arXiv subjects

P. Talou

Publications and source records attributed to P. Talou.

17 recordsLinked to original sources

arXiv2026

New Procedure for the Evaluation of Fission Product Yields: Application to the Spontaneous Fission of $^{252}$Cf

Over the last decade, there has been significant improvement in the understanding and modeling of the decay of fission fragments by both prompt and delayed emission. These model improvements open the door for performing consistent evaluations across multiple fission observables, providing not only mean values but also covariances between observables. One such model is the Hauser-Feshbach Fission Fragment Decay model implemented in $\texttt{BeoH}$, which uses distributions of initial conditions of fission fragments to perform a Hauser-Feshbach decay for prompt neutron and $\gamma$-ray emission and evaluated decay data to calculate cumulative fission product yields. This manuscript describes a new evaluation procedure for independent and cumulative fission product yields, including full correlations among the fission products. We use a Bayesian Kalman filter to fit both experimental cumulative fission product yields and those from the ENDF/B-VIII.0 evaluated library, producing mean values and covariances. In addition to comparing the fission products from these optimizations, we calculate prompt and delayed neutron and $\gamma$-ray multiplicities using the fitted parameters and compare to some available experimental data. We see reasonable agreement, even when these quantities are not included in the optimization.

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arXiv2023

The Los Alamos evaluation of $^{239}$Pu neutron-induced reactions in the fast energy range

A major revision of the evaluation of $^{239}$Pu neutron-induced reaction cross sections is reported in the fast energy range. The evaluation starts at 2.5 keV incident neutron energy and has been extended up to 30 MeV. Several other notable changes are included in this evaluation since the release of ENDF/B-VIII.0 including the adoption of the Standards fission cross section, inclusion of new radiative capture data of Mosby et al., inclusion of the (n,2n) data of Meot et al., in addition to advances in the treatment of reaction modeling. In contrast to previous evaluation efforts, this evaluation is reproducible with detailed information stored chronologically utilizing a Git repository. The final evaluation results have been compiled into an ENDF-formatted file, which has been processed successfully through NJOY, checked for internal consistency, benchmarked versus older evaluations and validated against a suite of critical assemblies and pulsed-spheres.

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arXiv2022

Nuclear data activities for medium mass and heavy nuclei at Los Alamos

Nuclear data is critical for many modern applications from stockpile stewardship to cutting edge scientific research. Central to these pursuits is a robust pipeline for nuclear modeling as well as data assimilation and dissemination. We summarize a small portion of the ongoing nuclear data efforts at Los Alamos for medium mass to heavy nuclei. We begin with an overview of the NEXUS framework and show how one of its modules can be used for model parameter optimization using Bayesian techniques. The mathematical framework affords the combination of different measured data in determining model parameters and their associated correlations. It also has the advantage of being able to quantify outliers in data. We exemplify the power of this procedure by highlighting the recently evaluated 239-Pu cross section. We further showcase the success of our tools and pipeline by covering the insight gained from incorporating the latest nuclear modeling and data in astrophysical simulations as part of the Fission In R-process Elements (FIRE) collaboration.

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arXiv2021

Angular momentum removal by neutron and $\gamma$-ray emissions during fission fragment decays

We investigate the angular momentum removal from fission fragments (FFs) through neutron and $\gamma$-ray emission, where we find that about half the neutrons are emitted with angular momenta $\ge 1.5\hbar$ and that the change in angular momentum after the emission of neutrons and statistical $\gamma$ rays is significant, contradicting usual assumptions. Per fission event, in our simulations, the neutron and statistical $\gamma$-ray emissions change the spin of the fragment by 3.5 -- 5~$\hbar$, with a large standard deviation comparable to the average value. Such wide angular momentum removal distributions can hide any underlying correlations in the fission fragment initial spin values. Within our model, we reproduce data on spin measurements from discrete transitions after neutron emissions, especially in the case of light FFs. The agreement further improves for the heavy fragments if one removes from the analysis the events that would produce isomeric states. Finally, we show that while in our model the initial FF spins do not follow a saw-tooth like behavior observed in recent measurements, the average FF spin computed after neutron and statistical $\gamma$ emissions exhibits a shape that resembles a saw tooth. This suggests that the average FF spin measured after statistical emissions is not necessarily connected with the scission mechanism as previously implied.

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arXiv2020

Fission Fragment Decay Simulations with the CGMF Code

The CGMF code implements the Hauser-Feshbach statistical nuclear reaction model to follow the de-excitation of fission fragments by successive emissions of prompt neutrons and $\gamma$ rays. The Monte Carlo technique is used to facilitate the analysis of complex distributions and correlations among the prompt fission observables. Starting from initial configurations for the fission fragments in mass, charge, kinetic energy, excitation energy, spin, and parity, $Y(A,Z,KE,U,J,\pi)$, CGMF samples neutron and $\gamma$-ray probability distributions at each stage of the decay process, conserving energy, spin and parity. Nuclear structure and reaction input data from the RIPL library are used to describe fission fragment properties and decay probabilities. Characteristics of prompt fission neutrons, prompt fission gamma rays, and independent fission yields can be studied consistently. Correlations in energy, angle and multiplicity among the emitted neutrons and $\gamma$ rays can be easily analyzed as a function of the emitting fragments.

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arXiv2020

Extension of the Hauser-Feshbach Fission Fragment Decay Model to Multi-Chance Fission

The Hauser-Feshbach fission fragment decay model, $\mathtt{HF^3D}$, which calculates the statistical decay of fission fragments, has been expanded to include multi-chance fission, up to neutron incident energies of 20 MeV. The deterministic decay takes as input pre-scission quantities - fission probabilities and the average energy causing fission - and post-scission quantities - yields in mass, charge, total kinetic energy, spin, and parity. From these fission fragment initial conditions, the full decay is followed through both prompt and delayed particle emissions, allowing for the calculation of prompt neutron and $\gamma$ properties, such as multiplicity and energy distributions, both independent and cumulative fission yields, and delayed neutron observables. In this work, we describe the implementation of multi-chance fission into the $\mathtt{HF^3D}$ model, and show an example of prompt and delayed quantities beyond first-chance fission, using the example of neutron-induced fission on $^{235}$U. This expansion represents significant progress in consistently modeling the emission of prompt and delayed particles from fissile systems.

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arXiv2020

Quantifying Uncertainties on Fission Fragment Mass Yields With Mixture Density Networks

Probabilistic machine learning techniques can learn both complex relations between input features and output quantities of interest as well as take into account stochasticity or uncertainty within a data set. In this initial work, we explore the use of one such probabilistic network, the Mixture Density Network (MDN), to reproduce fission yields and their uncertainties. We study mass yields for the spontaneous fission of $^{252}$Cf, exploring the number of training samples needed for converged predictions, how different levels of uncertainty propagate from the training set to the MDN predictions, and how well physical constraints of the yields - such as normalization and symmetry - are upheld by the algorithm. Finally, we test the ability of the MDN to interpolate between and extrapolate beyond samples in the training set using energy-dependent mass yields for the neutron-induced fission on $^{235}$U. The MDN provides a reliable way to include and predict uncertainties and is a promising path forward for supplementing sparse sets of nuclear data.

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arXiv2020

Correlations Between Fission Fragment and Neutron Anisotropies in Neutron-Induced Fission

Several sources of angular anisotropy for fission fragments and prompt neutrons have been studied in neutron-induced fission reactions. These include kinematic recoils of the target from the incident neutron beam and the fragments from the emission of the prompt neutrons, preferential directions of the emission of the fission fragments with respect to the beam axis due to the population of particular transition states at the fission barrier, and forward-peaked angular distributions of pre-equilibrium neutrons which are emitted before the formation of a compound nucleus. In addition, there are several potential sources of angular anisotropies that are more difficult to disentangle: the angular distributions of prompt neutrons from fully accelerated fragments or from scission neutrons, and the emission of neutrons from fission fragments that are not fully accelerated. In this work, we study the effects of the first group of anisotropy sources, particularly exploring the correlations between the fission fragment anisotropy and the resulting neutron anisotropy. While kinematic effects were already accounted for in our Hauser-Feshbach Monte Carlo code, $\mathtt{CGMF}$, anisotropic angular distributions for the fission fragments and pre-equilibrium neutrons resulting from neutron-induced fission on $^{233,234,235,238}$U, $^{239,241}$Pu, and $^{237}$Np have been introduced for the first time. The effects of these sources of anisotropy are examined over a range of incident neutron energies, from thermal to 20 MeV, and compared to experimental data from the Chi-Nu liquid scintillator array. The anisotropy of the fission fragments is reflected in the anisotropy of the prompt neutrons, especially as the outgoing energy of the prompt neutrons increases, allowing for an extraction of the fission fragment anisotropy to be made from a measurement of the neutrons.

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arXiv2019

Prompt Neutron Multiplicity Distributions Inferred from $\gamma$-ray and Fission Fragment Energy Measurements

We propose a novel method to extract the prompt neutron multiplicity distribution, $P(\nu)$, in fission reactions based on correlations between prompt neutrons, $\gamma$ rays, and fragment kinetic energy arising from energy conservation. In this approach, only event-by-event measurements of the total $\gamma$-ray energy released as a function of the total kinetic energy (TKE) of the fission fragments are performed, and no neutron detection is required. Using the $\texttt{CGMF}$ fission event generator, we illustrate the method and explore the accuracy of extracting the neutron multiplicity distribution when taking into account the energy resolution and calibration of the energy measurements. We find that a TKE resolution of under 2 MeV produces reasonably accurate results, independent of typical $\gamma$-ray energy measurement resolution.

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arXiv2018

High resolution measurement of tagged two-neutron energy and angle correlations in Cf-252(sf)

Background: Spontaneous fission events emit prompt neutrons correlated with one another in emission angle and energy. Purpose: We explore the relationship in energy and angle between correlated prompt neutrons emitted from 252Cf spontaneous fission. Methods: Measurements with the Chi-Nu array provide experimental data for coincident neutrons tagged with a fission chamber signal with 10 degree angular resolution and 1 ns timing resolution for time-of-flight energy calculations. The experimental results are compared to simulations produced by the fission event generators CGMF, FREYA, and MCNPX-POLIMI IPOL(1)=1. Results: We find that the measurements and the simulations all exhibit anisotropic neutron emission, though differences exist between fission event generators. Conclusions: This work shows that the dependence of detected neutron energy on the energy of a neutron detected in coincidence, although weak, is non-negligible, indicating that there may be correlations in energy between two neutrons emitted in the same fission event.

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arXiv2017

Correlated Prompt Fission Data in Transport Simulations

Detailed information on the fission process can be inferred from the observation, modeling and theoretical understanding of prompt fission neutron and $\gamma$-ray~observables. Beyond simple average quantities, the study of distributions and correlations in prompt data, e.g., multiplicity-dependent neutron and \gray~spectra, angular distributions of the emitted particles, $n$-$n$, $n$-$\gamma$, and $\gamma$-$\gamma$~correlations, can place stringent constraints on fission models and parameters that would otherwise be free to be tuned separately to represent individual fission observables. The FREYA~and CGMF~codes have been developed to follow the sequential emissions of prompt neutrons and $\gamma$-rays~from the initial excited fission fragments produced right after scission. Both codes implement Monte Carlo techniques to sample initial fission fragment configurations in mass, charge and kinetic energy and sample probabilities of neutron and $\gamma$~emission at each stage of the decay. This approach naturally leads to using simple but powerful statistical techniques to infer distributions and correlations among many observables and model parameters. The comparison of model calculations with experimental data provides a rich arena for testing various nuclear physics models such as those related to the nuclear structure and level densities of neutron-rich nuclei, the $\gamma$-ray~strength functions of dipole and quadrupole transitions, the mechanism for dividing the excitation energy between the two nascent fragments near scission, and the mechanisms behind the production of angular momentum in the fragments, etc. Beyond the obvious interest from a fundamental physics point of view, such studies are also important for addressing data needs in various nuclear applications. (See text for full abstract.)

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arXiv2016

Late Time Emission of Prompt Fission Gamma Rays

The emission of prompt fission $\gamma$ rays within a few nanoseconds to a few microseconds following the scission point is studied in the Hauser-Feshbach formalism applied to the deexcitation of primary excited fission fragments. Neutron and $\gamma$-ray evaporations from fully accelerated fission fragments are calculated in competition at each stage of the decay, and the role of isomers in the fission products, before $\beta$-decay, is analyzed. The time evolution of the average total $\gamma$-ray energy, average total $\gamma$-ray multiplicity, and fragment-specific $\gamma$-ray spectra, is presented in the case of neutron-induced fission reactions of $^{235}$U and $^{239}$Pu, as well as spontaneous fission of $^{252}$Cf. The production of specific isomeric states is calculated and compared to available experimental data. About 7% of all prompt fission $\gamma$ rays are predicted to be emitted between 10 nsec and 5 $\mu$sec following fission, in the case of $^{235}$U and $^{239}$Pu $(n_{\rm th},f)$ reactions, and up to 3% in the case of $^{252}$Cf spontaneous fission. The cumulative average total $\gamma$-ray energy increases by 2 to 5% in the same time interval. Finally, those results are shown to be robust against significant changes in the model input parameters.

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arXiv2015

Random-matrix approach to the statistical compound nuclear reaction at low energies using the Monte-Carlo technique

Using a random-matrix approach and Monte-Carlo simulations, we generate scattering matrices and cross sections for compound-nucleus reactions. In the absence of direct reactions we compare the average cross sections with the analytic solution given by the Gaussian Orthogonal Ensemble (GOE) triple integral, and with predictions of statistical approaches such as the ones due to Moldauer, to Hofmann, Richert, Tepel, and Weidenm\"{u}ller, and to Kawai, Kerman, and McVoy. We find perfect agreement with the GOE triple integral and display the limits of validity of the latter approaches. We establish a criterion for the width of the energy-averaging interval such that the relative difference between the ensemble-averaged and the energy-averaged scattering matrices lies below a given bound. Direct reactions are simulated in terms of an energy-independent background matrix. In that case, cross sections averaged over the ensemble of Monte-Carlo simulations fully agree with results from the Engelbrecht-Weidenm\"{u}ller transformation. The limits of other approximate approaches are displayed.

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arXiv2015

Benchmarking Nuclear Fission Theory

We suggest a small set of fission observables to be used as test cases for validation of theoretical calculations. The purpose is to provide common data to facilitate the comparison of different fission theories and models. The proposed observables are chosen from fission barriers, spontaneous fission lifetimes, fission yield characteristics, and fission isomer excitation energies.

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arXiv2007

Emission of Scission Neutrons in the Sudden Approximation

At a certain finite neck radius during the descent of a fissioning nucleus from the saddle to the scission point, the attractive nuclear forces can no more withstand the repulsive Coulomb forces producing the neck rupture and the sudden absorption of the neck stubs by the fragments. At that moment, the neutrons, although still characterized by their pre-scission wave functions, find themselves in the newly created potential of their interaction with the separated fragments. Their wave functions become wave packets with components in the continuum. The probability to populate such states gives evidently the emission probability of neutrons at scission. In this way, we have studied scission neutrons for the fissioning nucleus $^{236}$U, using two-dimensional realistic nuclear shapes. Both the emission probability and the distribution of the emission points relative to the fission fragments strongly depend on the quantum numbers of the pre-scission state from which the neutron is emitted. In particular it was found that states with $Ωπ$ = 1/2+ dominate the emission. Depending on the assumed pre- and post-scission configurations and on the emission-barrier height, 30 to 50% of the total scission neutrons are emitted from 1/2+ states. Their emission points are concentrated in the region between the newly separated fragments. The upper limit for the total number of neutrons per scission event is predicted to lie between 0.16 and 1.73 (depending on the computational assumptions).

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arXiv1998

Time-dependent properties of proton decay from crossing single-particle metastable states in deformed nuclei

A dynamical study of the decay of a metastable state by quantum tunneling through an anisotropic, non separable, two-dimensional potential barrier is performed by the numerical solution of the time-dependent Schrodinger equation. Initial quasi- stationary proton states are chosen in the framework of a deformed Woods-Saxon single-particle model. The decay of two sets of states corresponding to true and quasi levels-crossing is studied and the evolution of their decay properties as a function of nuclear deformation is calculated around the crossing point. The results show that the investigation of the proton decay from metastable states in deformed nuclei can unambiguously distinguish between the two types of crossing and determine the structure of the nuclear states involved.

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arXiv1998

Proton Decay from Excited States in Spherical Nuclei

Based on a single particle model which describes the time evolution of the wave function during tunneling across a one dimensional potential barrier we study the proton decay of $^{208}$Pb from excited states with non-vanishing angular momentum $\ell$. Several quantities of interest in this process like the decay rate $λ$, the period of oscillation $T_{osc}$, the transient time $t_{tr}$, the tunneling time $t_{tun}$ and the average value of the proton packet position $ r_{av} $ are computed and compared with the WKB results.

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