Recoil Gold Atoms from Fast-Neutron Bombardment

Gold foils were irradiated with fast neutrons from a Pu-Be neutron source and the gold atoms knocked out of these foils were collected on aluminum foils. Sets of Au-Al foils were placed at various angles around the Pu-Be source. Some foil sets had the gold foil nearer the neutron source; some had the aluminum foil nearer. In this way, both the ejection of gold atoms from open-ended displacement spikes and the sputtering of gold atoms by fast neutrons could be studied. As was expected, no 198Au was detected on the Al foils because of the small (n,γ) cross section of 197Au. After neutron activation in a nuclear reactor, all the Al foils emitted the 0.412-MeV gamma ray of 198Au. Calculations showed that each Al foil had approximately 1012 197Au atoms on it. Since the Pu-Be neutron source strength was known, the number of gold atoms ejected per open-ended displacement spike was calculated and found to be of the order of magnitude of 106. Focusing collisions may account for the large number of Au atoms per open-ended displacement spike. The sputtering ratio was found to be about 0.5 gold atoms sputtered per incident neutron.