Independent yields of fast-neutron fission ofTh232: Observation of proton odd-even effect and neutron shell effect on the yields

Abstract

Elemental yields in the fission of ${}_{}{}^{232}{}_{}{}^{}\mathrm{Th}$ induced by reactor neutrons were measured and found to disclose a pronounced odd-even effect. Independent and cumulative yields of isotopes of Kr, Xe, Sn, and Sb were measured and the independent yields of isotopes of Rb, Cs, and Sb were subsequently derived. A comparison of the yield values obtained with the calculated "normal" yields reveals a constant enhancement of products with an even number of protons, relative to those with an odd number. An average amplitude of 30(±12)% in the fluctuation of proton pairing was established for all isotopes measured, except for the doubly magic ${}_{}{}^{132}{}_{}{}^{}\mathrm{Sn}$, which shows an extreme effect, apparently due to its closed neutron shell. The odd-even effect is discussed in connection with the mass distribution, excitation, and total kinetic energies. The neutron shell effect on the yields is discussed with regard to the primary mass split and the neutron emission probabilities.