  Targets consisting of 3,4He implanted into thin aluminum foils (approximately
100, 200 or 600 ug/cm^2) were prepared using intense (a few uA) 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
ug/cm^2) 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 (approximately 6 x 10^17 helium/cm^2). 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(a,g)7Be reaction and the associated S34
astrophysical cross section factor (S-factor).
