Comparison of Yields from Fission ofU233,U234,U235,U236, andU238with 14.8-MeV Neutrons

Abstract

We analyzed mass-yield distributions from fission of $^{233}\mathrm{U}$, $^{234}\mathrm{U}$, $^{235}\mathrm{U}$, $^{236}\mathrm{U}$, and $^{238}\mathrm{U}$ with 14.8-MeV neutrons to study the details of the fission distribution as a function of target mass number at a fairly low excitation energy. The fission yields of individual products, both cumulative and independent, vary smoothly and as expected with uranium target mass number. The over-all distribution of yields becomes significantly narrower as the uranium mass number is increased. The average neutron emission associated with the very-low-yield products is about 3.5, significantly less than the value of about 4.5 for the fission fragments as a whole. This difference may be due to an increase in energy required for reactions leading to the low-yield products. The peak/valley ratio increases by a factor of more than 2 as the mass number is increased from 233 to 238, and decreases linearly with increasing excitation energy, with the even-mass and odd-mass uranium isotopes defining lines that are approximately parallel and separated by 1.4 MeV. This difference can be explained either by an odd-even effect in the neutron-evaporation/fission competition as a function of target mass number, or by an odd-even effect in the ratio of symmetric to asymmetric fission as a function of mass number. One consequence of the rapid change in peak/valley ratio with mass number is that fission of $^{232}\mathrm{U}$ with 14.8-MeV neutrons would be expected to produce a flat-topped mass-yield distribution, with a peak/valley ratio of about unity, while fission of lower-mass uranium isotopes would be expected to be predominantly symmetric. The average value of $\frac{\ensuremath{\Delta}{Z}_{p}}{\ensuremath{\Delta}A}$ was found to be -0.16 \ifmmode\pm\else\textpm\fi{} 0.02 for the shielded product $^{96}\mathrm{Nb}$ and -0.20 \ifmmode\pm\else\textpm\fi{} 0.01 for $^{136}\mathrm{Cs}$. This difference may be due to the change in prefission neutron emission with uranium mass number. All of the available data on fractional chain yields for fission of $^{236}\mathrm{U}^{*}$ at ${E}^{*}=6.4 \mathrm{and} 21.3$ MeV were used to estimate $\ensuremath{\Delta}{Z}_{p}(A)$ for $\ensuremath{\Delta}{E}^{*}=15$ MeV. The scatter in the $\ensuremath{\Delta}{Z}_{p}$ values is too large to allow a smooth curve to be drawn relating $\ensuremath{\Delta}{Z}_{p}$ as a function of mass number. It was found, however, that $\ensuremath{\Delta}{Z}_{p}$ values are larger on the average for heavy-fragment products ($\ensuremath{\Delta}{Z}_{p}=0.49\ifmmode\pm\else\textpm\fi{}0.05$) than for light-fragment products ($\ensuremath{\Delta}{Z}_{p}=0.26\ifmmode\pm\else\textpm\fi{}0.03$) over this range of excitation energy. The well-known ${Z}_{p}(A)$ function for thermal-neutron fission of $^{235}\mathrm{U}$, together with the $\frac{\ensuremath{\Delta}{Z}_{p}}{\ensuremath{\Delta}A}$ and $\frac{\ensuremath{\Delta}{Z}_{p}}{\ensuremath{\Delta}{E}^{*}}$ relationships reported here, can be used to estimate ${Z}_{p}$ values and fractional chain yields for fission of the various uranium isotopes with 14.8-MeV neutrons.