Yields and Angular Distributions of Some Gamma-Neutron Processes

Measurements of $\gamma -n$ yields for 53 elements have been made using betatron bremsstrahlung with maximum energies of 18 Mev and 22 Mev. The neutron flux was observed by a rhodium detector and compared with a standard Ra-Be neutron source. In the region from $Z=22\mathrm{}$ to $Z=83\mathrm{}$ the $\gamma -n$ yield in neutrons per mole per $r$ of radiation is observed to be 2270. $Z^{2.1}$ when the bremsstrahlung spectrum has a 22-Mev maximum energy. The regularity of neutron yield as a function of atomic number indicates a definite similarity in the $\gamma -n$ cross-section curves for neighboring heavy elements. The trend of the observed yields is in rough agreement with the yields predicted by Goldhaber and Teller. Estimates are made of the fractions of thorium and uranium atoms which undergo photo-fission.