Fission of pre-actinide nuclei. The (p,f) reaction in the proton energy region 24--30 MeV

We have used the Alma Ata isochronous cyclotron to measure the integrated and differential cross sections for fission by protons of energy 24--30 MeV for 15 target nuclei in the region W--Bi. The experimental results, analyzed together with similar data for (..cap alpha..,f) reactions, are used to determine the fission barriers and to study the dependence of the fission probability on the introduced angular momentum. An anomalous effect of angular momentum on the fissility of a nucleus, observed in energy regions near threshold, is observed and interpreted.