Incident Neutron Energy Range Research Articles

A detector system for `absolute' measurements of fission cross sections at n_TOF in the energy range below 200 MeV

A new measurement of the 235U(n,f) cross section was performed at the neutron time-of-flight facility n_TOF at CERN. The experiment focused on neutron energies from 20 MeV to several hundred MeV, and was normalized to neutron scattering on hydrogen. This is a measurement first of its kind at this facility, in an energy range that was until now not often explored, so the detector development phase was crucial for its success. Two detectors are presented, a parallel plate fission chamber (PPFC) and a recoil proton telescope (RPT), both dedicated to perform measurements in the incident neutron energy range from 30 MeV to 200 MeV. The experiment was designed to minimize statistical uncertainties in the allocated run time. Several efforts were made to ensure that the systematic effects were understood and under control. The results show that the detectors are suited for measurements at n_TOF above 30 MeV, and indicate the path for possible future lines of development.

Correlations between energy and γ -ray emission in Pu239 ( n , f )

We study $\ensuremath{\gamma}$-ray emission following $^{239}\mathrm{Pu}(n,f)$ over an incident neutron energy range of $2<{E}_{i}<40$ MeV. We present the first experimental evidence for positive correlations between the total angular momentum generated in fission and the excitation energy of the compound nucleus prior to fission. The $\ensuremath{\gamma}$-ray multiplicity increases linearly with incident energy below the second-chance fission threshold with a slope of $0.085\ifmmode\pm\else\textpm\fi{}0.010\phantom{\rule{4pt}{0ex}}{\mathrm{MeV}}^{\ensuremath{-}1}$. This linear trend appears to hold for the average excitation energy of the compound nucleus between $9<\ensuremath{\langle}{E}_{x}\ensuremath{\rangle}<19$ MeV. Most of the multiplicity increase comes from an enhancement around a $\ensuremath{\gamma}$-ray energy of 0.7 MeV, which we interpret as stretched quadrupole $\ensuremath{\gamma}$ rays that indicate an increase in total fission-fragment angular momentum with excitation energy.

The Covariance of PFNS Results from the Chi-Nu Experiment

The prompt fission neutron spectrum (PFNS) from neutron-induced fission is a fundamental quantity for the behavior of nuclear reactors, and has been measured many times on a wide variety of nuclei and covering different ranges of incident and emitted neutron energies. However, results from past measurements are frequently called into question in modern nuclear data evaluations because of a lack of thorough experimental documentation and incomplete uncertainty analyses. The Chi-Nu experiment at Los Alamos National Laboratory was designed to produce high-precision measurements of the PFNS of major actinides over a wide range of incident and emitted neutron energies, and with the documentation and covariance analysis required to ensure that the results of this experiment maintain their impact long into the future, thereby avoiding this pitfall of past measurements. In this work we describe the Chi-Nu experiment along with summaries of the treatment of and methods developed to address two important components of the analysis of Chi-Nu data: random-coincidence backgrounds and MCNP simulations. Furthermore, we describe the first results for correlations not just between all data points collected on a single target nucleus, but also between all data points from separate Chi-Nu measurements on 235U and 239Pu. These correlations are important for accurately calculating ratios of the PFNS from one actinide to another, which are rare and can be informative for nuclear data evaluation efforts.

Stopping Force Analysis of 235U Elemental Fission Product Yields for En = 0.11–92.4 MeV

Most evaluated elemental fission product yield distributions are not experimentally measured. Instead, the majority of evaluated distributions are based on analytic expressions of the Zp-model for relevant cumulative yields. Here we report independent elemental fission product yield distributions of a 235U target for incident neutron energies ranging from 0.11 MeV through 92.4 MeV. Atomic numbers are calculated by an approach that combines a 2E analysis with a stopping force analysis method, developed within this paper. These analyses are applied to more than 6.1 × 106 fission fragment ionization tracks captured within the NIFFTE (Neutron Induced Fission Fragment Tracking Experiment) collaboration fission time projection chamber (fissionTPC). A 3-Z resolution was obtained with the fissionTPC spatial and energy resolutions. Tabulated results are presented for the atomic yield and experimentally derived Zp values as a function of pre-neutron-emission fragment masses for the complete range of incident neutron energies. The stopping-force-derived Zp values tend to support the unchanged charge distribution theory within uncertainty.

Correlated n−γ angular distributions from the Q=4.4398 MeV C12(n,n′γ) reaction for incident neutron energies from 6.5 MeV to 16.5 MeV

Neutron scattering cross sections and angular distributions represent one of the most glaring sources of uncertainty in calculations of nuclear systems. Even simple nuclei like $^{12}\mathrm{C}$ show indications of errors in nuclear databases for scattering reactions. Measurements of inelastic neutron scattering have historically measured either the scattered neutrons or the nuclear deexcitation $\ensuremath{\gamma}$ emission. Only a very small number of experiments attempted correlated measurements of both the neutron and $\ensuremath{\gamma}$ data simultaneously, even though these $n\text{\ensuremath{-}}\ensuremath{\gamma}$ correlations could be essential for understanding particle transport in nuclear systems. In this work we describe a measurement of the $n, \ensuremath{\gamma}$, and correlated $n\text{\ensuremath{-}}\ensuremath{\gamma}$ angular distributions from the $Q=4.4398 \mathrm{MeV}$ $^{12}\mathrm{C}(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ reaction in a single experiment using an EJ-309 liquid scintillator detector array with wide angular coverage, and with a continuous incident neutron energy range from 6.5 to 16.5 MeV. We also provide a thorough covariance description of these results, including normalization of the probability distribution. While the measured $n$ distributions agree well with the relatively large number of available literature measurements, there are comparatively very few measurements of the $\ensuremath{\gamma}$ distributions from this reaction. However, our data support the presence of a nonzero ${a}_{4}$ Legendre polynomial component of the $\ensuremath{\gamma}$ angular distribution suggested in past measurements, which is currently not incorporated in the ENDF/B-VIII.0 library despite the use of these same literature data for evaluation of the $^{12}\mathrm{C}(n,{n}^{\ensuremath{'}}\ensuremath{\gamma})$ cross section. The correlated $n\text{\ensuremath{-}}\ensuremath{\gamma}$ distribution measurements are limited to three measurements at incident neutron energies near 14 MeV. Our results do not generally agree with any of these literature measurements. We observe clear indications of significant changes in the $n$ distribution for specific $\ensuremath{\gamma}$-detection angles and vice versa especially near thresholds for other reaction channels, which shows the potential for significant bias in experiments that, for example, tag on inelastic scattering using a single or small number of $\ensuremath{\gamma}$-detection angles and could impact particle transport calculations.

C13(n,2nγ)C12 γ -ray production in the 14–16 MeV incident neutron energy range

$\ensuremath{\gamma}$-ray emission from $^{12}\mathrm{C}$ and $^{13}\mathrm{C}$ samples irradiated with deuterium-tritium fusion neutrons was experimentally measured at the Omega Laser Facility and at the Ohio University Edwards Accelerator Laboratory. The intent of these measurements was to determine the feasibility of using $^{13}\mathrm{C}$-based plastic ablators with embedded $^{12}\mathrm{C}$ layers for ``dark mix'' diagnosis of inertial confinement fusion implosions. Spectrally resolved measurements at Ohio University identified significant 4.44-MeV $\ensuremath{\gamma}$-ray emission from the $^{13}\mathrm{C}(n,2n\ensuremath{\gamma})^{12}\mathrm{C}$-L1 reaction channel. The recorded 4.44-MeV $^{13}\mathrm{C}$ signal was compared against emission from an identically irradiated $^{12}\mathrm{C}$ target with known $^{12}\mathrm{C}$($n,{n}^{\ensuremath{'}}\ensuremath{\gamma})^{12}\mathrm{C}$-L1 cross section, which resulted in an average $^{13}\mathrm{C}(n,2n\ensuremath{\gamma})^{12}\mathrm{C}$-L1 cross section of 117 $\ifmmode\pm\else\textpm\fi{}$ 17 mb over the incident neutron energy distribution range from 14.4 to 15.8 MeV. Integrated $^{13}\mathrm{C}$ $\ensuremath{\gamma}$-ray signals above 2.9 MeV recorded with the Gas Cherenkov Detector at Omega exceeded MCNP6.1 predictions by a factor of 3. The additional signal was attributed to 4.44-MeV $\ensuremath{\gamma}\mathrm{s}$ resulting in an inferred $^{13}\mathrm{C}(n,2n\ensuremath{\gamma})^{12}\mathrm{C}$-L1 cross section of $95\ifmmode\pm\else\textpm\fi{}11$ mb at 14.1-MeV average incident neutron energy. As a result, the $^{13}\mathrm{C}$-based ``dark mix'' diagnostic concept was deemed infeasible.

Development of Two Software Packages for Connecting with the MCNPX and NJOY Nuclear Codes to Directly Obtain and Extract Multi-Group Nuclear Cross Sections for Any Isotope

In this research, two software packages are developed to be connected with MCNP and NJOY to obtain and extract nuclear cross sections. The first (main) software package was coupled with the MCNPX code to automatically run it to obtain cross sections of isotopes for multi-group and a wide range of incident neutron energies. It was also coupled with the second software package entitled AutoNJOY, which is directly coupled with the NJOY code, to automatically run it and obtain the cross sections of isotopes at any temperature for any reaction. The results obtained by the AutoNJOY were automatically transferred to the first software package, and the results were compared with those obtained by the MCNPX code, and the graphs of isotope cross sections were plotted in energy. The first software package is also able to analyze the microscopic cross sections obtained by the AutoNJOY and draws the graph of the cross section vs. energy. It is also able to be connected to the ACE library directly and reads microscopic cross sections of all the elements and their isotopes that have the various ZAID. It was observed that from the thermal neutron range to the fast, the cross sections of isotopes obtained by the AutoNJOY were nearly similar to the results obtained by the first software package, and the derived graphs of both methods approved with each other.

Measurements of neutron capture cross-sections for 187Re isotope around 14 MeV neutrons and theoretical calculations of excitation functions

The cross-sections of the 187Re(n,γ)188mRe and 187Re(n,γ)188gRe reactions were measured at the four neutron energies of 13.5, 14.1, 14.4 and 14.8 MeV through the activation method and using off-line γ-ray spectrometry. The previously developed formulas were used to subtract the effects of γ rays with close energies and the contribution from the excited state. A high-resolution γ-ray spectrometer with high-purity germanium detector was used to measure the induced γ activities. The simulation computations of the excitation functions concerning the above-mentioned two nuclear reactions were performed by adopting the nuclear theoretical model program system Talys-1.9. The theoretical excitation curves in the incident neutron energy range from 2.53 × 10−8 to 20 MeV were acquired. The fitting line of our results in the neutron energy range of 13.5–14.8 MeV, within experimental uncertainty, agrees very well with the theoretical excitation curve based on the computer code Talys-1.9.

Cross section measurements on zirconium isotopes for ~14 MeV neutrons and their theoretical calculations of excitation functions * *Supported by the National Natural Science Foundation of China (11575090)

The cross sections for the 94Zr(n,d*)93m+gY, 96Zr(n,γ)97Z, 96Zr(n,2n)95Zr, 90Zr(n,α)87mSr, 94Zr(n,α)91Sr,90Zr(n,p)90mY, 92Zr(n,p)92Y, and 94Zr(n,p)94Y reactions have been measured in the neutron energy range of 13.5-14.8 MeV by means of the activation technique. The neutrons were produced via the D-T reaction. A high-purity germanium detector with high energy resolution was used to measure the induced γ activities. In combination with the nuclear reaction theoretical models, the excitation curves of the above-mentioned eight nuclear reactions within the incident neutron energy range from the threshold to 20 MeV were obtained by adopting the nuclear theoretical model program system Talys-1.9. The resulting experimental cross sections were analyzed and compared with the experimental data from published studies. Calculations were performed using Talys-1.9 and are in agreement with our experimental results, previous experimental values, as well as results of the theoretical excitation curves at the corresponding energies. The theoretical excitation curves generally match the experimental data well.

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.

Investigation of the Reaction Cross-Section for Production the Radioactive Isotopes Used in Fabricating The Nuclear Batteries

Beta-decay radioactive isotopes have been widely used as a high-energy source in nuclear batteries. The aim of this study is to improve the evaluations of nuclear reactions for the production of some radionuclides with possible use in nuclear batteries. In particular, the 38Ar(n,γ)39Ar, 59Co(n,γ)60Co, 112Cd(n,γ)113mCd, 151Eu(n,γ)152Eu, 153Eu(n,γ) 154Eu, 193Os(n,γ) 194Os and 203Tl(n,γ) 204Tl neutron capture reactions have been studied for an incident neutron energy range from 10-6 eV up to 2 MeV. The evaluations of the capture cross sections are based on the nuclear model calculations with the latest release code EMPIRE-3.2.2 (Malta) and compared with the available experimental ones. The TENDL nuclear data library, which provides the output of TALYS nuclear model code system have been also presented along with the extracted data for some interactions with no experimental support. Simulation of the beta particles transportation and the generated bremsstrahlung intensity through silicon carbide semiconductor has been also investigated.

Determining the average prompt-fission-neutron multiplicity for Pu239(n,f) via a Pu240(α,α′f) surrogate reaction

The average prompt-fission-neutron multiplicity $\bar{\nu}$ is of significance in the areas of nuclear theory, nuclear nonproliferation, and nuclear energy. In this work, the surrogate-reaction method has been used for the first time to indirectly determine $\bar{\nu}$ for $^{239}$Pu($n$,$f$) via $^{240}$Pu($\alpha$,$\alpha^{\prime}f$) reactions. A $^{240}$Pu target was bombarded with a beam of 53.9-MeV $\alpha$ particles. Scattered $\alpha$ particles, fission products, and neutrons were measured with the NeutronSTARS detector array. Values of $\bar{\nu}$ were obtained for a continuous range of equivalent incident neutron energies between 0.25--26.25~MeV, and the results agree well with direct neutron measurements.

Fission fragment angular anisotropy in neutron-induced fission of U235 measured with a time projection chamber

Fission fragment angular distributions can provide an important constraint on fission theory, improving predictive fission codes, and are a prerequisite for a precise ratio cross section measurement. Available anisotropy data is sparse, especially at neutron energies above 5 MeV. For the first time, a three-dimensional tracking detector is employed to study fragment emission angles and provide a direct measurement of angular anisotropy. The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has deployed the fission time projection chamber (fissionTPC) to measure nuclear data with unprecedented precision. The fission fragment anisotropy of $^{235}$U has been measured over a wide range of incident neutron energies from 180 keV to 200 MeV; a careful study of the systematic uncertainties complement the data.

Cross section and neutron angular distribution measurements of neutron scattering on natural iron

New measurements of the neutron scattering double differential cross section of iron were carried out at the neutron time-of-flight facilities GELINA and nELBE. A neutron spectrometer consisting of an array of up to 32 liquid organic scintillators was employed, which was designed to measure the scattering differential cross section at eight scattering angles and to simultaneously determine the integral cross section via numerical quadrature. The separation of elastic from inelastic scattering was achieved by analyzing the time-of-flight-dependent light-output distributions to determine the scattered neutron energy. The method was validated by studying elastic scattering on carbon and it was proved to work well for the determination of the elastic cross section. Here, the possibility to extend it to inelastic scattering was investigated too. For these experiments a sample of natural iron was used and the results cover the incident neutron energy range from 2 to 6 MeV. Both the differential and the integral elastic cross sections were produced for $^{\mathrm{nat}}\mathrm{Fe}$, while for inelastic scattering, partial angular distributions for scattering from the first excited level of $^{56}\mathrm{Fe}$ could be determined.

Calculation and Evaluations for n+64,66,67,68,70,natZn Reactions

All cross sections, angular distributions, energy spectra, and double-differential cross sections are consistently calculated and analyzed for n+64,66,67,68,70,natZn reactions over the incident neutron energy range from 0.001 to 200 MeV. The optical model, preequilibrium and equilibrium reaction theories, and the distorted wave Born approximation theory are used. Theoretical calculated results are compared with the existing experimental data and the evaluated results in JEFF-3.2 and JENDL-4.0.

Accurate U238 ( n, 2n ) U237 reaction cross-section measurements from 6.5 to 14.8 MeV

The cross section for the $^{238}\mathrm{U}(n,2n)^{237}\mathrm{U}$ reaction has been measured in the incident neutron energy range from 6.5 to 14.8 MeV in small energy steps using an activation technique. Monoenergetic neutron beams were produced via the $^{2}\mathrm{H}(d,n)^{3}\mathrm{He}$ and $^{3}\mathrm{H}(d,n)^{4}\mathrm{He}$ reactions. $^{238}\mathrm{U}$ targets were activated along with Au and Al monitor foils to determine the incident neutron flux. The activity of the reaction products was measured in TUNL's low-background counting facility using high-resolution $\ensuremath{\gamma}$-ray spectroscopy. The results are compared with previous measurements and latest data evaluations. Statistical-model calculations, based on the Hauser-Feshbach formalism, have been carried out using the ${\mathrm{CoH}}_{3}$ code and are compared with the experimental results. The present self-consistent and high-quality data are important for stockpile stewardship and nuclear forensic purposes as well as for the design and operation of fast reactors.

Radiative neutron capture cross section from U236

The $^{236}\mathrm{U}(n,\ensuremath{\gamma})$ reaction cross section has been measured for the incident neutron energy range from 10 eV to 800 keV by using the Detector for Advanced Neutron Capture Experiments (DANCE) $\ensuremath{\gamma}$-ray calorimeter at the Los Alamos Neutron Science Center. The cross section was determined with the ratio method, which is a technique that uses the $^{235}\mathrm{U}(n,f)$ reaction as a reference. The results of the experiment are reported in the resolved and unresolved resonance energy regions. Individual neutron resonance parameters were obtained below 1 keV incident energy by using the $R$-matrix code sammy. The cross section in the unresolved resonance region is determined with improved experimental uncertainty. It agrees with both ENDF/B-VII.1 and JEFF-3.2 nuclear data libraries. The results above 10 keV agree better with the JEFF-3.2 library.

Point-by-point model calculation of the prompt neutron multiplicity distribution ν(A) in the incident neutron energy range of multi-chance fission

Prompt neutron multiplicity distributions ν(A) are required for prompt emission correction of double energy (2E) measurements of fission fragments to determine pre-neutron fragment properties. The lack of experimental ν(A) data especially at incident neutron energies (En) where the multi-chance fission occurs impose the use of ν(A) predicted by models. The Point-by-Point model of prompt emission is able to provide the individual ν(A) of the compound nuclei of the main and secondary nucleus chains undergoing fission at a given En. The total ν(A) is obtained by averaging these individual ν(A) over the probabilities of fission chances (expressed as total and partial fission cross-section ratios). An indirect validation of the total ν(A) results is proposed. At high En, above 70 MeV, the PbP results of individual ν(A) of the first few nuclei of the main and secondary nucleus chains exhibit an almost linear increase. This shape is explained by the damping of shell effects entering the super-fluid expression of the level density parameters. They tend to approach the asymptotic values for most of the fragments. This fact leads to a smooth and almost linear increase of fragment excitation energy with the mass number that is reflected in a smooth and almost linear behaviour of ν(A).

Absolute measurement of thePu242neutron-capture cross section

The absolute neutron-capture cross section of $^{242}\mathrm{Pu}$ was measured at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array along with a compact parallel-plate avalanche counter for fission-fragment detection. The first direct measurement of the $^{242}\mathrm{Pu}(n,\ensuremath{\gamma})$ cross section was made over the incident neutron energy range from thermal to $\ensuremath{\approx}6$ keV, and the absolute scale of the $(n,\ensuremath{\gamma})$ cross section was set according to the known $^{239}\mathrm{Pu}(n,f)$ resonance at ${E}_{n,R}=7.83$ eV. This was accomplished by adding a small quantity of $^{239}\mathrm{Pu}$ to the $^{242}\mathrm{Pu}$ sample. The relative scale of the cross section, with a range of four orders of magnitude, was determined for incident neutron energies from thermal to $\ensuremath{\approx}40$ keV. Our data, in general, are in agreement with previous measurements and those reported in ENDF/B-VII.1; the $^{242}\mathrm{Pu}(n,\ensuremath{\gamma})$ cross section at the ${E}_{n,R}=2.68$ eV resonance is within $2.4%$ of the evaluated value. However, discrepancies exist at higher energies; our data are $\ensuremath{\approx}30%$ lower than the evaluated data at ${E}_{n}\ensuremath{\approx}1$ keV and are approximately $2\ensuremath{\sigma}$ away from the previous measurement at ${E}_{n}\ensuremath{\approx}20$ keV.

Radiative neutron capture cross sections onLu176at DANCE

The cross section of the neutron capture reaction $^{176}\mathrm{Lu}(n,\ensuremath{\gamma})$ has been measured for a wide incident neutron energy range with the Detector for Advanced Neutron Capture Experiments at the Los Alamos Neutron Science Center. The thermal neutron capture cross section was determined to be $(1912\ifmmode\pm\else\textpm\fi{}132)$ b for one of the Lu natural isotopes, $^{176}\mathrm{Lu}$. The resonance part was measured and compared to the Mughabghab's atlas using the $R$-matrix code, sammy. At higher neutron energies the measured cross sections are compared to ENDF/B-VII.1, JEFF-3.2, and BRC evaluated nuclear data. The Maxwellian averaged cross sections in a stellar plasma for thermal energies between 5 keV and 100 keV were extracted using these data.