Hauser-Feshbach Statistical Model Research Articles

Thermonuclear 28P($p$, gamma)29S reaction rate and astrophysical implication in ONe nova explosion

An accurate 28P($p$, gamma )29S reaction rate is crucial to defining the nucleosynthesis products of explosive hydrogen burning in ONe novae. Using the recently released nuclear mass of 29S, together with a shell model and a direct capture calculation, we reanalyzed the 28P($p$, gamma )29S thermonuclear reaction rate and its astrophysical implication. We focus on improving the astrophysical rate for 28P($p$, gamma )29S based on the newest nuclear mass data. Our goal is to explore the impact of the new rate and associated uncertainties on the nova nucleosynthesis. We evaluated this reaction rate via the sum of the isolated resonance contribution instead of the previously used Hauser-Feshbach statistical model. The corresponding rate uncertainty at different energies was derived using a Monte Carlo method. Nova nucleosynthesis is computed with the 1-D hydrodynamic code SHIVA. The contribution from the capture on the first excited state at 105.64 keV in 28P is taken into account for the first time. We find that the capture rate on the first excited state in 28P is up to more than 12 times larger than the ground-state capture rate in the temperature region of 2.5$ $K to 4$ $K resulting in the total 28P($p$, gamma )29S reaction rate being enhanced by a factor of up to 1.4 at $ $K. In addition, the rate uncertainty has been quantified for the first time. It is found that the new rate is smaller than the previous statistical model rates, but it still agrees with them within uncertainties for nova temperatures. The statistical model appears to be roughly valid for the rate estimation of this reaction in the nova nucleosynthesis scenario. Using the 1-D hydrodynamic code SHIVA, we performed the nucleosynthesis calculations in a nova explosion to investigate the impact of the new rates of 28P($p$, gamma )29S. Our calculations show that the nova abundance pattern is only marginally affected if we use our new rates with respect to the same simulations but statistical model rates. Finally, the isotopes whose abundance is most influenced by the present 28P($p$, gamma )29S uncertainty are 28Si, $^ $S, $^ $Cl, and 36Ar, with relative abundance changes at the level of only 3<!PCT!> to 4<!PCT!>.

Study of 113In(α, α) elastic scattering to determine α-optical potential relevant for astrophysical γ-process

Abstract The α-optical potential is one of the key input parameters used to measure the reaction rate of the (γ, α)-process using the Hauser-Feshbach statistical model and the principle of detailed balance. α-elastic scattering experiment on 113In p-nucleus was carried out in the energy range E lab = 26−32 MeV. The vacuum evaporation technique was used to prepare the 113In target (∼86 μg cm−2). An energy-dependent local optical potential parameters set was obtained by analysing the experimental elastic scattering angular distribution data. The local potential parameters are extrapolated for lower energies and are used to calculate the 113In(α, γ) reaction cross-section.

Connection of four-dimensional Langevin model and Hauser-Feshbach theory to describe statistical decay of fission fragments

ABSTRACT We have developed a novel theoretical method to obtain independent fission product yields and prompt neutron observables by connecting mass and total kinetic energy (TKE) distributions calculated by a four-dimensional Langevin dynamical model to a Hauser-Feshbach statistical decay model. In the Langevin calculations, mass distributions corresponding to the standard I and II modes were obtained separately and superposed to reproduce the fission fragment yield of spontaneous fission of 238 , 240 , 242 Pu and thermal neutron-induced fission of 239 Pu . This was achieved by using different neck parameters for these two modes in the two-center shell model shape parametrization, and a systematics of the superposing ratio was obtained as a function of N − Z / A of the fissioning nuclei. The Hauser-Feshbach calculations were performed using a nuclear reaction code TALYS for 239 Pu n , f reaction in the incident energy range from thermal up to 5 MeV , and the calculated prompt fission observables were compared with experimental and evaluated data. Although further improvements are needed for the most important nuclides, it turned out that the present methodology has the capability to prepare fission-related nuclear data for nuclides for which measurements are difficult.

TALYS calculations of prompt fission observables and independent fission product yields for the neutron-induced fission of ^{235}U

The TALYS nuclear reaction code’s Hauser-Feshbach statistical decay model has been adapted in order to calculate prompt fission neutron and gamma -ray observables by iterating over deexciting fission fragments. Several fission fragment generators such as GEF, HF^{3}D, and SPY were employed to provide TALYS with databases. These databases contain standardized tables with fission fragment yields, mean excitation energies and their widths, and average total kinetic energy, as a function of charge and mass number of primary fission fragments. The resulting calculations, including prompt particle multiplicities, spectra, average energies, and independent fission product yields, were compared with experimental and evaluated data. This work first outlines the new methodology implemented in TALYS and examines the effects of three important parameters on the final evaporation data. Furthermore, the neutron-induced fission of ^{235}U is investigated in detail as a function of incident energy. The results from TALYS, with input from GEF and HF^{3}D, were compared with available experimental data and the results of the stand-alone GEF code. The proposed methodology contributes to an improved capability to model the fission process.

New 26P(p, γ)27S Thermonuclear Reaction Rate and Its Astrophysical Implications in the rp-process

Accurate nuclear reaction rates for 26P(p, γ)27S are pivotal for a comprehensive understanding of the rp-process nucleosynthesis path in the region of proton-rich sulfur and phosphorus isotopes. However, large uncertainties still exist in the current rate of 26P(p, γ)27S because of the lack of nuclear mass and energy level structure information for 27S. We reevaluate this reaction rate using the experimentally constrained 27S mass, together with the shell model predicted level structure. It is found that the 26P(p, γ)27S reaction rate is dominated by a direct capture reaction mechanism despite the presence of three resonances at E = 1.104, 1.597, and 1.777 MeV above the proton threshold in 27S. The new rate is overall smaller than the other previous rates from the Hauser–Feshbach statistical model by at least 1 order of magnitude in the temperature range of X-ray burst interest. In addition, we consistently update the photodisintegration rate using the new 27S mass. The influence of new rates of forward and reverse reaction in the abundances of isotopes produced in the rp-process is explored by postprocessing nucleosynthesis calculations. The final abundance ratio of 27S/26P obtained using the new rates is only 10% of that from the old rate. The abundance flow calculations show that the reaction path 26P(p, γ)27S(β +,ν)27P is not as important as previously thought for producing 27P. The adoption of the new reaction rates for 26P(p, γ)27S only reduces the final production of aluminum by 7.1% and has no discernible impact on the yield of other elements.

Nuclear many-body effects on particle emission following muon capture on Si28 and Ca40

Background: Muon captures on nuclei have provided us with plenty of knowledge of nuclear properties. Recently, this reaction attracts attention in electronics, because charged particle emissions following muon capture on silicon become to trigger non-negligible soft errors in memory devices.Purpose: To date, there is no theoretical framework based on the nuclear structure that describes a muon capture reaction followed by particle emissions comprehensively. The purpose of this work is to develop a new method that considers the nuclear many-body correlation for the accurate understanding of the soft errors in memory devices.Method: We combined the second Tamm-Dancoff approximation that is used to estimate muon capture rates with the two-component exciton model, the model describing particle emission from the pre-equilibrium state. For particle evaporation from the compound state, the Hauser-Feshbach statistical models were applied. We chose $^{28}\mathrm{Si}$ and $^{40}\mathrm{Ca}$ to check the performance of the framework.Result: We paid attention to the muon capture rates, the particle emission spectra, and the multiplicities that have a close interrelation with each other. We found that the nuclear many-body correlations including two-particle two-hole excitations is a key to explaining them simultaneously.Conclusion: The present study showed that the combination of the microscopic approach of muon capture and the two-component exciton model of particle emission is an effective tool to describe particle emission following the muon captures, giving the nuclear structure information additionally. For a finer understanding of particle emission following muon capture and a validation of the present framework, further experimental studies on particle emission spectra are highly expected.

Statistical (n,gamma ) cross section model comparison for short-lived nuclei

Neutron-capture cross sections of neutron-rich nuclei are calculated using a Hauser–Feshbach model when direct experimental cross sections cannot be obtained. A number of codes to perform these calculations exist, and each makes different assumptions about the underlying nuclear physics. We investigated the systematic uncertainty associated with the choice of Hauser-Feshbach code used to calculate the neutron-capture cross section of a short-lived nucleus. The neutron-capture cross section for ^{73}hbox {Zn}(n,gamma )^{74}hbox {Zn} was calculated using three Hauser-Feshbach statistical model codes: TALYS, CoH, and EMPIRE. The calculation was first performed without any changes to the default settings in each code. Then an experimentally obtained nuclear level density (NLD) and gamma -ray strength function (gamma hbox {SF}) were included. Finally, the nuclear structure information was made consistent across the codes. The neutron-capture cross sections obtained from the three codes are in good agreement after including the experimentally obtained NLD and gamma hbox {SF}, accounting for differences in the underlying nuclear reaction models, and enforcing consistent approximations for unknown nuclear data. It is possible to use consistent inputs and nuclear physics to reduce the differences in the calculated neutron-capture cross section from different Hauser-Feshbach codes. However, ensuring the treatment of the input of experimental data and other nuclear physics are similar across multiple codes requires a careful investigation. For this reason, more complete documentation of the inputs and physics chosen is important.

Influence of pre-scission neutron emission on high-energy U238 fission studied by the Langevin approach

Background: The information about the pre-scission emitted neutrons is crucial for the fission dynamics study such as fission time scale and dissipation mechanism. Purpose: The aim is to investigate the influence of the pre-scission neutron emission on the fragments mass, total kinetic energy, and the deformation distribution at the scission point in $^{238}\mathrm{U}(n,f)$ fissions at 110, 325, and 500 MeV. Method: A three-dimensional Langevin approach considering nucleus elongation, deformation, and mass asymmetry is applied to simulate fission dynamics. The Hauser-Feshbach statistical decay model is coupled to simulate the pre-scission neutron emission. Results: The properties of the pre-scission emitted neutron are first investigated. It is found that the multiplicity and the average kinetic energy increase with increasing excitation energy. The energy carried away by neutron emission is not negligible and it leads to the multichance fission, especially for the 110 MeV case. The fragments mass, total kinetic energy, and the deformation distribution at the scission point are then calculated with and without coupling the Hauser-Feshbach statistical decay model. It is found that the calculations partially reproduce the leftward shift of the experimental fragment mass distribution at high excitation energy. The pre-scission neutron emission also causes a reduction of the average total kinetic energy by about 2 MeV and a leftward shift of the distribution. The deformation distribution shows that about 75% of the fragments are produced in spherical and prolate ellipsoid shapes. No significant influence on deformation at the scission point was found. Conclusions: For high-energy fission, the energy reduction by pre-scission neutron emission should be considered, which will lead to the multichance fission. The pre-scission neutron emission is one of the reasons for the decrease of the average mass and the leftward shift of the fission fragments mass and total kinetic energy distribution.

Production of high-energyγrays in proton- andα-induced reactions

The high-energy $\ensuremath{\gamma}$ rays have been measured in proton and $\ensuremath{\alpha}$-induced reactions on medium mass target nuclei $^{115}\mathrm{In}$ and $^{112}\mathrm{Sn}$, respectively. Theoretical analyses of the spectra have been performed within the Hauser-Feshbach statistical model and the Akkermans-Gruppelaar exciton model formalisms. It is observed that the proton-induced reaction has significant contribution from the direct-semidirect capture in the region ${E}_{\ensuremath{\gamma}} \ensuremath{\approx}10--20$ MeV. The $^{4}\mathrm{He}$-ion-induced reaction could be described reasonably well by the results of the statistical model calculations. A reduced level density parameter, as compared to that used for the proton-induced reaction, was required to explain the high-energy $\ensuremath{\gamma}$-ray spectrum in the $^{4}\mathrm{He}$-ion-induced reaction.

Statistical Hauser-Feshbach Model Description of (n,α) Reaction Cross Sections for the Weak s-Process

The (n,α) reaction contributes in many processes of energy generation and nucleosynthesis in stellar environment. Since experimental data are available for a limited number of nuclei and in restricted energy ranges, at present only theoretical studies can provide predictions for all astrophysically relevant (n,α) reaction cross sections. The purpose of this work is to study (n,α) reaction cross sections for a set of nuclei contributing in the weak s-process nucleosynthesis. Theory framework is based on the statistical Hauser-Feshbach model implemented in TALYS code with nuclear masses and level densities based on Skyrme energy density functional. In addition to the analysis of the properties of calculated (n,α) cross sections, the Maxwellian averaged cross sections are described and analyzed for the range of temperatures in stellar environment. Model calculations determined astrophysically relevant energy windows in which (n,α) reactions occur in stars. In order to reduce the uncertainties in modeling (n,α) reaction cross sections for the s-process, novel experimental studies are called for. Presented results on the effective energy windows for (n,α) reaction in weak s-process provide a guidance for the priority energy ranges in the future experimental studies.

β -delayed neutron-emission and fission calculations within relativistic quasiparticle random-phase approximation and a statistical model

Background: $\ensuremath{\beta}$-delayed neutron emission and fission are essential in $r$-process nucleosynthesis. Although the number of experimental studies covering $r$-process nuclei has recently increased, the uncertainties of $\ensuremath{\beta}$-delayed neutron emission and fission are still large for $r$-process simulations.Purpose: Our aim is to introduce a theoretical framework for the description of $\ensuremath{\beta}$-delayed neutron-emission and fission rates based on relativistic nuclear energy density-functional and statistical models and investigate their properties throughout the nuclide map.Methods: To obtain $\ensuremath{\beta}$ strength functions, the relativistic proton-neutron quasiparticle random-phase approximation is employed. Particle evaporations and fission from highly excited nuclear states are estimated by the Hauser-Feshbach statistical model. $\ensuremath{\beta}$-delayed neutron branching ratios ${P}_{n}$ are calculated and compared with experimental data, and the $\ensuremath{\beta}$-delayed fission branching ratio ${P}_{f}$ are also assessed by using different fission barrier data.Results: Calculated ${P}_{n}$ are in a good agreement with the experimental data and the root mean square deviation is comparable to results of preceding works. It is found that energy withdrawal by $\ensuremath{\beta}$-delayed neutron-emission sensitivity varies ${P}_{n}$, especially for nuclei near the neutron drip line. ${P}_{f}$ depend sensitively on fission barrier data. It is found that not only the barrier height but also the number of barrier humps is important to evaluate ${P}_{f}$.Conclusions: The framework introduced in this work provides an improved theoretical description of the $\ensuremath{\beta}$-delayed neutron emission and fission. Since ${P}_{f}$ as well as ${P}_{n}$ depend strongly on fission barrier information, four kinds of fission barrier data are used in this work to allow further sensitivity studies of the $r$-process nucleosynthesis on the nuclear fission. More studies on fission barrier are highly requested to assess the role of $\ensuremath{\beta}$-delayed fission in the $r$-process study. A complete set of calculated data for $\ensuremath{\beta}$-delayed neutron emission and fission are summarized as a table in supplemental material for its use in $r$-process studies as well as to complement a part of nuclear data in which no experimental data are available.

Simulating low-energy neutrino interactions with MARLEY

Monte Carlo event generators are a critical tool for the interpretation of data obtained by neutrino experiments. Several modern event generators are available which are well-suited to the GeV energy scale used in studies of accelerator neutrinos. However, theoretical modeling differences make their immediate application to lower energies difficult. In this paper, I present a new event generator, MARLEY, which is designed to better address the simulation needs of the low-energy (tens of MeV and below) neutrino community. The code is written in C++14 with an optional interface to the popular ROOT data analysis framework. The current release of MARLEY (version 1.2.0) emphasizes simulations of the reaction 40Ar(νe,e−)40K⁎ but is extensible to other channels with suitable user input. This paper provides detailed documentation of MARLEY's implementation and usage, including guidance on how generated events may be analyzed and how MARLEY may be interfaced with external codes such as Geant4. Further information about MARLEY is available on the official website at http://www.marleygen.org. Program summaryProgram title:MARLEY 1.2.0CPC Library link to program files:https://doi.org/10.17632/4v7zxnc8j3.1Developer's respository link:http://github.com/MARLEY-MC/marleyCode Ocean capsule:https://codeocean.com/capsule/9868179Licensing provisions: GNU General Public License 3.0Programming language: C++14External routines/libraries used: GNU Scientific Library [1,2] (required), ROOT [3,4] (optional)Nature of problem: Simulation of neutrino-nucleus scattering events at energies of tens-of-MeV and belowSolution method: Initial two-to-two scattering kinematics are sampled using the allowed approximation differential cross section and tables of precomputed nuclear matrix elements. Subsequent de-excitations of the remnant nucleus are simulated using a Monte Carlo implementation of the Hauser-Feshbach statistical model and tabulated γ-ray decay schemes for discrete nuclear levels.Additional comments including restrictions and unusual features: Input data are provided with the code that are suitable for producing simulations of the charged-current reaction 40Ar(νe,e−)40K⁎, coherent elastic neutrino-nucleus scattering on spin-zero target nuclei, and neutrino-electron elastic scattering on any atomic target. Preparation of new reaction input files (whose format is documented in Appendix B) would enable other reaction channels and nuclear targets to be handled by the existing code framework. Although there is no maximum neutrino energy enforced by the code itself, realistic neutrino-nucleus scattering events may be generated up to roughly 50 MeV. Above this energy, the effects of forbidden nuclear transitions, which are neglected in the current treatment of the cross sections (see section 2.1), become increasingly important.

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 γ 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,π), CGMF samples neutron and γ-ray probability distributions at each stage of the decay process, conserving energy, spin and parity. Nuclear structure and reaction input data from the RIPL3 library are used to describe fission fragment properties and decay probabilities. Characteristics of prompt fission neutrons, prompt fission γ rays, and independent fission yields can be studied consistently. Correlations in energy, angle and multiplicity among the emitted neutrons and γ rays can be easily analyzed as a function of the emitting fragments. In its current version, CGMF is limited to certain spontaneous fission reactions on Pu and Cf isotopes, as well as neutron-induced fission reactions up to 20 MeV on U, Np, and Pu isotopes. Program summaryProgram Title:CGMF 1.1CPC Library link to program files:https://doi.org/10.17632/jrtt73npzw.1Licensing provisions: BSD-3 clauseProgramming language: C++Nature of problem: Modeling of fission fragment decaySolution method: Monte Carlo implementation of the Hauser-Feshbach statistical theory of nuclear reactions to describe the de-excitation of fission fragments on an event-by-event basis.Additional comments including restrictions and unusual features: spontaneous fission reactions for 238,240,242,244Pu and 252,254Cf; neutron-induced fission reactions from thermal up to 20 MeV for n+233,234,235,238U, n+237Np, and n+239,241Pu. Binary fission only (no ternary fission).

Measurements of 160Dy (p, γ) at Energies Relevant for the Astrophysical γ Process

Abstract Rare information on photodisintegration reactions of nuclei with mass numbers A ≈ 160 at astrophysical conditions impedes our understanding of the origin of p-nuclei. Experimental determination of the key (p, γ) cross sections has been playing an important role in verifying nuclear reaction models and providing rates of relevant (γ, p) reactions in the γ process. In this paper we report the first cross-section measurements of 160Dy(p, γ)161Ho and 161Dy(p, n)161Ho in the beam energy range of 3.4–7.0 MeV, partially covering the Gamow window. Such determinations are possible by using two targets with various isotopic fractions. The cross-section data can put a strong constraint on the nuclear level densities and gamma strength functions for A ≈ 160 in the Hauser–Feshbach statistical model. Furthermore, we find the best parameters for TALYS that reproduce the available A ∼ 160 data, 160Dy(p, γ)161Ho and 162Er(p, γ)163Tm, and recommend the constrained 161Ho(γ, p)160Dy reaction rates over a wide temperature range for γ process network calculations. Although the determined 161Ho(γ, p) stellar reaction rates at the temperature of 1 to 2 GK can differ by up to one order of magnitude from the NON-SMOKER predictions, it has a minor effect on the yields of 160Dy and accordingly the p-nuclei, 156,158Dy. A sensitivity study confirms that the cross section of 160Dy(p, γ)161Ho is measured precisely enough to predict yields of p nuclei in the γ process.

Investigating the Ag109(p,γ)Cd110 reaction and its underlying nuclear physics

Background: The nucleosynthesis of neutron-deficient $p$ nuclei remains an unsolved puzzle in nuclear astrophysics. Most likely, huge networks containing hundreds of nuclear reactions are responsible for the creation of this group of nuclei. In reality, many of the relevant reaction rates cannot be studied experimentally but need to be estimated using global and robust theoretical approaches. The underlying nuclear physics that enters these calculations is often still not constrained well enough, especially for nuclei further off the valley of stability.Purpose: Here, we complete the systematic measurement of radiative proton-capture reactions on stable Ag isotopes. The results will be used to test existing theoretical models and are crucial to constrain underlying nuclear physics properties. For this purpose, total cross sections of the $^{109}\mathrm{Ag}(p,\ensuremath{\gamma})^{110}\mathrm{Cd}$ reaction have been measured at proton energies between 2.5 and 5.0 MeV.Method: The cross-section measurements have been carried out by means of in-beam $\ensuremath{\gamma}$-ray spectroscopy and the observation of all ground-state transitions in $^{110}\mathrm{Cd}$. In general, the total cross sections depend strongly on the $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}$-SF) and the nuclear level density (NLD). While the former one is taken from the systematics in other even-even Cd isotopes, the NLD has been constrained by renormalizing microscopic, tabulated values onto the low-lying cumulative number of levels.Results: For the first time, $^{109}\mathrm{Ag}(p,\ensuremath{\gamma})^{110}\mathrm{Cd}$ reaction cross sections have been reported over a wide range of beam energies. The total cross-section results are in good agreement with a rather limited, formerly published dataset. A convincing agreement with recent Hauser-Feshbach calculations is received when using existing descriptions for the $\ensuremath{\gamma}$-SF and a renormalized, microscopic NLD model.Conclusion: The measured cross sections confirmed the reliability of existing theoretical models used in Hauser-Feshbach statistical model calculations. Coherent results are obtained for the statistical nuclear physics properties in the even-even Cd isotopes as well as for the derived experimental cross sections.

Nuclear de-excitations in low-energy charged-current νe scattering on Ar40

Background: Large argon-based neutrino detectors, such as those planned for the Deep Underground Neutrino Experiment, have the potential to provide unique sensitivity to low-energy (few to tens of MeV) electron neutrinos produced by core-collapse supernovae. Despite their importance for neutrino energy reconstruction, nuclear de-excitations following charged-current $\nu_e$ absorption on $^{40}$Ar have never been studied in detail at supernova energies. Purpose: I develop a model of nuclear de-excitations that occur following the $^{40}\mathrm{Ar}(\nu_e,e^{-})^{40}\mathrm{K}^*$ reaction. This model is applied to the calculation of exclusive cross sections. Methods: A simple expression for the inclusive differential cross section is derived under the allowed approximation. Nuclear de-excitations are described using a combination of measured $\gamma$-ray decay schemes and the Hauser-Feshbach statistical model. All calculations are carried out using a novel Monte Carlo event generator called MARLEY (Model of Argon Reaction Low Energy Yields). Results: Various total and differential cross sections are presented. Two de-excitation modes, one involving only $\gamma$-rays and the other including single neutron emission, are found to be dominant at few tens-of-MeV energies. Conclusions: Nuclear de-excitations have a strong impact on the achievable energy resolution for supernova $\nu_e$ detection in liquid argon. Tagging events involving neutron emission, though difficult, could substantially improve energy reconstruction. Given a suitable calculation of the inclusive cross section, the MARLEY nuclear de-excitation model may readily be applied to other scattering processes.

Astrophysical production of 146Sm in nuclear p - processes

The large time of life of 146Sm suggests the possibility to use this p - nuclide as astrophysical chronometer to study the geochemical galactic evolution. Due to the high temperature and large densities of gamma quanta, neutrons and protons in stellar environment 146Sm nucleus can be obtained in (γ,n), (n,2n), (p,2n) processes on 147Sm.The knowledge of corresponding cross sections of gamma rays, neutrons and alpha induced processes is of a great importance for the explanation of (146Sm/144Sm) ratio uncertainties observed on the Earth, meteorites, Moon and other celestial bodies.Cross sections of (γ,n), (n,2n), (α,γ) processes induced by fast gamma rays, neutrons and alphas on 147Sm and 142Nd, from threshold up to 25 MeV were evaluated and predicted in the frame of Hauser-Feshbach statistical model by using Talys software and the own computer programs. For each nuclear reaction contribution of direct, compound and pre-equilibrium mechanisms were determined. Theoretical evaluations are compared with existing experimental data. Parameters of optical potential in the incident and emergent channels and of nuclear densities were extracted. Calculated cross sections together with corresponding nuclear data were used in the evaluation of astrophysical rates necessary in the determination of elemental abundances as needed by nuclear astrophysical networks.

Using CGMF to estimate corrections for fission yields measured via γ-ray spectroscopy

Fission product yields have been inferred using γ-ray spectroscopy for several decades. Typically, these efforts have focused on even-Z even-A fission products as their nuclear structure are less complicated. To further simplify the situation, it is often assumed that no side-feeding to the ground-state occurs and multiplicity cuts have a negligible effect on the inferred yields. Using CGMF, a Hauser-Feshbach statistical decay model for the primary fission fragments, we estimate the impact of these assumptions and determine corrections for specific fission product yields. We report on these corrections and investigate their sensitivity to various nuclear parameters, specifically the spin distribution of the fission fragments and the assumed nuclear structure. Our results indicate that even in the simplest of cases, say the 2+ → 0+ transitions in even-Z even-A fragments, average level corrections are on the order of 75%.

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.

Effects of the nuclear structure of fission fragments on the high-energy prompt fission γ -ray spectrum in U235(nth,f)

The prompt fission γ-ray energy spectrum for cold-neutron-induced fission of U235 was measured in the energy range Eγ=0.8–20MeV, by gaining a factor of about 105 in statistics compared to the measurements performed so far. The spectrum exhibits local bump structures at Eγ≈4MeV and ≈6MeV, and also a broad one at ≈15MeV. In order to understand the origins of these bumps, the γ-ray spectra were calculated using a statistical Hauser-Feshbach model, taking into account the deexcitation of all the possible primary fission fragments. It is shown that the bump at ≈4MeV is created by the transitions between the discrete levels in the fragments around Sn132, and the bump at ≈6MeV mostly comes from the complementary light fragments. It is also indicated that a limited number of nuclides, which have high-spin states at low excitation energies, can contribute to the bump structure around Eγ≈15MeV, induced by the transition feeding into the low-lying high-spin states.