Neutron Capture Data Research Articles

Oklo natural fission reactors and dynamical models of dark energy.

Bounds on the cosmological variation of the fine structure constant α inferred from Oklo neutron capture dataare sometimes taken cum grano salis. It is possible to quantify uncertainties related to the treatment of excitation, deformation and the Coulomb interaction. On the basis of this analysis, it is concluded that Oklo data imply the relative change in α over the last 1.9 billion years is < 0.01ppm (95% CL). Accommodation of this constraint represents a challenge to dark energy models that predict that fundamental constants do change.

PyEGAF: An open-source Python library for the Evaluated Gamma-ray Activation File

The Evaluated Gamma-ray Activation File (EGAF) is one of the most comprehensive resources for thermal neutron-capture data. This database contains data from prompt gamma activation analysis measurements carried out in a consistent manner using the same experimental configuration at the Budapest Research Reactor for 245 isotopes. Although these valuable datasets have been freely available for many years, one of the drawbacks is the outdated and cryptic Evaluated Nuclear Structure Data File (ENSDF) format that is currently adopted for dissemination, making it difficult for users unfamiliar with the format to access and utilize the data contained therein. Furthermore, the ENSDF format does not readily lend itself to modern computational technologies and a parser is required to interpret the complicated mixed-record format. To help overcome these challenges, we have developed a translator to convert the ENSDF-formatted datasets into an open standard JavaScript Object Notation (JSON) format enabling accessibility to applications using different programming languages running in different environments. To compliment this effort, we have also developed an open-source software package implemented in Python, pyEGAF, that is designed to interact with the JSON data structures for general purpose access, manipulation, and analysis of the neutron-capture γ-ray data in EGAF. The new format, together with the pyEGAF library, greatly enhances access to the wider applications community where EGAF data may be useful or is required.

A Case Study on Deep Learning applied to Capture Cross Section Data Analysis

A good data analysis of neutron cross section measurements is necessary for generating high quality and reliable nuclear databases. Artificial intelligence techniques, and in particular deep learning, have proven to be very useful for pattern recognition and data analysis, and thus may be used in the field of experimental nuclear physics. In this publication, we train a neural network in order to improve the capture-to-background ratio of neutron capture data of measurements performed in the time-of-flight facility n_TOF at CERN with the so-called Total Absorption Calorimeter. The evaluation of this deep learning-based method on accurate Monte Carlo simulated measurements with 197Au and 239Pu samples suggests that the capture-to-background ratio can be increased 5 times above the standard method.

Constraining the nuclear spin distribution using improved ^{197}Au neutron resonance parameters

New neutron transmission data at resonance energies using a ^{197}Au sample were measured using an early version of the Device for Indirect Capture Experiments on Radionuclides (DICER), which is under development at the Los Alamos Neutron Science Center (LANSCE). These data were combined with previous neutron transmission and capture data in a simultaneous R-matrix analysis to extract improved neutron resonance parameters for this nuclide. As a result, total radiation widths, varGamma _{gamma }, were obtained for 33 J=1 and 44 J=2^{197}Au+n resonances. varGamma _{gamma } distributions for these two spins states were compared to distributions calculated according to the nuclear statistical model using published nuclear level density (NLD) and photon strength functions (PSF) measured using the Oslo technique. The calculated distributions were found to be narrower and the average values for the two spins states closer together than the data. The calculation can be brought into agreement with the data by substantial modifications to the spin distribution in ^{198}Au as a function of excitation energy. As far as we know, the spin distribution currently is otherwise poorly constrained. The modified spin distribution changes the shapes of the NLD and PSF extracted using the Oslo technique and so could have broad implications.

Capitalizing on nuclear data libraries’ comprehensiveness to obtain solar system r-process abundances

The recent observation of neutron stars merger by the laser interferometer gravitational-wave observatory collaboration and the measurements of the event’s electromagnetic spectrum as a function of time for different wavelengths have altered profoundly our understanding of the r-process site as well as considerably energized nuclear astrophysics research efforts. R-process abundances are a key element in r-process simulations, as a successful calculation must account for these abundances in the final debris of a stellar cataclysmic event. In this article, mankind’s complete knowledge of neutron cross sections obtained over the last 80 years, as encapsulated in the latest release of the evaluated nuclear data file (ENDF/B) library, is used to obtain solar system r-process abundances in a comprehensive data approach. ENDF/B cross sections have been successfully used for decades in nuclear power and defense applications and are now used to obtain r-process abundances in a fully traceable and documented way. This article r-process abundances provide complementary insights on the astrophysical events, overall quality of neutron capture data in the astrophysical region of temperatures, and demonstrate issues with the s-process contribution subtraction procedure.

Big-Bang Nucleosynthesis after Planck

We assess the status of big-bang nucleosynthesis (BBN) in light of the final Planck data release and other recent developments, and in anticipation of future measurements. Planck data from the recombination era fix the cosmic baryon density to 0.9% precision, and now damping tail measurements determine the helium abundance and effective number of neutrinos with precision approaching that of astronomical and BBN determinations respectively. All three parameters are related by BBN . In addition, new high-redshift measurements give D/H to better precision than theoretical predictions, and new Li/H data reconfirm the lithium problem. We present new 7Be(n,p)7Li rates using new neutron capture measurements; we have also examined the effect of proposed changes in the d(p,γ)3He rates. Using these results we perform a series of likelihood analyses. We assess BBN/CMB consistency, with attention to how our results depend on the choice of Planck data, as well as how the results depend on the choice of non-BBN, non-Planck data sets. Most importantly the lithium problem remains, and indeed is more acute given the very tight D/H observational constraints; new neutron capture data reveals systematics that somewhat increases uncertainty and thus slightly reduces but does not essentially change the problem. We confirm that d(p,γ)3He theoretical rates brings D/H out of agreement and slightly increases 7Li new experimental data are needed at BBN energies. Setting the lithium problem aside, we find the effective number of neutrino species at BBN is Nν = 2.86 ± 0.15. Future CMB Stage\ obreakdash-4 measurements promise substantial improvements in BBN parameters: helium abundance determinations will be competitive with the best astronomical determinations, and Neff will approach sensitivities capable of detecting the effects of Standard Model neutrino heating of the primordial plasma.

Measurement of the Ge70(n,γ) cross section up to 300 keV at the CERN n_TOF facility

Neutron capture data on intermediate mass nuclei are of key importance to nucleosynthesis in the weak component of the slow neutron capture processes, which occurs in massive stars. The ($n,\gamma$) cross section on $^{70}$Ge, which is mainly produced in the $s$~process, was measured at the neutron time-of-flight facility n_TOF at CERN. Resonance capture kernels were determined up to 40~keV neutron energy, and average cross sections up to 300~keV. Stellar cross sections were calculated from $kT=5$~keV to $kT=100$ keV and are in very good agreement with a previous measurement by Walter and Beer (1985), and recent evaluations. Average cross sections are in agreement with Walter and Beer (1985) over most of the neutron energy range covered, while being systematically smaller for neutron energies above 150~keV. We have calculated isotopic abundances produced in $s$-process environments in a 25 solar mass star for two initial metallicities (below solar, and close to solar). While the low metallicity model reproduces best the solar system germanium isotopic abundances the close to solar model shows a good global match to solar system abundances between mass numbers A=60-80.

Neutron capture data for unstable nuclei – Astrophysical quests and experimental challenges

The abundances of the chemical elements heavier than iron can be attributed in about equal parts to the r and s processes, which are taking place in supernova explosions and during the He and C burning phases of stellar evolution, respectively. So far, quantitative studies of the r-process are out of reach, because it involves reactions on extremely short-lived neutron-rich nuclei. On the contrary, the situation for the s-process is far advanced, thanks to a comprehensive database of experimental (n,γ) cross sections for most isotopes along the reaction path from 12C to the Pb/Bi region. For the stable isotopes last gaps in the data are presently closed, but further studies are clearly needed to reach the required accuracy and to resolve remaining discrepancies. The quest for cross sections of unstable isotopes remains a persisting challenge though. In particular, nuclei which act as branching points are of prime interest, because they provide key information on the deep stellar interior. While the activation method is limited to a few exceptional branch-point nuclei, successful measurements via the time-of- flight technique are depending on intense pulsed neutron sources and elaborate methods for sample production. Current developments in Europe are providing promising perspectives in both areas.

Neutron capture measurements and resonance parameters of dysprosium

Neutron capture yields of dysprosium isotopes ( 161Dy, 162Dy, 163Dy, and 164Dy were measured using the time-of-flight method with a 16 segment sodium iodide multiplicity detector. The measurements were made at the 25m flight station at the Gaerttner LINAC Center at Rensselaer Polytechnic Institute. Resonance parameters were obtained using the multilevel R-matrix Bayesian code SAMMY. The neutron capture data for four enriched dysprosium isotopes and one natural dysprosium sample were sequentially fitted. New resonances not listed in ENDF/B-VII.1 were observed. There were 29 and 17 new resonances from 161Dy and 163Dy isotopes, respectively. Six resonances from 161Dy isotope, two resonances from 163Dy, and four resonances from 164Dy were not observed. The capture resonance integrals of each isotope were calculated with the resulting resonance parameters and those of ENDF/B-VII.1 in the energy region from 0.5eV to 20MeV and were compared to the capture resonance integrals with the resonance parameters from ENDF/B-VII.1. A resonance integral value of the natural dysprosium calculated with present resonance parameters was $ 1405 \pm 3.5$ barn. The value is $ \sim 0.3\%$ higher than that obtained with the ENDF/B-VII.1 parameters. The distributions of the present and ENDF/B-VII.1 neutron widths were compared to a Porter-Thomas distribution. Neutron strength functions for 161Dy and 163Dy were calculated with the present resonance parameters and both values were in between the values of “Atlas of Neutron Resonances” and ENDF/B-VII.1. The present radiation width distributions of 161Dy and 163Dy were fitted with the $ \chi^{2}$ distribution by varying the degrees of freedom.

Developments in capture-γlibraries for nonproliferation applications

The neutron-capture reaction is fundamental for identifying and analyzing the γ-ray spectrum from an unknown assembly because it provides unambiguous information on the neutron-absorbing isotopes. Nondestructive-assay applications may exploit this phenomenon passively, for example, in the presence of spontaneous-fission neutrons, or actively where an external neutron source is used as a probe. There are known gaps in the Evaluated Nuclear Data File libraries corresponding to neutron-capture γ-ray data that otherwise limit transport-modeling applications. In this work, we describe how new thermal neutron-capture data are being used to improve information in the neutron-data libraries for isotopes relevant to nonproliferation applications. We address this problem by providing new experimentally-deduced partial and total neutron-capture reaction cross sections and then evaluate these data by comparison with statistical-model calculations.

Neutron Capture Measurements and Resonance Parameters of Gadolinium

Neutron capture measurements were performed with the time-of-flight method at the Gaerttner LINAC Center at Rensselaer Polytechnic Institute (RPI) using isotopically enriched gadolinium (Gd) samples (155Gd, 156Gd, 157Gd, 158Gd, and 160Gd). The neutron capture measurements were made at the 25-m flight station with a 16-segment sodium iodide multiplicity detector. After the data were collected and reduced to capture yields, resonance parameters were obtained by a combined fitting of the neutron capture data for five enriched Gd isotopes and one natural Gd sample using the multilevel R-matrix Bayesian code SAMMY. A table of resonance parameters and their uncertainties is presented. We observed 2, 169, 96, and 1 new resonances in 154Gd, 155Gd, 157Gd, and 158Gd isotopes, respectively. Resonances in the ENDF/B-VII.0 evaluation that were not observed in the current experiment and could not be traced to a literature reference were removed. This includes 11 resonances from the 156Gd isotope, 1 resonance from 157Gd, 1 resonance from 158Gd, and 6 resonances from the 160Gd isotope.The resulting resonance parameters were used to calculate the capture resonance integrals in the energy region from 0.5 eV to 20 MeV and were compared to calculations obtained when using the resonance parameters from ENDF/B-VII.0 and previous RPI results. The present parameters gave a resonance integral value of 395 ± 2 b, which is ∼0.8% higher and ∼1.7% lower than that obtained with the ENDF/B-VII.0 parameters and with the previous RPI parameters, respectively.

Distribution of total radiation widths for neutron resonances of Pt isotopes

High quality neutron capture and transmission data were measured on isotopically enriched 192,194,195,196 Pt and natural Pt samples at ORELA. R -matrix analysis of this data revealed resonance parameters for 159, 413, 423, 258, and 11 neutron resonances for neutron energies below 5.0, 16.0, 7.5, 16.0, and 5.0 keV for 192,194,195,196,198 Pt+n, respectively. Earlier analysis of data on reduced neutron widths, Γ0 n , showed that the distributions of Γ0 n for 192,194 Pt deviate significantly from the Porter-Thomas distribution (PTD) predicted by random matrix theory. In this contribution we report on preliminary results of the analysis of distribution of total radiation widths, Γγ , in 192,194,195,196 Pt+n reactions. Comparison of experimental data with predictions made within the nuclear statistical model indicates that standard models of Photon Strength Functions (PSFs) and Nuclear Level Density predict Γγ distributions which are too narrow. We found that satisfactory agreement between experimental and simulated distributions can be obtained only by a strong suppression of the PSFs at low γ -ray energies and/or by violation of the usual assumption that primary transitions from neutron resonances follow the PTD. The shape of PSFs needed for reproduction of our Γγ data also nicely reproduces spectra from several (n,γ ) experiments on the neighbor nuclide 198 Au.

Astrophysical quests for neutron capture data of unstable nuclei

Astrophysical quests for neutron capture data of unstable nuclei

Experimental neutron capture data of 58Ni from the CERN n_TOF facility

The neutron capture cross section of 58 Ni was measured at the neutron time of flight facility n_TOF at CERN, from 27 meV to 400 keV neutron energy. Special care has been taken to identify all the possible sources of background, with the so-called neutron background obtained for the first time using high-precision GEANT4 simulations. The energy range up to 122 keV was treated as the resolved resonance region, where 51 resonances were identified and analyzed by a multilevel R -matrix code SAMMY. Above 122 keV the code SESH was used in analyzing the unresolved resonance region of the capture yield. Maxwellian averaged cross sections were calculated in the temperature range of kT = 5 – 100 keV, and their astrophysical implications were investigated.

Experimental neutron capture data of58Ni from the CERN n_TOF facility

The $^{58}$Ni $(n,\gamma)$ cross section has been measured at the neutron time of flight facility n_TOF at CERN, in the energy range from 27 meV up to 400 keV. In total, 51 resonances have been analyzed up to 122 keV. Maxwellian averaged cross sections (MACS) have been calculated for stellar temperatures of kT$=$5-100 keV with uncertainties of less than 6%, showing fair agreement with recent experimental and evaluated data up to kT = 50 keV. The MACS extracted in the present work at 30 keV is 34.2$\pm$0.6$_\mathrm{stat}\pm$1.8$_\mathrm{sys}$ mb, in agreement with latest results and evaluations, but 12% lower relative to the recent KADoNIS compilation of astrophysical cross sections. When included in models of the s-process nucleosynthesis in massive stars, this change results in a 60% increase of the abundance of $^{58}$Ni, with a negligible propagation on heavier isotopes. The reason is that, using both the old or the new MACS, 58Ni is efficiently depleted by neutron captures.

Photoneutron cross sections for118--124Sn and theγ-ray strength function method

Photoneutron cross sections were measured for ${}^{118}$Sn, ${}^{119}$Sn, ${}^{120}$Sn, ${}^{122}$Sn, and ${}^{124}$Sn near neutron threshold with quasi-monochromatic laser-Compton scattering $\ensuremath{\gamma}$ rays. A systematic analysis of the present photoneutron data and existing neutron-capture data is made using the $\ensuremath{\gamma}$-ray strength function on the basis of the HFB + QRPA model of $E1$ strength supplemented with a pygmy dipole resonance, which was deduced from a previous study on ${}^{116}$Sn and ${}^{117}$Sn. Radiative neutron capture cross sections for two radioactive nuclei, ${}^{121}$Sn and ${}^{123}$Sn, are deduced through the $\ensuremath{\gamma}$-ray strength function method.

Astrophysical reaction rates forNi58,60(n,γ) from new neutron capture cross section measurements

New neutron capture cross sections of $^{58,60}\mathrm{Ni}$ were measured in the energy range from 100 eV to 600 keV using the Oak Ridge Electron Linear Accelerator. The combination of these new neutron capture data with previous transmission data allowed a resonance analysis up to 900 keV using $\mathcal{R}$-matrix theory. The theoretically determined direct capture cross sections were included in the analyses. From these resonance parameters and the direct capture contribution, new ($n$,$\ensuremath{\gamma}$) astrophysical reaction rates were determined over the entire energy range needed by the latest stellar models describing the so-called weak $s$ process.

Experimental study of the electric dipole strength in the even Mo nuclei and its deformation dependence

Two methods based on bremsstrahlung were applied to the stable even Mo isotopes for the experimental determination of the photon strength function covering the high excitation energy range above 4 MeV with its increasing level density. Photon scattering was used up to the neutron separation energies ${S}_{n}$ and data up to the maximum of the isovector giant resonance (GDR) were obtained by photoactivation. After a proper correction for multistep processes the observed quasicontinuous spectra of scattered photons show a remarkably good match to the photon strengths derived from nuclear photoeffect data obtained previously by neutron detection and corrected in absolute scale by using the new activation results. The combined data form an excellent basis to derive a shape dependence of the $E1$ strength in the even Mo isotopes with increasing deviation from the $N=50$ neutron shell (i.e., with the impact of quadrupole deformation and triaxiality). The wide energy coverage of the data allows for a stringent assessment of the dipole sum rule and a test of a novel parametrization developed previously which is based on it. This parametrization for the electric dipole strength function in nuclei with $A&gt;80$ deviates significantly from prescriptions generally used previously. In astrophysical network calculations it may help to quantify the role the p-process plays in cosmic nucleosynthesis. It also has impact on the accurate analysis of neutron capture data of importance for future nuclear energy systems and waste transmutation.

Photoneutron cross sections forZr96: A systematic experimental study of photoneutron and radiative neutron capture cross sections for zirconium isotopes

Photoneutron cross sections were measured for $^{96}\mathrm{Zr}$ near neutron threshold with quasimonochromatic laser-Compton-scattering $\ensuremath{\gamma}$-ray beams. A systematic analysis of photoneutron and radiative neutron capture data for zirconium isotopes within the statistical model calculation leads to a unified picture of low-energy $\ensuremath{\gamma}$-ray strengths for zirconium isotopes that is described by the HFB $+$ QRPA model of $E1$ strength supplemented with an extra $\ensuremath{\gamma}$ strength attributed to a giant $M1$ resonance. Results of the systematic analysis including radiative neutron capture cross sections for radioactive $^{95}\mathrm{Zr}$ and $^{93}\mathrm{Zr}$ nuclei are presented.

R-Matrix Analysis of 232Th Neutron Transmissions and Capture Cross Sections in the Energy Range Thermal to 4 keV

Neutron resonance parameters of 232Th were obtained from the Reich-Moore SAMMY analysis of high-resolution neutron transmission measurements performed at the Oak Ridge Electron Linear Accelerator (ORELA) by Olsen in 1981, along with the high-resolution neutron capture measurements performed in 2005 at the Geel Linear Accelerator (GELINA, Belgium) by Schillebeeckx and at the n-TOF facility (CERN, Switzerland) by Aerts. The ORELA data were analyzed previously by Olsen with the Breit-Wigner multilevel code SIOB, and the results were used in the ENDF/B-VI evaluation. In the new analysis of the Olsen neutron transmissions by the modern computer code SAMMY, better accuracy is obtained for the resonance parameters by including in the experimental database the recent experimental neutron capture data. The experimental database and the method of analysis are described in the report. The neutron transmissions and the capture cross sections calculated with the resonance parameters are compared to the experimental values. A description is given of the statistical properties of the resonance parameters. The new evaluation results in a decrease in the capture resonance integral.