Relative Photofission Yields of Several Fissionable Materials

Relative photofission yields of ${\mathrm{U}}^{238}$, ${\mathrm{U}}^{236}$, ${\mathrm{U}}^{235}$, ${\mathrm{U}}^{234}$, ${\mathrm{U}}^{233}$, ${\mathrm{Th}}^{232}$, ${\mathrm{Np}}^{237}$, and ${\mathrm{Pu}}^{239}$ were measured at three betatron energies with a photomultiplier fission fragment detector. Results at betatron energies of 17 and 20 Mev are in agreement and give the following relative fission values: ${\mathrm{Th}}^{232}$, 0.31; ${\mathrm{U}}^{238}$, 1.00; ${\mathrm{U}}^{236}$, 1.43; ${\mathrm{U}}^{235}$, 2.40; ${\mathrm{U}}^{234}$, 1.82; ${\mathrm{U}}^{233}$, 2.54; ${\mathrm{Np}}^{237}$, 2.40; ${\mathrm{Pu}}^{239}$, 3.17. The relative ($\gamma , f$) yields are empirically correlated with the nuclear parameter $\frac{Z^2}{A}$. Some comments are made on the competition between fission and neutron emission.