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[en] We have measured neutron capture cross sections intended to address defense science problems including mix and the Quantification of Margins and Uncertainties (QMU), and provide details about statistical decay of excited nuclei. A major part of this project included developing the ability to produce radioactive targets. The cross-section measurements were made using the white neutron source at the Los Alamos Neutron Science Center, the detector array called DANCE (The Detector for Advanced Neutron Capture Experiments) and targets important for astrophysics and stockpile stewardship. DANCE is at the leading edge of neutron capture physics and represents a major leap forward in capability. The detector array was recently built with LDRD money. Our measurements are a significant part of the early results from the new experimental DANCE facility. Neutron capture reactions are important for basic nuclear science, including astrophysics and the statistics of the γ-ray cascades, and for applied science, including stockpile science and technology. We were most interested in neutron capture with neutron energies in the range between 1 eV and a few hundred keV, with targets important to basic science, and the s-process in particular. Of particular interest were neutron capture cross-section measurements of rare isotopes, especially radioactive isotopes. A strong collaboration between universities and Los Alamos due to the Academic Alliance was in place at the start of our project. Our project gave Livermore leverage in focusing on Livermore interests. The Lawrence Livermore Laboratory did not have a resident expert in cross-section measurements; this project allowed us to develop this expertise. For many radionuclides, the cross sections for destruction, especially (n,γ), are not well known, and there is no adequate model that describes neutron capture. The modeling problem is significant because, at low energies where capture reactions are important, the neutron reaction cross sections show resonance behavior or follow 1/v of the incident neutrons. In the case of odd-odd nuclei, the modeling problem is particularly difficult because degenerate states (rotational bands) present in even-even nuclei have separated in energy. Our work included interpretation of the γ-ray spectra to compare with the Statistical Model and provides information on level density and statistical decay. Neutron capture cross sections are of programmatic interest to defense sciences because many elements were added to nuclear devices in order to determine various details of the nuclear detonation, including fission yields, fusion yields, and mix. Both product nuclei created by (n,2n) reactions and reactant nuclei are transmuted by neutron capture during the explosion. Very few of the (n,γ) cross sections for reactions that create products measured by radiochemists have ever been experimentally determined; most are calculated by radiochemical equivalences. Our new experimentally measured capture cross sections directly impact our knowledge about the uncertainties in device performances, which enhances our capability of carrying out our stockpile stewardship program. Europium and gadolinium cross sections are important for both astrophysics and defense programs. Measurements made prior to this project on stable europium targets differ by 30-40%, which was considered to be significantly disparate. Of the gadolinium isotopes, 151Gd is important for stockpile stewardship, and 153Gd is of high interest to astrophysics, and nether of these (radioactive) gadolinium (n,γ) cross sections have been measured. Additional stable gadolinium isotopes, including 157,160Gd are of interest to astrophysics. Historical measurements of gadolinium isotopes, including 152,154Gd, had disagreements similar to the 30-40% disagreements found in the historical europium data. Actinide capture cross section measurements are important for both Stockpile Stewardship and for nuclear forensics. We focused on the 242mAm(n,γ) measurement, as there was no existing capture measurement for this isotope. The cross-section measurements (cross section vs. En) were made at the Detector for Advanced Neutron Capture Experiments. DANCE is comprised of a highly segmented array of barium fluoride (BaF2) crystals specifically designed for neutron capture-gamma measurements, using small radioactive targets (less than one milligram). A picture of half the array, along with a photo of one crystal, is shown in Fig. 1. DANCE provides the world's leading capability for measurements of neutron capture cross sections with radioactive targets. The DANCE is a 4π calorimeter and uses the intense spallation neutron source the Lujan Center at the Los Alamos National Laboratory. The detector array consists of 159 barium fluoride crystals arranged in a sphere around the target
[en] Neutron and proton resonances provide detailed level density information. However, due to experimental limitations, some levels are missed and some are assigned incorrect quantum numbers. The standard method to correct for missing levels uses the experimental widths and the Porter-Thomas distribution. Analysis of the spacing distribution provides an independent determination of the fraction of missing levels. We have derived a general expression for such an imperfect spacing distribution using the maximum entropy principle and applied it to a variety of nuclear resonance data. The problem of spurious levels has not been extensively addressed
[en] Precise gamma-ray thermal neutron capture cross sections have been measured at the Budapest Reactor for all elements with Z=1-83, 92 except for He and Pm. These measurements and additional data from the literature been compiled to generate the Evaluated Gamma-ray Activation File (EGAF), which is disseminated by LBNL and the IAEA. These data are nearly complete for most isotopes with Z<20 so the total radiative thermal neutron capture cross sections can be determined directly from the decay scheme. For light isotopes agreement with the recommended values is generally satisfactory although large discrepancies exist for 11B, 12, 13C, 15N, 28, 30Si, 34S, 37Cl, and 40, 41K. Neutron capture decay data for heavier isotopes are typically incomplete due to the contribution of unresolved continuum transitions so only partial radiative thermal neutron capture cross sections can be determined. The contribution of the continuum to the neutron capture decay scheme arises from a large number of unresolved levels and transitions and can be calculated by assuming that the fluctuations in level densities and transition probabilities are statistical. We have calculated the continuum contribution to neutron capture decay for the palladium isotopes with the Monte Carlo code DICEBOX. These calculations were normalized to the experimental cross sections deexciting low excitation levels to determine the total radiative thermal neutron capture cross section. The resulting palladium cross sections values were determined with a precision comparable to the recommended values even when only one gamma-ray cross section was measured. The calculated and experimental level feedings could also be compared to determine spin and parity assignments for low-lying levels
[en] An accurate value of the nuclear resonance spacing is crucial for determination of level densities. Level densities are key input for the calculation of nuclear reaction rates and cross sections. This paper discusses various effects that can adversely impact the average level spacing, with a special emphasis on the issue of quantum number assignment. The most striking property of spacings of resonances with the same quantum number is level repulsion. We investigate how a simple test based on level repulsion can be used in the identification of spin misassingment and provide new experimental verification of the proposed test. Proton resonances obtained in the 44Ca+p reaction are used as an example. In addition, s-wave neutron resonances in the 238U+n reaction are considered.
[en] We have measured precise thermal neutron capture γ-ray cross sections σγ for all stable Palladium isotopes with the guided thermal neutron beam from the Budapest Reactor. The data were compared with other data from the literature and have been evaluated into the Evaluated Gamma-ray Activation File (EGAF). Total radiative neutron capture cross-sections σ0 can be deduced from the sum of transition cross sections feeding the ground state of each isotope if the decay scheme is complete. The Palladium isotope decay schemes are incomplete, although transitions deexciting low-lying levels are known for each isotope. We have performed Monte Carlo simulations of the Palladium thermal neutron capture de-excitation schemes using the computer code DICEBOX . This program generates a level scheme where levels below a critical energy Ecrit are taken from experiment, and those above Ecrit are calculated by a random discretization of an a priori known level density formula ρ(E, Jπ). Level de-excitation branching intensities are taken from experiment for levels below Ecrit and the capture state, or calculated for levels above Ecrit assuming an a priori photon strength function and applying allowed selection rules and a Porter-Thomas distribution of widths. The calculated feeding to levels below Ecrit can then be normalized to the measured cross section deexciting those levels to determine the total radiative neutron cross-section σ0. In this paper we have measured σ0[102Pd(n,γ)] = 0.9 ± 0.3 b, σ0[104Pd(n,γ)] = 0.61 ± 0.11 b, σ0[105Pd(n,γ)] = 21.1 ± 1.5 b, σ0[106Pd(n,γ)] = 0.36 ± 0.05 b, σ0[108Pd(n,γ)(0)] = 7.6 ± 0.6 b, σ0[108Pd(n,γ)(189)] = 0.185 ± 0.011 b, and σ0[110Pd(n,γ)] = 0.10 ± 0.03 b. We have also determined from our statistical calculations that the neutron capture state in 107Pd is best described as 2+(60%)+3+(40%). Agreement with literature values was excellent in most cases. We found significant discrepancies between our results for 102Pd and 110Pd and earlier values that could be resolved by re-evaluation of the earlier results
[en] Level densities and radiative strength functions in 171Yb and 170Yb nuclei have been measured with the 171Yb(3He,3He(prime) γ)171Yb and 171Yb(3He, αγ)170Yb reactions. A simultaneous determination of the nuclear level density and the radiative strength function was made. The present data adds to and is consistent with previous results for several other rare earth nuclei. The method will be briefly reviewed and the result from the analysis will be presented. The radiative strength function for 171Yb is compared to previously published work.
[en] Level densities and radiative strength functions in 171Yb and 170Yb nuclei have been measured using the 171Yb(3He3Heγ)171Yb and 171Yb(3He,αγ)170Yb reactions. New data on 171Yb are compared to a previous measurement for 171Yb from the 172Yb(3He,αγ)171Yb reaction. Systematics of level densities and radiative strength functions in 170,171,172Yb are established. The entropy excess in 171Yb relative to the even-even nuclei 170,172Yb due to the unpaired neutron quasiparticle is found to be approximately 2kB. Results for the radiative strength function from the two reactions lead to consistent parameters characterizing the ''pygmy'' resonances. Pygmy resonances in the 170,172Yb populated by the (3He,α) reaction appear to be split into two components for both of which a complete set of resonance parameters are obtained
[en] Neutron and proton resonances provide detailed level density information. Due to experimental limitations some fraction of the resonances are not observed, and this missing fraction must be determined. The standard correction for missing levels uses the experimental widths and the Porter-Thomas distribution. Analysis of the spacing distribution should yield equivalent information. A general expression for an imperfect spacing distribution (with a fraction of levels randomly missing) was obtained with the maximum entropy principle. This formulation was tested extensively with numerical data and then applied to proton and neutron resonance data sets. Since in Random Matrix Theory the widths and spacings are not correlated, this method complements the conventional approach that considers only the widths. The two analysis methods are compared
[en] The preequilibrium reaction mechanism makes an important contribution to neutron-induced reactions above En ∼ 10 MeV. The preequilibrium process has been studied exclusively via the characteristic high energy neutrons produced at bombarding energies greater than 10 MeV. They are expanding the study of the preequilibrium reaction mechanism through γ-ray spectroscopy. Cross-section measurements were made of prompt γ-ray production as a function of incident neutron energy (En = 1 to 250 MeV) on a 48Ti sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center facility. The prompt-reaction γ rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Neutron energies were determined by the time-of-flight technique. The γ-ray excitation functions were converted to partial γ-ray cross sections taking into account the dead-time correction, target thickness, detector efficiency and neutron flux (monitored with an in-line fission chamber). Residual state population was predicted using the GNASH reaction code, enhanced for preequilibrium. The preequilibrium reaction spin distribution was calculated using the quantum mechanical theory of Feshback, Kerman, and Koonin (FKK). The multistep direct part of the FKK theory was calculated for a one-step process. The FKK preequilibrium spin distribution was incorporated into the GNASH calculations and the γ-ray production cross sections were calculated and compared with experimental data. The difference in the partial γ-ray cross sections using spin distributions with and without preequilibrium effects is significant
[en] Prompt γ-ray production cross section measurements were made as a function of incident neutron energy (En = 1 to 35 MeV) on an enriched (95.6%) 150Sm sample. Energetic neutrons were delivered by the Los Alamos National Laboratory spallation neutron source located at the Los Alamos Neutron Science Center (LANSCE) facility. The prompt-reaction γ rays were detected with the large-scale Compton-suppressed Germanium Array for Neutron Induced Excitations (GEANIE). Above En ∼ 8 MeV the pre-equilibrium reaction process dominates the inelastic reaction. The spin distribution transferred in pre-equilibrium neutron-induced reactions was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). These preequilibrium spin distributions were incorporated into the Hauser-Feshbach statistical reaction code GNASH and the γ-ray production cross sections were calculated and compared with experimental data. Neutron inelastic scattering populates 150Sm excited states either by (1) forming the compound nucleus 151Sm* and decaying by neutron emission, or (2) by the incoming neutron transferring energy to create a particle-hole pair, and thus initiating the pre-equilibrium process. These two processes produce rather different spin distributions: the momentum transfer via the pre-equilibrium process tends to be smaller than in the compound reaction. This difference in the spin population has a significant impact on the γ-ray de-excitation cascade and therefore in the partial γ-ray cross sections. The difference in the partial γ-ray cross sections using spin distributions with and without preequilibrium effects was significant, e.g., for the 558-keV transition between 8+ and 6+ states the calculated partial γ-ray production cross sections changed by 70% at En = 20 MeV with inclusion of the spin distribution of pre-equilibrium process