Evidence for isomeric electron induced fission of 232Th

Fragment angular distributions and yields were measured for the electrofission of 232Th in the electron energy Ee range 4.54-6.64 MeV. The behaviour of the angular distribution with changing Ee and the observation of an isotropic angular distribution for Ee=4.54 MeV together with a shelf in the electrofission yield for Ec< ≅ 5.2 MeV, indicates the existence of a fission isomer in the second well of the potential energy curve. An inner barrier height of ≅ 5 MeV and a height of the second minimum Vparallel<4 MeV can be deduced from our measurements. (orig.)