Monte Carlo Hauser-Feshbach Research Articles

Recent developments in the modeling of (n,γ) reactions with FIFRELIN

Nowadays, the accuracy of experimental results relies more and more on detailed simulations. In the STEREO experiment, the interaction of neutrinos in the liquid scintillator is signed by a n-capture on a Gd atom. The FIFRELIN predictions of the Gd γ-cascades were shown to significantly improve the Data/MC agreement. In the CRAB method, lately proposed to calibrate cryogenic particle detectors at low energy (100 eV), the FIFRELIN cascades of the W and Ge isotopes played a central role in the feasibility study of the method. The FIFRELIN code employs a Monte Carlo Hauser-Feshbach framework based on Bečvár’s algorithm. A sample of nuclear level schemes is generated for a specific isotope of interest, taking into account the uncertainties from nuclear structure. In this work, new improvements on the FIRELIN de-excitation process are reported. Angular correlations of γ-rays in the de-excitation process have been implemented in order to provide a more accurate description of the γ-ray cascades. The anisotropy of the γ-rays with respect to the axis of a previously emitted γ-ray is modeled using the angular correlation formalism, which requires input of the spins and multipolarities of the states involved in the FIFRELIN cascade. Furthermore, the simulation of primary γ-rays emitted from (n,γ) reactions has been updated using the EGAF database.

Systematic measurements of prompt fission γ rays – and what they tell us about fission fragment de-excitation

In a recent measurement of prompt γ rays from the spontaneous fission of 252Cf an experimental setup was chosen that allowed the study of angular correlations between γ rays and the nuclei from which they were emitted. The observed angular distribution of prompt fission γ rays was then analyzed to estimate the relative contributions from radiation of different multipolarity. Preliminary results are presented and compared to previous angular correlationmeasurements from different fission processes and recent results from calculations with the Monte Carlo Hauser-Feshbach code FIFRELIN.

Prompt gamma rays from 252Cf(sf) and their angular distributions

In a recent measurement of prompt γ rays from the spontaneous fission of 252Cf an experimental setup was chosen that allowed the study of angular correlations between γ rays and the nuclei from which they were emitted. The observed angular distribution of prompt fission γ rays was then analyzed to estimate the relative contributions from radiation of different multipolarity. First preliminary results are presented and compared to previous angular correlation measurements from different fission processes and recent results from calculations with the Monte Carlo Hauser-Feshbach code FIFRELIN.

Comprehensive modeling of prompt fission neutrons and γ rays in the spontaneous fission of 252Cf

We present a comprehensive set of calculations performed with the Monte Carlo Hauser-Feshbach code CGMF of the prompt fission neutrons and γ rays emitted in the spontaneous fission of Cf-252. This reaction has been studied in depth over the years and provides an almost perfect test for the assumptions, parameters and output of the CGMF code. Here we present results for prompt neutron observables beyond the ubiquitous average prompt fission neutron spectrum and multiplicity. In particular, we compare CGMF calculations to experimental data on neutron data per fragment mass split, neutron-light fragment and neutron-neutron angular distributions, and on the time dependence of the average prompt γ-ray multiplicity. Finally, we briefly discuss the recent integration of CGMF into the MCNP6.2 transport code.

Properties of prompt-fissionγrays

In a Monte Carlo Hauser-Feshbach statistical framework, we describe spectra, average multiplicities, average energy, and multiplicity distributions of the prompt $\ensuremath{\gamma}$ rays produced in the thermal neutron-induced fission of $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$, and the spontaneous fission of $^{252}\mathrm{Cf}$. Comparisons against recent experimental data show reasonable agreement in all cases investigated, after adjustment of the initial spin distribution in the fission fragments. In particular, when we include in the calculation the Doppler broadening we obtain a qualitatively good description of the measured low-energy spectra, where contributions from collective discrete transitions in specific fragments can be identified. At higher energies, both the calculated neutron and $\ensuremath{\gamma}$-ray spectra are softer than experimental data. The impact of selected model parameters on the prompt neutron and $\ensuremath{\gamma}$-ray spectra is analyzed. Finally, we present the prompt $\ensuremath{\gamma}$ spectrum and multiplicity distribution for the neutron-induced fission of $^{235}\mathrm{U}$ for 5.5 MeV neutron incident energy, just below the threshold for second-chance fission.

Modeling the Emission of Prompt Fission γ Rays for Fundamental Physics and Applications

Prompt fission γ rays are calculated through Monte Carlo Hauser-Feshbach simulations of the decay of the primary fragments formed in the fission process. The CGMF code is used to track individual decay histories for specific initial fission fragment configu- rations, sampling emission probability distributions for both neutron and γ-ray emissions at each stage of the evaporation process. The prompt fission γ-ray spectrum averaged over all initial fission fragments is inferred as well as the average total number of emitted photons 〈Nγ〉. In addition, the probability distribution P〈Nγ〉 and individual spectra corresponding to specific fission frag- ments are computed. Results for several fission reactions of interest, 252Cf (sf), nth+239Pu, nth+235U, are presented and compared with recent experimental data.

Promptγ-ray production in neutron-induced fission of239Pu

Background: The prompt gamma-ray spectrum from fission is important for understanding the physics of nuclear fission, and also in applications involving fission. Relatively few measurements of the prompt gamma spectrum from ${}^{239}$Pu($n,f$) have been published.Purpose: This experiment measured the multiplicity, individual gamma energy spectrum, and total gamma energy spectrum of prompt fission gamma rays from ${}^{239}$Pu($n,f$) in the neutron energy range from thermal to 30 keV, to test models of fission and to provide information for applications.Method: Gamma rays from neutron-induced fission of ${}^{239}$Pu were measured using the DANCE gamma-ray calorimeter. Fission events were tagged by detecting fission products in a parallel-plate avalanche counter in the center of DANCE. The measurements were corrected for detector response using a geant4 model of DANCE. A detailed analysis for the gamma rays from the 1${}^{+}$ resonance complex at 10.93 eV is presented.Results: A six-parameter analytical parametrization of the fission gamma-ray spectrum was obtained. A Monte Carlo Hauser-Feshbach calculation provided good general agreement with the data, but some differences remain to be resolved.Conclusions: An analytic parametrization can be made of the gamma-ray multiplicity, energy distribution, and total-energy distribution for the prompt gamma rays following neutron-induced fission of ${}^{239}$Pu. This parametrization may be useful for applications. Modern Monte Carlo Hauser-Feshbach calculations can do a good job of calculating the fission gamma-ray emission spectrum, although some details remain to be understood.

Monte Carlo Hauser-Feshbach predictions of prompt fissionγrays: Application tonth+235U,nth+239Pu, and252Cf (sf)

The prompt neutron and $\ensuremath{\gamma}$ emission from primary fission fragments are calculated for thermal neutron induced fission of ${}^{235}$U and ${}^{239}$Pu and for spontaneous fission of ${}^{252}$Cf using a Monte Carlo Hauser-Feshbach approach for the evaporation of the excited fission fragments. Remaining free model parameters, such as excitation energy sharing and initial spin distribution, are determined by comparison of the neutron emission characteristics with experimental data. Using the obtained parameters the $\ensuremath{\gamma}$-ray characteristics, e.g., $\ensuremath{\gamma}$ spectrum, multiplicity distribution, average multiplicity and energy, and multiplicity distribution, are calculated and compared with available experimental data.

Monte Carlo Simulation for Statistical Decay of Compound Nucleus

We perform Monte Carlo simulations for neutron and γ-ray emissions from a compound nucleus based on the Hauser-Feshbach statistical theory. This Monte Carlo Hauser-Feshbach (MCHF) method calculation, which gives us correlated information between emitted particles and γ-rays. It will be a powerful tool in many applications, as nuclear reactions can be probed in a more microscopic way. We have been developing the MCHF code, CGM, which solves the Hauser-Feshbach theory with the Monte Carlo method. The code includes all the standard models that used in a standard Hauser-Feshbach code, namely the particle transmission generator, the level density module, interface to the discrete level database, and so on. CGM can emit multiple neutrons, as long as the excitation energy of the compound nucleus is larger than the neutron separation energy. The γ-ray competition is always included at each compound decay stage, and the angular momentum and parity are conserved. Some calculations for a fission fragment 140Xe are shown as examples of the MCHF method, and the correlation between the neutron and γ-ray is discussed.

Systematic Quantification of Uncertainties for Evaluated Prompt Fission Neutron Spectra and Multiplicities

Uncertainties associated with evaluated average prompt fission neutron spectra and multiplicities are obtained for a suite of actinides in the Los Alamos model formalism. Systematics for the model input parameters are taken from the literature and used as prior values in a Bayesian updating procedure. Posterior systematics as well as associated posterior uncertainties are inferred. In addition, cross-isotope correlations are evaluated for the first time. The quantification of uncertainties associated with advanced Monte Carlo Hauser-Feshbach calculations of prompt fission neutron spectra is also discussed.

Statistical (?) decay of light hot nuclei

The reaction 12 C+ 12 C at 95 MeV beam energy has been measured using the GARFIELD+RCo apparatuses at Laboratori Nazionali di Legnaro LNL - INFN, Italy, in the framework of an experimental campaign proposed by the NUCL-EX collaboration. The aim is to progress in the understanding of statistical properties of light nuclei at excitation energies above particle emission thresholds, by measuring exclusive fusion-evaporation data. A theoretical study of the system, performed with a newly developed Monte Carlo Hauser-Feshbach code, is shown, together with preliminary results of the data analysis.

Recent Advances in Nuclear Fission Theory: Pre- and Post-Scission Physics

Recent advances in the modeling of the nuclear fission process for data evaluation purposes are reviewed. In particular, it is stressed that a more comprehensive approach to fission data is needed if predictive capability is to be achieved. The link between pre- and post-scission data is clarified, and a path forward to evaluate those data in a consistent and comprehensive manner is presented. Two examples are given: (i) the modeling of fission cross-sections in the R-matrix formalism, for which results for Pu isotopes from 239 to 242 are presented; (ii) the modeling of prompt fission neutrons in the Monte Carlo Hauser-Feshbach framework. Results for neutron-induced fission on {sup 235}U are discussed.