X-Ray Yield Curves forγ−nReactions

Yield curves for the reactions ${\mathrm{C}}^{12}(\gamma ,n){\mathrm{C}}^{11}$ and ${\mathrm{Cu}}^{63}(\gamma ,n){\mathrm{Cu}}^{62}$ have been taken with x-rays up to 100 Mev. The induced radioactivity at each energy is plotted per unit x-ray intensity as measured by a $r$-meter thimble jacketed by ⅛ in. of Pb. Both yield curves are similar to the photo-fission yield curves, the x-ray yield increasing to a maximum and then slowly decreasing as the x-ray energy is increased. With simple assumptions regarding the generation of ionizing secondaries in the Pb walls of the monitor and assumption of a constant intensity x-ray spectrum, these curves can be analyzed. The relative cross section is found to have a maximum at approximately 22 Mev for ${\mathrm{Cu}}^{62}$ and 30 Mev for ${\mathrm{C}}^{11}$, decreasing to negligible values at high quantum energies. This decrease in cross section can be attributed to competition from multiple disintegrations. These reactions provide detectors sensitive only to part of the x-ray spectrum. Absorption curves have been taken in Pb using uranium photo-fission and in Cu and Pb using the ${\mathrm{Cu}}^{63}(\gamma ,n){\mathrm{Cu}}^{62}$ reaction as detectors. The resulting absorption coefficients compare favorably with theoretical values.