Monte Carlo approach to sequential neutron emission from fission fragments

The prompt fission neutron spectrum N(E) plays an important role in various nuclear technologies, both in energy and nonenergy applications. From a more fundamental point of view, an accurate knowledge of N(E) can shed some light on the nuclear fission process itself. The Los Alamos (or Madland-Nix) model (1) has been commonly and successfully used over the years to predict the spectrum and the average number of prompt neutrons, ¯ ν, as funtions of both the fissioning nucleus and its excitation energy. It simulates the deexcitation of the fission fragments by evaporating neutrons having the Weisskopf spectrum, with an assumption of simple triangular- shaped initial nuclear temperature distribution. However, the Los Alamos model cannot predict more specific physical quantities, such as the prompt neutron multiplicity distribution P (ν), because it does not follow the neutron evaporation process in detail, but instead averages over the decay chain. The present work is an attempt to go a step beyond the Los Alamos model and look in more detail at the fission fragment decay process. We extended the Los Alamos model (1) by implementing a Monte Carlo simulation of the statistical decay (Weisskopf- Ewing) of the fission fragments (FF) by sequential neutron emission. To simulate the initial distribution of total excitation energy (TXE) possible in the FF, we use experimental data on the total kinetic energy distributions P (TKE), nuclear masses, neutron separation, and kinetic energies (in the case of neutron induced fission) when available and results from calculations otherwise. To simulate the decay of each fission fragment, one needs to know how the TXE is distributed in the two fragments. Two different hypotheses for partitioning the TXE are considered: (i) both fission fragments have the same nuclear temperature as the fissioning compound nucleus temperature and (ii) the TXE is split according to experimental values of the average total number of emitted neutrons (¯ νexp(A)), average

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