Systematics of Photoproton Reactions

The photoproton yields from the elements Ta, Pt, Pb, W, and Au have been determined for betatron bremsstrahlung bombardment at 22-Mev peak energy by the use of zinc sulfide detectors and a 40-μsec betatron pulse duration. The yields, as measured in photoprotons per mole per roentgen, are as follows: Ta, 5.7×${10}^4$; Pt, 2.9×${10}^4$; Pb, 5.8×${10}^4$; W, 5.2×${10}^4$ and Au, 1.9×${10}^4$. These values, along with previous determinations, permit a study of general behavior of photoproton yields for all $Z$ values throughout the periodic table. Comparisons with calculations based on the evaporation model show good agreement with experimental trends up to a $Z$ of 50, after which the measured yields are too high by factors ranging from 10 to ${10}^4$. Calculations based on the direct photoelectric process give better agreement.