Rare Nuclear Reactions Induced by 14.7-MeV Neutrons

Activation cross sections for 14.7±0.2-MeV neutrons were measured for [$(n, n\alpha )+(n, \alpha n)$] reactions with ${\mathrm{Cu}}^{65}$, ${\mathrm{Zn}}^{70}$, ${\mathrm{Ga}}^{71}$, and ${\mathrm{Nb}}^{93}$, while upper limits were set for this reaction for ${\mathrm{V}}^{51}$, ${\mathrm{Ge}}^{76}$, ${\mathrm{Br}}^{81}$, ${\mathrm{Rb}}^{87}$, ${\mathrm{Ag}}^{107}$, ${\mathrm{Ag}}^{109}$, ${\mathrm{In}}^{115}$, ${\mathrm{Au}}^{197}$, and ${\mathrm{Tl}}^{203}$. Cross-section limits also were set for ($n, 2p$) reactions on ${\mathrm{Si}}^{29}$, ${\mathrm{K}}^{41}$, ${\mathrm{Sc}}^{45}$, ${\mathrm{Ti}}^{50}$, ${\mathrm{V}}^{51}$, ${\mathrm{Mn}}^{55}$, ${\mathrm{As}}^{75}$, ${\mathrm{Y}}^{89}$, ${\mathrm{Nb}}^{93}$, ${\mathrm{Cs}}^{133}$, ${\mathrm{La}}^{139}$, ${\mathrm{Pr}}^{141}$, and ${\mathrm{Tb}}^{159}$; and for ($n$, ${\mathrm{He}}^3$) reactions on ${\mathrm{Sc}}^{45}$, ${\mathrm{Nb}}^{93}$, ${\mathrm{Au}}^{197}$, and ${\mathrm{Tl}}^{205}$. Cross sections were determined for ($n, \gamma$) reactions on ${\mathrm{Y}}^{89}$, ${\mathrm{Nb}}^{93}$, and ${\mathrm{Pr}}^{141}$, and upper limits were set for ($n, 3n$) reactions with ${\mathrm{Pr}}^{141}$, ${\mathrm{Au}}^{197}$, and ${\mathrm{Tl}}^{203}$. The [$(n, np)+(n, pn)$] reaction was detected with ${\mathrm{Ni}}^{58}$, but not with ${\mathrm{Mo}}^{92}$. A value of 520±120 mb was established for the former, and an upper limit set for the latter. Absolute disintegration rates were obtained by both beta and gamma spectroscopy counting methods. Extensive use of radiochemical separation was made in order to isolate the low-yield rare reaction products from large target samples. Statistical theory was employed (using parameters selected from a critical analysis of the literature) to make theoretical cross section estimates for many of the reactions studied. The theoretical cross sections for ($n, \alpha n$) reactions agree remarkably well with the experimentally determined [$(n, n\alpha )+(n, \alpha n)$] cross section sums, thereby suggesting that the path for this reaction may be predominantly the ($n, \alpha n$) process. For certain cases, statistical theory predicts appreciable ($n, 2p$) cross sections at 14.7 MeV; e.g., for ${\mathrm{Cr}}^{50}$, ${\mathrm{Ni}}^{58}$, ${\mathrm{Kr}}^{78}$, and ${\mathrm{Mo}}^{92}$. It is suggested that in studies of emitted proton spectra which show an "excess" of low-energy protons, a contribution may be present from the ($n, 2p$) as well as the ($n, \mathrm{np}$) reaction. A new gamma at 176±4 keV in about 5% of 1.8-day ${\mathrm{Sc}}^{48}$ decays is confirmed.