Improved Cumulative Fission Yield Measurements with Fission Spectrum Neutrons on 235U

Abstract

Two isotopically-characterized targets of high-purity (99.84±0.03%) 235U-oxide and a neutron dosimetry package were exposed to a pulsed neutron irradiation using the Godiva critical assembly at the National Criticality Experiments Research Center. Following irradiation, the fissioned targets were counted with a broad-energy germanium detector. Estimates of the total fissions for each target were made using measurements of well-known fission products. The total fission values were then used to measure the fission yields of 21 fission products with half-lives between 636 and 119,700 seconds. Mean values of seventeen of the measured fission products fell within the 95% confidence level of the Evaluated Nuclear Data Files VII.1/b (ENDF/B-VII.1) using the larger of the two uncertainties: 84gBr, 89Rb, 91Sr, 93Y, 97Zr, 101Mo, 105Ru, 128Sn, 129Sb, 130gSb, 131Sb, 131mTe, 133I, 135I, 141Ba, 142Ba, and 142La. Seven isotopes were measured to better precision than is listed in the ENDF/B-VII.1 database: 89Rb, 93Y, 128Sn, 129Sb, 130gSb, 131mTe, and 133I. Four of the measured isotopes were found to be statistically different from the ENDF/B-VII.1 database: 94Y, 134Te, 134I, and 138Xe. Two of the isotopic analyses conducted in this paper fit the parent and daughter data simultaneously. The parent-daughter pairs measured in this way were the 134Te/I and 142Ba/La pairs. The fission yield results obtained in this way are unique, especially for 134I, because the parent and daughter half-lives are so close. Given the limited number of direct measurements of the 134Te/I pair from fast neutron induced fission and the seemingly high estimate for 134Te from previous work, it is likely that the value in this paper is more accurate; however, additional measurements of this pair are needed to validate this assertion.