Thresholds for (p, n) Reactions on 26 Intermediate-Weight Nuclei

Abstract

Protons with energies below 5 MeV, produced by either a 3-MV or a 5.5-MV Van de Graaff accelerator, were used to find ($p, n$) thresholds and threshold limits for 26 nuclei with $37\le A\le 112$. Data are presented from several independent measurements which were made over a period of about five years; the present values supersede those presented in earlier abstracts by Trail and Johnson and by Johnson and Galonsky. Energy calibrations are based on one or more of the following absolute standards: ${}_{}{}^7{}_{}{}^{}\mathrm{Li}(p, n)$, 1880.7±0.4 keV; ${}_{}{}^{11}{}_{}{}^{}\mathrm{B}(p, n)$ 3016.4±1.5; ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(p, n)$, 4234.4±1.0 keV; and ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}(p, \alpha \gamma )$, 872.5±0.4, 934.1±0.9, 1346.6±1.1, and 1373.5±0.6 keV. Neutrons were detected in each experiment by several B${\mathrm{F}}_3$ counters in $4\pi$ geometry. The yields near threshold have been interpreted in terms of the Hauser-Feshbach statistical theory of the compound nucleus, and in most cases there is good agreement with the predictions for the ground-state transitions. The targets and the corresponding negative $Q$ values in keV for these ground-state transitions are as follows: ${}_{}{}^{37}{}_{}{}^{}\mathrm{Cl}$, 1596.9±2.5; ${}_{}{}^{41}{}_{}{}^{}\mathrm{K}$, 1209.7±1.5; ${}_{}{}^{49}{}_{}{}^{}\mathrm{Ti}$, 1383.6±1.0; ${}_{}{}^{51}{}_{}{}^{}\mathrm{V}$, 1533.7±1.8; ${}_{}{}^{53}{}_{}{}^{}\mathrm{Cr}$, 1380.4±1.6; ${}_{}{}^{55}{}_{}{}^{}\mathrm{Mn}$, 1014.4±0.8; ${}_{}{}^{57}{}_{}{}^{}\mathrm{Fe}$, 1619±2; ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}$, 1855.3±1.6; ${}_{}{}^{61}{}_{}{}^{}\mathrm{Ni}$, 3024±4; ${}_{}{}^{65}{}_{}{}^{}\mathrm{Cu}$, 2135.8±1.7; ${}_{}{}^{67}{}_{}{}^{}\mathrm{Zn}$, 1783.3±1.4; ${}_{}{}^{68}{}_{}{}^{}\mathrm{Zn}$, 3707±5; ${}_{}{}^{69}{}_{}{}^{}\mathrm{Ga}$, 3006±4; ${}_{}{}^{70}{}_{}{}^{}\mathrm{Zn}$, 1439±3; ${}_{}{}^{71}{}_{}{}^{}\mathrm{Ga}$, 1018±2; ${}_{}{}^{74}{}_{}{}^{}\mathrm{Ge}$, 3348±5; ${}_{}{}^{75}{}_{}{}^{}\mathrm{As}$, 1647.3±1.1; ${}_{}{}^{80}{}_{}{}^{}\mathrm{Se}$, 2653±3; ${}_{}{}^{106}{}_{}{}^{}\mathrm{Pd}$, 3754±13; ${}_{}{}^{108}{}_{}{}^{}\mathrm{Pd}$, 2670±100; and ${}_{}{}^{112}{}_{}{}^{}\mathrm{Cd}$, 3400±20. In addition, three thresholds were observed for which the comparison of the observed yield with the predictions indicates that the reactions proceed to the excited states in the residual nuclei. The three targets and the corresponding negative $Q$ values in keV are as follows: ${}_{}{}^{73}{}_{}{}^{}\mathrm{Ge}$, 1189±15; ${}_{}{}^{89}{}_{}{}^{}\mathrm{Y}$, 4207±6; ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}$, 2720±100. The fact that the ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(p, n){}_{}{}^{93}{}_{}{}^{}\mathrm{Mo}$ threshold to the 1.48-MeV state was observed indicates that lower states in ${}_{}{}^{93}{}_{}{}^{}\mathrm{Mo}$ have $J<~\frac{5}{2}$. For three other targets the yield curves showed some indication of a threshold; however, comparisons with the theory in these cases indicate that only the following upper limits can be set to $-Q$ in keV; ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}(p, n){}_{}{}^{48}{}_{}{}^{}\mathrm{Sc}$, <640; ${}_{}{}^{82}{}_{}{}^{}\mathrm{Se}(p, n){}_{}{}^{82}{}_{}{}^{}\mathrm{Br}$, <920; ${}_{}{}^{93}{}_{}{}^{}\mathrm{Nb}(p, n){}_{}{}^{93}{}_{}{}^{}\mathrm{Mo}$, <1290. A comparison with the theory indicates that the observed yield above 650 keV for ${}_{}{}^{48}{}_{}{}^{}\mathrm{Ca}$ must be due to a transition to an excited state rather than the ground state of ${}_{}{}^{48}{}_{}{}^{}\mathrm{Sc}$.