Nuclear-charge distribution near symmetry for thermal-neutron-induced fission of 235U.

Abstract

Available data concerning nuclear-charge distribution for thermal-neutron-induced fission of $^{235}\mathrm{U}$ have been evaluated for the region near symmetry (104\ensuremath{\le}A\ensuremath{\le}129, 42\ensuremath{\le}Z\ensuremath{\le}50). Evaluated data or averages are listed and are interpreted by the use of empirical ${Z}_{P}$ and ${A}_{P}^{\mathcal{'}}$ models. The parameters and functions derived for the two models are presented and are compared with each other and with the data. Both models represent most data satisfactorily, and they permit the calculation of more reliable estimates of unmeasured independent-fission-product yields near symmetry than has been possible in the past. The two models show the same major trends, so the physical phenomena deduced from the trends are not model dependent. The independent yields of fission products with Z values of 50 and 42 are much larger than those of products with Z values of 43 through 49. The yields for products with Z values of 49 and 43 are larger than those of products with Z values of 44 through 48. These yield differences indicate that the stable filled, or nearly filled, 50-proton shell in the heavy fragment causes the fission probability to increase rapidly with mass number in the range from 120 to 130 between improbable symmetric and probable asymmetric fission. The lack of even-odd-proton and -neutron effects near symmetry indicates that excitation energies are higher for $^{236}\mathrm{U}$ nuclei undergoing symmetric fission than for those undergoing asymmetric fission.