Fission modelling with FIFRELIN

International audienceThe nuclear fission process gives rise to the formation of fission fragments and emission of particles ($n$, $\gamma$, $e$^−$). The particle emission from fragments can be prompt and delayed. We present here the methods used in the FIFRELIN code, which simulates the prompt component of the de-excitation process. The methods are based on phenomenological models associated with macroscopic and/or microscopic ingredients. Input data can be provided by experiment as well as by theory. The fission fragment de-excitation can be performed within Weisskopf (uncoupled neutron and gamma emission) or a Hauser-Feshbach (coupled neutron/gamma emission) statistical theory. We usually consider five free parameters that cannot be provided by theory or experiments in order to describe the initial distributions required by the code. In a first step this set of parameters is chosen to reproduce a very limited set of target observables. In a second step we can increase the statistics to predict all other fission observables such as prompt neutron, gamma and conversion electron spectra but also their distributions as a function of any kind of parameters such as, for instance, the neutron, gamma and electron number distributions, the average prompt neutron multiplicity as a function of fission fragment mass, charge or kinetic energy, and so on. Several results related to different fissioning systems are presented in this work. The goal in the next decade will be i) to replace some macroscopic ingredients or phenomenological models by microscopic calculations when available and reliable, ii) to be a support for experimentalists in the design of detection systems or in the prediction of necessary beam time or count rates with associated statistics when measuring fragments and emitted particle in coincidence iii) extend the model to be able to run a calculation when no experimental input data are available, iv) account for multiple chance fission and gamma emission before fission, v) account for the scission neutrons. Several efforts have already been made to replace macroscopic ingredients and phenomenology by microscopic ingredients provided in various nuclear parameter libraries such as electric dipole photon strength functions or HFB level densities. First results relative to theses aspects are presented in this work

Impact of FIFRELIN input parameters on fission observables

Evaluated nuclear data are essential for nuclear reactor studies. In order to significantly improve the precision of nuclear data, more and more fundamental fission models are used in the evaluation processing. Therefore, tests of fission models become a central issue. In this framework, FIFRELIN (FIssion Fragments Evaporation Leading to an Investigation of Nuclear data) is a Monte Carlo code developed in order to modelize fission fragments de-excitation through the emission of neutrons, γ and conversion e-. To be performed, a FIFRELIN calculation relies on several models such as gamma strength function and nuclear level density and of more empirical hypothesis such as total excitation energy repartition or angular momentum given by the fission reaction. Moreover, pre-emission mass yield and kinetic energy distribution per mass are necessary to process the simulation. A set of five free parameters are chosen to reproduce a target observable. Often this observable corresponds to the mean neutron multiplicity for heavy and light fragment. In this work, the impact of the set of parameters on different output observables (neutron emission probability, neutron multiplicity as function of the fission fragment mass) is investigated

Influence of scission neutrons on the prompt fission neutron spectrum calculations

The calculation of the Prompt Fission Neutron Spectrum (PFNS) was performed using the FIFRELIN Monte Carlo code simulating the de-excitation of the whole fission fragments. This de-excitation is governed by the Hauser-Feshbach statistical model, which has the advantage to take into account the conservation laws for the energy, spin and parity of the initial and final states. In this way, the competition between prompt neutron and prompt gamma emission can be properly accounted for. Assuming that the prompt neutron emission comes only from an evaporation process of the fully accelerated fission fragments, our calculations are not able to reproduce satisfactorily the experimental data. In this context, we have added an additional source of neutrons that may arise during the sudden rupture of the neck (the so-called scission neutrons). Applied in the case of the spontaneous fission of 252Cf, our PFNS calculations show a very good agreement with the Mannhart evaluation by accounting for a 2% scission neutron contribution

Staggering of angular momentum distribution in fission

We review here the role of angular momentum distributions in the fission process. To do so the algorithm implemented in the FIFRELIN code [?] is detailed with special emphasis on the place of fission fragment angular momenta. The usual Rayleigh distribution used for angular momentum distribution is presented and the related model derivation is recalled. Arguments are given to justify why this distribution should not hold for low excitation energy of the fission fragments. An alternative ad hoc expression taking into account low-lying collectiveness is presented as has been implemented in the FIFRELIN code. Yet on observables currently provided by the code, no dramatic impact has been found. To quantify the magnitude of the impact of the low-lying staggering in the angular momentum distribution, a textbook case is considered for the decay of the 144Ba nucleus with low excitation energy

Calculation of the fission observables in the resolved resonance energy region of the 235U(n,f) reaction

Measurement of the fission fragments in coincidence with the emitted prompt neutrons was undertaken recently, at JRC-Geel institute, for the 235U(n,f) reaction in the resolved resonance energy region, up to 160 eV incident neutron energy. From this experimental work, fluctuations of several fission observables (mass yields, average total kinetic energy T̅K̅E̅, average prompt neutron multiplicity v̅P) were clearly observed. In the present work, these experimental pre-neutron fission fragment mass and kinetic energy distributions were used as input data for the FIFRELIN Monte Carlo code. By adopting the Hauser-Feshbach statistical model, the code simulates the de-excitation of the fission fragments. Four free parameters are available in the code: two of them (called RTmin and RTmax) govern at the scission point the sharing of the total available excitation energy between the two nascent fission fragments, while the two others (called σL and σH) assign the initial fission fragment spins. In this way, fission observables (prompt particles energy spectra and multiplicities, delayed neutrons multiplicity,. . . ) and correlations between them can be predicted and investigated. Here, these four free parameters were tuned in order to reproduce the average prompt neutron multiplicity at the resonance En=19.23 eV, resonance for which the experimental statistical uncertainty on v̅P is the lowest one. Then, the calculations were perfomed for all resonances by keeping the same set of free parameters. We show that the calculated fluctuations of v̅P in the resonances can rather be well reproduced by considering only the fluctuations of the pre-neutron mass yields and kinetic energy. In addition, from our calculation procedure, other fission observables fluctuations can also be predicted

Prompt particle emission in correlation with fission fragments

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

Prompt particle emission in correlation with fission fragments

International audienc

Summation calculation of delayed neutron yields for

Summation calculations have been performed in order to compare the quality of several nuclear data libraries. The objective was to obtain the average delayed neutron yield, as well as the average delayed-neutron half-life for different fissioning systems (235U, 238U and 239Pu) at different energies (thermal and fast) by using microscopic data. Each quantity is presented with a first evaluation of the uncertainty, computed under the assumption that the variables are all independent of each other

Implementation of a multi-chance fission model in FIFRELIN for 235U(n,f) reaction

International audienceNeutron emission is a physical process that can occur at various stages of nuclear fission. The main source of neutron emission comes from primary fission fragments after full acceleration. Another source of neutron emission comes from the de-excitation of the nucleus prior to scission (scission neutrons are neglected in this work). This type of event can occur if the energy of the incoming neutron is above a threshold energy dependent on nuclear properties (neutron binding energy Sn and fission barrier height Bf). This type of de-excitation initiates a competition between fission and neutron emission. This physical mechanism is referred to as multi-chance fission and becomes significant for incident neutron kinetic energies higher than the fission barrier height of the residual nucleus (approximately 5 MeV). Until now, this phenomenon has not been considered in the FIFRELIN code, limiting its scope of application. In this context, we propose a method to model multi-chance fission for the 235U(n,f) reaction. It is based on evaluated microscopic partial fission cross-sections used to calculate multi-chance fission probabilities