Angular and Energy Distributions of Photoprotons from Aluminum and Tantalum

Foils of aluminum and tantalum were irradiated at energies from 25 to 65 Mev in the x-ray beam of the Iowa State College synchroton. Photoprotons produced at various angles were recorded in nuclear emulsions. Those emitted with low energies appear isotropically distributed and have energy distributions characteristic of statistical evaporation processes. The higher-energy protons from aluminum have an angular distribution of the form ${(sin\theta +psin\theta cos\theta )}^2$ with $p\approx 0.7$, whereas those from tantalum have a nearly pure ${sin}^2\theta$ distribution. Their energy distributions can be represented in the form ${E_p}^{-n}$, where $E_p$ is the proton energy. For aluminum, sharp increases in the exponent $n$ occur at a value of $E_p$ slightly greater than half the maximum photon energy of the irradiation. The increases are considerably less in the case of tantalum. The aluminum results are qualitatively consistent with Levinger's quasi-deuteron model of the nucleus. None of the proposed photon interaction models gives a good explanation of the tantalum results.