nested within a stack of pure polyethylene blocks. The two sides and back were stacked with neutron absorbing borated poly blocks to set boundary condition.” - **Source:** [Koeth, Gilde & Moroch, Measurement of Neutron Diffusion Time from Fast Pulsed Systems (draft, 2020)](https://cyclotroninfo.com/library/#lib-279) — p. 3 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A simple, buildable moderator/detector assembly: plain polyethylene where you want thermalization, borated polyethylene where you want the diffusion problem bounded. Reported as this source's construction. Draft report. **dg-1780** · applicability level 3 · detectors, shielding, beam-measurement · https://cyclotroninfo.com/design-guide/shielding/#dg-1780 - **Rule (this site's paraphrase of the source):** (draft report) Binning cyclotron-pulse-to-neutron-detection intervals over a 1 hour acquisition, or 720,000 cyclotron pulses, gave the Rutgers/UMD group an exponential fit with a measured neutron diffusion time of approximately 94 microseconds (Fig. 6 states "Fit tau: 93.5752 microseconds", data of Dec 28, 2019). The measured path was target, through the chamber wall, through approximately 8 inches of air, then diffusing through the polyethylene before entering the 3He — a process the authors presume is dominated by the time spent in the polyethylene and which is long compared to the 10 microsecond RF pulse. - **Formula:** `fitted exponential diffusion time tau ≈ 94 us (Fig. 6 fit value 93.5752 us)` - **Source quote (verbatim):** “The neutron propagation from the target, through the chamber wall, through approximately 8 inches of air, and then finally diffusing through the polyethylene before entering the 3He is the measured quantity. That process, presumably dominated by the duration spent in the polyethylene is long compared to the 10us RF pulse (the time window in which a neutron could be produced). The multichannel analyzer's binning created a histogram of cyclotron pulse-neutron detection time intervals over a 1-hour period of acquisition, or 720,000 cyclotron pulses. Figure 6 shows a fit to the data, yielding a measured diffusion time of approximately 94us.” - **Source:** [Koeth, Gilde & Moroch, Measurement of Neutron Diffusion Time from Fast Pulsed Systems (draft, 2020)](https://cyclotroninfo.com/library/#lib-279) — p. 4 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The headline number — but it is an effective decay constant of the complete target-to-detector timing distribution in this one assembly (chamber wall, ~8 inches of air, then the polyethylene), which the authors presume is polyethylene-dominated. What transfers is the strategy: delayed counting can separate neutron events from the RF transient — with the usable quiet window measured on each machine, not assumed from the 94 µs. 720,000 pulses in one hour is consistent with the 200 pps quoted earlier in the draft. Fit value read from the rendered Fig. 6 image (p.5). Draft report. **dg-1926** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1926 - **Rule (this site's paraphrase of the source):** At 1 ft from the anode, the BRH-tested cold-cathode tubes gave 6.7-15 R/h (tubes #1 and #3 on a 3-inch coil), 68-75.5 R/h (tube #2 on the 3-inch coil) and 42-217 R/h on a 6-inch coil; tube #2 on the 6-inch coil (about 73 kVp and 0.20 mA per the report's approximate Table 2 operating conditions) read 178-217 R/h across radials A-C. - **Formula:** `Tube #2, 6-inch coil: 208 / 217 / 178 R/h on radials A/B/C at 1 ft; derived: ~1e3 R/h per mA at 1 ft (~1e2 R/h per mA at 1 m) at ~73 kVp` - **Source quote (verbatim):** “Table 1 is a listing of exposure levels measured from three tubes operated from different high voltage sources. Readings are shown in the region of maximum exposure.” - **Source:** [Properzio, Woodcock & Heidersdorf, X-Ray Emission from Cold-Cathode Gas Discharge Tubes Used for Educational Purposes — BRH/DEP 70-1, Bureau of Radiological Health, US Public Health Service (1970)](https://cyclotroninfo.com/library/#lib-289) — p. 20 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Normalising tube #2 (derived arithmetic, not a source figure): about 210 R/h / 0.20 mA is roughly 1,000 R/h per mA at 1 ft, or roughly 100 R/h per mA at 1 m by inverse square, at ~73 kVp with only glass-wall filtration. That is an operating-point-specific comparison figure for a struck surface at a comparable voltage, not an upper bound: output need not scale with average current across waveforms, and other target materials and geometries can exceed it. A metal chamber wall attenuates more than thin glass, but by how much is a calculation or measurement for the actual wall thickness, material, spectrum and penetrations. **dg-1927** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1927 - **Rule (this site's paraphrase of the source):** Along the main radials of BRH tube #2 (6-inch coil), exposure rate fell from 240-281 R/h at 1 ft to 55-65 R/h at 2 ft, 12.6-15.8 R/h at 4 ft, 2.4-3.5 R/h at 8 ft and 0.53-0.77 R/h at 16 ft; the authors judged the falloff in general agreement with the inverse-square law and noted day-to-day output variation. - **Formula:** `Table 3, radial A: 240, 56.1, 12.6, 2.89, 0.530 R/h at 1, 2, 4, 8, 16 ft; derived 1->16 ft ratio ~360-450 (radials A-C) vs 256 for 1/r^2` - **Source quote (verbatim):** “A study using tube #2 and the large (6-inch) induction coil was made to determine exposure levels at various distances from the tube along the radials diagrammed in figure 7.” - **Source:** [Properzio, Woodcock & Heidersdorf, X-Ray Emission from Cold-Cathode Gas Discharge Tubes Used for Educational Purposes — BRH/DEP 70-1, Bureau of Radiological Health, US Public Health Service (1970)](https://cyclotroninfo.com/library/#lib-289) — p. 25 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Distance is a strong and cheap lever at these energies: going from 1 ft to 16 ft cut the rate by a factor of about 360-450 here (derived from the table), somewhat more than the 256 that inverse square alone predicts; the authors call the falloff in general agreement with inverse square, and the excess may reflect air attenuation of soft photons or day-to-day output scatter (an interpretation, not the source's). A layout that keeps operators several metres from the electrodes gains a large factor before any shielding is added. **dg-1928** · applicability level 4 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1928 - **Rule (this site's paraphrase of the source):** In the BRH cold-cathode tube tests, the major output went into the hemisphere facing the target anode; the authors conclude that the hemisphere behind the anode is attenuated by a factor of up to several hundred by the anode and its stem (in Table 3 the rear radials D and E read 2.84 and 5.45 R/h at 1 ft against 240-281 R/h in front). - **Formula:** `Table 3 at 1 ft: D 2.84, E 5.45 vs A-C 240-281 R/h -> rear/front ~1/45 to 1/100 (derived); authors' conclusion: up to several hundred` - **Source quote (verbatim):** “The output in the hemisphere behind the target anode is attenuated by a factor of up to several hundred by the anode and anode stem.” - **Source:** [Properzio, Woodcock & Heidersdorf, X-Ray Emission from Cold-Cathode Gas Discharge Tubes Used for Educational Purposes — BRH/DEP 70-1, Bureau of Radiological Health, US Public Health Service (1970)](https://cyclotroninfo.com/library/#lib-289) — p. 37 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** In these tubes the struck electrode itself acted as a shield, making the emission strongly directional toward the side the electrons arrived from. The surfaces stray electrons hit in a tabletop machine (dee faces, liner, chamber wall) can likewise make emission highly directional, but the BRH pattern is not transferable to a chamber's geometry: survey all around the chamber and every penetration rather than assuming a hot direction. **dg-1937** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1937 - **Rule (this site's paraphrase of the source):** Fewell and Shuping's measured output of a tungsten-anode mammography tube (Dynamax 69M, inherent filtration ~0.64 mm Al), read from their Fig. 6 at 1 m with no added filter, is roughly 1.6 mR/mAs at 30 kVp, about 3.3 at 40, about 5.7 at 50 and about 8 at 60 kVp (the curve ends near 9 at about 67 kVp); with 3.05 mm Al added it reaches about 4.4 mR/mAs at 80 kVp. - **Formula:** `W anode, 0.0 mm Al added, 1 m: ~1.6 (30), ~3.3 (40), ~5.7 (50), ~8 (60 kVp) mR/mAs (graph readings, +/-0.3); derived: 1 mR/mAs at 1 m = 3.6 R/h per mA` - **Source quote (verbatim):** “The data are presented graphically by plotting mR/mAs as a function of kVp for various values of x-ray beam filtration.” - **Source:** [Fewell & Shuping, Handbook of Mammographic X-Ray Spectra — HEW Publication (FDA) 79-8071, Bureau of Radiological Health (1978)](https://cyclotroninfo.com/library/#lib-290) — p. 12 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Converting units (derived), 1 mR/mAs at 1 m is 3.6 R/h per mA, so this lightly filtered tungsten target at 50 kV gives about 20 R/h per mA at 1 m — a benchmark for that tube (tungsten, ~0.64 mm Al inherent filtration, its target angle and geometry), not a bound: a less filtered line of sight or a different angular distribution can give more. For an accelerator surface struck by 30–60 keV electrons, use a conservative spectrum and transport calculation (target material, angle, filtration, distance, electron current) and verify by survey; the thick-target Z-proportional yield rule is an efficiency heuristic, not a dose bound. The curves start near 20–25 kVp. **dg-1942** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1942 - **Rule (this site's paraphrase of the source):** The 1964 PHS population-exposure survey (reprinted in the 1970 Radiological Health Handbook) gives mean output at 12 inches of 78.6 mR/mAs at 50 kVp with 0.5 mm Al total filtration, falling to 19.0 at 2.5 mm Al and 9.3 at 4.5 mm Al for nondental units; at 100 kVp the same filtrations give 208, 83.6 and 53.2 mR/mAs. Dental units at 50 kVp gave 91.1 mR/mAs at 0.5 mm Al. - **Formula:** `nondental, 12 in: 50 kVp 78.58 (0.5 mm Al) / 18.99 (2.5) / 9.29 (4.5); 100 kVp 208.36 / 83.55 / 53.15 mR/mAs; dental 50 kVp 91.11 (0.5 mm Al)` - **Source quote (verbatim):** “Mean milliroentgens per milliampere-second at 12 inches by kilovolt peak and filtration categories for nondental X-ray units” - **Source:** [Bureau of Radiological Health, Radiological Health Handbook, Revised Edition — US Public Health Service (January 1970)](https://cyclotroninfo.com/library/#lib-291) — p. 170 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Moved to 1 m by inverse square (factor 0.093, derived), the 50 kVp / 2.5 mm Al entry is about 1.8 mR/mAs, which agrees with the NBS Handbook 76 Table 7 value at the same filtration. For this 50 kVp survey category, increasing aluminium filtration from 0.5 to 4.5 mm reduced the reported mean output by about a factor of 8; an accelerator wall must be evaluated with its own material and spectrum. **dg-1943** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1943 - **Rule (this site's paraphrase of the source):** J. R. Cameron's curves in the 1970 Radiological Health Handbook give output at 40 inches falling steeply with total filtration: at 40 kVp from about 7-8 mR/mAs near 0.6 mm Al to about 0.35 at 4.5 mm Al, and at 150 kVp from about 60 to about 13 mR/mAs; at 2.5 mm Al and 40 inches the companion chart reads about 1 (40 kVp), 2 (50 kVp) and 19 mR/mAs (150 kVp). - **Formula:** `40 in: 40 kVp ~7.5 (0.6 mm Al) -> ~0.35 (4.5 mm Al); 150 kVp ~60 -> ~13 mR/mAs; at 2.5 mm Al (PDF p. 171): ~1 / ~2 / ~19 mR/mAs at 40 / 50 / 150 kVp (graph readings)` - **Source quote (verbatim):** “mR/mAs vs. Filtration for various kVp x rays” - **Source:** [Bureau of Radiological Health, Radiological Health Handbook, Revised Edition — US Public Health Service (January 1970)](https://cyclotroninfo.com/library/#lib-291) — p. 172 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Soft beams are filtered far more than hard ones in these tube spectra: by the graph, 4 mm of aluminium removes about 95 % at 40 kVp and about 80 % at 150 kVp (readings for the stated filtration interval). For a chamber, compute transmission from the actual spectral distribution and wall material, keeping the endpoint and any hard component; specify shielding by the attenuation required, in whatever material provides it, rather than assuming a few millimetres of wall suffices or that lead is categorically needed. **dg-1944** · applicability level 2 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1944 - **Rule (this site's paraphrase of the source):** For radiation-field limiters, NCRP Report 33 (reprinted in the 1970 Radiological Health Handbook) lists the lead needed to cut a useful beam to 5 percent as 0.10 mm at 60 kVp (HVL 1.2 mm Al) and 0.16-0.35 mm at 100 kVp (HVL 1.0-3.0 mm Al), rising to 1.0 mm at 200 kVp; the figures are approximate broad-beam values and apply also to the lead equivalent of lead rubber and lead glass. - **Formula:** `5% transmission: 60 kVp 0.10 mm Pb; 100 kVp 0.16 / 0.25 / 0.35 mm Pb for HVL 1/2/3 mm Al; 140 kVp 0.7; 200 kVp 1.0 mm Pb` - **Source quote (verbatim):** “The third column refers to lead or to the required equivalent lead thickness of lead-containing materials (e.g. lead rubber, lead glass, etc.).” - **Source:** [Bureau of Radiological Health, Radiological Health Handbook, Revised Edition — US Public Health Service (January 1970)](https://cyclotroninfo.com/library/#lib-291) — p. 168 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Twenty-fold attenuation at 60-100 kV takes only tenths of a millimetre of lead, which is why thin lead sheet, lead tape or lead-glass viewports are practical for a tabletop chamber. When specifying a lead-glass viewport, ask for its lead-equivalent thickness at the operating kV rather than its glass thickness. **dg-1945** · applicability level 2 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1945 - **Rule (this site's paraphrase of the source):** Broad-beam half-value layers at high filtration (Table 12 of the 1970 Radiological Health Handbook, identical to NBS Handbook 76 Table 12) are 0.05 mm lead / 0.51 cm concrete at 50 kVp, 0.18 mm / 1.27 cm at 70 kVp, 0.24 mm / 1.8 cm at 100 kVp and 0.3 mm / 2.3 cm at 150 kVp; one tenth-value layer equals 3.33 HVL. - **Formula:** `HVL Pb (mm) 0.05 / 0.18 / 0.24 / 0.27 / 0.3 at 50 / 70 / 100 / 125 / 150 kVp; concrete (cm) 0.51 / 1.27 / 1.8 / 2.0 / 2.3; TVL = 3.33 HVL` - **Source quote (verbatim):** “One tenth-value layer is equivalent to 3.33 half-value layers.” - **Source:** [Bureau of Radiological Health, Radiological Health Handbook, Revised Edition — US Public Health Service (January 1970)](https://cyclotroninfo.com/library/#lib-291) — p. 167 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Combined with an output estimate, these give a first barrier estimate under a constant-HVL assumption: n = log2(required reduction) HVLs — a factor of 1,000 is about 10 HVL, i.e. about 0.5 mm of lead at 50 kV or 1.8 mm at 70 kV (derived). Polyenergetic beams harden as shielding is added, so successive HVLs and TVLs grow and broad-beam buildup adds; for a real barrier use transmission curves or first/equilibrium TVLs for the kVp, filtration and geometry, include penetrations, and verify the installed barrier by survey. The 1975 NBS Handbook 114 table gives 0.15 mm at 70 kV (dg-1956). **dg-1946** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1946 - **Rule (this site's paraphrase of the source):** NBS Handbook 76 bases barrier design on the finding that, at a given kilovoltage and with minimum added filtration, any X-ray machine's exposure rate is nearly constant in roentgens per milliampere-minute at 1 m, and that barrier thickness is essentially independent of the machine's added filtration because that filtration is small next to the barrier. - **Formula:** `dose rate(r) ~ K0(kVp) x I(mA) / r(m)^2, K0 in R/mA-min at 1 m` - **Source quote (verbatim):** “The filtration added to the useful beam in an X-ray machine is always small in comparison with the attenuation afforded by the barrier, and hence the barrier thickness required at a given kilovoltage is essentially independent of any changes in half-value-layer caused by added filtration in the machine. Thus, it is sufficient, for the purposes of protection calculations, to establish transmission curves specified in kilovolts under conditions of minimum added filtration. It has also been found that at any given kilovoltage and with minimum added filtration the exposure dose rate produced by any X-ray machine is nearly a constant when expressed in terms of roentgens per milliampere-minute at a distance of 1 m.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 52 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** This normalisation suggests a screening estimate for an accelerator electrode (an interpretation, not the handbook's): treat it as a tube at the greatest potential difference an electron can traverse (the dee-to-dee difference for push-pull dees), with the stray electron current as tube current and a conservative high-Z thick-target output, read R/mA-min at 1 m and scale by current and 1/r² from the nearest emitting region — then validate the source term and the finished shielding by measurement or a validated transport calculation. The handbook's curves start at 50 kVp, so lower electrode voltages are an extrapolation. **dg-1947** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1947 - **Rule (this site's paraphrase of the source):** NBS Handbook 76 tabulates design X-ray output K0 at 1 m as 0.05, 0.1, 0.4, 2, 20 and 280 R/mA-min at 50, 70, 100, 250, 1,000 and 2,000 kV, with unshielded distances to reach 10 mR/week in the environs; for a weekly WUT of 2 mA-min the distances are 11, 15 and 30 ft at 50, 70 and 100 kV. - **Formula:** `K0 (R/mA-min at 1 m): 0.05 (50 kV), 0.1 (70), 0.4 (100), 2 (250), 20 (1,000), 280 (2,000); WUT = 2 -> 11 / 15 / 30 ft at 50 / 70 / 100 kV (environs)` - **Source quote (verbatim):** “For design purposes only, the maximum permissible exposure is taken to be 10 mr/wk.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 34 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The K0 row is the output the handbook's distance tables assume (filtration not stated in the table): 0.05 R/mA-min at 50 kV is about 0.8 mR/mAs at 1 m (derived), below the same handbook's measured diagnostic averages at 2.5 mm Al (dg-1948: about 1.6–1.7 mR/mAs at 1 m by inverse square) and well below Fewell and Shuping's unfiltered tungsten reading (dg-1937: about 5.7). These are tube-specific calibration data, not bounds for an accelerator electrode; a conservative source term comes from worst-case measurement or a validated model. The 10 mR/week basis is the handbook's 1961 design value. **dg-1948** · applicability level 2 · safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1948 - **Rule (this site's paraphrase of the source):** NBS Handbook 76 gives average diagnostic output, measured in air at 2.5 mm Al total filtration, of 1.8, 2.8, 4.2, 5.8, 8.0, 9.8 and 15.2 R per 100 mA-s at 12 inches for 50, 60, 70, 80, 90, 100 and 125 kVp, falling to 0.2-1.7 at 36 inches; its dental table gives 12 R/mA-min at 50 kVp, 4 inches, 1.5 mm Al. - **Formula:** `R/100 mA-s at 12 in: 1.8 (50), 2.8 (60), 4.2 (70), 5.8 (80), 8.0 (90), 9.8 (100), 15.2 (125 kVp); 2.5 mm Al` - **Source quote (verbatim):** “Measured in air with total filtration equivalent to 2.5 mm Al.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 36 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** In this tabulated tube-and-filtration configuration, output roughly doubles from 50 to 70 kVp and doubles again by about 100 kVp at fixed current — calibration context for how strongly voltage drives output. A cyclotron electrode behind an aluminium-equivalent wall needs its own target-and-filtration model or measurement before these numbers can be said to represent it. **dg-1949** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1949 - **Rule (this site's paraphrase of the source):** Size a primary barrier by NBS Handbook 76's equation K = P d^2 / (W U T), with P the weekly permissible exposure, d in metres, W the workload in mA-min/week, U the use factor and T the occupancy factor, then read the barrier thickness where the kV transmission curve reaches K; the handbook's worked example (100 kVp, 10 m, W = 1,000, U = 1/4, T = 1, P = 0.1 R/week) gives K = 0.04 and 1 1/2 in of concrete or 0.4 mm of lead. - **Formula:** `K = P d^2/(W U T); example: 0.1 x 10^2/(1,000 x 1/4 x 1) = 0.04 R/mA-min at 1 m (arithmetic checked) -> 1 1/2 in concrete or 0.4 mm Pb (100 kVp)` - **Source quote (verbatim):** “The smaller the product of WUT and the greater the distance, the larger the permitted value of K.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 58 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** For a tabletop accelerator, W is stray-electron current integrated over weekly operating minutes (mA-min), U is about 1 for an electrode radiating in all directions (an explicitly conservative choice), and T follows the room layout. P in this method is exposure in R/week, and the handbook's values (0.1 R/week controlled, 0.01 R/week environs) are 1961 design values: a present-day design constraint is expressed in dose quantities and must be converted to a quantity compatible with the transmission curves, with an ALARA design goal below any legal limit rather than the limit itself. **dg-1950** · applicability level 3 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1950 - **Rule (this site's paraphrase of the source):** NBS Handbook 76 estimates the concrete equivalent of other wall materials by density alone (concrete inches = density x thickness in inches / 2.35), noting that for materials of higher atomic number than concrete this underestimates the equivalent and so errs toward extra protection. - **Formula:** `t_concrete_eq(in) = rho(g/cc) x t(in) / 2.35` - **Source quote (verbatim):** “Concrete equivalent in inches is equal to the density of the material in question multiplied by the thickness of the material in inches and divided by 2.35.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 59 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The same density scaling credits existing structure toward the barrier — in the handbook's example 1 in of sand plaster (1.54 g/cc) counts as 0.65 in of concrete — for verified homogeneous construction in the low-kV range the method was written for; joints, voids and penetrations are not credited. A steel chamber wall is higher-Z than concrete, so density scaling understates it at low kV, while at higher energies energy-dependent transmission data are needed. Confirm the completed barrier by survey. **dg-1951** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1951 - **Rule (this site's paraphrase of the source):** For secondary barriers NBS Handbook 76 counts leakage-radiation HVLs separately from scatter, taking 90-degree scatter as about 0.1 percent of the incident exposure (so K may be 1,000 times larger) with the useful-beam curve below 500 kV; if the two thicknesses are about equal add 1 HVL to the larger, and if they differ by at least 3 HVL the thicker alone suffices. - **Formula:** `scatter at 90 deg ~0.1% -> K_scatter = 1,000 P d^2/(W T) (<=500 kV, useful-beam curve); equal thicknesses: +1 HVL; >=3 HVL apart: thicker alone` - **Source quote (verbatim):** “If the barrier thicknesses for leakage and scattered radiations are found to be approximately the same, 1 hvl should be added to the larger one to obtain the required total secondary barrier thickness.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 61 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Stray-electron X-rays from a cyclotron electrode are closest to 'leakage' in this vocabulary (an analogy, not the handbook's), with scatter from the chamber, magnet and bench a second component. The handbook's add-one-HVL shortcut is for comparable paths; a strict design sums the separately attenuated contributions against the constraint — even three HVLs apart the smaller path still adds about 12 %, i.e. about 0.17 HVL more — and the 0.1 % scatter figure carries the handbook's field-size, distance and material conditions, so a bench installation uses geometry-specific scatter data. **dg-1952** · applicability level 2 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1952 - **Rule (this site's paraphrase of the source):** NBS Handbook 76's Fig. 3 lead transmission curves (50-200 kVp, pulsed waveform, 0.5 mm Al at 50-125 kVp) fall from about 1 R/mA-min at 1 m to 1e-5 R/mA-min in about 0.5 mm of lead at 50 kVp, about 1.6 mm at 70 kVp and about 3 mm at 100 kVp (graph readings); the handbook notes that the 50 and 70 kVp curves were obtained by interpolation and extrapolation of Braestrup's 1944 data. - **Formula:** `lead for K = 1e-5 R/mA-min at 1 m: ~0.5 mm (50), ~1.6 mm (70), ~3 mm (100), ~3.9 mm (150), ~6.2 mm (200 kVp) (graph readings)` - **Source quote (verbatim):** “The curves at 50 and 70 kvp were obtained by interpolation and extrapolation of available data (Braestrup, 1944).” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 53 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The low-kV curves are the ones a tabletop designer reads, and the least directly measured part of the figure (interpolated and extrapolated from 1944 data). Prefer modern transmission data or a validated calculation for the actual spectrum and geometry; any engineering margin added is an independent assumption, stated as such, and the installed barrier is verified by worst-case survey. **dg-1953** · applicability level 4 · shielding, rf · https://cyclotroninfo.com/design-guide/shielding/#dg-1953 - **Rule (this site's paraphrase of the source):** In footnotes to its shielding tables for busy 250 kVp and 300 kVp therapeutic installations (Tables 7 and 8), NBS Handbook 76 states that constant potentials may require 15 to 25 percent more lead and 5 to 15 percent more concrete than the tabulated thicknesses, which are computed for pulsating potentials; the handbook gives no such factor for its lower-kV tables. - **Formula:** `constant potential: +15-25% Pb, +5-15% concrete vs pulsating (footnote to the 250 and 300 kVp tables only)` - **Source quote (verbatim):** “Constant potentials may require 15 to 25 percent larger thicknesses of lead and 5 to 15 percent larger thicknesses of concrete than those given here for pulsating potentials.” - **Source:** [NCRP Report No. 26, Medical X-Ray Protection Up to Three Million Volts — NBS Handbook 76, National Bureau of Standards (1961)](https://cyclotroninfo.com/library/#lib-292) — p. 47 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The factor is the handbook's for 250-300 kVp and is not stated for lower voltages; using it at tens of kV is an extrapolation. A d-c electrode or a well-filtered supply produces more photons near the endpoint than a pulsating one at the same peak voltage, so an electrode held at steady d-c (extractor, deflector, rectifier stack) is the constant-potential case; an RF dee, near peak for only part of each cycle, plausibly sits closer to the pulsating case (an interpretation, not the source's). **dg-1954** · applicability level 2 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1954 - **Rule (this site's paraphrase of the source):** Under the 1975 non-medical X-ray standard (ANSI N543, NBS Handbook 114), a Protective Installation, which needs no occupancy restriction outside, must hold exposure at any accessible point 2 in (5 cm) from the enclosure surface to 0.5 mR in any hour; the standard notes this low level demands more shielding, usually a small cost for low-energy sources and small enclosures. - **Formula:** `Protective: <= 0.5 mR/h at 5 cm from enclosure surface` - **Source quote (verbatim):** “The exposure at any accessible region 2 in (5 cm) from the outside surface of the enclosure cannot exceed 0.5 mR in any 1 hour.” - **Source:** [ANSI N543-1974, General Safety Standard for Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies up to 10 MeV — NBS Handbook 114, National Bureau of Standards (1975)](https://cyclotroninfo.com/library/#lib-293) — p. 18 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** A cabinet-style shield around a tabletop chamber is the 'small enclosure, low energy' case the standard describes; its 0.5 mR in any hour at 5 cm is a historical leakage benchmark, and the remark about cost is the handbook's own. Whether occupancy or access controls can be relaxed depends on the complete installation meeting the regulations in force (enclosure construction, interlocks, operating conditions, current dose limits), not on this one figure. **dg-1955** · applicability level 3 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1955 - **Rule (this site's paraphrase of the source):** For an Enclosed Installation, NBS Handbook 114 (1975) allows 10 mR/h at 1 ft from the enclosure in occupied areas and 100 mR/h in normally unoccupied areas, to be met for any tube at any rated mA and kV, with no removable filter in place and the source oriented for the highest rate; it puts the shielding saving over Protective class at as much as 4.3 HVL. - **Formula:** `Enclosed: <= 10 mR/h (occupied) / 100 mR/h (unoccupied) at 30 cm; Protective vs Enclosed: up to 4.3 HVL more` - **Source quote (verbatim):** “For x-ray installations, this exposure limitation shall be met for any x-ray tube to be used in the enclosures and operating at any specified mA and kV rating within the manufacturer's published recommendations.” - **Source:** [ANSI N543-1974, General Safety Standard for Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies up to 10 MeV — NBS Handbook 114, National Bureau of Standards (1975)](https://cyclotroninfo.com/library/#lib-293) — p. 19 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The test condition is the transferable part: survey at the highest electrode voltage and stray current the machine can reach, without temporary shielding, and in the worst orientation. For an accelerator that means the maximum RF drive and the worst conditioning state, not the typical operating point. **dg-1956** · applicability level 2 · shielding, materials · https://cyclotroninfo.com/design-guide/shielding/#dg-1956 - **Rule (this site's paraphrase of the source):** NBS Handbook 114 (after NCRP Report 34) gives broad-beam HVL / TVL at high attenuation of 0.05 / 0.16 mm lead at 50 kV, 0.15 / 0.5 mm at 70 kV and 0.24 / 0.8 mm at 100 kV (concrete 0.17 / 0.6, 0.33 / 1.1 and 0.6 / 2.0 in), and warns that at low attenuation the values are significantly less. - **Formula:** `Pb HVL/TVL (mm): 0.05/0.16 (50 kV), 0.15/0.5 (70), 0.24/0.8 (100), 0.29/0.95 (150), 0.48/1.6 (200); concrete (in): 0.17/0.6, 0.33/1.1, 0.6/2.0, 0.88/2.9, 1.0/3.3` - **Source quote (verbatim):** “Approximate values obtained at high attenuation for the indicated peak voltage values under broad-beam conditions; with low attenuation these values will be significantly less.” - **Source:** [ANSI N543-1974, General Safety Standard for Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies up to 10 MeV — NBS Handbook 114, National Bureau of Standards (1975)](https://cyclotroninfo.com/library/#lib-293) — p. 52 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Use TVLs when a large reduction is needed: three TVLs give a nominal factor of 1,000 under the table's stated broad-beam, high-attenuation conditions (about 0.5 mm of lead at 50 kV, 2.4 mm at 100 kV, derived). These NCRP 34 values differ from the 1961 NBS Handbook 76 table at low kV (lead HVL 0.15 versus 0.18 mm at 70 kV; concrete 0.33 versus 0.5 in), and the two tables state different beam conditions ('high attenuation' versus 'high filtration'), so they are not directly comparable; taking the larger is the conservative choice, and a fabricated shield with seams and penetrations needs calculation or measurement for its actual spectrum and geometry. **dg-1957** · applicability level 2 · shielding, fabrication · https://cyclotroninfo.com/design-guide/shielding/#dg-1957 - **Rule (this site's paraphrase of the source):** NBS Handbook 114 defines a protective tube housing as one whose leakage at 1 m from the target does not exceed 1 R in an hour at maximum rating, averaged over 100 cm^2 (NBS Handbook 76 set the diagnostic-type housing at 100 mR in an hour at 1 m), and gives unshielded controlled-area distances for 100 mR/week: at WUT = 2 mA-min/week, 3, 5 and 9 ft at 50, 70 and 100 kV. - **Formula:** `housing leakage: protective <= 1 R/h at 1 m (avg 100 cm^2); diagnostic-type (NBS HB 76) <= 100 mR/h at 1 m` - **Source quote (verbatim):** “radiation at a distance of one meter from the target does not exceed one roentgen in one hour when the tube is operated at its maximum rating.” - **Source:** [ANSI N543-1974, General Safety Standard for Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies up to 10 MeV — NBS Handbook 114, National Bureau of Standards (1975)](https://cyclotroninfo.com/library/#lib-293) — p. 51 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** These are historical component-housing limits — a tube housing presumes separate installation shielding and controls — not accessible-surface targets for a chamber: exposure near 1 R/h would exceed modern area constraints quickly. Set accessible-area targets from the jurisdiction's current dose and occupancy limits, calculate the full source and shield geometry (hardening, buildup, distributed sources, penetrations, streaming — not a simple subtraction of wall HVLs), and survey with enough spatial resolution to find narrow hot spots; the 100 cm² averaging provision is a housing-qualification convention, not licence to average away a leak. **dg-1958** · applicability level 4 · shielding, safety · https://cyclotroninfo.com/design-guide/shielding/#dg-1958 - **Rule (this site's paraphrase of the source):** NBS Handbook 111 (ANSI N43.2-1971) accepts a primary-beam trap located 5 cm from the focal spot of a diffraction or fluorescence tube if it is equivalent to at least 1.5-1.7 mm of lead at 50 kVp, 5.6-5.9 mm at 70 kVp or 7.7-7.9 mm at 100 kVp, for anode currents of 20-160 mA; the target is 0.25 mrem/h. - **Formula:** `Table 2, lead at 5 cm: 20 mA 1.5 / 5.6 / 7.7 mm; 40 mA 1.6 / 5.8 / 7.9; 80 mA 1.6 / 5.9; 160 mA 1.7 mm (50 / 70 / 100 kVp)` - **Source quote (verbatim):** “inherent shielding of the trap or barrier is at least equivalent to the thickness of lead specified in table 2 for the maximum rated anode current and potential.” - **Source:** [ANSI N43.2-1971, Radiation Safety for X-Ray Diffraction and Fluorescence Analysis Equipment — NBS Handbook 111, National Bureau of Standards (1972)](https://cyclotroninfo.com/library/#lib-294) — p. 14 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The table shows kV, not current, dominating the lead requirement for these analytical tubes: an eightfold rise in current adds about 0.2 mm, while 50 to 70 kV adds about 4 mm — calibration context for a small, intense source, not an upper bound for an accelerator, whose loss targets, voltage waveforms, duty factors and source distributions differ. Derive shielding from the machine's credible electron energy, current and duty factor, loss target and geometry, then survey worst case. **dg-1959** · applicability level 2 · shielding, chamber · https://cyclotroninfo.com/design-guide/shielding/#dg-1959 - **Rule (this site's paraphrase of the source):** NBS Handbook 111 requires the chamber walls of an enclosed-beam X-ray system to limit the dose rate 5 cm from their outer surface to 0.25 mrem/h in normal operation, and limits unwanted radiation from components such as high-voltage rectifiers to 10 mrem per week at 5 cm from the generator cabinet (an average of 0.25 mrem/h only if exposure is uniform over a 40-hour week). - **Formula:** `enclosed-beam chamber: <= 0.25 mrem/h at 5 cm; HV-rectifier radiation: <= 10 mrem/week at 5 cm (= 0.25 mrem/h x 40 h)` - **Source quote (verbatim):** “inherent shielding of the chamber walls shall be sufficient to limit the dose rate in all regions 5 cm from its outer surface to 0.25 mrem/hr during normal operation.” - **Source:** [ANSI N43.2-1971, Radiation Safety for X-Ray Diffraction and Fluorescence Analysis Equipment — NBS Handbook 111, National Bureau of Standards (1972)](https://cyclotroninfo.com/library/#lib-294) — p. 15 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** The 0.25 mrem/h at 5 cm figure is a well-defined surface target for an evacuated chamber that encloses its own X-ray sources, checkable with a survey meter whose reference point is held 5 cm from the wall. It is the 1971 standard's figure for analytical equipment, not a requirement written for accelerators. **dg-1963** · applicability level 3 · fabrication, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1963 - **Rule (this site's paraphrase of the source):** NBS Handbook 111 advises inspecting lead shielding parts after reassembly for damage or distortion that could open a leak, and mounting them so they cannot cold-flow under their own weight; it reports that most severe injuries occur during non-routine operations such as repair and alignment. - **Source quote (verbatim):** “After reassembly the x-ray system shall be checked by the radiation protection officer or other person in charge. Particular attention should be given to the alignment of shielding, shutters and collimators. Lead parts should be inspected for damage or distortion which could result in radiation leakage and should be mounted in such a way that they will not cold-flow because of their own weight. […] Most severe injuries occur during non-routine operations such as repair and alignment [9].” - **Source:** [ANSI N43.2-1971, Radiation Safety for X-Ray Diffraction and Fluorescence Analysis Equipment — NBS Handbook 111, National Bureau of Standards (1972)](https://cyclotroninfo.com/library/#lib-294) — p. 16 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Lead sheet wrapped on a chamber or hung as a curtain can creep or distort under sustained load, opening seams at overlaps and fasteners. Support lead on a rigid backing, overlap joints, and re-survey after any maintenance that disturbs the shielding. **dg-1980** · applicability level 3 · physics-theory, modeling, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1980 - **Rule (this site's paraphrase of the source):** Berger and Seltzer's electron tables run from 10 keV upward and are least certain at the low end: when the electron velocity falls to about the atomic-electron velocity their stopping-power formula ceases to be valid, so the range integral is closed by linear interpolation to zero below 1 keV; their 10 keV ranges exceed Nelms' (5 keV cut) by 12% for Pb, 6% Ag, 2% Al and 1% H2, with differences of at most 0.5% at 50 keV. - **Formula:** `range r = integral_0^T [ (1/rho) dE/dx ]_tot^-1 dT (c.s.d.a.)` - **Source quote (verbatim):** “The ranges of Nelms are on the low side, ours on the high side, and the discrepancies give an indication of the error likely to result from lack of knowledge of low-energy stopping behavior.” - **Source:** [Berger & Seltzer, Tables of Energy Losses and Ranges of Electrons and Positrons — NASA SP-3012 (1964)](https://cyclotroninfo.com/library/#lib-298) — p. 19 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Use these tables for the keV end of a low-energy electron beamline (ranges, stopping, bremsstrahlung yield), not for the tens-to-hundreds-of-eV regime they do not cover. The 10 keV range discrepancy the authors quantify is for H2, Al, Ag and Pb (1–12 %); for neon, argon and air it is unquantified here — compare against modern ESTAR-class data before assigning a percentage. **dg-1983** · applicability level 4 · physics-theory, beam-dynamics, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1983 - **Rule (this site's paraphrase of the source):** SP-3012 Table I, argon (page image read): at 10 keV the collision stopping power is 14.57 MeV cm^2/g, c.s.d.a. range 4.050 x 10^-4 g/cm^2 and radiation yield 4.425 x 10^-4 (twice neon's); at 20 keV the range is 1.324 x 10^-3 g/cm^2. - **Formula:** `rho_Ar(1 Pa, 293 K) ~ 1.64e-8 g/cm^3` - **Source quote (verbatim):** “0.010 1.457E 01 1.112E-02 1.458E 01 4.050E-04 4.425E-04” - **Source:** [Berger & Seltzer, Tables of Energy Losses and Ranges of Electrons and Positrons — NASA SP-3012 (1964)](https://cyclotroninfo.com/library/#lib-298) — p. 72 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Referee arithmetic for argon at 1 Pa and 293 K (~1.64 x 10^-8 g/cm^3): 10 keV path length ~250 m, energy loss ~0.24 eV per cm. The higher radiation yield than neon matters only for X-ray estimates; about 4 x 10^-4 of a stopped 10 keV beam's energy goes to bremsstrahlung in argon itself. **dg-1984** · applicability level 4 · physics-theory, safety, shielding · https://cyclotroninfo.com/design-guide/shielding/#dg-1984 - **Rule (this site's paraphrase of the source):** SP-3012 Table I, air (page image read): at 10 keV the collision stopping power is 19.70 MeV cm^2/g, c.s.d.a. range 2.892 x 10^-4 g/cm^2 and radiation yield 1.463 x 10^-4; at 20 keV the range is 9.805 x 10^-4 g/cm^2. - **Formula:** `rho_air(1 atm, 20 C) = 1.205e-3 g/cm^3` - **Source quote (verbatim):** “0.010 1.970E 01 5.012E-03 1.971E 01 2.892E-04 1.463E-04” - **Source:** [Berger & Seltzer, Tables of Energy Losses and Ranges of Electrons and Positrons — NASA SP-3012 (1964)](https://cyclotroninfo.com/library/#lib-298) — p. 130 - **Editorial note, tabletop extrapolation (written by this site, not taken from the source):** Referee arithmetic at atmospheric density (1.205 x 10^-3 g/cm^3): 10 keV electrons travel about 2.4 mm of path in air and 20 keV about 8.1 mm (c.s.d.a. path, so projected penetration is less), so a beam vented or escaping through a thin window stops within millimetres of air. The 1.46 x 10^-4 radiation yield is for electrons stopped in air only; bremsstrahlung from the exit window, the target or any other higher-Z material intercepting the beam is assessed separately, and the 20 keV range is the table's own row (page image). > **Page provenance.** Last reviewed: 23 August 2026 · 80 source-extract rules from 19 cited documents · [Methodology](https://cyclotroninfo.com/about/methodology/) · [Report a correction](mailto:info@cyclotroninfo.com?subject=Correction%3A%20Cyclotron%20Shielding%20Design%3A%2080%20Sourced%20Rules) · [Changelog](https://cyclotroninfo.com/changelog/)