Design Guide › Shielding
Shielding design rules
51 of the guide’s 1374 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.
To combine this tag with another (rules carrying both), use the filterable view: /design-guide/?domain=shielding and add a second chip. Related domains, by how often they share a rule with this one: Safety (24), Detectors (4), Fabrication (3), Beam measurement (2), Materials (2).
Verify before use. Every rule here is a source extract in the vocabulary of the editorial 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.
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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).
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 cmSource, quote & tabletop applicability
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.
Boom, Toth & Zucker, Residual Radiation of the LRL 184-inch Cyclotron — ORNL-3158 (1961) — p. 18
Tabletop: ENERGY SCOPE: a sub-MeV reference machine or next machine produces no residual gamma fields to shield; 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 Pb around a NaI detector against room background (0.6 cm/HVL at 662 keV Cs-137 scale).
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Assume your shielding estimate will prove low and your experiment space too small; design margin and expansion room in from the start because every predecessor facility needed both.
Source, quote & tabletop applicability
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.
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 184
Tabletop: 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.
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Build the shield estimate as an explicit chain — dose limit, source term, attenuation, secondary buildup — and at each approximation record which direction the error runs, keeping the net conservative but only to within the precision of the input data.
Source, quote & tabletop applicability
we have neglected both the secondary production and target attenuation; this results in a conservative estimate still within the precision of other data.
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 189
Tabletop: Scale-free methodology (their 810-MeV cascade physics is not): a next machine's neutron estimate should carry the same per-step conservatism bookkeeping — deliberate, directional, and not stacked beyond what the data precision justifies.
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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 & tabletop applicability
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
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 186-188
Tabletop: At amateur energies the split maps to fast neutrons (thickness/moderation) vs capture gammas and activation (materials choice near the target); the sort-by-question habit is scale-free.
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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 & tabletop applicability
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.
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 199
Tabletop: Scale-free research method, and a pointer: reactor-shielding texts (Price, Horton and Spinney 1957) remain the right source for any amateur duct/labyrinth question the accelerator corpus lacks.
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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) (~0.1 per 90-deg bend with an extended leg), legs must never sight intense sources, and parallel ducts sit several diameters apart.
T_total = product(T_leg_i) * product(T_bend_j); T_bend ~ (1/3)csc(theta); extended entering leg adds ~3xSource, quote & tabletop applicability
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
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 200-204
Tabletop: The transmission algebra is for low-energy neutrons and is scale-free; their 27-ft legs are not. Any next-machine cable/utility penetration or entry labyrinth can be sized with exactly this product-of-elements method.
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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 & tabletop applicability
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
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 204
Tabletop: Scale-free insurance move that costs nothing at design time — pairs with the 'initial shielding always proves inadequate' rule above.
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Use stepped (labyrinth) joints on shield doors and plugs so ordinary construction tolerances are acceptable, and compensate any thickness lost to mechanisms (wheels, tracks) with locally denser material.
Source, quote & tabletop applicability
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.
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 206
Tabletop: Their 800-ton plug is 810-MeV scale; the stepped-joint principle sizes down directly to block-wall doorways and removable concrete/poly plugs around a benchtop target station.
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Trade shielding construction methods on delivered cost per attenuation: their study found solid low-strength concrete walls cheaper (about 2/3 the cost) than cored walls with compacted rock fill, and earth-over-arch the cheapest roof.
Source, quote & tabletop applicability
solid concrete walls can be placed for about 2/3 the cost of walls cored with compacted rock fill.
Oak Ridge National Laboratory, A Proposal for the Mc² Isochronous Cyclotron — ORNL-3540 (1963) — p. 242
Tabletop: 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.
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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 & tabletop applicability
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.
Tabletop: Directly relevant to the plan's facility question and to any MeV-class educational machine - a basement corner with earth on two sides replaces feet of poured concrete, and siting next to existing utilities/controls is a cost line the ORNL study treated as seriously as the magnet.
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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 & tabletop applicability
A considerable amount of undesirable radiation will be produced by that part of the beam intercepted by the slit system.
Tabletop: DIRECT and cheap to honor at layout time, nearly impossible later. Even at 150-170 keV the slit is the hottest x-ray point on the line (thick-target bremsstrahlung at full beam power); a next machine should treat every defining aperture as a shielded component.
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Set flight-path length against the room, not just the resolution equation: paths were kept under 1.2 m so that neutrons scattered from the concrete floor (4 ft below the beam pipe) arrive outside the window, letting the counter run unshielded; the massive Pb + LiH-paraffin shield was left unused partly because its effect on counting efficiency had never been measured.
Source, quote & tabletop applicability
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
Fulbright et al., A Fast Neutron Time of Flight System for Use with Cyclotrons — NYO-9360 (1962) — p. 12
Tabletop: 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.
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Size shielding for the SECONDARY radiation, not the primary beam: the interaction of the beam with the target, the accelerator structure, or the shielding itself most often determines the type and magnitude of shielding required.
shield for secondaries (X-rays, neutrons) produced where the beam is lost, not for the primary ionsSource, quote & tabletop applicability
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.
Tabletop: For the reference machine and a next machine the primary protons never leave the chamber; the machine's entire external radiation field IS secondary — dee-gap electron bremsstrahlung today, 11B(p,alpha) products and any (p,n)-capable contaminants at a next machine's energies.
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On positive-ion machines below 50 MeV, ignore primary-particle bremsstrahlung (it scales ~1/M^2 of the projectile mass) and look instead for the three real X-ray sources: characteristic X-rays from inner-shell vacancies, nuclear deexcitation, and bremsstrahlung from stray electrons.
bremsstrahlung ~ 1/M^2 -> proton bremsstrahlung negligible; hazard = characteristic X-rays + stray-electron bremsstrahlungSource, quote & tabletop applicability
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.
Tabletop: Confirms the program's standing model that dee-voltage electrons, not the proton beam, are THE radiation hazard on a sub-MeV proton cyclotron.
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Lacking design detail, estimate 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; the authors label the assumption unreliable but note it still predicts "very considerable" X-ray production.
I_e(back-streaming) ~ 0.2 * I_ion at E ~ V_terminal/3, as a bounding source-term assumptionSource, quote & tabletop applicability
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.
Tabletop: Directly usable bounding recipe for a next machine's hazard analysis: treat the machine as an electron gun of 0.2x the circulating/source current at ~1/3 of peak dee voltage (multipactor and secondary electrons play the "back-streaming" role), then look up kV X-ray-tube output data for that current and voltage.
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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.
shield(ion machine) ~ shield(electron machine, 90 degrees, same I and E)Source, quote & tabletop applicability
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.
Tabletop: The conservative sizing pattern for any product-machine enclosure — bound the cyclotron by an equivalent electron machine at dee voltage and shield for that; at <=13 kV the "shield" is millimeters of steel/leaded glass, which is why chamber walls suffice on the reference machine.
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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%).
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 & tabletop applicability
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
Tabletop: Sets the scaling logic (efficiency ~ Z and E) even though sub-MeV values must be extrapolated downward: keep stray electrons landing on LOW-Z surfaces (Al, graphite) rather than W/steel to cut X-ray yield several-fold — an argument for aluminum dee/liner surfaces on product machines.
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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.
few-keV electrons -> dipole pattern, max at 90 degrees to electron path; MeV electrons -> forward-peakedSource, quote & tabletop applicability
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.
Tabletop: For dee-gap electrons accelerated axially/radially in the chamber, expect the soft X-ray leakage to peak SIDEWAYS from the electron paths — survey all around the chamber midplane and windows, not just along any assumed "beam" direction.
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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 1-40 MeV electrons only; below ~1 MeV take the source-term assumptions from this chapter but the output numbers from kV X-ray-tube data.
D(behind x cm concrete) = TableII-3(T,x) * W(kW)/R(m)^2, valid 5.5-40.5 MeV; below ~1 MeV use X-ray-tube output tablesSource, quote & tabletop applicability
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.
Tabletop: Scope honestly: this is this collection's only full X-ray shielding workflow, but its curves START at ~1 MeV. For the 5-13 kV dee upgrade, pair its 0.2*I/(V/3) source assumption with NCRP-49-class tube output data (R/mA-min at 1 m vs kVp) instead of Table II-3.
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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.]
sigma(p,nuclear) ~ 0 below ~0.1 MeV; barrier penetration grows sharply with E thereafterSource, quote & tabletop applicability
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.
Tabletop: Closes the neutron question for the reference machine at ~150 keV-class energies EXCEPT via the light-nuclei exceptions the chapter waves off — which here are exactly the deliberate 11B(p,alpha) target and any deuterium contamination (see Ch. IV rule).
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(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 only becomes the DOMINANT channel about 1 MeV above threshold (an emitted neutron faces no Coulomb barrier).
E_thr(p,n) > 0.78 MeV (stable targets), ~MeV for light nuclei; n-channel dominant at E > E_thr + ~1 MeVSource, quote & tabletop applicability
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.
Tabletop: 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 by >600 keV of margin; the audit must be redone if energy ever approaches ~1.9 MeV (7Li(p,n) threshold 1.88 MeV).
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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.
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 & tabletop applicability
H2(gamma,n)H1 ... 2.23 ... C12(gamma,n)C11 ... 18.7 ... O16(gamma,n)O15 ... 16.3
Tabletop: Fully closed at the photon energies of the reference machine and a next machine (<=keV-class bremsstrahlung, 429 keV 11B(p,alpha) line region), but the table is the permanent reference for why nothing photonuclear can happen on these machines — useful verbatim in the product hazard analyses.
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Neutron shielding is slow-down-then-capture: light nuclei (hydrogen) dominate energy loss, so concrete far outperforms lead, and a facility shielded in concrete for X-rays "generally contains adequate neutron shielding in the process" — the serious neutron problem arises only when the neutron hazard exceeds the photon hazard (proton and deuteron machines).
concrete X-ray shield ~ adequate neutron shield (rule of thumb, <30 MeV); capture gammas must be shielded in turnSource, quote & tabletop applicability
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.
Tabletop: For any future neutron-capable operation (a deuterium species test, or a >1.9 MeV machine) the shielding material answer is already decided — hydrogenous concrete/HDPE, not lead — and the X-ray shield does NOT automatically cover it, because the reference machine accelerates protons.
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For first-pass neutron shield sizing use the reactor-derived removal-cross-section method: treat penetration as exp(-Sigma_r * x) with the empirically chosen removal cross section (roughly 3/4 of the total cross section at 8 MeV; Sigma_r ~ 0.094 cm^-1 for ordinary or barytes concrete), and reinforce thicknesses in the forward direction where high-energy anisotropy defeats the method.
phi(x) = phi_0 exp(-Sigma_r x); sigma_removal ~ 0.75*sigma_total @8 MeV; Sigma_r(concrete) ~ 0.0942-0.0945 /cmSource, quote & tabletop applicability
Experimental removal cross sections are roughly three-quarters of the total cross section for 8 MeV neutrons. For hydrogen this fraction is somewhat larger.
Tabletop: The one-line neutron shield calculator for any contingency planning — e.g. a D-D contamination source term attenuates ~10x per 24 cm of concrete; keep safety factors for cross-section uncertainty as the chapter directs.
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Worked pattern for a neutron shield: 20-MeV protons on an optimized Cu target make ~6.5e10 n/s per uA; taking the flux at the shield, demanding six orders of magnitude attenuation, and inverting exp(-Sigma_r x) gives 146 cm of barytes (or ordinary) concrete — and a one-step inverse-square correction (144 cm) shows the slab-normal assumption is already good.
Y(20 MeV p on Cu) ~ 6.5e10 n/s/uA; x = ln(attenuation)/Sigma_r -> 146 cm for 1e6Source, quote & tabletop applicability
This means the shield must reduce the fast neutron flux by six orders of magnitude. Therefore e-Sigma_r*x = 10-6 ... X = 146 cm.
Tabletop: The template to copy for any neutron-capable scenario: source yield -> 1/4pi*r^2 flux at shield -> required attenuation from the dose criterion -> x = ln(A)/Sigma_r. Also the scale anchor for why amateur neutron machines are enclosure-limited: five FEET of concrete for a 1-mA 20-MeV machine.
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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 the source gas or beam-loaded surfaces makes neutrons with no threshold protection; every other common neutron-producing reaction is endoenergetic.
D(d,n)He3 Q=+3.27 MeV, T(d,n)He4 Q=+17.6 MeV -> no energy threshold; all common (p,n)/(gamma,n) are threshold-gatedSource, quote & tabletop applicability
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.
Tabletop: 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. It keeps a neutron survey requirement honest even though the expected yield at tabletop beam densities is tiny.
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Induced activity around an accelerator is a two-step process (beam makes neutrons/photons in the target; those activate surroundings), and because absorption probability goes as 1/v the THERMAL cross section — not the fast one — should be used when estimating what gets activated.
activation A0 = M*phi*sigma_thermal*(1-exp(-lambda*t_irr)); slowing-down activation negligible by comparisonSource, quote & tabletop applicability
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.
Tabletop: The correct bookkeeping if a neutron-capable operation is ever run — inventory surrounding materials (Cu 3.9 b, W 34 b, Au 96 b thermal per Table IV-2) against thermal flux; also why activation on today's neutron-free machines is nil.
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In ordinary concrete the only activation products that matter are Na-24 (15 h) and perhaps K-42 (12.4 h) — 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 the day but decays by a factor of several thousand overnight.
concrete activation governed by Na-24 (15 h) / K-42 (12.4 h); 3-5 day cooldown -> negligibleSource, quote & tabletop applicability
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.
Tabletop: Ready-made cooldown-scheduling logic for any future neutron-producing facility work; for the current machines it documents why the basement structure cannot become activated.
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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.
thermal flux ~1/3 per 10 cm concrete -> ~2/3 of activation in first 10 cm -> sacrificial inner block rowSource, quote & tabletop applicability
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.
Tabletop: 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.
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Long-lived photon-produced isotopes in shielding (Na-22, 2.6 y, from long bremsstrahlung irradiation above threshold) cannot be waited out — if large quantities build up the activated concrete must be physically removed, so plan wall design for that contingency up front.
above-threshold gamma flux + years of operation -> Na-22 inventory -> removable-wall contingency in designSource, quote & tabletop applicability
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.
Tabletop: Scope closed for the machine's photon energies (needs >12.4-MeV photons), but the design principle — never pour a monolithic shield you might someday have to demolish as radwaste — transfers to every enclosure decision.
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Flux-density-to-dose conversion for neutrons (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 QF peak (~11) near 0.5-1 MeV makes fast neutrons ~35x more restrictive per unit flux than thermal.
100 mrem/40h flux limits: 680 (thermal), 19 (1 MeV), 17 (10 MeV) n/cm2-sSource, quote & tabletop applicability
2.5 x 10-8 (thermal) 2 680 ... 5 x 10-1 11 27 ... 1 11 19
Tabletop: The number that turns any future neutron survey reading into a stay-time — a D-D contamination field of even a few n/cm2-s at 2.45 MeV is already a nontrivial fraction of a 1972 occupational week (modern limits tighten this ~5x for the public).
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Estimate X-ray streaming through a maze/labyrinth by following successive 90-degree Compton scatters: assume conservatively that 0.05 of the incident energy scatters into one steradian at each bounce, I_p = (I_1/r_n^2) * prod[0.05*S_j*cos45/r_i^2]; the method matched a Co-60 measurement within a factor of ~2 (0.65 calculated vs 1 mr/hr measured on a 6-ft three-legged maze).
I_p = I_1/r_n^2 * prod_i [0.05 * S_i * cos45 / r_i^2] per 90-deg scatter legSource, quote & tabletop applicability
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.
Tabletop: Scales down perfectly — the same hand calculation sizes a cable/vacuum-line penetration dogleg or an instrument port baffle in a product-machine enclosure, where a straight-through hole would be the dominant leak.
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Permit NO line-of-sight path for radiation through any access route or penetration, and then still evaluate the scatter path through the maze — geometry (not material) is the streaming problem, and the maze delay must be made at least as long as a heavy door would impose.
no line-of-sight through any penetration; scatter path evaluated per the 0.05/sr ruleSource, quote & tabletop applicability
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.
Tabletop: The audit rule for every feedthrough, window, and joint in an enclosure — check sight-lines from the X-ray source point (dee gap) outward, then bound the one-bounce leakage.
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For neutron streaming through mazes use the albedo chain phi_p = (phi_s/(4pi r_n^2)) * prod[beta*Omega_i]: neutron albedo runs from 0.66 for thermal down to ~0.05 for fast (0.4 is a conservative fast-neutron choice), and boron-loaded concrete on the shield-wall surface absorbs thermal neutrons instead of reflecting them.
albedo beta = 0.66 (thermal) -> ~0.05 (fast); 0.4 conservative; boron loading cuts thermal reflectionSource, quote & tabletop applicability
albedo for neutrons (from 0.66 for thermal neutrons to approx. 0.05 for fast neutrons)
Tabletop: Contingency reference only at current energies, but the boron-surface trick (borated HDPE sheet lining a duct) is the cheap fix if any future neutron source term streams through an enclosure penetration.
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Massive shielding doors carry their own hazards: slow travel with great momentum (engineer the stopping to avoid trapping personnel or cracking walls), and every door must be manually openable from BOTH inside and outside after a loss of power; doors must shield at least as well as the adjoining wall.
door shielding >= wall; manual egress inside+outside under power loss; engineered decelerationSource, quote & tabletop applicability
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.
Tabletop: Scale-invariant egress principle — even an interlocked benchtop lid or a walk-in enclosure door must never imprison anyone on power loss; spring-return or manually liftable closures only.
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Choose the resonator mode/geometry so that tuning and mechanical elements sit outside the main vacuum chamber, shielded from both magnetic field and radiation; iron housings (2 in. at Nevis) can finish the magnetic shielding of moving parts.
half-wave resonator puts voltage node / tuner outside chamber; 2-in. Fe housing shields rotorsSource, quote & tabletop applicability
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
Tabletop: 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.
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Line surfaces struck by lost beam with non-activating material (Nevis used marble pole liners) so that stray protons deposit in low-activation stone rather than in iron and copper.
marble (CaCO3) liners over pole/sector iron in beam-loss regionsSource, quote & tabletop applicability
We expect to use marble pole liners where possible, as in the past, to reduce sector iron, etc., activation
Tabletop: Below ~few-MeV protons activation is negligible, so this is a higher-energy note — but the general idea (choose what lost beam hits) already applies to sputter contamination and outgassing on any machine.
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Buy shielding with geometry before mass: aim the primary beam stop away from occupied areas, take secondary beams off at ~90 degrees where neutron spectra are soft, and put multiple bends between production targets and experimenters.
beam stop aimed away + 90-degree takeoff + >=2 bends per secondary lineSource, quote & tabletop applicability
Since the underground beam stop is aimed away from the experimental areas, this greatly eases shielding, and subsequent background problems
Tabletop: 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; a first-class rule for any facility layout.
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For forward-cone neutrons above ~100 MeV, attenuation flattens to roughly a factor 2 per 6 in. of iron (or ~18 in. ordinary concrete), while at >=90 degrees the softer (<100 MeV) spectrum gains nearly a factor 10 per 6 in. Fe — shield thickness must be budgeted per direction.
>100 MeV forward cone: x2 per ~6 in. Fe (~18 in. concrete); >=90 deg: ~x10 per 6 in. FeSource, quote & tabletop applicability
require - 6 in. Fe (or the equivalent) for each factor of 2 attenuation
Tabletop: Pure high-energy datum — no tabletop relevance except as a worked example of directional shielding budgets, but it anchors the energy scaling.
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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).
HVT(ordinary concrete, cyclotron-target fast neutrons) ~ 10 cmSource, quote & tabletop applicability
fixes the half-value thicknesses of ordinary concrete for neutrons from cyclotron targets at approximately 10 cm
Tabletop: The corpus's first concrete (literal) shielding number for MeV-class cyclotron neutrons — below the (p,n)/(d,n) thresholds the reference machine produces none, but this is the sizing constant the moment any future machine or D-beam work crosses into neutron production; measured for far harder spectra than an amateur will make, hence conservative.
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Measure shield attenuation with the machine itself as the source: place a slab of the candidate material (3 ft x 3 ft x thickness) in front of a detector recessed in a cavity in a thick concrete "igloo", and normalize every detector reading to a fixed beam monitor so source fluctuations divide out of the attenuation curve.
attenuation = (detector/monitor) vs slab thickness; slab 3'x3', detector in 1.5-inch cubical cavitySource, quote & tabletop applicability
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]
Tabletop: 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.
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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 & tabletop applicability
The monitor and detector employed were aluminum-walled ionization chambers, with DC Amplification, indicating on microammeters placed outside the magnetic field of the cyclotron.
Tabletop: Analog meter movements, photomultipliers, and many GM counters misread in a stray field of even tens of gauss; separate the sensing volume from the readout and keep the readout outside the fringe field.
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Expect transition (buildup) effects at the front face of any shield: attenuation only becomes exponential after the radiation reaches equilibrium with the secondaries it generates in the absorber, so fit half-value thicknesses to the displaced linear portion of the curve, never to the first readings behind thin layers.
fit exponential slope only beyond the equilibrium (buildup) depth; extrapolation of the linear portion back to zero is displaced from the no-absorber readingSource, quote & tabletop applicability
The transition effects occur as the neutron beam approaches equilibrium with the secondary and scattered particles produced in the absorbing medium.
Tabletop: A dosimeter reading just behind the first inches of shielding measures the buildup region, not the attenuation slope; thin-shield tests systematically misestimate what a thick shield will do in either direction.
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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.
dose behind paraffin layer = up to 2x dose entering it, when preceded by high-Z materialSource, quote & tabletop applicability
Paraffin yields a transition increase of 60% following Fe, and of 100% following Pb with similar geometry.
Tabletop: When adding polyethylene or paraffin outside a metal chamber wall, a survey reading taken between the layers or behind too thin a hydrogenous layer can exceed the bare-wall reading; the hydrogen layer must be thick enough to absorb the recoil protons it creates.
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Threshold-activation detectors sandwiched between absorber slabs give cleaner attenuation half-values than ionization chambers: they are insensitive to the low-energy scattered background, immune to magnetic fields and electronics drift, and yield exponential curves with better precision.
activation of a threshold-reaction foil vs absorber depth -> half-value thickness; Moyer used C12(n,2n)C11, threshold ~20 MeVSource, quote & tabletop applicability
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.
Tabletop: 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.
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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 & tabletop applicability
Because of the geometry employed, these measurements are neither a true determination of pure scattering nor pure absorption.
Tabletop: Handbook removal cross sections assume good (poor-geometry-corrected) conditions; a home measurement in tight geometry will read more optimistic than broad-beam reality because scattered radiation misses a small detector.
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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.
sigma per nucleon decreases with Z (shadow effect); merit ~ density x (hydrogen + light-element fraction)Source, quote & tabletop applicability
one should seek substances which combine high density with low atomic number. Among convenient and practical materials none would seem better than concrete.
Tabletop: The shadow-effect argument is a >100 MeV argument, but the conclusion strengthens at low energy where hydrogen elastic scattering dominates moderation - concrete, water, and polyethylene beat any metal for neutron shielding per dollar and per pound.
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Shield for machine-generated loss points, not just the target: beam grazing the interior of the dee sprayed fast neutrons in considerable intensity through 180 degrees of azimuth, in addition to the forward cone from the probe target.
Source, quote & tabletop applicability
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.
Tabletop: Wherever beam is lost - dee edges, septum, probe stalk, chamber wall - is a source; a survey plan that only looks downstream of the target will miss most of the emission solid angle.
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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 & tabletop applicability
Measurements with a BF3 proportional counter have indicated diffusion of slow neutrons through various access openings from the enclosure.
Tabletop: Cable ways, viewport lines-of-sight, and door gaps are the paths that matter once any bulk shielding exists; thermal-neutron instruments answer a different question than fast-neutron ones and both belong in a survey.
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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.
report dose rate AND beam current together; dose scales linearly with current at fixed geometrySource, quote & tabletop applicability
These quoted measurements are made with Al-walled ionization chambers, and correspond to a deuteron beam of about 0.2 x 10-6 amp.
Tabletop: 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.
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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 & tabletop applicability
It is designed so as to take maximum advantage of naturally occurring shielding of a small ravine.
Tabletop: Physics transfers even if the scale does not: mass is mass, and cheap mass (earth berms, water tanks, basement corners) is legitimate neutron/gamma shielding for a D-D-capable machine. Spec detail: PDF p.128.