The mass difference $K^{40}-{\mathrm{Ar}}^{40}$ and the electron-capture (EC) decay energy of ${\mathrm{K}}^{40}$ to the first excited state (2+) of ${\mathrm{Ar}}^{40}$ have been obtained from a measurement of the $Q$ value of the exothermic reaction ${\mathrm{K}}^{40}(n_{\mathrm{th}},p){\mathrm{Ar}}^{40}$. The following reactions have been observed and their $Q$ values determined: ${\mathrm{K}}^{40}(n,p_0){\mathrm{Ar}}^{40}$ (2.270±0.005 MeV), ${\mathrm{K}}^{40}(n,p_1){\mathrm{Ar}}^{40}$ (0.811±0.005 MeV), ${\mathrm{K}}^{40}(n,\alpha ){\mathrm{Cl}}^{37}$ (3.866±0.007 MeV), and, for comparison, ${\mathrm{Cl}}^{35}\mathrm{}(n,p)\mathrm{}{\mathrm{S}}^{35}$ (0.612±0.004 MeV). From the reaction ${\mathrm{K}}^{40}(n,p_0){\mathrm{Ar}}^{40}$ the values 1.488±0.005 MeV and 27±5 keV, respectively, have been obtained for the ${\mathrm{K}}^{40}$-${\mathrm{Ar}}^{40}$ mass difference and the EC decay energy of ${\mathrm{K}}^{40}$. The cross sections of the observed reactions were determined relative to the ${\mathrm{N}}^{14}\mathrm{}(n,p)\mathrm{}{\mathrm{C}}^{14}$ reaction cross section, yielding the values: ${\mathrm{K}}^{40}(n,p_0){\mathrm{Ar}}^{40}$ (4.30±0.34 b), ${\mathrm{K}}^{40}(n,p_1){\mathrm{Ar}}^{40}$ (0.127±0.016 b), ${\mathrm{K}}^{40}(n,\alpha ){\mathrm{Cl}}^{37}$ (0.388±0.032 b), and ${\mathrm{Cl}}^{35}\mathrm{}(n,p)\mathrm{}{\mathrm{S}}^{25}$ (0.446±0.040 b).

• Received 24 August 1967

DOI:https://doi.org/10.1103/PhysRev.165.1184