Further Radiochemical Studies of Fission of theU236*Compound Nucleus

Abstract

Fission-product yields relative to the fission yield of Mo" were measured for the fission of Th 232 with He 4 ions of energies ranging from threshold to 45 MeV and for the fission of U 235 with 14-MeV neutrons. The linear relationships between fission yields predicted by the two-mode fission hypothesis and previously observed for fission of U 236 * at low excitation energies break down at the higher energies in many cases. The fission yield of Mo" and the total fission cross section were evaluated as a function of He 4 ion energy.

Fission Mass Yield Dependence on Angular Momentum

“…) Yields are given in Table I for all the individual fission products measured. Some disagreements exist with earlier measurements of the U 236 compound nucleus yields 7 ' 19 40 Our smallest mass number case of Th 232 0,/) has symmetry at mass 114 after neutron emission, but no enhanced yield of symmetric fission is indicated by the neighboring Ag lf3 yields in Table I. For the next heavier case, mass distributions from Th 282 (a,/) derived from some measurements of fragment energies 12 also gave no indication of an additional peak for symmetric fission.…”

“…3 for common excitation energy E® of the compound nucleus U 236 . The symmetric fission yield has been observed previously to increase exponentially [17][18][19] with energy to 15-MeV excitation and then linearly 18,19 with energy. There is evidence of a slight dip in the symmetric fission yield at the 20-MeV excitation energy onset of (x,2nf) fissions.…”

“…1, is relatively large for fissions produced at the 23.43-MeV and approximately 16-MeV excitation energies of first-and second-chance fissions, respectively. However, because of the exponentially de-creasing yields [17][18][19] below about 15-MeV excitation energy, the symmetric fission yield of third-chance fission induced at approximately 8 MeV is relatively very low. The combination of partial cross sections and yields into the combined yields 8 to be greater than unity.…”

Mass spectrometric determination of Rb fission yields from 3He and 4He induced fission of 238U and 232Th

“…The fission of 'j8U and ' "Th by moderate energy helium ions MeV) has been studied previously by both radiochemical (1)(2)(3)(4)(5)(6) and mass spectrometric techniques (7,8), to determine cumulative and independent yields of various fission products. Cumulative yields of fission products at or near stability are used to determine the m a~s -~i e l d curves whereas independent yields are used to determine charge distributions.…”