The Photodisintegration ofHe4Nuclei by X-Rays from a 100-Mev Betatron

The photodisintegration of ${\mathrm{He}}^4$ nuclei has been studied in a cloud chamber, with a 100-Mev betatron as a source of x-rays. The predominant reactions observed are ${\mathrm{He}}^4(\gamma ,p){\mathrm{H}}^3$ and ${\mathrm{He}}^4(\gamma ,n){\mathrm{He}}^3$. The angles, $\theta$, at which the protons from the ($\gamma ,p$) reaction are emitted with respect to the x-ray beam are consistent with a ${sin}^2\theta$ distribution. The measured average energy of the x-ray photons producing the $\gamma -p$ disintegration is approximately 27 Mev. The total integrated photodisintegration cross section is 0.1±0.05 Mev-barn; this value is based on an earlier determination of the absolute nitrogen cross section, obtained by a direct comparison of the number of nuclear disintegrations with the number of position-electron pairs produced in the gas in the cloud chamber.