Nuclear-Charge Distribution of Fission-Product Chains of Mass Numbers 131-133

Abstract

New information on the nature of the nuclear charge distribution in thermal-neutron fission of $^{235}\mathrm{U}$ has been obtained from measurements of fission yields of the principal constituents in each of three adjacent mass chains. A regular variation in the width of the charge-distribution curves with mass number has been shown, and values of the most probable nuclear charge (${Z}_{p}$) have been determined. No effects attributable to the 50-proton or 82-neutron closed shells have been observed in the charge-distribution curves examined. Independent and cumulative fission yield values were determined for the principal constituents in each of the three mass chains 131, 132, and 133. Values measured are: $^{131}\mathrm{Sn}$, (1.28\ifmmode\pm\else\textpm\fi{}0.21)% cumulative; $^{131}\mathrm{Sb}$, (1.66\ifmmode\pm\else\textpm\fi{}0.40)% independent; $^{132}\mathrm{Sn}$, (0.59\ifmmode\pm\else\textpm\fi{}0.17)% cumulative; $^{132}\mathrm{Sb}$, (2.76\ifmmode\pm\else\textpm\fi{}0.35)% independent; $^{132}\mathrm{Te}$, (0.86\ifmmode\pm\else\textpm\fi{}0.10)% independent; $^{133}\mathrm{Sn}$, 0.015% cumulative; $^{133}\mathrm{Sb}$, (3.05\ifmmode\pm\else\textpm\fi{}0.39)% independent; $^{133m}\mathrm{Te}$, (2.19\ifmmode\pm\else\textpm\fi{}0.33)% independent; $^{133g}\mathrm{Te}$, (2.07\ifmmode\pm\else\textpm\fi{}0.31)% independent; and $^{133}\mathrm{I}$, (0.17\ifmmode\pm\else\textpm\fi{}0.06)% independent. The charge distribution curve for each mass chain was established from the fission yield data. The data were fitted to the Gaussian relationship, $P(Z)=A\mathrm{exp}\ensuremath{-}\frac{{(Z\ensuremath{-}{Z}_{p})}^{2}}{c}$. The widths of the curves as given by values of $c$ for mass chains 131, 132, and 133 are, respectively: 1.10\ifmmode\pm\else\textpm\fi{}0.05, 0.74\ifmmode\pm\else\textpm\fi{}0.08, and 0.57\ifmmode\pm\else\textpm\fi{}0.05. The data show a monotonic decrease in the width of the charge distribution curves with increasing fission-product mass number. Implications of this observation are discussed in respect to previous assumptions of a single charge distribution curve for all fission-product masses in low-energy fission. Values of ${Z}_{p}$ as given by the Gaussian curves for mass chains 131, 132, and 133 are, respectively: 50.72\ifmmode\pm\else\textpm\fi{}0.07, 51.07\ifmmode\pm\else\textpm\fi{}0.06, and 51.59\ifmmode\pm\else\textpm\fi{}0.04. These values are compared with published values derived from assumptions of a single charge distribution curve for all mass chains. Predicted effects of nuclear shells on charge distribution have been examined in the light of the present data. No effect was detected in mass chains 131, 132, and 133. New half-life values determined are: $^{131}\mathrm{Sn}$, 1.32\ifmmode\pm\else\textpm\fi{}0.23 min; $^{132}\mathrm{Sn}$, 1.00\ifmmode\pm\else\textpm\fi{}0.17 min; $^{132}\mathrm{Sb}$, 3.13\ifmmode\pm\else\textpm\fi{}0.33 min; and $^{133}\mathrm{Sb}$, 2.67\ifmmode\pm\else\textpm\fi{}0.33 min. The half-life of $^{133}\mathrm{Sn}$ was estimated to be 55 sec.