Subthreshold photofission of even-even nuclei

Within quantum-mechanical fission theory, the angular distributions of fragments originating from the subthreshold photofission of the even-even nuclei Th-232, U-234, U-236, U-238, Pu-238, Pu-240, and Pu-242 are analyzed for photon energies below 7 MeV. Special features of various fission channels are assessed under the assumption that the fission barrier has a two-humped shape. It is shown that the maximum value of the relative orbital angular momentum L-m of fission fragments can be found upon taking into account deviations from the predictions of A. Bohr's formula for the angular distributions of fission fragments. The result is L-m approximate to 30. The existence of an "isomeric shelf" for the angular distributions of fragments from U-236 and U-238 photofission in the low-energy region is confirmed. (c) 2005 Pleiades Publishing, Inc.