Helium gas at atmospheric pressure was irradiated with 26-, 29-, 32-, and 40-Mev bremsstrahlung spectra. The energy and angular distributions of the protons produced by the photodisintegration of ${\mathrm{He}}^4$ were studied using nuclear emulsions to detect the charged particles. Angular distributions of the protons are fitted to a differential cross section of the form $\sigma \left({\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}\right)\sim \left(\frac{a}{b}\right)+{{sin}^2\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}[1+(\frac{\gamma }{b}\left){cos\theta }_{\mathrm{c}.\mathrm{m}.\mathrm{}}\right]$. Over the energy range studied, the interference term may increase slowly with energy but is approximately constant ($\frac{\gamma }{b}=0.2\mathrm{}\pm 0.1$); the isotropic term increases rapidly with energy from 0 to 0.33±0.2 at a mean photon energy of 32.5 Mev. The ($\gamma , p$) cross section reaches a peak value of about 1.8 mb at about 26 Mev. $\int {20}^{40}\sigma \mathrm{dE}=0.016\mathrm{}\pm 0.005$ Mev barns.

• Received 18 May 1954

DOI:https://doi.org/10.1103/PhysRev.96.1306