Search

Energy production in Type Ia x-ray bursts depends on a number of nuclear reactions that become efficient in a hot environment (up to 2 GK). Model sensitivity studies have been performed in an effort to better identify the reactions that have the largest effect, but these efforts are hampered by the high level of complexity of the astrophysical model and large nuclear physics uncertainties. In a recent study, the $^{24}\mathrm{Mg}(\ensuremath{\alpha},p)^{27}\mathrm{Al}$ reaction was found to significantly affect the energy generation in x-ray bursts. This manuscript reports the first study of the $^{24}\mathrm{Mg}(\ensuremath{\alpha},{p}_{1,2}\ensuremath{\gamma})^{27}\mathrm{Al}$ reaction at energies relevant for x-ray bursts. The branches to the $^{27}\mathrm{Al}$ excited states increase to a small degree the estimates of the total astrophysical $^{24}\mathrm{Mg}(\ensuremath{\alpha},p)^{27}\mathrm{Al}$ reaction rate.

Although the prompt fission neutron spectrum (PFNS) is an essential component of neutron-driven systems that has been measured for decades, there are still multiple glaring unknowns regarding the PFNS of major actinides in the fission neutron incident energy range, specifically with regard to multichance fission and pre-equilibrium neutron emission processes. The only impactful experimental $^{239}\mathrm{Pu}$ PFNS measurements included in recent nuclear data evaluations were measured over a limited outgoing neutron energy range at thermal and 1.5-MeV average incident neutron energy, while other potentially impactful measurements have been shown to contain errors that resulted in either large uncertainty increases or in complete exclusion from nuclear data evaluation. We report here a measurement of the $^{239}\mathrm{Pu}$ PFNS over a wide range of incident neutron energy (1--20 MeV) and three orders of magnitude in outgoing neutron energy (0.01--10 MeV) resulting from the Chi-Nu experiment at the Los Alamos Neutron Science Center. These results are the combination of separate PFNS measurements in the same experimental area, one using a Li-glass and the other a liquid scintillator detector array. Covariances between all PFNS data points from each detector and within each incident energy range were generated between all other data in both detector arrays and within all other incident neutron energy bins, yielding a single covariance matrix for all 1300 PFNS data points reported here. These covariances are based on a thorough assessment of systematic bias and uncertainties associated with the measurement, PFNS extraction technique, combination of data from each detector type, and other aspects of the analysis. The existence of covariances between PFNS data points in different incident neutron energy ranges yielded covariances between average PFNS energy values at each incident energy to be reported here as well, which allowed for firm statements to be made regarding a shape of a purely experimental mean PFNS energy trend for the first time. Although minor PFNS shape differences exist between the results reported here and recent nuclear data evaluations, the ENDF/B-VIII.0 and JEFF-3.3 PFNS evaluations agree reasonably well with the present results from 1-to 10-MeV incident neutron energy, which spans the well-measured 1.5-MeV incident neutron energy PFNS from Lestone and Shores as well as the onset of second-chance fission. However, while the pre-equilibrium component of the PFNS above 12-MeV incident neutron energy roughly agrees in position and magnitude with ENDF/B-VIII.0 and JEFF-3.3, clear differences relating to the relative magnitude of third-chance fission PFNS features are present in the PFNS shape and in the mean PFNS energy trends.

Prompt γ-ray spectra were measured for neutron-induced fission of $^{239,241}$Pu with incident neutron energy from thermal to about 100 keV and spontaneous fission of $^{240,242}$Pu using the Detector for Advanced Neutron Capture Experiments (DANCE) array in coincidence with the detection of fission fragments by a parallel-plate avalanche counter. The unfolded prompt fission γ-ray spectra can be reproduced reasonably well by Monte Carlo Hauser-Feschbach statistical model for neutron-induced fission channel but not for the spontaneous fission channel. However, this entrance-channel dependence of the prompt fission γ-ray emission can be described qualitatively by the model due to the very different fission-fragment mass distributions and a lower average fragment spin for spontaneous fission. A supportive evidence is provided by the unfolded 2-D spectrum of total γ-ray energy vs multiplicity where the γ-ray multiplicity distribution has a tail extended to higher multiplicity for neutron-induced fission channel.

To enhance the capabilities of the DANCE array, a new detector array NEUANCE was developed to enable simultaneous measurements of prompt fission neutrons and γ rays. NEUANCE was designed and constructed using 21 stilbene organic scintillator crystals. It was installed in the central cavity of the DANCE array. Signals from the 160 BaF 2 detectors of DANCE and the 21 detectors of NEUANCE were merged into a newly designed high-density high-throughput data acquisition system. The excellent pulse shape discrimination properties of stilbene enabled detection of neutrons with energy thresholds as low as 30–40 keVee. A fission reaction tagging method was developed using a NEUANCE γ -ray or neutron signal. The probability of detecting a neutron from the spontaneous fission of 252Cf using NEUANCE is ∼ 47%. New correlated data for prompt fission neutrons and prompt fission γ rays were obtained for 252 Cf using this high detection efficiency experimental setup. Average properties of prompt fission neutron emission as a function of prompt fission γ -ray quantities were also obtained, suggesting that neutron and γ -ray emission in fission are correlated.

New prompt fission neutron spectrum measurements are reported for 235U( n , f ) reactions induced by neutrons with energies from 0.7 to 20 MeV. These measurements cover outgoing neutron energies from 2.5 MeV down to 10 keV, using an array of 6Li-glass scintillators for neutron detection and a double time-of-flight technique. The neutrons were produced at the Weapons Neutron Research facility of the Los Alamos Neutron Science Center. A detailed MCNP® model of the experimental equipment and the surrounding room was used to interpret the experimental results. Backgrounds were measured in situ, making use of the time-dependent singles rates of the various detectors with asynchronous readout from waveform digitizers. The results presented here have been included in a re-evaluation of the fission neutron spectra for this fissioning system, a description of which is presented elsewhere in this issue.

The 239Pu(n,γ) cross section has been measured from 10 eV to 1.3 MeV as part of an experimental campaign using the Detector for Advanced Neutron Capture Experiments (DANCE). The work represents a significant advance in experimental technique, with improved systematic uncertainties in key regions in the keV to MeV regime. In general the results of prior work are confirmed with improved uncertainties, particularly at the highest incident neutron energies.

A new measurement is presented of the 6 Li(n, α )t cross section from 245 keV to 10 MeV using a 252Cf fission chamber with 6 LiI(Eu) and Cs 2 LiYCl 6 :Ce (CLYC) scintillators which act as both target and detector. Neutron energies are determined from the time of flight (TOF) method using the signals from spontaneous fission and reaction product recoil. Simulations of neutron downscatter in the crystals and fission chamber bring 6 Li(n, α )t cross section values measured with the 6 LiI(Eu) into agreement with previous experiments and evaluations, except for two resonances at 4.2 and 6.5 MeV introduced by ENDF/B-VII.1. Suspected neutron transport modeling issues cause the cross section values obtained with CLYC to be discrepant above 2 MeV.

Calculations of detector response functions are complicated because they include the intricacies of signal creation from the detector itself as well as a complex interplay between the detector, the particle-emitting target, and the entire experimental environment. As such, these functions are typically only accessible through time-consuming Monte Carlo simulations. Furthermore, the output of thousands of Monte Carlo simulations can be necessary in order to extract a physics result from a single experiment. Here we describe a method to obtain a full description of the detector response function using Monte Carlo simulations. We also show that a response function calculated in this way can be used to create Monte Carlo simulation output spectra a factor of ∼ 1000 × faster than running a new Monte Carlo simulation. A detailed discussion of the proper treatment of uncertainties when using this and other similar methods is provided as well. This method is demonstrated and tested using simulated data from the Chi-Nu experiment, which measures prompt fission neutron spectra at the Los Alamos Neutron Science Center.

Properties of fission in $^{233}\mathrm{U}$ were measured at the Los Alamos Neutron Science Center at incident neutron energies from thermal to 40 MeV. Fragments were observed in coincidence by using a twin ionization chamber with Frisch grids. The average total kinetic energy released and fragment mass yields were determined by using the double energy analysis method based on conservation of mass and momentum. The experimental method was validated by using $^{232}\mathrm{Th}$ and absolute energy was calibrated by using thermal-neutron--induced fission of $^{235}\mathrm{U}$. This work incorporates novel applications of multi-chance fission channel cross sections and fission models to account for the complexities introduced by prompt neutron emission at high energy and extends results up to higher incident neutron energies than previously measured. Accurate experimental measurements of these parameters are necessary to better understand the fission process in isotopes central to the thorium fuel cycle.

The Neutron Induced Fission Fragment Tracking Experiment (NIFFTE) collaboration has performed measurements with a fission time projection chamber (fissionTPC) to study the fission process by reconstructing full three-dimensional tracks of fission fragments and other ionizing radiation. The amount of linear momentum imparted to the fissioning nucleus by the incident neutron can be inferred by measuring the opening angle between the fission fragments. Using this measured linear momentum, fission fragment angular distributions can be converted to the center-of-mass frame for anisotropy measurements. Angular anisotropy is an important experimental observable for understanding the quantum mechanical state of the fissioning nucleus and vital to determining detection efficiency for cross section measurements. Neutron linear momentum transfer to fissioning $^{235}$U, $^{238}$U, and $^{239}$Pu and fission fragment angular anisotropy of $^{235}$U and $^{238}$U as a function of neutron energies in the range 130 keV--250 MeV are presented.

The neutron total cross sections $\sigma_{tot}$ of $^{16,18}$O, $^{58,64}$Ni, $^{103}$Rh, and $^{112,124}$Sn have been measured at the Los Alamos Neutron Science Center (LANSCE) from low to intermediate energies (3 $\leq E_{lab} \leq$ 450 MeV) by leveraging waveform-digitizer technology. The $\sigma_{tot}$ relative differences between isotopes are presented, revealing additional information about the isovector components needed for an accurate optical-model description away from stability. Digitizer-enabled $\sigma_{tot}$-measurement techniques are discussed and a series of uncertainty-quantified dispersive optical model (DOM) analyses using these new data is presented, validating the use of the DOM for modeling light systems ($^{16,18}$O) and systems with open neutron shells ($^{58,64}$Ni and $^{112,124}$Sn). The valence-nucleon spectroscopic factors extracted for each isotope reaffirm the usefulness of high-energy proton reaction cross sections for characterizing depletion from the mean-field expectation., Comment: 28 pages, 20 figures

Two measurements of fission γ rays were performed with the DANCE and NEUANCE arrays using the reactions 235 U(n, f) and 252 Cf(sf). Utilizing the fast time response of the detectors and a method for estimating the accidental background, we obtained the energy spectrum of the late prompt fission γ rays as a function of the time since fission. The experimental results are compared with predictions of the code CGMF folded with GEANT4 simulations of the detector response.

Background: The rapid neutron capture process is one of the main nucleosynthesis processes of elements heavier than Fe. Uncertainties in nuclear properties, such as masses, half-lives, and $\ensuremath{\beta}$-delayed neutron probabilities can cause orders of magnitude of variation within astrophysical $r$-process simulations. Presently, theoretical models are used to make global predictions of various nuclear properties for the thousands of nuclei required for these simulations, and measurements are required to benchmark these models, especially far from stability.Purpose: $\ensuremath{\beta}$-decay strength distributions can be used to not only inform astrophysical $r$-process simulations, but also to provide a stringent test for theoretical calculations. The aim of this work is to provide accurate strength distributions for $^{69,71}\mathrm{Co}\phantom{\rule{4pt}{0ex}}\ensuremath{\beta}$ decay.Method: The technique of total absorption spectroscopy was used to measure the $\ensuremath{\beta}$ decay of $^{69,71}\mathrm{Co}$ for the first time at the National Superconducting Cyclotron Laboratory. The ions were implanted in a double-sided silicon strip detector at the center of the Summing NaI(Tl) detector and identified using standard particle identification methods. The response of the detection system to the $\ensuremath{\beta}$-decay electron and subsequent $\ensuremath{\gamma}$-ray radiation was fit to the observed experimental data using a ${\ensuremath{\chi}}^{2}$-minimization technique.Results: $\ensuremath{\beta}$-feeding intensities and Gamow-Teller strength distributions were extracted from the fits of the experimental data. The $\ensuremath{\beta}$-decay intensities show that there is a large percentage of feeding to levels above 2 MeV, which have not been observed in previous studies. The resultant $\ensuremath{\beta}$-feeding intensities and Gamow-Teller strength distributions were compared to shell model and quasiparticle random phase approximation (QRPA) calculations.Conclusions: Comparing experimentally determined $\ensuremath{\beta}$-decay strength distributions provides a test of models, which are commonly used for global $\ensuremath{\beta}$-decay properties for astrophysical calculations. This work highlights the importance of performing detailed comparisons of models to experimental data, particularly far from stability and as close to the $r$-process path as possible.

Background: The recent observation of a neutron-star merger finally confirmed one astrophysical location of the rapid neutron-capture process (r-process). Evidence of the production of $Al140$ nuclei was seen, but there is still little detailed information about how those lighter elements are produced in such an environment. Many of the questions surrounding the $A\ensuremath{\approx}80$ nuclei are likely to be answered only when the nuclear physics involved in the production of r-process nuclei is well understood. Neutron-capture reactions are an important component of the r-process, and neutron-capture cross sections of r-process nuclei, which are very neutron rich, have large uncertainties.Purpose: Indirectly determine the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$.Methods: The nuclear level density (NLD) and $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$) of $^{74}\mathrm{Zn}$ were determined following a total absorption spectroscopy (TAS) experiment focused on the $\ensuremath{\beta}$ decay of $^{74}\mathrm{Cu}$ into $^{74}\mathrm{Zn}$ performed at the National Superconducting Cyclotron Laboratory. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were used as inputs in a Hauser-Feshbach statistical model to calculate the neutron-capture cross section and reaction rate.Results: The NLD and $\ensuremath{\gamma}\mathrm{SF}$ of $^{74}\mathrm{Zn}$ were experimentally constrained for the first time using $\ensuremath{\beta}$-delayed $\ensuremath{\gamma}$ rays measured with TAS and the $\ensuremath{\beta}$-Oslo method. The NLD and $\ensuremath{\gamma}\mathrm{SF}$ were then used to constrain the neutron-capture cross section and reaction rate for the $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ reaction.Conclusions: The uncertainty in the neutron-capture cross section and reaction rate of $^{73}\mathrm{Zn}(n,\ensuremath{\gamma})^{74}\mathrm{Zn}$ calculated in TALYS was reduced to under a factor of 2 from a factor of 5 in the cross section and a factor of 11 in the reaction rate using the experimentally obtained NLD and $\ensuremath{\gamma}\mathrm{SF}$.

The physical properties of neutrons emitted from neutron-induced fission are fundamental to our understanding of nuclear fission. However, while state-of-the-art fission models still incorporate isotropic fission neutron spectra, it is believed that the preequilibrium prefission component of these spectra is strongly anisotropic. The lack of experimental guidance on this feature has not motivated incorporation of anisotropic neutron spectra in fission models, though any significant anisotropy would impact descriptions of a fissioning system. In the present work, an excess of counts at high energies in the fission neutron spectrum of ^{239}Pu is clearly observed and identified as an excess of the preequilibrium prefission distribution above the postfission neutron spectrum. This excess is separated from the underlying postfission neutron spectrum, and its angular distribution is determined as a function in incident neutron energy and outgoing neutron detection angle. Comparison with neutron scattering models provides the first experimental evidence that the preequilibrium angular distribution is uncorrelated with the fission axis. The results presented here also impact the interpretation of several influential prompt fission neutron spectrum measurements.

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $\ensuremath{\gamma}$-emission probability at very low $\ensuremath{\gamma}$ energies. In this work, we present measurements of the $\ensuremath{\gamma}\text{-decay}$ strength of $^{70}\mathrm{Ni}$ over the wide range $1.3\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}8$ MeV. A significant enhancement is found in the $\ensuremath{\gamma}\text{-decay}$ strength for transitions with ${E}_{\ensuremath{\gamma}}l3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\ensuremath{\gamma}$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation, and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.

A single, unifying measurement of the $^{239}\mathrm{Pu}$ capture cross section from 1 keV to 1.3 MeV has been performed for the first time using the Detector for Advanced Neutron Capture Experiments (DANCE) at the Los Alamos Neutron Science Center (LANSCE). The experimental method combines a prior experiment's characterization of prompt fission $\ensuremath{\gamma}$ rays in conjunction with a fission tagging detector with a separate experiment using a thick $^{239}\mathrm{Pu}$ sample to extract the neutron capture cross section in ratio to $^{239}\mathrm{Pu}(n,f)$. We have made new predictions of the capture cross section taking into account recent results for the $M1$ scissors mode present in other actinides. The results show deviations from current evaluations which are 30% higher at the highest energies, and will be used to improve calculations relevant for several applications.

Different aspects of the nuclear fission process have been studied at Los Alamos Neutron Science Center (LANSCE) using various instruments and experimental techniques. Properties of the fragments emitted in fission have been investigated using Frisch-grid ionization chambers, a Time Projection Chamber (TPC), and the SPIDER instrument which employs the 2v-2E method. These instruments and experimental techniques have been used to determine fission product mass yields, the energy dependent total kinetic energy (TKE) release, and anisotropy in neutron-induced fission of U-235, U-238 and Pu-239.

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.

The $^{242}\mathrm{Am}$ metastable isomer's neutron-induced destruction mechanisms were studied at the Los Alamos Neutron Science Center using the Detector for Advanced Neutron-Capture Experiments array with a compact parallel-plate avalanche counter. New $^{242\mathrm{m}}\mathrm{Am}$ neutron-capture cross sections were determined from 100 meV to 10 keV, and the absolute scale was set with respect to a concurrent measurement of the well-known $^{242\mathrm{m}}\mathrm{Am}$ neutron-induced-fission cross section. The new fission cross section spans an energy range from 100 meV to 1 MeV and was normalized to the ENDF/B-VII.1 evaluated cross section to set the absolute scale. Our $^{242\mathrm{m}}\mathrm{Am}(n,f)$ cross section agrees well with the cross section of Browne et al. [Phys. Rev. C 29, 2188 (1984)] over this large energy interval. The new neutron-capture cross section measurement complements and agrees well with our recent results reported below 1 eV in Buckner et al. [Phys. Rev. C 95, 024610 (2017)]. This new work comprises the most comprehensive study of $^{242\mathrm{m}}\mathrm{Am}(n,\ensuremath{\gamma})$ above thermal energy. Neutron-induced resonance energies and parameters were deduced with the sammy $R$-matrix code for incident neutron energies up to 45 eV, and the new average ${\mathrm{\ensuremath{\Gamma}}}_{\ensuremath{\gamma}}$ is $13%$ higher than the evaluated average $\ensuremath{\gamma}$ width.

The neutron-induced reaction cross sections of $^{242m}\mathrm{Am}$ were 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. A new neutron-capture cross section was determined, and the absolute scale was set according to a concurrent measurement of the well-known $^{242m}\mathrm{Am}(n,f)$ cross section. The ($n,\ensuremath{\gamma}$) cross section was measured from thermal energy to an incident energy of 1 eV at which point the data quality was limited by the reaction yield in the laboratory. Our new $^{242m}\mathrm{Am}$ fission cross section was normalized to ENDF/B-VII.1 to set the absolute scale, and it agreed well with the ($n,f$) cross section reported by Browne et al. (1984) from thermal energy to 1 keV. The average absolute capture-to-fission ratio was determined from thermal energy to ${E}_{n}=0.1$ eV, and it was found to be 26(4)$%$ as opposed to the ratio of $19%$ from the ENDF/B-VII.1 evaluation.

Neutron capture and β− -decay are competing branches of the s-process nucleosynthesis path at 85 Kr [1], which makes it an important branching point. The knowledge of its neutron capture cross section is therefore essential to constrain stellar models of nucleosynthesis. Despite its importance for different fields, no direct measurement of the cross section of 85 Kr in the keV-regime has been performed. The currently reported uncertainties are still in the order of 50% [2, 3]. Neutron capture cross section measurements on a 4% enriched 85 Kr gas enclosed in a stainless steel cylinder were performed at Los Alamos National Laboratory (LANL) using the Detector for Advanced Neutron Capture Experiments (DANCE). 85 Kr is radioactive isotope with a half life of 10.8 years. As this was a low-enrichment sample, the main contaminants, the stable krypton isotopes 83 Kr and 86 Kr, were also investigated. The material was highly enriched and contained in pressurized stainless steel spheres.

The Chi-Nu experiment aims to accurately measure the prompt fission neutron spectrum for the major actinides. At the Los Alamos Neutron Science Center (LANSCE), fission can be induced with neutrons ranging from 0.7 MeV and above. Using a two arm time-of-flight (TOF) technique, the fission neutrons are measured in one of two arrays: a 22-6 Li glass array for lower energies, or a 54-liquid scintillator array for outgoing energies of 0.5 MeV and greater. Presented here are the collaboration's preliminary efforts at measuring the 235 U PFNS.

The absolute neutron-capture cross section of 242 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. During target fabrication, a small amount of 239 Pu was added to the active target so that the absolute scale of the 242 Pu(n,γ) cross section could be set according to the known 239 Pu(n,f) resonance at En,R = 7.83 eV. The relative scale of the 242 Pu(n,γ) cross section covers four orders of magnitude for incident neutron energies from thermal to ≈ 40 keV. The cross section reported in ENDF/B-VII.1 for the 242 Pu(n,γ) En,R = 2.68 eV resonance was found to be 2.4% lower than the new absolute 242 Pu(n,γ) cross section.

The average Total Kinetic Energy (TKE ) release and fission-fragment yields in neutron-induced fission of 235 U and 238 U was measured using a Frisch-gridded ionization chamber. These observables are important nuclear data quantites that are relevant to applications and for informing the next generation of fission models. The measurements were performed a the Los Alamos Neutron Science Center and cover E n = 200 keV – 30 MeV. The double-energy (2E) method was used to determine the fission-fragment yields and two methods of correcting for prompt-neutron emission were explored. The results of this study are correlated mass and TKE data.

The CIELO collaboration has studied neutron cross sections on nuclides that significantly impact criticality in nuclear technologies – 16 O, 56 Fe, 235,8 U and 239 Pu – with the aim of improving the accuracy of the data and resolving previous discrepancies in our understanding. This multi-laboratory pilot project, coordinated via the OECD/NEA Working Party on Evaluation Cooperation (WPEC) Subgroup 40 with support also from the IAEA, has motivated experimental and theoretical work and led to suites of new evaluated libraries that accurately reflect measured data and also perform well in integral simulations of criticality.

Neutron-capture reactions play an important role in heavy element nucleosynthesis, since they are the driving force for the two processes that create the vast majority of the heavy elements. When a neutron capture occurs on a short-lived nucleus, it is extremely challenging to study the reaction directly and therefore the use of indirect techniques is essential. The present work reports on such an indirect measurement that provides strong constraints on the 68Ni(n, γ)69Ni reaction rate. This is done by populating the compound nucleus 69Ni via the β decay of 69Co and measuring the γ-ray deexcitation of excited states in 69Ni. The β-Oslo method was used to extract the γ-ray strength function and the nuclear level density. In addition the half-life of 69Co was extracted and found to be in agreement with previous literature values. Before the present results, the 68Ni(n, γ)69Ni reaction was unconstrained and the purely theoretical reaction rate was highly uncertain. The new uncertainty on the reaction rate based on the present experiment (variation between upper and lower limit) is approximately a factor of 3. The commonly used reaction libraries JINA-REACLIB and BRUSLIB are in relatively good agreement with the experimental rate. The impact of the new rate on weak r-process calculations is discussed. This research was first published in Journal of Physics G: Nuclear and Particle Physics. © IOP Publishing.

The fragment mass and kinetic energy in neutron-induced fission of $^{238}\mathrm{U}$ has been measured for incident energies from 1 to 30 MeV at the Los Alamos Neutron Science Center. The change in mass distributions over this energy range were studied, and the transition from highly asymmetric to more symmetric mass distributions is observed. A decrease in average total kinetic energy $(\overline{\mathrm{TKE}})$ with increasing excitation energy is observed, consistent with previous experimental work. Additional structure at multichance fission thresholds is present in the $\overline{\mathrm{TKE}}$ data. The correlations between fragment masses and total kinetic energy and how that changes with excitation energy of the fissioning compound nucleus were also measured. The fission mass yields and average total kinetic energy are important for fission-based technologies such as nuclear reactors to understand nuclear waste generation and energy output when developing new and advanced concepts. The correlations between fragment mass and kinetic energy are needed both as input for theoretical calculations of the deexcitation process in fission fragments by prompt radiation emission and for validating advanced theoretical fission models describing the formation of the primordial fragments.

The $\ensuremath{\beta}$-decay intensity of $^{70}\mathrm{Co}$ was measured for the first time using the technique of total absorption spectroscopy. The large $\ensuremath{\beta}$-decay $Q$ value [12.3(3) MeV] offers a rare opportunity to study $\ensuremath{\beta}$-decay properties in a broad energy range. Two surprising features were observed in the experimental results, namely, the large fragmentation of the $\ensuremath{\beta}$ intensity at high energies, as well as the strong competition between $\ensuremath{\gamma}$ rays and neutrons, up to more than 2 MeV above the neutron-separation energy. The data are compared to two theoretical calculations: the shell model and the quasiparticle random phase approximation (QRPA). Both models seem to be missing a significant strength at high excitation energies. Possible interpretations of this discrepancy are discussed. The shell model is used for a detailed nuclear structure interpretation and helps to explain the observed $\ensuremath{\gamma}$-neutron competition. The comparison to the QRPA calculations is done as a means to test a model that provides global $\ensuremath{\beta}$-decay properties for astrophysical calculations. Our work demonstrates the importance of performing detailed comparisons to experimental results, beyond the simple half-life comparisons. A realistic and robust description of the $\ensuremath{\beta}$-decay intensity is crucial for our understanding of nuclear structure as well as of $r$-process nucleosynthesis.

The average total kinetic energy $(\overline{TKE})$ in $^{239}\mathrm{Pu}(n,f)$ has been measured for incident neutron energies between 0.5 and 50 MeV. The experiment was performed at the Los Alamos Neutron Science Center (LANSCE) using the neutron time-of-flight technique, and the kinetic energy of fission fragments post-neutron emission was measured in a double Frisch-gridded ionization chamber. This represents the first experimental study of the energy dependence of $\overline{TKE}$ in $^{239}\mathrm{Pu}$ above neutron energies of 6 MeV.

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.

Nuclear reactions where an exotic nucleus captures a neutron are critical for a wide variety of applications, from energy production and national security, to astrophysical processes, and nucleosynthesis. Neutron capture rates are well constrained near stable isotopes where experimental data are available; however, moving far from the valley of stability, uncertainties grow by orders of magnitude. This is due to the complete lack of experimental constraints, as the direct measurement of a neutron-capture reaction on a short-lived nucleus is extremely challenging. Here, we report on the first experimental extraction of a neutron capture reaction rate on ^{69}Ni, a nucleus that is five neutrons away from the last stable isotope of Ni. The implications of this measurement on nucleosynthesis around mass 70 are discussed, and the impact of similar future measurements on the understanding of the origin of the heavy elements in the cosmos is presented.

The goal of the Chi-Nu experiment at the Los Alamos Neutron Science Center is to measure the prompt fission neutron spectra from major actinides using a double time-of-flight method with a pulsed, white incoming neutron source. Fission events are detected with a parallel-plate avalanche counter and outgoing neutrons are detected with either a 6Li-glass or liquid scintillator detector array for low- or high-energy neutrons, respectively. A detector response matrix for the interaction of neutrons with the experimental environment for neutrons measured with the Chi-Nu 6Li-glass detector array has been calculated to obtain a full understanding of the measured Chi-Nu data and also to allow for nearly instantaneous production of simulated Chi-Nu data spectra. Prompt fission neutron spectra corresponding to 19 incoming neutron energy ranges from 0.7-20 MeV have been extracted using the ratio-of-ratios method with Chi-Nu 6Li-glass data on the neutron-induced fission of 235U.

We review the role of the (n , γ f ) process in the low-energy neutron-induced fission reaction of 239 Pu. Recent measurements of the average total γ -ray energy released in this reaction were performed with the Detector for Advanced Neutron Capture Experiments (DANCE) at Los Alamos. Significant fluctuations of this quantity in the resonance region below 100 eV can be interpreted by invoking the presence of the indirect (n, γ f ) process. Modern calculations of the probability for such an event to occur are presented.