Abstract: (D, T) neutron radioactive sputtering yields

Abstract

(D, T) neutrons have sufficient kinetic energy for various nuclear reactions such as (n,α), (n,p), and (n,2n) to occur. A series of experiments have been performed using a 14.8 MeV (d, t) neutron source in which the radioactive daughters ejected from the near surface region of the target material have been collected and measured. The geometry of the experimental arrangement was such that radioactive daughters ejected in the forward direction (neutron current direction) and backward direction were collected. For example, the forward radioactive sputtering ratio for 92mNb produced by the 93Nb(n,2n) reaction from cold-worked niobium foil is (1.06±0.06) 10−7 radioactive atoms/neutron, and the backward sputtering ratio for 92mNb is (7.58±1.09)10−10 atoms/neutron. For annealed gold the forward sputtering ratio for 196Au produced by the 197Au(n, 2n) reaction is (1.05±0.06)10−7 atoms/neutron, and the backward sputtering ratio for 196Au is (2.83±0.09)10−9 atoms/neutron. These sputtering ratios are the slopes of measured yields versus measured fluence curves and are obtained by a linear least-squares fit to the data. The quoted errors are the standard deviations calculated by the linear regression routine. Figure 1 is a bar graph summary of the forward radioactive sputtering ratios obtained for a large variety of metals and also for 316 stainless steel. The graph includes the reaction producing the radioactive recoil and the half-life of the radioactive daughter. It can be seen that the sputtering ratio for all of the materials studied range from the mid-10−8 to mid-10−7 atoms/neutron range. We have not observed any statistical differences in the radioactive sputtering ratios between annealed and cold worked niobium or between annealed and cold worked 316 stainless steel. In addition, it is possible to determine an effective emission range of the radioactive recoil. Since the metal targets used in the experiments are thicker than heavy-ion ranges in metals, an effective emission range can be defined simply as the product of the ratio of the number of radioactive recoil atoms on the collector to the number of recoil atoms in the target and the thickness of the foil. As an example, the forward effective range of the 92mNb recoil ion in niobium is 410 Å, while the backward effective range is only about 3 Å. For such small ranges, elemental surface and near surface composition such as oxide and carbide layers may produce errors in the results because of uncertainties in the parent atom density. Complete details of this experiment and data analysis are being submitted to Physical Review or the Journal of Applied Physics.