Prompt Fission Neutron Multiplicity Research Articles

Prompt fission neutron multiplicity in the 239Pu(n, f) reaction and its energy dependence

Measuring prompt fission neutrons to high precision is an experimental challenge, especially for radioactive fissioning nuclides. However, accurate average multiplicities, , and kinetic energy distributions of prompt fission neutrons are essential for fundamental and applied nuclear physics. We present here a recent measurement of the 239Pu (n,f) as a function of the incident-neutron energy, over the range 1-700 MeV. The measurement was performed with a cutting-edge setup and an innovative technique, which allowed to minimize and account for the main sources of bias. An unprecedented precision was therefore achieved. Our data are compared to GEF predictions as well as to evaluated libraries. For the first time, at low energies, the ENDF/B-VIII.0 nuclear data evaluation is validated with an independent measurement and the evaluated uncertainty reduced by up to 60%. This work paves the way to precisely measure prompt fission neutron multiplicities on highly radioactive nuclei.

Producing Evaluation-quality 239Pu Average Prompt Fission Neutron Multiplicities using a Correlated Fission Model

An evaluation of the average prompt fission neutron multiplicity, ν¯ p, of 239Pu(n,f) is shown. This evaluation includes (a) the correlated fission model CGMF, and (b) a detailed analysis of past and recently published experimental data. Using CGMF-calculated ν¯ pas prior enables to link, through the use of evaluated model input parameters, ν¯ p to other fission observables such as the prompt fission neutron spectrum (PFNS), pre-neutron emission fission yields as a function of mass, and the average total kinetic energy of the fragments. These evaluated parameters produce realistic predictions of many fission observables, while the evaluated ν¯ p agrees well (χ2 ≈ 1) with data. Moreover, with the new evaluated ν¯ p, the effective neutron multiplication factor of fast Pu ICSBEP critical assemblies are predicted with a mean bias of 58 pcm compared to 18 pcm with ENDF/B-VIII.0, when paired with a new 239Pu PFNS and fission cross section. Due to these encouraging validation results, the evaluated ν¯ p is currently part of a release candidate for the 239Pu ENDF/B-VIII.1 file. Hence, a correlated fission model was used for the first time for evaluating ν¯ p that is of evaluation quality. This is an important step towards consistent evaluations of prompt fission observables.

Expansion of the surrogate method to measure the prompt fission neutron multiplicity for Pu241

Fission neutron multiplicity data are imperative for nuclear models that support criticality safety, nuclear nonproliferation, and stockpile stewardship. This work presents the first experimental measurement to determine the average neutron multiplicity ($\overline{\ensuremath{\nu}}$) and the neutron multiplicity distribution using a surrogate reaction. $^{241}\mathrm{Pu}$ has a short half-life of 14.3 yr. To circumvent experimental difficulties, inelastic scattering of 53.96-MeV $\ensuremath{\alpha}$ particles on $^{242}\mathrm{Pu}$ served as a surrogate for $^{241}\mathrm{Pu}(n,f)$. The present text reports and discusses the average neutron multiplicity, in addition to the neutron multiplicity moments and distribution for equivalent neutron energies up to 20 MeV. The $\overline{\ensuremath{\nu}}$ measured in this experiment agrees with previous measurements until 12 MeV, where discrepancies arise due to preequilibrium neutron emission.

Reexamining the role of the ( n,γf ) process in the low-energy fission of U235 and Pu239

The $(n,\gamma f)$ process is reviewed in light of modern nuclear reaction calculations in both slow and fast neutron-induced fission reactions on $^{235}$U and $^{239}$Pu. Observed fluctuations of the average prompt fission neutron multiplicity and average total $\gamma$-ray energy below 100 eV incident neutron energy are interpreted in this framework. The surprisingly large contribution of the M1 transitions to the pre-fission $\gamma$-ray spectrum of $^{239}$Pu is explained by the dominant fission probabilities of 0$^+$ and $2^+$ transition states, which can only be accessed from compound nucleus states formed by the interaction of $s$-wave neutrons with the target nucleus in its ground state, and decaying through M1 transitions. The impact of an additional low-lying M1 scissors mode in the photon strength function is analyzed. We review experimental evidence for fission fragment mass and kinetic energy fluctuations in the resonance region and their importance in the interpretation of experimental data on prompt neutron data in this region. Finally, calculations are extended to the fast energy range where $(n,\gamma f)$ corrections can account for up to 3\% of the total fission cross section and about 20\% of the capture cross section.

Studying fission neutrons with 2E-2v and 2E

This work aims at measuring prompt-fission neutrons at different excitation energies of the nucleus. Two independent techniques, the 2E-2v and the 2E techniques, are used to map the characteristics of the mass-dependent prompt fission neutron multiplicity, v(A), when the excitation energy is increased. The VERDI 2E-2v spectrometer is being developed at JRC-GEEL. The Fission Fragment (FF) energies are measured using two arrays of 16 silicon (Si) detectors each. The FFs velocities are obtained by time-of-flight, measured between micro-channel plates (MCP) and Si detectors. With MCPs placed on both sides of the fission source, VERDI allows for independent timing measurements for both fragments. 252Cf(sf) was measured and the present results revealed particular features of the 2E-2v technique. Dedicated simulations were also performed using the GEF code to study important aspects of the 2E-2v technique. Our simulations show that prompt neutron emission has a non-negligible impact on the deduced fragment data and affects also the shape of v(A). Geometrical constraints lead to a total-kinetic energy-dependent detection efficiency. The 2E technique utilizes an ionization chamber together with two liquid scintillator detectors. Two measurements have been performed, one of 252Cf(sf) and another one of thermal-neutron induced fission in 235U(n,f). Results from 252Cf(sf) are reported here.

The charge polarization and its impact on prompt fission neutron multiplicity

Prompt neutrons emitted from fission fragments of actinides show a specific induced-energy dependence. The mass distribution of prompt neutrons emitted from light fragments are independent of induced energies, while the number of prompt neutrons originating from heavy fragments increases with increasing induced energy. The understanding of the mechanism of this behavior has been a long standing problem, and cannot be solved unless we assume very artificial conditions such as fragment-mass dependent temperatures. Therefore, the neutron multiplicity has been used as fitted information in major models for prompt neutron spectra. In this paper, we evaluate the prompt neutron multiplicity as a function of the fission-fragment product of the n-induced fission of U-235 based on the fission configuration derived from the Langevin equation. We found that the charge polarization, which is the deviation from the unchanged charge distribution (UCD) assumption, can directly produce the specific induced-energy dependence of prompt neutrons without extra assumptions.

Analysis of prompt fission neutrons in235U(nth,f) and fission fragment distributions for the thermal neutron induced fission of234U

This paper presents the ongoing analysis of two fission experiments. Both projects are part of the collaboration between the nuclear reactions group at Uppsala and the JRC-IRMM. The first experiment deals with the prompt fission neutron multiplicity in the thermal neutron induced fission of 235 U(n,f). The second, on the fission fragment properties in the thermal fission of 234 U(n,f).The prompt fission neutron multiplicity has been measured at the JRC-IRMM using two liquid scintillators in coincidence with an ionization chamber. The first experimental campaign focused on 235 U(n th ,f) whereas a second experimental campaign is foreseen later for the same reaction at 5.5 MeV. The goal is to investigate how the so-called sawtooth shape changes as a function of fragment mass and excitation energy. Some harsh experimental conditions were experienced due to the large radiation background. The solution to this will be discussed along with preliminary results.In addition, the analysis of thermal neutron induced fission of 234 U(n,f) will be discussed. Currently analysis of data is ongoing, originally taken at the ILL reactor. The experiment is of particular interest since no measurement exist of the mass and energy distributions for this system at thermal energies. One main problem encountered during analysis was the huge background of 235 U(n th ,f). Despite the negligible isotopic traces in the sample, the cross section difference is enormous. Solution to this parasitic background will be highlighted.

A new neutron counter for fission research

A new neutron counter for research experiments on nuclear fission has been developed. This instrument is designed for the detection of prompt fission neutrons within relatively high levels of gamma and neutron background. It is composed of a set of 3Heproportional counters arranged within a block of polyethylene which serves as moderator. The detection properties have been studied by means of Monte Carlo simulations and experiments with radioactive sources. These properties are confirmed by an experiment on neutron-induced fission of 238Uat the WNR facility of the Los Alamos Neutron Science Center during which the mean prompt fission neutron multiplicity, or ν¯ has been measured from 1 to 20MeV of incident neutron energy.

Multi-modal calculations of prompt fission neutrons from 238U(n, f) at low induced energy

Properties of prompt fission neutrons from 238U(n, f) are calculated for incident neutron energies below 6 MeV using the multi-modal model, including the prompt fission neutron spectrum, the average prompt fission neutron multiplicity, and the prompt fission neutron multiplicity as a function of the fission fragment mass v(A) (usually named “sawtooth" data) The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental fission fragment data. The predicted results are in good agreement with the experimental data.

Investigation of phenomenological models for the Monte Carlo simulation of the prompt fission neutron andγemission

A Monte Carlo simulation of the fission fragment deexcitation process was developed in order to analyze and predict postfission-related nuclear data which are of crucial importance for basic and applied nuclear physics. The basic ideas of such a simulation were already developed in the past. In the present work, a refined model is proposed in order to make a reliable description of the distributions related to fission fragments as well as to prompt neutron and $\ensuremath{\gamma}$ energies and multiplicities. This refined model is mainly based on a mass-dependent temperature ratio law used for the initial excitation energy partition of the fission fragments and a spin-dependent excitation energy limit for neutron emission. These phenomenological improvements allow us to reproduce with a good agreement the $^{252}\mathrm{Cf}$(sf) experimental data on prompt fission neutron multiplicity $\overline{\ensuremath{\nu}}(A)$, $\overline{\ensuremath{\nu}}(TKE)$, the neutron multiplicity distribution $P(\ensuremath{\nu}),$ as well as their energy spectra $N(E)$, and lastly the energy release in fission.

Multi-modal analysis for low energy neutron-induced fission of 235U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.

Prompt fission neutron multiplicity and spectrum calculations for thermal and fast neutron induced reactions on 233,231Pa nuclei

The prompt fission neutron multiplicity and spectra of 233Pa(n, f) and 231Pa(n, f) were calculated for incident neutron energies up to 50 MeV and 30 MeV, respectively using refined neutron evaporation models. Those models have been already successfully applied to calculate the prompt neutron multiplicities and spectra for many actinides. The unique experimental data concerning the fission fragment mass distribution and total kinetic energy for the 234,233,232Pa fissioning nuclei were exploited in the frame of the Point-by-Point treatment. The total average prompt neutron multiplicity, the fission fragment pair multiplicity and the multiplicity dependence on fragment mass (sawtooth) for the reactions 233,232,231Pa(n th, f) and the average values of the model parameters for 234-232Pa fissioning nuclei were calculated. For the other Pa fissioning nuclei involved in the studied reactions relevant parameters were established following systematic trends. Due to a total lack of experimental prompt fission neutron multiplicity and spectrum data for Pa isotopes, the present results were compared with systematic trends following from neighboring actinides. The calculated prompt fission neutron multiplicity as well as the prompt γ-ray energy of the 233,231Pa(n, f) reactions are obtained in very good agreement with all systematic trends.

Experimental prompt fission neutron “sawtooth” data described by the “point by point” model

The experimental data concerning the prompt fission neutron number as function of the fission fragment mass (currently named “sawtooth” data) when they exist, allow a more refined verification of the “point by point” model and in the same time the validation of the methods used to determine the model parameters corresponding to the fission fragment pairs covering the entire fission fragment range. The prompt neutron multiplicities of each fission fragment pair provided by the “point by point” model are compared with the experimental data concerning the sum of neutrons emitted by the light and heavy fragments forming a pair obtained from experimental sawtooth data. The available fission fragment pair multiplicity and sawtooth experimental data for the 233,235,238U(n, f) and 237Np(n, f) reactions as well as for the spontaneous fission of 252Cf are well described by the “point by point” model results, proving again that this treatment is a powerful tool for prompt fission neutron multiplicity and spectrum evaluation purposes.

Inner barrier shape symmetries in 237Np neutron data evaluation up to 40 MeV incident energy

The 237Np neutron induced cross-section evaluation up to 40 MeV incident energy is made in consistent way, based on models with all parameters in agreement with experimental data, theoretical and statistical considerations. The resulted cross-sections describe very well the existing experimental data without any re-normalization and adjustments. The shape symmetry change at the inner fission barrier for the Np isotopes is determined on the basis of an original systematic, the fission level density and the fission barrier parameters being obtained according with these symmetries. The prompt fission neutron multiplicity and spectra of n + 237Np are calculated with a refined model that takes into account the entire fission fragment range with a point-by-point treatment. In the incident energy range where multiple fission chances are involved the average values of the model parameters are determined by point-by-point treatment. The prompt fission neutron multiplicity and spectra, as well as the average prompt gamma-ray energy are obtained in very good agreement with the existing experimental data. The systematic behaviours of the model parameters as function of quantities in connection with the respective Np fissioning nuclei are also obtained in the frame of this work.

A refined model for 235U( [formula omitted]) prompt fission neutron multiplicity and spectrum calculation with validation in integral benchmarks

The prompt fission neutron multiplicity and spectra of the n + U 235 reaction are calculated for the first time with a refined model that takes into account the entire fission fragment range with a point-by-point treatment. At higher incident neutron energies, the treatment of neutrons evaporated prior to scission is improved by the use of the ( n , x n ) spectra obtained in the frame of the cross-section calculation (compound nucleus and pre-equilibrium mechanisms). The new prompt fission neutron multiplicity and spectra up to 30 MeV incident neutron energy, included in the U-235 BRC/JEFF3.1t evaluation, lead to very good K eff results with the fast and thermal integral benchmarks.

Prompt fission neutron multiplicity and spectrum model for 30–80 MeV neutrons incident on 238U

The improved Los Alamos model is developed for the first time in order to provide prompt fission neutron multiplicity, prompt fission neutron spectra and other quantities at high incident neutron energies where the fission of secondary compound nuclei formed by charged particle emission occurs. In this model (exemplified by the n+ 238U reaction up to 80 MeV incident energy) the fission of the secondary nuclei formed by proton emission, neutron evaporation from the nuclei formed by proton emission, deuteron emission, alpha emission and neutron evaporation from the nuclei formed by alpha emission is taken into account. Input model parameters and related excitation energy dependences are determined using available experimental information and systematics as well as total and partial neutron induced fission cross-sections and their ratios obtained separately from a recent evaluation performed up to medium energies. Our present model predictions are in good agreement with the measured prompt neutron spectra and multiplicities.

Prompt fission neutron multiplicity and spectrum model for 30?80 MeV neutrons incident onB238U

The improved Los Alamos model is developed for the first time in order to provide prompt fission neutron multiplicity, prompt fission neutron spectra and other quantities at high incident neutron energies where the fission of secondary compound nuclei formed by charged particle emission occurs. In this model (exemplified by the n+238U reaction up to 80 MeV incident energy) the fission of the secondary nuclei formed by proton emission, neutron evaporation from the nuclei formed by proton emission, deuteron emission, alpha emission and neutron evaporation from the nuclei formed by alpha emission is taken into account. Input model parameters and related excitation energy dependences are determined using available experimental information and systematics as well as total and partial neutron induced fission cross-sections and their ratios obtained separately from a recent evaluation performed up to medium energies. Our present model predictions are in good agreement with the measured prompt neutron spectra and multiplicities.

Prompt fission neutron multiplicity and spectrum evaluation for 235U(n, f) in the frame of the multi-modal fission model

The multi-modal fission approach, in the frame of an improved version of the Los Alamos model, is used for the evaluation of the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U. The three most dominant fission modes (standard I, standard II and superlong) are taken into account. The multi-modal parameters entering the multiplicity and spectrum model are determined on the basis of experimental data measured at IRMM. The total prompt fission neutron multiplicity and spectra, both obtained as superposition of the modal contributions are in good agreement with the experimental data. Also, the average prompt gamma-ray energy for each fission mode is obtained and the total prompt gamma-ray energy is in good agreement with the existing experimental data, too. The agreement of the prompt neutron spectrum with the experimental data is improved especially in the lower fission neutron energy region of the spectrum when the neutron emission anisotropy effect is taken into account.

Prompt fission neutron multiplicity and spectra evaluation in the frame of the multi-modal fission model for 237Np(n,f) and 238U(n,f)

The multi-modal fission model approach was used for the evaluation of the prompt fission neutron multiplicity and spectra for the neutron induced fission of 237Np and 238U. The model parameters are based on experimental data obtained at IRMM.The three most dominant fission modes (standard I, standard II and superlong) are taken into account. For each fission mode the prompt neutron multiplicity and spectrum are calculated within an improved version of the Los Alamos model. The calculated total prompt neutron multiplicity and spectrum are in good agreement with the experimental data. Also, the average prompt gamma ray energy for each fission mode is obtained as well as the total prompt gamma ray energy. The total prompt gamma ray energy for 237Np(n,f) is in agreement with existing experimental data.

Prompt fission neutron multiplicity and spectrum evaluations for n+ 238U reaction

The prompt fission neutron multiplicity and spectra for n+ 238U reaction are calculated using an improved Los Alamos model which includes the linear relation between the average prompt gamma ray energy and the prompt neutron multiplicity and also the average fission fragment kinetic energy dependence on the incident neutron energy. The coefficients describing the quadratic variation of the fission fragment kinetic energy versus the incident energy are obtained by extrapolation of the data and procedure used for n+ 235U reaction. The inverse process compound nucleus cross-section of the fissioning nucleus is calculated using the coupled channel method. In the incident energy range where only the first fission chance is involved the comparison of present spectrum evaluation with spectrum calculation using multi-modal model is made too. The calculated prompt neutron multiplicity and spectra of 238U neutron induced fission are in good agreement with the experimental data for the entire incident energy range required in evaluations, proving the validity of the used procedure.