Experimental Photo-Fission Thresholds inU23592,U23892,U23392,Pu23994, andTh23290

An investigation of the photo-fission thresholds in ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}_{}^{235}{}_{}{}^{}$, ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}_{}^{238}{}_{}{}^{}$, ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}_{}^{233}{}_{}{}^{}$, ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}_{}^{239}{}_{}{}^{}$, and ${}_{90}{}^{}{}_{}{}^{}\mathrm{Th}_{}^{232}{}_{}{}^{}$ with a 20-Mev betatron is reported. A method of alpha-self-cancellation is described which permits the detection of fissions and the determination of threshold energies in the two highly alpha-active samples of ${}_{92}{}^{}{}_{}{}^{}\mathrm{U}_{}^{233}{}_{}{}^{}$ and ${}_{94}{}^{}{}_{}{}^{}\mathrm{Pu}_{}^{239}{}_{}{}^{}$. An investigation of the photo-neutron threshold in nitrogen and the energy calculation from mass data provided a check of the absolute energy scale obtained from voltage measurements on the betatron.