Semiempirical Description of the Mass Distribution for the²³⁸U(<i>n, f</i>) Process in the 1.5- to 15-MeV Energy Interval

Different empirical and semiempirical systematics have been developed to predict unmeasured fission product yields. One of these methods, originally proposed by Musgrove et al. and developed by Cook et al., is used to describe the energy dependence of the mass distribution in neutron-induced fission of 238U utilizing published yield data.The available measured cumulative yields of fission products are collected for monoenergetic 238U(n, f) processes. The mass distributions at approximate neutron energies of 1.5, 2.0, 3.0, 3.9, 5.2, 6.0, 7.0, 7.9, 9.0, and 14.7 MeV are fitted by the sum of five Gaussian functions. The energy dependence of the parameters of the Gaussian functions can also be described by semiempirical formulas. The 2σ error of the mass yields calculated by the fitted parameters can be estimated to be ∼10% in the peak regions and 20% in the valley region for the above neutron energies. The formulas with the given parameters can be useful in estimating unmeasured 238U fission product yields for any monoenergetic and nonmonoenergetic neutron irradiations in the range of 1.5 to 15 MeV. The method has been tested in a study of the 238U fission by neutrons having a Watt spectrum produced in the thermal fission of235U.