A comparison between two evaluations of neutron multiplicity distributions

Within MCNP6, users can use multiplicity distributions for spontaneous and neutron-induced fission from an evaluation performed by Lestone (LA-UR-05-0288). This evaluation assumes the multiplicity distributions are Gaussian and adjusts the standard deviations to reproduce measured 2nd and 3rd factorial moments. In MCNPX, users have the option of using multiplicity distributions for spontaneous fission using an evaluation of the available experimental data by Santi and Miller (LA-UR-07-6229). The spontaneous fission multiplicity distributions and corresponding factorial moments from these two evaluations are compared in the present paper. The differences are minor or within experimental errors for all but the spontaneous fission of 238Pu. The 238Pu evaluations are based on data obtained in 1956. We recommend that the 238Pu multiplicity distribution be re-measured. Given the good agreement between the two evaluations, the choice of evaluation will make little difference for the modeling of spontaneous fission. However, we suggest that the evaluation in LA-UR-05-0288 be adopted as the MCNP default because it also includes recommended distributions for neutron induced fission.