Threshold Measurements on the Nuclear Photo-Effect

The high voltage x-radiation from the betatron has been employed to produce ($\gamma ,n$) reactions in several elements of atomic number up to 47, the reaction observed by detection of induced radioactivity. The peak x-ray energy was controlled by integrating the voltage on the main coils with an RC circuit, which actuated the orbit expander at a predetermined electron energy. Upon irradiating samples at sequences of energies, measuring their activity with beta-counters, and plotting activities against peak energy, smooth curves were obtained from which thresholds were estimated. The accuracy of the method is limited by low activity in almost all cases and by uncertainties in energy output. The following thresholds in Mev have been measured: ${\mathrm{C}}^{11}$, 18.7 to 19.4; ${\mathrm{N}}^{13}$, 11.1±0.5; ${\mathrm{O}}^{15}$, 16.3±0.4; ${\mathrm{Fe}}^{53}$, 14.2±0.4; ${\mathrm{Cu}}^{62}$, 10.9±0.3; ${\mathrm{Zn}}^{63}$, 11.6±0.4; ${\mathrm{Se}}^{79\mathrm{o}\mathrm{r}81}$, 9.8±0.5 for the lower, short period isomer; ${\mathrm{Mo}}^{91\mathrm{o}\mathrm{r}93}$, 13.5±0.4; ${\mathrm{Ag}}^{108}$, 9.3±0.5; ${\mathrm{Ag}}^{106}$, 9.5. The first three are in fair agreement with other data, but because of exceptionally weak activities are not sufficiently reliable to afford a test of the method. Rough measurements on the uranium photo-fission threshold are also reported. Since the radiation from the betatron is continuous and its spectral distribution is unknown, the excitation curves are not susceptible to interpretation, and cross sections for the above reactions cannot be estimated at present.