Background: The probability per unit area for a radioactive 88Zr nucleus (t1/2 = 83.4 d) to capture neutrons across resonance energies-the neutron-capture resonance integral (I gamma )-is the largest ever reported [Shusterman et al., Phys. Rev. C 103, 024614 (2021)] with a value of 2.53(28) x 106 barns. This implies that the neutroncapture cross section is orders of magnitude larger than previously thought at energies relevant to explosive environments applications, such as nuclear astrophysics and radiochemical diagnostics of nuclear devices. Purpose: These applications require the shape of the capture cross section across a wide energy range which has never been measured before. The unexpectedly large values of both the thermal capture cross section and resonance integral could have a large impact on applications. Thus, the determination of the 88Zr neutron capture cross sections over a wide energy range is important on both fundamental and applied levels. was performed at the Device for Indirect Capture Experiments on Radionuclides (DICER) at the Los Alamos Neutron Science CEnter (LANSCE), from 0.0253 to 500 eV using the time of flight technique. Any resonances responsible for the large (n, gamma ) cross section (cr gamma ), will be reflected in the (n,tot) cross section. The (n,tot) data were used to extract the capture cross section as a function of energy. Results: The present work revealed a nuclear level in the n + 88Zr system at 171 meV above the neutron separation energy and a thermal neutron-capture cross section of 771(31) kb, which is in good agreement with the recently published value [Shusterman et al., Phys. Rev. C 103, 024614 (2021) and Shusterman et al., Nature (London) 565, 328 (2019)]. In contrast, the neutron-capture resonance integral extracted from the present data is 15.21(67) kb and is roughly a factor of 200 smaller than the literature value [Shusterman et al., Conclusions: The recently reported enormous thermal capture cross section was confirmed and is attributed to a resonance at 171 meV and bound state 174 eV below the neutron separation energy. Our obtained value of the resonance integral is 166 times smaller than reported and that can have a huge impact on applications.

With very few exceptions, direct measurements of neutron capture rates on radionuclides have not been possible. A number of indirect methods have been pursued such as the surrogate method [1], the γ-ray strength function method [2, 3], the Oslo method [4–7] and the β-Oslo method [8]. Substantial effort has been devoted to quantify the usually large systematic errors that accompany the results from these techniques. A new instrument has been developed at the Los Alamos Neutron Science Center (LANSCE) to provide more accurate data on several radionuclides relevant to nuclear criticality safety, radiochemical diagnostics, astrophysics, nuclear forensics and nuclear security, by measuring the transmission of neutrons through radioactive samples and studying resonance properties. The Device for Indirect Capture on Radionuclides (DICER) [9–13] and associated radionuclide production at the Isotope Production Facility (IPF), both at LANSCE, as well radioactive sample fabrication, have been under development the last few years. A description of the new apparatus, data on a few mid-weight stable isotopes and efforts on radionuclide measurements will be presented.

Capture-to-fission cross section ratios are used as an alternative to absolute cross section measurements. This is due to the simplification on the calculations and the reduction of the uncertainties with respect to an absolute measurement of the cross section by eliminating experimental complications like self-absorption, beam/target overlap and non-uniformities. Different capture-to-fission reactions have been measured through the years at the Los Alamos Neutron Science Center (LANSCE) at Los Alamos National Laboratory (LANL) using the Detector for Advanced Neutron Capture Experiments (DANCE) combined with different fission detectors: a Parallel Plate Avalanche Counter (PPAC) to detect fission fragments (FF), and the NEUtron detector array at dANCE (NEUANCE) to detect fission neutrons. As DANCE detects the γ-rays produced in capture and fission reactions, the fission instrument placed inside the DANCE cavity is used to tag the fission γ-rays for background identification and subtraction. Some examples of capture-to-fission ratio measurements performed with DANCE in the last years are the 233U, 235U and 239Pu. The measurement technique, the different setups, and other potential applications of the instruments will be described.

The presence of 240Pu in nuclear fuels for reactors has resulted in high uncertainties in the results of reactor and nuclear transmutation calculations because of deficiencies in 240Pu-related nuclear data. Specifically for the prompt fission neutron spectrum (PFNS) of 240Pu, there is only one neutron-induced, (𝑛,𝑓), measurement at 0.85 MeV incident neutron energy and only one complete spontaneous fission, (sf), measurement. This limited availability of data does not sufficiently guide nuclear data evaluations of these quantities. Here, we report on a measurement of both the 240Pu(sf) and the 240Pu(𝑛,𝑓) PFNS, both over the emitted neutron energy range of 0.79–10.0 MeV, and from incident neutron energies of 1.0–20.0 MeV for the (𝑛,𝑓) reaction. Measurements were made with a hemispherical array of liquid scintillators at the high-energy Los Alamos Neutron Science Center white neutron source at the Weapons Neutron Research facility as part of the joint LANL-LLNL Chi-Nu experimental campaign to measure actinide fission neutron spectra. These measurements are the first of their kind, and provide clear experimental evidence for second-chance fission, third-chance fission, and pre-equilibrium neutron emission processes in neutron-induced fission of 240Pu, and are the first ever measurements above 1 MeV incident neutron energy.3 MoreReceived 16 February 2024Accepted 29 April 2024DOI:https://doi.org/10.1103/PhysRevC.109.064611©2024 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasFissionNeutron physicsNuclear reactorsNucleon induced nuclear reactionsPhysical SystemsTransmutation & power generation with acceleratorsPropertiesA ≥ 220Nuclear Physics

Study. Each individual program is reported in detail in Commercial Scale Coal Test, Volumes 1 through 6. Supplementary work on the sampling and testing program for run-of-mine Illinois Basin coals has been reviewed in the Tri-State Synfuels Project Review Report, Volume 13, Coal Sampling and Testing, June 1982. The contents of this volume were revised in NOvember 1982 to take into account the completion of the export sample program and the wastewater treatability study. The export sample program had been interrupted by the demobilization activities but has now been completed. The wastewater treatability study was commenced prior to demobilization. However, due to the lengthy nature of the program, the study was only recent completed. The expeirmental results of both studies and design recommendations and conclusions are included in this volume.

The cross section for neutron interactions on argon is an important design and operational parameter for a number of neutrino, dark matter, and neutrinoless double beta decay experiments which use liquid argon as a detection or shielding medium. There is a discrepancy between the evaluated total cross section in the $20$ to $70~\rm$\,keV neutron kinetic energy region given in the ENDF database and a single measurement conducted by an experiment with a thin target (0.2 atoms/barn) optimized for higher cross sections. This gives rise to significant uncertainty in the interaction length of neutrons in liquid argon. This discrepancy is now resolved by new results presented here from the Argon Resonance Transport Interaction Experiment (ARTIE), a thick target experiment (3.3 atoms/barn) optimized for the small cross sections in this energy region.

The neutron capture cross section of Kr-83 has been measured via the time-of-flight technique between 25 meV and 500 keV. The experiment used the DANCE array at the Los Alamos National Laboratory. Maxwellian Averaged Cross Sections have been derived for a range of stellar temperatures and are found to be in good agreement with previous data. The impact of the new cross sections on stellar nucleosynthesis has been investigated.

The thermal neutron capture cross section of 88 Zr was recently reported to be the second largest in nature with the largest resonance integral measured. Presumably, these very large values are caused by a resonance or resonances very near thermal energy. Determining their energies and widths, and hence the shape of the cross section away from thermal energies,is useful for applications. The short half-life (83.4 days) and associated large background, renders direct measurements of the neutron capture cross section impossible using current techniques. However, it is possible to measure the total neutron cross section, and hence the resonance properties, using the newly commissioned Device for Indirect Capture Experiments on Radionuclides (DICER) at the Los Alamos Neutron Science Center (LANSCE). Transmission measurements are utilized as a surrogate method to perform capture measurements. The 88 Zr needed for a DICER measurement was produced at the Isotope Production Facility (IPF) and cleanly separated from the production target material. A description of the new instrument, efforts and preliminary results on 88 Zr will be presented.

Uranium-233 plays an important role in the Th-U fuel cycle, which has been proposed as an alternative to the U-Pu fuel cycle due to its reduced amount of transuranium elements. The available experimental 233U(n,γ) cross section data in the literature are scarce, [1–3]. In 2008, the 233U(n,γ) cross section was investigated at LANL using the DANCE detector combined with a PPAC, however the statistics in the keV regime were inadequate for a reliable extraction of the cross section at 100 keV. An accurate measurement of the 233U(n,γ) cross section is required by the NCSP to complete the neutron-induced cross section data; a new evaluation reported the need of 233U capture data. The challenge in this measurement lies in the difficulty of measuring capture cross section data due to the competing capture and fission channels. Fission reactions are around one order of magnitude more likely than capture for 233U. The accuracy in the capture cross section measurement relies on the discrimination between the γ-rays produced in capture and fission reactions, for which an experimental setup combining capture and fission detectors is needed. Following this requirement, a new measurement has been performed at LANSCE combining the γ-ray array DANCE with the neutron detector NEUANCE to identify fission and neutron-capture events. This measurement will provide results of the 233U capture-to-fission ratio in the Resolved and Unresolved Resonance regions.

With the recent emergence of fast nuclear reactors, there has been a corresponding increasing interest in $^{238}\mathrm{U}$-related nuclear data. However, while existing literature data span much of the energy ranges of interest for the prompt fission neutron spectrum (PFNS) for neutron-induced fission of $^{238}\mathrm{U}$, most literature data sets are highly correlated, and thus new, independent measurements of this quantity are needed. In this work, we report the results of a new measurement of the $^{238}\mathrm{U}$ PFNS at the Los Alamos Neutron Science Center for incident neutron energies from 1.5--20.0 MeV, and outgoing neutron energies of 0.01--10.0 MeV. With some notable exceptions, the present results generally agree with existing literature data, especially with regard to features relating to multichance fission and pre-equilibrium features in the PFNS, thus adding confidence to existing nuclear data evaluations and filling in gaps of knowledge at previously unmeasured incident neutron energies. This result is the third in a series of PFNS measurements by the Chi-Nu collaboration now spanning all three major actinides, $^{239}\mathrm{Pu}$, $^{235}\mathrm{U}$, and $^{238}\mathrm{U}$. Thus, for the first time, we report reliable experimental PFNS ratios and average PFNS energy comparisons for measurements of all three of these isotopes including accurate correlations between the different, but correlated experiments.

The neutron capture cross section of ^83 Kr has been measured via the time-of-flight technique between 25 meV and 500 keV. The experiment used the DANCE array at the Los Alamos National Laboratory. Maxwellian Averaged Cross Sections have been derived for a range of stellar temperatures and are found to be in good agreement with previous data. The impact of the new cross sections on stellar nucleosynthesis has been investigated.

Uranium-233 plays an important role in the Th-U fuel cycle, which has been proposed as an alternative to the U-Pu fuel cycle due to its reduced amount of transuranium elements. The available experimental U-233(n,gamma) cross section data in the literature are scarce, [1-3]. In 2008, the U-233(n,gamma) cross section was investigated at LANL using the DANCE detector combined with a PPAC, however the statistics in the keV regime were inadequate for a reliable extraction of the cross section at 100 keV. An accurate measurement of the U-233(n,gamma) cross section is required by the NCSP to complete the neutron-induced cross section data; a new evaluation reported the need of U-233 capture data. The challenge in this measurement lies in the difficulty of measuring capture cross section data due to the competing capture and fission channels. Fission reactions are around one order of magnitude more likely than capture for U-233. The accuracy in the capture cross section measurement relies on the discrimination between the gamma-rays produced in capture and fission reactions, for which an experimental setup combining capture and fission detectors is needed. Following this requirement, a new measurement has been performed at LANSCE combining the gamma-ray array DANCE with the neutron detector NEUANCE to identify fission and neutron-capture events. This measurement will provide results of the U-233 capture-to-fission ratio in the Resolved and Unresolved Resonance regions.

The neutron-induced capture-to-fission cross section ratio of 233U has been measured at the Los Alamos Neutron Science Center at Los Alamos National Laboratory in the energy range from 0.7 eV to 250 keV. The detector setup combines the Detector for Advanced Neutron Capture Experiments (DANCE) to measure & gamma; rays generated from both capture and fission reactions, and the neutron detector array at DANCE to measure fission neutrons. This is the first measurement of the capture-to-fission ratio between 2 and 30 keV. The evaluations are in good agreement with the results in the resolved resonance region. In both the unresolved resonance region and the fast neutron region, a lower capture-to-fission ratio is obtained in this work from 10 to 150 keV compared to current evaluations, while good agreement with the experimental data and the evaluations is found above 150 keV. Statistical model calculations were performed to compare with the experimental data. Significantly reduced (r & gamma; ) was required to reproduce the measured data.

The Chi-Nu project to measure prompt fission neutron energy spectra for the major actinides has now completed two measurements, for neutron-induced fission of 239 Pu and 235 U for incident neutron energies from 1 to 20 MeV, and has almost completed the prompt fission neutron spectrum measurement for 238 U. In addition, similar data have been taken for spontaneous fission in some other isotopes, and a measurement of fission neutron spectra from 240 Pu( n , f ) is in progress. These measurements are done at the same facility, with the same equipment and analyzed in a similar way. A useful way to look at such data is to examine ratios of the prompt fission neutron spectra among actinides, as some of the experimental uncertainties involved in these data are the same from isotope to isotope, making the ratios of fission neutron spectra less dependent on the experimental and analysis details. We discuss here the ratios of prompt fission neutron spectra between 239 Pu( n , f ) and 235 U( n , f ) , as well as the evolution of the mean fission neutron energy for 239 Pu( n , f ) and 235,238 U( n , f ) with increasing incident neutron energy.

Background: The statistical approach is usually applied for the description of electromagnetic decay of the nucleus with the exception of the lowest excitation energies as well as for the calculation of the interaction of photons with nuclei, in particular the reaction cross sections. This concept employs nuclear level density (NLD) and photon strength functions (PSFs).Purpose: While PSFs and NLD of some well-deformed rare-earth nuclei were measured by several methods, sometimes with conflicting results, the PSFs of 168Er were addressed only by (gamma , gamma') experiments. On the other hand, the low-lying levels of 168Er are well studied, including the isomeric state at 1094 keV, which enables various tests of the statistical approach.Methods: The gamma rays following radiative neutron capture on a 167Er sample were measured with the highly segmented gamma-ray calorimeter Detector for Advanced Neutron Capture Experiments at the Los Alamos Neutron Science Center. The gamma-ray energy spectra for different multiplicities (multistep cascade, or MSC, spectra) were gathered for many s-wave resonances of both possible spins. Moreover, we were able to detect the decay of the short-lived isomer and deduce the isomeric ratio for a few resonances.Results: Analysis of the MSC spectra within the statistical model enabled us to draw conclusions about dipole PSFs, in particular on the properties of the scissors mode, and NLD. The spectra can be well reproduced with phenomenological PSFs models but not with any of several models based on quasiparticle random-phase approximation (QRPA) calculations with different interactions. We showed that nonstatistical effects in feeding of the isomeric state play a role up to excitation energies of at least about 2 MeV. Conclusions: Deduced parameters of the scissors mode were found to be similar to those of neighbor well -deformed even-even Gd and Dy nuclei. Models like that of Kadmenskij, Markushev, and Furman (KMF) or like the modified generalized Lorentzian (MGLO) model provide a good description of experimental spectra. In contrast to several previous analyses of well-deformed rare-earth isotopes, we were able to match the experimental isomeric ratio with statistical model simulations.