Targets design rules

Expect the internal/in-vacuum components that the beam actually strikes to be the hottest objects in the building — Crocker probe targets and the exit strip read >10,000 r/hr against ~500 r/hr for the deflector — and design their removal paths and storage shielding first.

safetytargets dg-874

Evaporate boron from a COVERED slotted boat machined from spectroscopic-grade carbon rod; the cover both cuts radiative heat loss (boron needs white heat) and stops the charge scattering out of the boat during heating.

targetsfabricationmaterials dg-875

Working boron-evaporation parameters: pressure below 1e-5 mm Hg, ~280 A at 8 V (~2.2 kW) through the carbon boat to white heat, deposition onto 3.25 x 4-in glass plates; endpoint is visual — the charge stays darker than the boat until just before vaporizing, then brightens and disappears in seconds.

targetsfabricationvacuum dg-876

Budget boron-evaporation boats as consumables — the slot clogs with boron carbide and a boat survives at most two evaporations — so machine boats in batches before a target campaign.

targetsfabricationmaterials dg-877

Recover self-supported films by float-off: pre-clean the glass in Calgonite (detergent) solution, tilt ~20 deg, run warm tap water in slowly, cut the floating film to size, and lift it on 0.030-in aluminum frames; add a pinch of detergent to cut surface tension and pause before lifting clear so trapped water drains.

targetsfabrication dg-878

When a single-thickness film is too fragile, bring the frame up under the middle of the floating film so it folds double over the frame and the two layers adhere — Hoke and Newman's double 50-100 ug/cm2 enriched-B10 targets were far easier to make than single 25-50 films.

targetsfabrication dg-879

For water-sensitive evaporated films (e.g. magnesium), first arc-coat the soap-primed glass with 5-10 ug/cm2 of carbon as a parting and backing layer, and ramp evaporator current slowly (10-20 min) so the charge outgasses quietly instead of spattering; conserve enriched isotope with a glass recovery hood over the source.

targetsfabricationvacuum dg-880

Measure internal-beam energy with photographic film behind a stepped absorber folded from aluminum foil: expose ~0.1 uA-sec (arc OFF, source-off current control, field deliberately detuned to throttle intensity), leave an uncovered film strip as an intensity reference, and read densitometer values against a range-energy scale.

beam-measurementtargets dg-882

Map the beam on an internal target by sectioning the target itself: an array of thin strips (17 carbon foils, 1/32 x 5.5 in), pre-scored, bombarded once (15 min at 15 uA), then snapped into 1/2-in pieces and counted individually — yielding full 2-D isointensity contours of the beam spot from a single bombardment.

beam-measurementtargets dg-886

Braze beryllium (and similar hard-to-wet) targets in a vacuum furnace, not with flux: flux brazing left inclusions that impaired heat transfer and cost 40-60% of target efficiency as poorly bonded Be eroded; sandwiching 0.006-in. aluminum-silicon alloy and vacuum-furnace brazing gave 100% bonding without tinning (86-inch neutron targets).

targetsfabricationmaterials dg-941

Thick-target neutron yield planning number: bombarding beryllium with ~20-MeV-class protons on the 86-inch gave approximately 1 neutron per 50 protons — the machine served as a controlled fast-neutron source for a biology program on this basis.

targetssafety dg-942

Verify target areal-density uniformity before quantitative use: Harvard scanned a collimated beam across a carbon target and located the Bragg-curve tail at each point, resolving 0.2% density changes; reactor-grade rod stock showed 1%-per-quarter-inch gradients (2% near the edge) and was rejected in favor of pyrolytic graphite.

targetsbeam-measurement dg-1157

Start every target heat-load estimate from the deposited beam power P = I x dE (current times energy lost in the target). Corwin's worked example: 100 nA losing 43 keV in a 380 ug/cm2 PbCl2 target gives P = 0.0043 W into a 1 mm x 3 mm (0.03 cm2) beam spot. For a target thick enough to stop the beam, dE is the full beam energy.

targetsbeam-measurement dg-1158

Radiation limit (Corwin): a thin foil radiates from both faces, so the temperature needed to shed power P to surroundings at T0 follows P = e*sigma*2A*(T^4 - T0^4). For his 0.0043 W / 0.03 cm2 example, emissivity 1.0 requires 103 C but a more realistic e = 0.1 requires 345 C — and a thin film's effective emissivity may be lower still because the film is partly transparent.

targets dg-1159

Conduction limit (Corwin): heat conducted radially from a round beam spot of radius r_b to a target frame at radius r_t obeys P = 2*pi*k*h*dT / (1/2 + ln(r_t/r_b)) — note thickness h enters linearly. His example (h = 0.65 um, r_b = 0.1 cm, r_t = 0.64 cm): an insulator with k = 2 W/mC needs dT = 973 C to conduct the load, while a metal target (k ~ 200 W/mC) conducts the same load with only a ~12 C rise.

targetsmaterials dg-1160

Moving the target multiplies the survivable power: sweeping the beam spot around a circle enlarges both the radiating area and the conduction perimeter. Corwin's combined small-dT heat equation for a spot swept on a circle of radius r_c is P = [e*sigma*2*A_c*4*T0^3 + 2*pi*k*h/(1/4 + ln(r_t/r_c))]*dT; for his example (16 mm circumference, A_c = 48 mm2) the high-speed-rotation equilibrium is dT = 65 C versus ~1000 C stationary, and conduction alone then holds 400 C — below the salt's 501 C melting point.

targets dg-1161

Check the transient, not just the equilibrium: beam suddenly applied to cold material heats it at dT/dt = P/(m*c) before radiation/conduction respond — 1300 C/sec for Corwin's 11 ug spot (c = 0.068 cal/C g). At 1.9 turns/sec each part of the target sits in the beam only 29 ms and rises only 39 C per pass; the moving-target temperature is a sawtooth (65 to 105 C in his example). The spot's thermal recovery time is tau = r_b^2/alpha with alpha = k/(rho*c) the diffusivity (0.83 s in the example).

targets dg-1162

Thickness scaling of the two limits (Corwin): making the target thicker leaves the conduction limit's temperature unchanged in his per-thickness formulation while the radiation-limited temperature rises (more power deposited, same radiating area). Thin targets are radiation-limited; thick ones become conduction-limited.

targets dg-1163

Corwin's practical rotator: three targets on a fully adjustable, chamber-independent rotator turning 1.9 turns/sec through an O-ring shaft seal (a ferrofluidic feedthrough allows faster); reconciling Yntema's observations he notes carbon foil life improves both with motion AND with heating to ~400 C, foils thin above 450 C and thicken when cooler, and suspects damage is worst "when there is radiation from a single spot only".

targetsfabrication dg-1164

Process-selection ladder (Adair & Kobisk, ORNL/IRML): rolling is by far the most material-conserving route to thin metal foils; vacuum evaporation reaches ultrathin elemental and thin/thick compound films but with efficiencies as low as ~1%; electroplating, casting/pressing, CVD and sputtering fill the corners. Their Table 1 (PDF 18-23) lists, element by element, which method reaches which ug/cm2 range in which backed/self-supporting form — read it before choosing a technique.

targetsfabrication dg-1165

Roll metals inside a stainless-steel sandwich (Adair & Kobisk): consolidate the reduced metal into a bead, flatten in a hydraulic press, then roll between stainless sheets — the sandwich keeps the foil from adhering to the mill rolls and permits much thinner foils than bare rolling. Rolled foils are typically 1 x 1 inch.

targetsfabrication dg-1166

Evaporate expensive material from tubular crucibles (carbon, Mo, W, Ta) chosen for chemical compatibility with the evaporant, and expect only ~1% collection efficiency in ordinary geometry (Adair & Kobisk); electron-bombardment guns or RF heating serve the refractory and reactive cases, and vacuum reduction-distillation converts oxides directly to metal films.

targetsfabricationmaterials dg-1167

Characterize targets by areal density (ug/cm2 = atom count per area), never by linear thickness: microscopic voids and mixed crystal phases make any linear measurement converted through bulk density "grossly erroneous", while weight is directly proportional to the number of nuclei if stoichiometry is known (Adair & Kobisk).

targetsbeam-measurement dg-1168

Weighing discipline (Adair & Kobisk): direct weighing reaches ~0.5% for samples above ~1 mg even in a normal lab, and the NBS sequence-weighing scheme (sample and standard weights of similar mass weighed in a set order) cancels balance zero-point drift — an interlab check on 200+ boron and lithium crystals of 50-300 ug agreed to better than +-1% in almost every case.

targetsbeam-measurement dg-1169

Quartz crystal monitors are for in-situ rate and rough thickness only — calibrate them (ORNL used a vacuum microbalance; CBNM Mol made boron and uranium reference layers defined to +-0.3% that way), take the final thickness from direct weighing after removal, water-cool the crystal when the source environment exceeds ~300 C, and extend range for thick deposits with a rotating apertured wheel that samples the flux (Adair & Kobisk).

targetsbeam-measurementfabrication dg-1170

Measure self-supporting film thickness by charged-particle energy loss (Adair & Kobisk): collimated alphas (241Am) through the foil, spectrum shift on a calibrated MCA, areal density = dE / stopping power. Works from a few ug/cm2 to several mg/cm2 — but published stopping-power data carry ~+-10% accuracy, which bounds the absolute result. Fission fragments (252Cf) resolve much thinner foils; beta transmission covers 40-500 mg/cm2.

targetsbeam-measurementdetectors dg-1171

Quick semi-quantitative gauges (Adair & Kobisk): a calibrated light densitometer reads carbon foil areal density in the 5-50 ug/cm2 range (useless above ~40-45 ug/cm2 where transmission saturates); low-geometry alpha or gamma counting assays radioactive deposits to ~+-1% with geometry factors down to 1e-7 for hot samples.

targetsbeam-measurementdetectors dg-1172

Map uniformity, not just mean thickness (Adair & Kobisk): scan the foil with a collimated alpha (or beta) beam position by position and draw a thickness topograph — a rolled 58Ni foil profiled this way showed max deviation +-1.5%. For radioactive deposits, scan with a small aperture and a silicon detector.

targetsbeam-measurement dg-1173

Know what is in the target, not just how much (Adair & Kobisk): IRML characterized completed targets by elemental, spark-source and isotopic mass-spectrographic analysis — impurity species and concentration matter as much as thickness because contaminant reactions masquerade as signal.

targets dg-1174

Sputter from rolled isotopic foils when film properties matter (Adair & Kobisk): IRML modified a commercial sputtering system's target electrode from 12.7 cm down to 6.4 cm to accept small rolled isotope foils as sputter sources — "a very reproducible process" — trading deposition speed for material economy and reproducibility.

targetsfabrication dg-1175

Build the alpha source yourself (Thompson): expose an aluminum foil above an open 228Th bottle with the foil held at -300 V; recoil-ionized 220Rn plates onto the foil and decays to 212Pb (10.6 h half-life), giving alphas at 8786, 6090 and 6050 keV. ~10 h activation, ~24 h useful source life, no sealed-source procurement. Enclose the activator in a vented glove box because of the escaping radon.

targetsdetectorssafety dg-1176

Calibrate with two peaks and measure only shifts (Thompson): spread the 6050 and 8786 keV lines across the analyzer with a biased amplifier, compute keV/channel from their separation, then read target thickness from the channel shift of each peak with the target in place. Estimate fractional channel positions from adjacent-channel counts (peaks are Gaussian) — a 5-channel shift read to whole channels is only good to ~20%. Vacuum below 1e-4 torr, else detector-bias glow discharge can damage the detector.

targetsbeam-measurementdetectors dg-1177

Limits and free diagnostics of the alpha-loss method (Thompson): minimum measurable thickness ~ Tmin = 1.22*(30 + A) ug/cm2 (A = atomic number) for 10% accuracy at half-channel estimation on 512 channels; upper limit beyond 5 mg/cm2. Peak broadening beyond the no-target width flags nonuniformity; small UNSHIFTED satellite peaks flag pinholes. Composition must be known — use Bragg additivity for compounds.

targetsbeam-measurement dg-1178

Seed difficult condensers with a hexagonal-structure metal (Heagney & Heagney, MicroMatter): high-vapor-pressure hexagonal metals (Zn, Cd, Sb, As, Mg) stick poorly to amorphous substrates; pre-evaporating ~1 ug/cm2 of Be or Bi (both hexagonal, both mono-isotopic) from a side-by-side source — without breaking vacuum, to avoid oxidizing the seed — makes them condense uniformly with very high sticking coefficients.

targetsfabricationmaterials dg-1179

Reduce oxides in the evaporation boat with graphite (Heagney & Heagney): mix the metal oxide with spectroscopic-grade graphite powder, press to a pellet, heat in a tubular or e-beam boat — CO2 evolution on the pressure gauge tracks the reduction, and tens of mg reduce in tens of minutes while holding the chamber below 5e-5 torr. Works for Zn, Cd (which evaporate at reduction temperature), Fe, Cr.

targetsfabricationvacuum dg-1180

In-situ reduction for calcium-class reactive metals (Thomas, ANL): press CaCO3 (or CaO) with a reducing agent such as Zr into a pellet, evaporate from a closed tantalum boat with heat shield and collimator at only 3.0 cm source-substrate distance — routine targets from FEW-MILLIGRAM isotope samples onto substrates as thin as 15 ug/cm2 carbon. And soften the roughing sequence: an abrupt roughing-valve opening blew every thin carbon substrate off its frame in their new commercial evaporator.

targetsfabricationvacuum dg-1181

Sputter-yield systematics (Scaife, Hanley & Purser): threshold 10-40 eV; yield rises exponentially from ~100 to ~1000 eV; nearly linear above 1 keV; maximum near 25 keV for conductors (50-60 keV dielectrics), plateau beyond. At the 20-keV working point, yields run 2-10 atoms/Ar ion (silver ~9.5, refractory metals ~2) — and yields rarely differ between materials by more than ~10x, unlike evaporation rates.

targetsfabricationion-source dg-1182

Sputtering decouples deposition from vapor pressure (Scaife et al., after Wehner): at 2000 C the evaporation rates of aluminum and tungsten differ by NINE orders of magnitude, their sputter yields by a factor of two — so refractory metals, and mixtures whose components would fractionate in a melt, deposit as readily as anything else, with no crucible contact and hence no crucible contamination.

targetsfabricationmaterials dg-1183

The 10-eV ejection energy is why sputtered films are tough (Scaife et al.): sputtered atoms arrive at ~10 eV versus ~0.1 eV thermal — far above the ~0.1 eV adhesion energy — producing chemisorption-grade adherence (500 A Ti on glass resisted a scissors point), in-flight substrate cleaning, localized annealing, and self-supporting films with bulk-like strength and ductility. Corollary: the cleaning defeats some release agents — Teepol is scrubbed off, NaCl and BaCl still work.

targetsfabricationmaterials dg-1184

Sputter in high vacuum, not plasma pressure (Scaife et al.): focused-beam sputtering at 1e-6 torr keeps the sputtered atoms' mean free path at meters so they arrive with full energy; glow-discharge sputtering at 1e-3 torr (mean free path ~1 cm) thermalizes them in gas collisions, costing adhesion, strength and rate. Film purity follows too — ~100 ppm noble gas is the residue (Xe in Ta).

targetsvacuumfabrication dg-1185

Working numbers for a focused-ion-beam sputter rig (Scaife et al.): 2 mA of 20-25 keV Ar+ from a von Ardenne-type duoplasmatron through an einzel lens (beam kept under ~30% of lens diameter to dodge spherical aberration) erodes ~50 ug/sec total and deposits ~20 ug/cm2/min of Ti at 2.5 cm; usable targets from ~10 mg of source material; ~10% thickness uniformity over a 30-degree included angle statically, better with source/substrate rotation; deposition falls as 1/d^2 and the sputtered-atom lobe is slightly narrower than cosine, peaked near the source normal.

targetsfabricationion-source dg-1186

Contact evaporation for maximum recovery (Reynolds & Morgan, Liverpool/Manchester): place the substrate DIRECTLY on top of a resistance-heated tantalum tube source containing the charge, with stacked tantalum mesh discs inside acting as a multi-point source for uniformity — 200 ug of Cd isotope gave 200 ug/cm2 targets with >= 90% material recovery and ~10% uniformity over a cm2; heat to ~450 C and cool slowly so the glass slide does not crack.

targetsfabrication dg-1187

Store reactive targets under inert gas cradle-to-grave (Bonetti et al., Bruyeres-le-Chatel): lithium, calcium and rare-earth foils live in dessicated-argon glove boxes from fabrication onward and SHIP in argon-filled containers; electrodeposited targets are nonuniform (thickness variations up to 100%, asymmetric) while electrosprayed ones hold ~7% — know the uniformity signature of the process before trusting a target from it.

targetsmaterialsfabrication dg-1188

Vacuum storage beats atmosphere for degradable targets (Worthington, Jedlowski & Thomas, ANL): a 90-position storage wheel at < 5e-7 torr (diff-pumped, -35 to -40 C optically dense baffle against oil) with a screw-tip transfer rod and a double-valved transfer chamber moves hygroscopic or oxidizing targets evaporator-to-storage-to-beamline without ever seeing air; the transfer interlock is pumped below 0.05 torr before either valve opens, and an electronic interlock system guards against the system's one chronic accident — vacuum loss.

targetsvacuum dg-1189

The parting agent, not the evaporation, can set target nonuniformity (Abele et al., TU Muenchen): parting-agent crystallites run 100-2000 A with 50-1000 A surface roughness — the same order as a 10 ug/cm2 carbon or 100 ug/cm2 gold film (~500 A) — so the film is a replica of the crystallite field "however uniform an evaporation may be". A 1-mm-aperture alpha thickness scan averages right over it; the damage appears as excess energy straggling, growing sharply with target tilt angle.

targetsfabricationbeam-measurement dg-1190

Choose low-crystallite organic parting agents for resolution work (Abele et al.): among NaCl, betaine/sucrose, CsI, alanine and Teepol (= Lensodel), only Teepol — and nearly, alanine — kept measured straggling near the Vavilov (ideal-target) prediction at 30-50 degree tilt; NaCl and betaine replicas broadened it severely. Their QA method transfers whole: pass monoenergetic alphas through the finished target and compare the straggling width to Vavilov — "check the resolution of the target without using expensive beam time".

targetsfabricationbeam-measurement dg-1191

Proton targetry is forgiving; energy loss scales as the square of projectile charge (Erskine, ANL): with a proton beam even a leftover gold target gave 5.1 keV FWHM at 16 MeV (1 part in 3100) — "with a proton beam, targetry is just no problem" — because mean energy loss carries a Z_proj^2 factor (oxygen loses ~64x more than protons at equal energy) while straggling grows ~Z_proj; a 10% target nonuniformity that costs protons 1.9% in energy width costs a calcium beam 22%.

targetsbeam-measurement dg-1192

Backings are not free (Erskine): a carbon substrate "produces a lot of difficulty because of the contaminant reactions observed from carbon", carbon has ~2.5x gold's energy loss per ug/cm2, and reaction-site path-length compensation by tilting the target only works if the target is flat — bowing, wedge or microscopic roughness defeats it. His 48Ca best case stuck at 110 keV resolution from suspected calcium-layer nonuniformity no target change could beat.

targets dg-1193

Foil lifetime scales inversely with beam current DENSITY, not current (Yntema, ANL): he plots carbon stripper lifetimes as particle-uA-min per mm2 of actual beam spot (for an oscillated target, the spot size, not the swept area), against ion velocity (MeV/A); stationary unheated foils fall on a straight line in these variables. Minimum practical stripper foil ~3 ug/cm2.

targetsbeam-measurement dg-1194

Heat carbon foils to ~500 C under beam (Yntema): radiative heating of a carbon foil to ~500 C under a 4 MeV Ni+ beam extended life ~40x; adding slow motion (6x effective area) made it >200x an unheated stationary foil. Pre-annealing at 1000 C before use gave NO benefit — the heat must be present during bombardment. Foils also thicken under beam from hydrocarbon cracking (suppressed in 1e-9 torr vacuum at Harwell) plus beam-induced effects; oscillation moderates the thickening.

targetsvacuum dg-1195

Electrostatics can kill a foil instantly (Yntema): beam current flowing through a thin foil concentrates near the frame edge, and current + temperature gradients rupture foils at the holder; a charged insulator near the foil "can be blown off the frame almost instantaneously". A thin evaporated Au layer raised carbon foil life ~50% (ANL), while a thin Al layer DECREASED it substantially (Chalk River).

targetsfabrication dg-1196

Defocus and raster whenever the physics allows (Berry, ANL/Chicago): sweeping the beam at 1 kHz in x and y across a 25 mm2 aperture-defined area cut 5 ug/cm2 carbon foil breakage by 5-10x by making the current density uniform; a defining pre-aperture keeps beam off the foil holder ("better lifetime characteristics"); and a rotatable 23-foil carousel makes replacement cheaper than heroics — "defocus whenever possible is the moral".

targetsbeam-measurement dg-1197

Match the e-beam spot to the evaporant droplet (Maier-Komor, TU Muenchen): for small isotope charges (100 mg Mo = 3.5 mm droplet) the most efficient energy transfer is beam diameter = droplet diameter; a larger spot wastes power on the cooled crucible, a smaller one saturates (dense vapor above the impact point scatters the beam and burns power making secondary electrons and ions). ~2 kW reaches evaporation for such charges despite ~50% backscatter loss — Kanter/Sommerkamp data give total power absorption ~49% on a Ta sphere (PAC ~ 0.61 cos^1/2), worse for high Z.

targetsfabrication dg-1198

Regulate the e-gun supply or re-aim at every power change (Maier-Komor): drooping (resistance/inductance-limited) supplies sag up to 25% at full load; with magnetic deflection the spot radius goes as sqrt(voltage), so a 25-mm-radius gun's spot walks ~7 mm at full load — clean off the evaporant. Find the true spot by melting a hole in a copper foil laid in the crucible (electrostatic/permanent-magnet guns) or by maximizing crystal-monitor rate versus deflection (electromagnetic guns). Use water-cooled COPPER crucibles (W cracks, Mo gains little), cleaned of oxide, one dedicated crucible per isotope.

targetsfabrication dg-1199

Manage charge on insulating substrates during e-gun runs (Maier-Komor): a parting-agent-coated substrate is an insulator — start at a very low evaporation rate so the growing layer can discharge, or sparks crack the parting film and the young target layer; ground the substrate mount so the film edge makes contact. An INSULATED mount charges toward the gun's acceleration voltage and pulls plasma ions into the film, growing hillocks and craters; coat a glass bell jar's inside with metal or it charges to the acceleration voltage too.

targetsfabricationsafety dg-1200

Budget substrate heating from condensation and source radiation (Maier-Komor): condensation releases ~6e5 J/g-atom for low-vapor-pressure metals; at 10 cm throw a condensation rate of 5e-8 g/cm2 s (= 1e-2 torr source vapor pressure for a 3.5 mm droplet) keeps a 50 ug/cm2 NaCl parting layer under ~10 C/sec of heating. Radiant load depends on source temperature at equal vapor pressure — Mo radiates ~10x what Au does at 1e-2 torr — so cool the substrate or pulse the evaporation; and hold source vapor pressure below ~1e-6 torr-equivalent rates if contamination ratio matters.

targetsfabricationvacuum dg-1201

Sulphide (and volatile-compound) evaporation is a cleaning-and-preheat protocol, not just a boat temperature (Peck, Queen's): abrasive-clean the substrate (gently on gold — buried grit mimics a Si contaminant), water then ethanol rinse, pump while still alcohol-wet; chimney-topped Mo boat, substrate 6 cm above, charge pressed to a tablet to stop splashing; cold-trapped system at 5e-6 torr; hold the boat at cherry red (~600-900 C) one minute to pre-heat the substrate receptive, then bright red (~1100-1450 C) to deposit; close the high-vacuum valve during the evaporation and mind sulphide toxicity on venting.

targetsfabricationsafety dg-1202

Expect beam-induced foil failure to be MECHANICAL, not thermal: the beam spot thickens (carbon buildup and/or mass transport toward the spot), the film tightens, radial stress lines develop, and the foil tears across the thickened spot — so a foil breaks at its thickest part, at the bombarded/unbombarded transition.

targetsbeam-dynamics dg-1203

Foil-lifetime levers that cost nothing: start with the beam spot as large as possible and focus down slowly, use the largest practical foil diameter, and heat the foil evenly to reduce the temperature gradient (uneven heating drives the stress).

targetsbeam-measurement dg-1204

A thin metal backing can rescue an otherwise doomed foil: a 2 ug/cm2 carbon film backed with 10 ug/cm2 of gold survived beam exposure that tore an identical unbacked carbon foil at the beam spot.

targetsmaterials dg-1205

When forming a target compound by heating a deposit on a substrate (e.g. nitriding Ti on Ta), the reaction temperature has a two-sided window: too low leaves the reaction incomplete, too high diffuses the reactant into the substrate and smears the target thickness. For TiN on Ta the window is 750-800 C.

targetsmaterials dg-1206

Hydrogen tube-furnace reduction converts most common target oxides (CuO, Fe2O3, WO3, GeO2, PbO...) to metal with modest equipment, but the combustion boat must be chemically compatible (iron in a graphite boat forms carbide), the H2 must be deoxygenated and dried, and an oil trap should guard against flashback if the vent gas is burned.

targetsmaterialssafety dg-1207

Electrolytic reduction is the most material-conserving oxide-to-metal route for zinc and cadmium: better than 90% of the metal deposits on the cathode at 5-10 mA/cm2 from cyanide/hydroxide baths of a few ml, and the remainder is chemically recoverable.

targetsmaterials dg-1208

For a beam-durable deuterium target, evaporate titanium in a low-pressure D2 atmosphere (5e-3 torr, 2-3 h slow evaporation, backings at 200 C) rather than using deuterated polyethylene, which dies quickly in beam; assay the occluded deuterium by nuclear scattering, not by weight.

targetsmaterials dg-1209

When the target element is volatile or liquid, build the target from its most thermally stable compound rather than the metal or an amalgam — a sublimed HgS film tolerated ~20 particle-nA of heavy-ion beam where the amalgam target allowed ~1 particle-nA, and inhomogeneity shows immediately as a low-energy tail on the elastic peak.

targetsmaterials dg-1210

Cool a fragile target in use by conduction through its edges: connect the target edge to a chilled copper block with silver paint. A block at -80 C held a mercury-sulfide target near -50 C during bombardment.

targetsmaterials dg-1211

In reduction-distillation, pick a reductant of MODERATE oxygen affinity so the reaction speed is controllable by furnace temperature: tungsten powder reduces HgO smoothly at 500 C in ~10 min, while thermodynamically stronger reductants (Zr, Th) react explosively and scatter the charge.

targetsmaterialssafety dg-1212

Thick (beam-stopping) targets from powder: press 300-400 mg between two polished stainless ferrotype plates at ~3 tons/in2 into a ~2-cm disc of 100-150 mg/cm2; strengthen fragile pressed discs with a few drops of dilute polyethylene-in-xylene, and glove-box the pressing for air/water-sensitive compounds.

targetsfabrication dg-1213

Split any conductive target-holder ring when the target sits in an RF field, so the ring cannot carry induced eddy currents.

targetsrf dg-1214

Weighing is a poor thickness gauge below ~10 ug/cm2 — adsorption/desorption alone contributes ~0.5 ug/cm2 of error — so use optical transmittance with a double-reflectance correction (ln(I_T/I_0) = -mu*x + ln(1-R_R)); measure thin foils at short wavelengths and thick foils at long wavelengths where they still transmit.

targetsbeam-measurement dg-1215

Prevent stress failure of evaporated films by heating the substrate during deposition: film tension falls with substrate temperature, passes through zero, and can go compressive; the crossover scales with melting point (~210 C for Ni, ~100 C for Cu, ~300 C for Fe). Annealing AFTERWARD (<=400 C) does not remove the frozen-in stress.

targetsmaterialsfabrication dg-1216

A small bench-type hand-cranked rolling mill is sufficient for foils down to 1-5 mg/cm2 — large mills are not required; and when a small isotope quantity rolls non-uniform (thick center, thin edges, too few passes), press it between two polished cobalt-tungsten-carbide flats instead.

targetsfabrication dg-1217

Pack-rolling craft: 3-5 mil polished tungsten sheet makes a hard reusable pack (but not for the very thinnest foils); spring-steel sheet works alongside stainless; and when a soft metal (Pd) welds itself to the pack below ~500 ug/cm2, make three to five passes at the SAME mill setting before reducing further.

targetsfabrication dg-1218

Heating and moving a stripper/target foil multiply its life under intense beams: holding foil and frame at 450+-150 C gave ~4x lifetime, orbiting the foil at 1 rpm (beam walking around the foil) ~3x, and both together ~6x. Failure signature changes too — cold foils break suddenly and completely, heated foils tear gradually from an edge.

targets dg-1219

Beam duty-cycling extends foil life only if the off-periods are long enough for stress relaxation: equal on/off periods of 60 s helped noticeably, 6-s periods did almost nothing.

targets dg-1220

Rolled foils outlast evaporated foils under intense beams because their structure is crystalline: evaporated films (substrate <700 K) are partly amorphous with many Frenkel defects, and beam-driven phonon excitation reorders those regions, wrinkling and destroying the film; in a rolled foil displacing an atom costs the full Wigner energy (10-40 eV in metals).

targetsmaterials dg-1221

Rolling feedstock must be a clean solid bead: e-beam melt 50-500 mg portions in a water-cooled copper crucible, let the drop solidify slowly from the cooled side so impurities concentrate in a last-frozen "stalagmite", cut it off, and repeat (~10x for uranium). Avoid pressed-and-sintered powder (grain-boundary defects end rolling early) and arc melting (gas impurities: 1 ppm impurity in the arc gas is like working at 1e-3 torr).

targetsmaterialsfabrication dg-1222

Pack (sandwich) rolling jacket spec: bright-annealed, temper-passed stainless of 0.5 mm or thinner, LOW carbon (<0.03%, low grain disintegration) for most metals, or ~0.1%-C spring steel for metals less ductile than nickel; use vacuum-melted stainless for the inner jacket — ordinary cold-rolled band carries ingot-scale texture lines that slice thin foils into strips along the rolling direction.

targetsmaterialsfabrication dg-1223

Pack-rolling schedule: reduce ~3-10% per pass; when the jacket has grown to about twice its size, transfer the foil to a fresh jacket; below 5-10 mg/cm2 switch to a double sandwich — a 0.1-0.2 mm vacuum-melted inner jacket inside the 0.5 mm outer — and keep the same 2x-growth rule.

targetsfabrication dg-1224

Anneal rolled foils between resistively-heated tantalum sheets in good vacuum for ~30 min at a temperature chosen BELOW any phase transition of the metal (uranium: below 930 K at 1e-7 torr); etch oxide first with highest-purity dilute nitric acid — an electronegative foil getters every metal impurity out of a dirty acid.

targetsmaterialsfabrication dg-1225

The experiment chamber vacuum is part of the target lifetime budget for reactive metals: uranium foils that left the lab metallic were destroyed as oxide in 1-2 h of beam at 1e-5 torr chamber pressure (ion-getter-pump-like oxidation at the beam spot); adding a cryopump to reach 1e-6 torr extended life to many hours.

targetsvacuum dg-1226

Reactive sputtering produces target films that evaporation cannot match for durability: adherent, "even, tough" nitride films with free choice of backing material, good thickness control, and purity limited only by system cleanliness — suitable for water-cooled targets under 10 uA alpha beams at 2-5 MeV.

targetsmaterials dg-1227

Keep hydrocarbons out of any sputtering/discharge deposition system — they crack in the discharge and load the film with carbon or carbides — and condition before deposit: outgas substrates hot, pre-sputter 10-15 min with shutters CLOSED onto the shield to expose fresh cathode, dump the contaminated gas, then refill and open the shutters with the cold trap filled.

targetsvacuum dg-1228

An uncooled sputtering cathode heats up during long runs and the sputter rate climbs with it, so deposited thickness is NOT linear in time — either water-cool the cathode or calibrate thickness against weighed samples rather than clock time.

targetsbeam-measurement dg-1229

Protect oxidation-prone target layers as a sandwich: evaporate 0.01-0.2 mg/cm2 of carbon, titanium, nickel or gold over (and under) the active layer; layers thick enough to be their own heat sink (20-100 mg/cm2) can instead go bare onto ~2-mm copper chips.

targetsmaterials dg-1230

Refractory-metal (W) evaporation practice: spot-weld the outgassed isotope ball to a ground-flat tungsten rod pedestal in the water-cooled crucible and heat by electron bombardment from a loop filament (6 kV, 130 mA); higher evaporation rates give LESS stressed targets; and tank pressure above 4e-6 torr makes the films brittle with short shelf life. Best substrate: NaCl-coated 10-mil stainless at 400-600 F.

targetsmaterialsvacuum dg-1231

Inventory of solid-target failure modes under intense beams — and the escape hatch: beam-spot temperatures reach ~3000 C where heat conduction is poor (past most melting points), charge buildup breaks nonconductive targets electrostatically, and structural rearrangement causes strain, peeling, blistering and voids. A windowless supersonic gas jet (density knot of an underexpanded Laval nozzle) is the unbreakable alternative, tunable ~1-100 ug/cm2 by inlet pressure.

targetsvacuum dg-1232

Mount curl-prone foils on frames pre-coated with Canada Balsam dissolved in xylene: once the solvent dries the balsam stays tacky indefinitely at low vapor pressure and retains foils that would otherwise curl off on drying; keep a cover frame over freshly floated films until dry.

targetsfabricationvacuum dg-1233

Rollability of a brittle metal is set by parts-per-hundred-thousand purity: a few hundred ppm of almost any metal (or O, N, H) embrittles chromium; only the highest-purity reduction route (hydrogen) gives rollable material, and heat treatments at intermediate thicknesses prevent pinholes when rolling on below ~1 mg/cm2.

targetsmaterials dg-1234

Derive the gas-purity spec for hydrogen reduction from equilibrium thermodynamics, not habit: for Cr2O3 + H2 at 1400 K the equilibrium water ceiling is 540 ppm, and completing the reaction in reasonable time wants <=10 ppm — hence a palladium-diffusion purifier (<1 ppm H2O/O2) and 6 h at 1400 K.

targetsmaterials dg-1235

Thick carbon foils (1-8 mg/cm2) need no evaporator: settle 325-mesh GRAPHITE powder from an air suspension onto carbon-coated glass, then press at ~14 tons/in2 into a lustrous flexible film (uniformity <10%). Amorphous carbon is rejected on physics grounds — it will not bind, and its low thermal conductivity and high resistivity make it a poor accelerator target anyway.

targetsfabrication dg-1236

Ion-beam power density on a sputter target (or any small-spot bombarded surface) forces cooling: at ~10 kV and 2-4 mm focus the loading exceeds 100 W/cm2 and a low-conductivity material surface runs several hundred C — hot enough to oxidize reactive materials mid-deposition and spoil thickness reproducibility — so the material post must be cooled.

targetsmaterials dg-1237

For long uninterrupted deposition runs and reactive process gases, a cold-cathode Penning source beats a duoplasmatron: no filament to burn out, 165 h of stable output (0.2% stability at 0.58 mA) demonstrated, chemically resistant to O2/N2 operation, and total source power under 30 W — no cooling circuit needed.

ion-sourcetargets dg-1238

Electrodeposited metal targets are mechanically tougher in beam than evaporated ones of the same material; for platinum the workable compromise is ~250 mA/cm2 (higher current densities give spongy deposits) from a hexahydroxoplatinate bath, ~3.6 ug/cm2-min.

targetsmaterials dg-1239

Carbon foil breakage under ion beams depends only on TOTAL integrated fluence — it is independent of foil thickness (2-22 ug/cm2) and of beam current — following tau(p-uA-min/mm2) ~ A*E^1.15 (MeV/amu) with A ~20 for Ar (larger for lighter ions, ~60 for N; smaller for heavier, ~5 for Ni/Br). Therefore make stripper/window foils as thin as mechanics allow: thinner costs nothing in life and minimizes dispersion.

targetsbeam-dynamics dg-1240

Optical transmittance calibrates surface density for evaporated METAL films too (Al, Cr, Cu, Au, Ag, Sn, Ti; ~+-20% at 546.1 nm) — but gravimetric calibration fails on slides coated with soap-like parting agents, which lose weight in vacuum; reactive metals (Ti, Al, Cr) reproduce worst because of oxide/gas uptake.

targetsbeam-measurement dg-1241

Kill pinholes by fixing the SUBSTRATE, and map your evaporation boat before spending isotope: an hour of argon glow discharge "levels" a commercial copper foil enough to give pinhole-free 1.8 mg/cm2 films over 100 cm2, and weighed-square thickness maps show the yield/uniformity trade — a chimney boat close-in gives maximum thickness over one small spot, a central-hole boat at 15 cm gives ~80% edge uniformity.

targetsfabrication dg-1242

Load titanium with hydrogen (or deuterium/tritium) by heating to ~650 C in sub-atmospheric purified gas: outgas at 800 C in vacuum first, pass the gas through a deoxygenating cartridge AND a liquid-nitrogen trap (without the trap absorption simply fails), and meter the uptake as a pressure drop in a known volume; ~2 h absorb-and-cool per batch, reversible by pumping at 800 C.

targetsion-sourcematerials dg-1243

Size the target heat problem by straight beam-power arithmetic before any material choice: P(W) = particle rate x energy per particle. Folger (GSI) example: 3e11/s of 17.5 MeV/u 238-U carries ~208 W total; focused to ~0.2 cm^2 that is ~1 kW/cm^2 specific deposition.

targetsbeam-dynamics dg-1244

Rotate the target when average power exceeds what a static foil stands: Folger (GSI) ran 9 sector ("banana") targets covering 59.6% of a 97.4 cm circumference at 15.5 cm radius, spun at 666 rpm phase-locked to the beam macropulse (20 degrees per 5 ms pulse) so successive pulses hit different targets; ~1e17 particles were integrated without significant radiation damage.

targetsfabrication dg-1245

Interlock target rotation with the beam: a GSI rotating disc of 1.8 mg/cm^2 Au targets survived up to 1 uA of 15 MeV/u Au ions at 1333 rpm, but an identical target exposed to full beam with the drive motor switched off was destroyed — SEM showed zones of molten gold from thermal deposition. Damage manifests first as wrinkling from beam heating, then melting.

targetssafety dg-1246

Sandwich low-melting-point target metals (Pb, Bi) between carbon layers: GSI standard practice for high-current wheel targets, e.g. C/Bi/C at 0.03/0.5/0.03 mg/cm^2, extending stability and lifetime under bombardment.

targetsfabrication dg-1247

Multi-layer overcoats buy target lifetime through conduction, not just chemistry: the 1983 heavy-ion discussion (led by Kobisk) recorded that metal/carbon overcoat layers enhance thermal and electrical conductivity, shunting both heat and accumulated charge to the heavy frame around the target, significantly increasing lifetime; energy accumulation showed up as local melting, detected as broadening of scattering peaks.

targetsmaterials dg-1248

Protect reactive target metals with a thin sacrificial overcoat, sized by experiment: Argonne reported lithium (500 ug/cm^2 on pinhole-free Ni) protected by ca. 500 ug/cm^2 of copper — possibly as thin as 100 ug/cm^2 — which held oxygen/moisture attack off for about five minutes of air exposure; gold at 30-40 ug/cm^2 gave only marginal protection.

targetsmaterials dg-1249

Monitor target thickness and damage in-beam rather than trusting pre-weighing: GSI watched the elastic-scattering peak at 30 deg forward with a surface-barrier detector — FWHM measures target homogeneity, broadening flags damage — and calibrated the detectors against weighed standard targets. Per-target histories on a rotating wheel came from tagging each count with wheel position.

targetsbeam-measurement dg-1250

When resolution does not matter, defocus: Ford (ORNL/HHIRF) class-1 experiments ran rolled 0.5-5 mg/cm^2 targets at 0.5-5 electrical uA — all the beam the target withstands — and deliberately diffused the beam spot on the target to manage heating.

targetsbeam-dynamics dg-1251

For small-quantity evaporations the failure mode is a molten ball overheating the substrate: the 1983 general-targets discussion (led by Gursky) prescribes cooling the substrate or backing it with a heat sink, and notes ~25 mg of isotope suffices if the e-beam focus is very small. Guard against evaporating the water-cooled copper hearth itself when e-beam heating tiny charges.

targetsfabrication dg-1252

Balance substrate heating against water cooling with visible diagnostics: Hinn (U. Washington) deposited thick Si on 0.3 mil Cu foil clamped loosely to a water-cooled copper beam-stop block; if pitting or burn-up of the foil occurs, increase cooling; if the deposit curls as it thickens, increase heating by slowing the water flow. Substrate sat at 900-1000 C purely from 35 mm source proximity.

targetsfabrication dg-1253

Make thick elemental Si from enriched SiO2 by magnesium reduction in a closed crucible: 200 mg oxide + 170 mg freshly filed Mg (avoid excess Mg or Mg2Si forms), pressed into a capped Ta crucible, 1000 C for 1 h under vacuum, MgO leached 48 h in 2N HCl, product outgassed at 1000 C; overall yield ~70%, and 70-80 mg of reduced Si makes one 1 mg/cm^2 target (Hinn).

targetsfabrication dg-1254

Internally stressed deposits have a shelf life: Hinn's 1 mg/cm^2 Si targets slowly curled and fractured within about two weeks (trapped impurities producing stress); targets left on their copper substrate could be recovered by re-annealing, and finished targets were stored under vacuum or argon.

targetsmaterials dg-1255

Slackened stripper foils live ~10x longer than taut ones: ATLAS (Pardo) mounts 2 ug/cm^2 arc-evaporated carbon on a holder whose diameter is then reduced to slacken the foil, gaining approximately an order of magnitude in lifetime (Ni beams: ~5 particle-nA-hours on target, ~1.0 particle-uA-h on the foil). ORNL (Ford) got reliable mass production of slackened foils by mounting them still wet in an airstream so they slip on the frame.

targetsfabrication dg-1256

Treat stripper/degrader foils as magazine-fed consumables and design the changer in from the start: HHIRF's tandem carried a 180-foil magazine (5-10 ug/cm^2 glow-discharge carbon); a slackened foil under a 1 uA, 10 mm^2 127-I beam at 25 MV was expected to last only ~1 hour (Ford).

targetsfabrication dg-1257

A cyclotron can feed stripper foils through the dee itself: the Chalk River superconducting cyclotron mounts glow-discharge (cracked ethylene) carbon foils at fixed intervals on a continuous bicycle chain that passes down the hollow coaxial-tuner conductor and into the upper dee half via the dee stem; expected foil lifetimes minutes to hours; failed foils advance to a magazine replaceable through a vacuum lock. Foils must be flat — ripples increase effective source thickness and degrade the first-orbit definition (Gallant & Dmytrenko).

targetsdeefabrication dg-1258

Glow-discharge (cracked-hydrocarbon) carbon beats arc-evaporated carbon for foil lifetime only above ~10 ug/cm^2: at 2-5 ug/cm^2 all fabrication methods gave about the same lifetime (Pardo/ATLAS); the stripper-foil discussion (Adair) found published comparisons mostly invalid because beams and current densities differed, and called for same-beam side-by-side lifetime tests.

targetsmaterials dg-1259

The saddle-field ion source is the cold path to sputtered targets: its beam carries a high fraction of energetic neutrals (sputters insulators as well as conductors), focuses to ~2 mm (small isotope quantities), and heats the evaporant only ~10 C — no damage to substrate or release agent, so substrates can sit very close for high collection efficiency; no magnetic field, electrostatic only (G. Thomas, ANL).

targetsfabricationion-source dg-1260

Saddle-field sputter setup numbers (Thomas, ANL): gun at 30 degrees to the target surface and ~5 cm away (steeper angles back-sputter material into the gun); vacuum ~1e-5 Torr with high-purity argon at very low flow (chamber pressure rises only ~0.2e-5 Torr); ~2 mA at 6 kV; component alignment is critical — misalignment shorts the source; the beam is visible in the dark for alignment.

targetsfabrication dg-1261

Budget time, not power, for sputtered targets — deposition rates are tens of ug/cm^2 per hour: measured saddle-field rates were Au ~44, Sn ~14, W ~12.5, Ni 5-13, Fe 4-10, Si ~4 ug/cm^2/hr (Glover, Drinkwater, and ANL columns agreeing on Au). About half an hour to stabilize, then the rig runs virtually unattended for days.

targetsfabrication dg-1262

Focused-ion-beam sputtering is the scarce-isotope economizer: GSI (Folger) consumed only 2.3 mg of Zr to make five 0.1 mg/cm^2 targets on carbon backings with a 1 mA, 10 kV Ar+ beam focused to ~1 mm on the cathode (Sletten-type apparatus); self-supported 0.2-0.5 mg/cm^2 rare-earth sputter layers were routine after dissolving a copper substrate.

targetsfabrication dg-1263

Stretch fragile foils flat with O-ring compression against a polished reference surface: the Argonne stretcher (Radford) clamps the foil between an O-ring and a tapered ring, then a central tube with an optically polished top lifts the foil; tightening the screws compresses the O-ring and draws the foil taut across the polished face. Near-100% success on 2-10 mg/cm^2 gold; foils stayed stretched under beam heating and after evaporating target material onto them; foil-to-foil gap verified capacitively down to ~3 um.

targetsfabrication dg-1264

Thermally stretch polypropylene windows over a heated dome and metallize them gently: Chalk River stretched 25 um film over a Teflon-coated disc held with a deliberate radial gradient (edge 105 C, middle 115 C, center 125 C) to ~75 ug/cm^2; a 10 ug/cm^2 cellulose-nitrate undercoat raised heat resistance; then Cr 5 ug/cm^2 in two steps and Au 20 ug/cm^2 in three steps with cooling pauses — single-step evaporation ruptured the film by radiant heat (Dmytrenko et al.).

targetsdetectorsfabrication dg-1265

Outgas the substrate and dry the finished foil completely — water is the hidden stress agent: McMaster's self-supporting rare-earth targets (Yaraskavitch & Peng) required baking the glass slides at 400 C before depositing the ~25 ug/cm^2 NaCl parting layer (residual moisture caused self-support failure via high film stress) and, after float-off, flushing all water from the mounted foil with methanol drops or pinholes and breakage appeared on drying. Success rate ~80% for 100-300 ug/cm^2 Dy/Er/Gd/Yb.

targetsfabrication dg-1266

Rotate the substrate for uniformity: with the substrate offset r from the source axis and rotating, thickness variation across a 2 cm target falls below 1% at the optimum ratio r/h ~ 0.7 (Behrndt geometry), versus 23% for a static substrate at close range; Maier (Munich) measured ~1% in practice by Au x-ray fluorescence. Static close-crucible geometry still wins on economy: 186 ug/cm^2 collected per mg of evaporant at h = 15 mm.

targetsfabrication dg-1267

Store hygroscopic and oxidizing targets under vacuum with out-rush protection: Argonne's computer-controlled target store (Nardi & Worthington) keeps 100 targets in a vacuum chamber with transfer under vacuum to the experiment; on recovery from a fault it roughs through a restrictor valve first, specifically to prevent target damage by gas out-rush, and its legacy failure list is instructive — plugged water-cooling pipes, loosening thermocouple connectors, a valve slamming on the transfer rod.

targetsvacuum dg-1268

A solid target in a circulating (storage-ring) beam is thickness-capped by beam heating and thermal runaway: IUCF Cooler design (Lozowski) tolerates only ~1 ug/cm^2 (C) to ~1.5 ug/cm^2 (Bi) with the beam traversing ~1e6 times/s; thicker targets defeat the cooling and dump the stored beam. Proposed solid-target routes: graze the beam edge with the target edge, and use fibers/whiskers (7 um C fiber ~975 ug/cm^2, 0.5 um quartz ~80 ug/cm^2) as overcoated substrates.

targetsbeam-dynamics dg-1269

Chemical vapor deposition makes thick refractory films from milligram feedstock: Chalk River (Gallant) flowed dilute H2 + WF6 over an induction- or lamp-heated susceptor at ~500 C and produced good-quality tungsten films over 2 mg/cm^2 thick; only the susceptor reaches reaction temperature, so very small quantities of W, Ta, or Mo serve, and the H2 sweeps out the fluorine by-product. Flagged as nearly absent from target-lab practice at the time.

targetsfabrication dg-1270

Durable self-supported oxide targets by cation-loaded cellulose decomposition: Quinby (ORNL) soaked purified carboxy-methyl-cellulose (dialysis-tubing) membrane in a boiling nitrate solution of the element, then decomposed it flat in staged heating (weighted teflon/copper/quartz sandwich under a heat lamp, 230 C oven, then furnace oxidation); films of 100 ug/cm^2 to several mg/cm^2 were strong, near-transparent, weighable, and frame-mountable — cohesion attributed to chemical bonding of cations in the polymer, well below sintering temperatures.

targetsfabrication dg-1271

Actinide-alloy targets for in-beam work were arc-melted into cubic non-paramagnetic host intermetallics and mounted on thick brass holders serving as heat sinks; the Stony Brook fission-isomer team chose the UIr2 host to defeat paramagnetic relaxation, with radiation damage from recoil implantation the other standing obstacle (paraphrase only — journal reprint, no quotation).

targetsmaterials dg-1272