The artificial radioactivity produced in targets of Cu, Ni, Ag, Cr and Mo by bombardment with four-Mev protons has been studied. In Cu, the ($p,n$) reaction gives rise to the radioelements ${\mathrm{Zn}}^{63}$ (half-life 38.5 min., upper limit of positron spectrum 1.8 Mev) and ${\mathrm{Zn}}^{65}$. Two new radioactive isotopes of Cu are found in Ni bombarded with protons; they have tentatively been assigned as follows: ${\mathrm{Cu}}^{58}$, half-life 81 sec., and ${\mathrm{Cu}}^{60}$, half-life 7.9 minutes. Both emit positrons. Also produced in Ni are ${\mathrm{Cu}}^{61}$, both by the ($p,n$) and the ($p,\gamma$) reaction (threshold <1.8 Mev), and ${\mathrm{Cu}}^{64}$, by the ($p,n$) reaction (threshold apparently 2.1 Mev). Proton bombardment of Ag results in a radioactive Cd isotope which emits very soft negative electrons which are believed to be conversion electrons of a nuclear gammaray. The identification of the responsible Cd isotope and the mode of decay is discussed. The half-life of this radio-element is 6.7 hours; the threshold for the reaction is 2.4 Mev. 40-minute ${\mathrm{Mn}}^{51}$ is formed by proton bombardment of Cr. This radioelement emits positrons. Three unassigned activities produced in Mo have half-lives of 45 min., 2.7 hours, and 36.5 hours; the last emits negative electrons.

• Received 7 November 1938

DOI:https://doi.org/10.1103/PhysRev.55.113