Independent Yields of Krypton and Xenon Isotopes in Thermal-Neutron Fission ofU235. Observation of an Odd-Even Effect in the Element Yield Distribution

Abstract

The fractional and independent fission yields of krypton isotopes (masses 87 to 94) and xenon isotopes (masses 137-143) produced in thermal-neutron fission of $^{235}\mathrm{U}$ were measured using a mass separator operating in an on-line mode with a fission source exposed to time-controlled neutron fluxes. The noble-gas isotopes were collected simultaneously at given times and each mass was measured separately. The distribution of the isotopic yields of each element was found to be Gaussian, in general agreement with Wahl's estimates. The total independent yields of krypton and xenon, as well as their complementary elements, barium and strontium, were found to indicate an odd-even effect in the element yield distribution. Even-$Z$ elements are formed in higher yields than their odd-$Z$ neighbors (20-50% higher); the even-$Z$ yields are also higher than Wahl's normal element yields (\ensuremath{\sim}12% higher for Kr and Ba and \ensuremath{\sim}40% for Xe and Sr). Half-life values of 20 nuclides produced in the decay of krypton and xenon isotopes were measured as well.