Abstract
The de-excitation process of primary fission fragments can be simulated with the FIFRELIN Monte Carlo code leading to an estimation of prompt fission observables such as neutron/gamma multiplicities and spectra in correlation with fission fragments. De-excitation cascades are simulated using the notion of nuclear realization following Becvar terminology generalized to neutron/gamma coupled emission. A nuclear realization is a random set of nuclear levels (energy, spin, parity) in association with partial widths for neutron, gamma or electron emission. Experimental data related to electromagnetic transitions in the discrete level region are taken from RIPL-3 database. When nuclear level structure is completely unknown (in the continuum region), level density and strength function models are used. In between these regions, our partial knowledge of nuclear structure is completed by models up to a fixed maximum level density. In this way the whole available experimental information is accounted for. FIFRELIN is ruled by five free input parameters driving the excitation energy sharing, the rotational energy and the spin distribution of primary fission fragments. These five free parameters are determined to match a target observable such as the average total prompt neutron multiplicity (ν). Once this procedure is completed, the whole set of fission observables can be compared with experimental results. Obviously the number of observables obtained within this code is higher than what is available from measurements. This code can therefore provide useful insights into the compatibility between models and a whole set of fission observables. In the present work the influence of shell corrections is reported on level densities and prompt fission neutron spectra (PFNS). The impact of the input data such as primary fission fragment total kinetic energy (TKE) is also addressed. Average prompt neutron multiplicity as a function of TKE is also estimated for each mass split and compared to recent measurements. The presence of structures in the calculations (especially for light nuclei) is clearly related to the nuclear level scheme. Various situations occur and an overestimation (or underestimation) of the calculated number of emitted neutrons can be correlated to the light or heavy fragment of a pair and to a restricted energy range. In addition prompt fission gamma spectra (PFGS) are estimated for selected fragment mass ranges and compared to recent measurements. In this way the presence of specific gamma-ray transitions can be established.
Highlights
More and more experiments are performed in the field of nuclear physics to improve our knowledge of fission phenomenon and complete the international nuclear databases used in neutron/gamma transport codes
In this paper we have focused the discussion around the average prompt neutron multiplicity as a function of total kinetic energy for a given mass split (ν(TKE;A)) and prompt fission gamma spectra for a selected mass range (P F G S(A))
We have shown in this work that the impact of a recent modification in the code (Mengoni-Nakajima shell corrections from RIPL-3 database) allows reproducing level densities of 239U compound nucleus and improving prompt fission neutron spectrum calculations with FIFRELIN
Summary
More and more experiments are performed in the field of nuclear physics to improve our knowledge of fission phenomenon and complete the international nuclear databases used in neutron/gamma transport codes. The well known average prompt neutron multiplicity (ν) used to determine the effective multiplication factor in a reactor core can be measured as a function of the primary fission fragment mass ν(A) and total kinetic energy ν(TKE). Recent measurements were performed at IRMM concerning total kinetic energy dependence of the multiplicity for given mass splits ν(TKE;A) [1] and prompt fission gamma spectra for different mass ranges [2, 3]. At Lohengrin mass spectrometer of the Institut LaueLangevin (ILL) located at Grenoble (France), recent measurements of kinetic energy dependent isomeric ratio are achieved. More than a single average value or a global spectrum, these data allow us to constrain our models, selecting a parameter value among others or accepting/rejecting an hypothesis
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