The paper reports a study of certain induced radioactivities associated with potassium. It is shown that the half-life of ${\mathrm{K}}^{42}$ is 12.4±0.2 hours. In addition to the well-established deuteron and slow neutron reactions ${\mathrm{K}}^{42}$ is also formed by the reactions ${\mathrm{Sc}}^{45}+n^1\to {\mathrm{K}}^{42}+{\mathrm{He}}^4$ and ${\mathrm{Ca}}^{42}+n^1\to {\mathrm{K}}^{42}+{\mathrm{H}}^1$. This last appears to have a relatively small probability. No evidence was found for the reaction ${\mathrm{Ca}}^{44}+{\mathrm{H}}^2\to {\mathrm{K}}^{42}+{\mathrm{He}}^4$. Fast neutron irradiation of potassium yields active chlorine and ${\mathrm{A}}^{41}$ by the transmutations ${\mathrm{K}}^{39 or 41}+n^1\to {\mathrm{Cl}}^{36 or 38}+{\mathrm{He}}^4$; ${\mathrm{K}}^{41}+n^1\to {\mathrm{A}}^{41}+{\mathrm{H}}^1$. There was no indication of the presence of ${\mathrm{A}}^{39}$ in the activated samples. Alpha-particle bombardment of chlorine yields ${\mathrm{K}}^{38}$ according to the reaction ${\mathrm{Cl}}^{35}+{\mathrm{He}}^4\to {\mathrm{K}}^{38}+n^1$. This isotope of potassium decays with a half-life of ${7.7}_5$±${0.1}_5$ minutes, emitting positrons having a maximum energy of about 2 Mev and gamma-rays which may be annihilation radiation only. ${\mathrm{K}}^{38}$ is also obtained by deuteron bombardment of calcium thus: ${\mathrm{Ca}}^{40}+{\mathrm{H}}^2\to {\mathrm{K}}^{38}+{\mathrm{He}}^4$.

• Received 3 May 1937

DOI:https://doi.org/10.1103/PhysRev.51.1033