STM-D-0992Paper2006Published and peer-reviewed

In beam tests of implanted helium targets

J. E. McDonald · Ralph H. France III · R. A. Jarvis · M. W. Ahmed · M. A. Blackston · Th. Delbar · M. Gai · T. J. Kading · Y. Parpottas · B. A. Perdue · R. M. Prior · D. F. Rubin · M. C. Spraker · J. D. Yeomans · L. Weissman · H. R. Weller · E. L. Wilds Jr

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To measure a nuclear reaction you need a target made of the right element, and helium is awkward, because it is a gas that will not sit still. McDonald, France, Jarvis, Gai, Weller and their colleagues test the workshop answer: fire a low-energy helium beam into a very thin aluminium foil until the helium is buried inside the metal, then use the foil itself as the target. Their question is whether that buried gas stays put once a real experiment starts hitting it. It does. Bouncing proton and oxygen beams off the foils before, during and after bombardment, they measure the helium content and find it equals the dose that went in, and they see no loss of helium and no change in its depth profile after as much as two days under beam. They also map the ceiling: aluminium blisters above roughly six hundred thousand million million helium atoms per square centimetre, and implanting a second, deeper layer does not get past it.

Why it matters hereChapter 12 is about nuclei reacting inside a metal lattice, and every such experiment stands or falls on how much fuel the lattice actually holds and whether it stays there under beam. This paper is the measurement of exactly that, made with ordinary backscattering, and it also names the trap: the standard range code disagrees with the standard tabulations about how deep low-energy helium goes, so a target can be wrong before the physics begins.

What it claims

01Helium implanted into a thin aluminium foil makes a usable nuclear target: recoil measurements with a 15 MeV oxygen beam give an areal density of 3.4 plus or minus 0.3 times ten to the seventeenth helium-4 atoms per square centimetre, equal to the implanted fluence, and the extracted content is independent of scattering angle. An earlier report that only half the implanted dose is retained came from neglecting the centre-of-mass Jacobian, of order 0.5, in the yield formula.Section III; Figures 1 and 2
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02The implanted helium is stable under bombardment. The width of the recoil spectrum stayed constant through 18.4 hours of a 20 MeV oxygen beam at 50 particle nanoamperes, with no deterioration against cumulative dose, and a target implanted from both sides and then exposed for two days to an 80 particle nanoampere helium-4 beam showed no change in content, peak position or peak width when scanned across in eight steps.Section III; Figures 3, 4 and 7
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03The ceiling on loading is the blistering limit of about 6 times ten to the seventeenth helium atoms per square centimetre, and it cannot be beaten by stacking layers: two implantations, each individually below the limit, together exceeded it and gave broad low-statistics peaks because the layers did not stay distinct. Implanting from opposite sides of a 216 microgram per square centimetre foil does give two separate layers, seen as two peaks in 1.0 MeV proton backscattering, separated by 45 micrograms per square centimetre.Section II; Section III; Figure 6
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04The standard range codes disagree with the measurements in opposite directions. SRIM underestimates the range of very low energy alpha particles in aluminium — it predicts 92.8 micrograms per square centimetre at 50 keV against 142 from Northcliffe and Schilling and 141.8 from the NIST tabulation — while Northcliffe and Schilling and NIST both overestimate it. Trusting SRIM for 20 keV helium-4 in 80 to 100 microgram per square centimetre foils placed the helium at the far edge and only about 10 percent of the fluence was retained. SRIM’s prediction for the straggle, by contrast, matches the measured 27 keV peak width.Section III, the range comparison paragraphs
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05Beam heating broadens the depth profile without driving the gas out: a 45 keV implantation beam of 2 to 3 microamperes, depositing about 100 milliwatts, widened the first implanted layer from 27 to 46 keV while leaving the same amount of helium-3 in it, mirroring what the Weizmann group reported for a heated beryllium-7 implanted target.Section III, the two-layer profile analysis; Figure 7
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06The point of the exercise is a measurement: a helium-3 implanted target at 5 times ten to the seventeenth atoms per square centimetre with a modest 500 nanoampere helium-4 beam at 1.0 MeV should yield about 40 counts per hour of 2.016 MeV direct-capture gamma rays in a germanium detector of 2 percent efficiency, with an energy spread of about 30 keV, which is enough to measure the helium-3 plus alpha capture reaction and its astrophysical S-factor down to a centre-of-mass energy of about 400 keV.Section I; Section IV, Conclusion
Designed, not yet built

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Abstract

Targets consisting of 3,4He implanted into thin aluminum foils (approximately 100, 200 or 600 µg/cm2) were prepared using intense (a few µA) helium beams at low energy (approximately 20, 40 or 100 keV). Uniformity of the implantation was achieved by a beam raster across a 12 mm diameter tantalum collimator at the rates of 0.1 Hz in the vertical direction and 1 Hz in the horizontal direction. Helium implantation into the very thin (approximately 80-100 µg/cm2) aluminum foils failed to produce useful targets (with only approximately 10% of the helium retained) due to an under estimation of the range by the code SRIM. The range of low energy helium in aluminum predicted by Northcliffe and Shilling and the NIST online tabulation are observed on the other hand to over estimate the range of low energy helium ions in aluminum. An attempt to increase the amount of helium by implanting a second deeper layer was also carried out, but it did not significantly increase the helium content beyond the blistering limit (6 × 10^17 atoms/cm2). The implanted targets were bombarded with moderately intense 4He and 16O beams of 50-100 particle nA. Rutherford Back Scattering of 1.0 and 2.5 MeV proton beams and recoil helium from 15.0 MeV oxygen beams were used to study the helium content and profile before, during and after bombardments. We observed the helium content and profile to be very stable even after a prolonged bombardment (up to two days) with moderately intense beams of 16O or 4He. Helium implanted into thin (aluminum) foils is a good choice for thin helium targets needed, for example, for a measurement of the 3He(alpha, gamma)7Be reaction and the associated S34 astrophysical cross section factor (S-factor).

J. E. McDonald, Ralph H. France III, R. A. Jarvis, M. W. Ahmed, M. A. Blackston, Th. Delbar, M. Gai, T. J. Kading, Y. Parpottas, B. A. Perdue, R. M. Prior, D. F. Rubin, M. C. Spraker, J. D. Yeomans, L. Weissman, H. R. Weller and E. L. Wilds Jr, In beam tests of implanted helium targets, Journal of Instrumentation 1, P09003 (2006); preprint arXiv:nucl-ex/0608027, 16 August 2006. The beams came from the Wright Nuclear Structure Laboratory at Yale, the Triangle Universities Nuclear Laboratory at Duke and the Yale 1 MV teaching accelerator.

(Abstract only — see the rights note above. On this site, the companion craft of loading light nuclei into metals for low-energy fusion work is covered by the experimental techniques for electron-screening measurements at /library/stm-cd71abdcaa, the measurement of enhanced screening for deuterium in metals at /library/stm-2808bb6545, the deuterium-on-zirconium screening experiment at /library/stm-f5eb4f4d1a, and NASA Glenn’s overview of lattice confinement fusion, which turns a loaded lattice into a reaction volume, at /library/stm-e25595eb3b.)

The way in

https://doi.org/10.1088/1748-0221/1/09/p09003LICENCE. Published as Journal of Instrumentation volume 1, article P09003, September 2006, by IOP Publishing under its own licence with no Creative Commons statement; the IOP article page is closed to automated retrieval. The authors’ preprint, arXiv nucl-ex/0608027 version 1 of 16 August 2006, carries the arXiv non-exclusive distribution licence, which is also not a Creative Commons licence, so the sheet stays abstract-only. TEXT READ. The complete preprint was retrieved and read on 2026-09-08, and every claim below is located to its numbered sections and figures. ABSTRACT. The abstract reproduced below is the authors’ own preprint abstract, which is fuller than the version the registry record carried from the publisher. Superscript and subscript nuclide labels are flattened to plain text, so helium-3 and helium-4 appear as 3,4He and the reaction as 3He(alpha, gamma)7Be; nothing else is changed. AUTHORS AND YEAR. The seventeen authors are in the order printed on the paper. The registry record carried the second author as R H France; the full name Ralph H. France III is taken from the INSPIRE-HEP author record. The work was supported by four United States Department of Energy grants and was carried out at the University of Hartford, Georgia College and State University, the Laboratory for Nuclear Science at Avery Point of the University of Connecticut, Duke University and the Triangle Universities Nuclear Laboratory, the Universite Catholique de Louvain, Yale University and North Georgia College and State University.

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Lattice confinement fusion