EXCITATION-ENERGY DEPENDENCE OF FRAGMENT CHARACTERISTICS FOR THE PHOTOFISSION OF TH-232

Independent and cumulative product yields were measured for the photof ission of Th-232 with bremsstrahlung with endpoint energies 6.5, 7.0, 8.0, 11.0, 12.0, and 14.0 MeV, applying gamma spectrometric techniques on catcherfoils and pneumatically transported Th-232-samples. The ind ependent heavy fragment yields for the fission of the Th-232 compound nucleus at excitation energies in the vicinity of the fission barrier were deduced. Postneutron mass, isobaric charge, isotopic mass distrib utions, isotonic and elemental yield distributions and proton odd-even effects were obtained from these independent yields. In the mass dist ributions a maximum yield is observed for mass splits with heavy fragm ents in me region of A = 142, corresponding with a high production of Ba(Z = 56)- isotopes. A slightly increased yield is also observed for mass splits with heavy mass in the vicinity of A = 134. The latter eff ect increases with increasing compound nucleus excitation energy. The similarity between the mass distributions of the N = 142 fissioning sy stems Th-232, U-234 and Pu-236 is striking. For low excitation energy the proton odd-even effect in the element distributions amounts to 30% , while on the other hand no sizeable neutron odd-even effect could be deduced from the isotonic distributions. The proton odd-even effects remain constant up to compound nucleus excitation energies of about 7. 85 MeV. For higher compound nucleus excitation energies the proton odd -even effect drops rapidly. A possible explanation of these observatio ns in terms of pair breaking at the outer barrier is proposed. (C) 199 7 Elsevier Science B.V.