Historical research guide · 0992 · 2006

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

In one minute

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.

Connection

Chapter 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.

From the source

Helium 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
The 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
The 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
The 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
Beam 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
The 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

The way in

https://doi.org/10.1088/1748-0221/1/09/p09003

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

LICENCE. 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.

How to cite it

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 (2006) In beam tests of implanted helium targets doi:10.1088/1748-0221/1/09/p09003

Where it sits in the curriculum

Lattice confinement fusion