  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.

Nuclear Theory