Proton Induced Radioactivities III. Zinc and Selenium Targets

Zinc bombarded with 6.5-Mev protons shows activities of half-lives 18.0±0.5 min., 72±4 min., 9.4±0.2 hr., and 84.4±2.0 hr., corresponding to the known periods of ${\mathrm{Ga}}^{70}$, ${\mathrm{Ga}}^{68}$, ${\mathrm{Ga}}^{66}$, and ${\mathrm{Ga}}^{67}$, respectively. In addition, a new period of 48±2-min. half-life is observed and assigned to ${\mathrm{Ga}}^{64}$. Thick target excitation curves are given for ${\mathrm{Ga}}^{64}$, ${\mathrm{Ga}}^{68}$, and ${\mathrm{Ga}.\mathrm{}}^{70}$. Selenium bombarded with 6.3-Mev protons shows activities of half-lives 6.3±0.2 min., 17.4±0.5 min., 4.4±0.3 hr., and 33±1 hr., corresponding to the known periods of ${\mathrm{Br}}^{78}$, ${\mathrm{Br}}^{80}$ (2) and ${\mathrm{Br}}^{82}$, respectively. The previously observed discrepancy between the thresholds for the two ${\mathrm{Br}}^{80}$ activities and their maximum $\beta$-ray energies has been confirmed. Thick and thin targets excitation curves were obtained. The cross section for the production of the ${\mathrm{Br}}^{80}$ isomers by the ${\mathrm{Se}}^{80}$ ($p-n$) reaction are 0.82 and 0.22×${10}^{-25\mathrm{}}$ ${\mathrm{cm}}^2$ for the short and long periods, respectively, at a proton energy of 6.3 Mev. The ratio of these two cross sections rises from 3.6 at 6.3 Mev to over 200 at 3.2 Mev.