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One crucial issue for applications of inorganic scintillators in future HEP experiments is radiation damage in a severe radiation environment, such as the HL-LHC. While radiation damage induced by ionization dose is well understood, investigations are on-going to understand radiation damage induced by hadrons, including both charged hadrons and neutrons. Aiming at understanding neutron induced radiation damage in fast inorganic scintillators, BaF_2, LYSO/LFS and PWO crystals were irradiated at LANSCE by a combination of particles, including neutrons, protons and γ-rays. The results indicate that LYSO/LFS and BaF_2 crystal plates are radiation hard up to 4 × 10^(15) fast neutrons/cm^2.

The capabilities of 7 Li-enriched Cs 2 7 LiYCl 6 (C 7 LYC) scintillation detectors for fast neutron spectroscopy are explored in benchmark experiments that exploit its excellent pulse-shape discrimination between neutrons and γ rays, and its unprecedented ≈ 10% energy resolution for fast neutrons in the few MeV range, obtained through the 35 Cl(n,p) reaction. Energy- and angle-resolved elastic and inelastic neutron scattering cross-section measurements of 56 Fe(n,n’) were performed at Los Alamos National Laboratory with a pulsed white neutron source and an array of 1 ″ × 1 ″ C 7 LYC crystals. The results convincingly establish the utility of this dual n/ γ scintillator for fast neutron spectroscopy. Intrinsic efficiency measurements of both 1 ″ × 1 ″ and the first ever 3 ″ × 3 ″ C 7 LYC crystal have been initiated, using mono-energetic fast neutron beams at UMass Lowell generated via the 7 Li(p,n) reaction. The spectroscopic capabilities and potential of C 7 LYC are discussed in the context of developing this emerging scintillator for targeted science applications.

A new neutron counter for research experiments on nuclear fission has been developed. This instrument is designed for the detection of prompt fission neutrons within relatively high levels of gamma and neutron background. It is composed of a set of 3He proportional counters arranged within a block of polyethylene which serves as moderator. The detection properties have been studied by means of Monte Carlo simulations and experiments with radioactive sources. These properties are confirmed by an experiment on neutron-induced fission of 238U at the WNR facility of the Los Alamos Neutron Science Center during which the mean prompt fission neutron multiplicity, or ν ¯ has been measured from 1 to 20 MeV of incident neutron energy.

The prompt neutron spectrum from neutron-induced fission needs to be known in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security application. A program to measure this spectrum for neutron-induced fission of 239 Pu is underway at the Los Alamos Neutron Science Center. The goal is to obtain data on the shape of the spectrum with a small uncertainty over the emitted neutron energy range of 100 keV to 12 MeV with additional data below and above this range. The incident neutron energy range will be from 0.5 to 30 MeV. The status of this program including results of initial experimental measurements is described here.

The prompt γ-ray spectra from 235U(n,f) at incident energies of 1 to 20 MeV and for 235Cf(s.f.) were measured up to 4 MeV in coincidence with two parallel plate avalanche counters and the liquid scintillator array FIGARO. The unfolded γ-ray spectra from 1–4 MeV using the single value decomposition and iterative Bayesian techniques for the incident neutron energy regions corresponding to the first, second and third chance fissions were found to nearly identical and similar to the distribution from the spontaneous fission of 252Cf. General agreement with the γ-ray distributions from fission was also found with previous measurements.

Many isotopes in nuclear materials exhibit strong peaks in neutron absorption cross sections in the epithermal energy range (1–1000 eV). These peaks (often referred to as resonances) occur at energies specific to particular isotopes, providing a means of isotope identification and concentration measurements. The high penetration of epithermal neutrons through most materials is very useful for studies where samples consist of heavy-Z elements opaque to X-rays and sometimes to thermal neutrons as well. The characterization of nuclear fuel elements in their cladding can benefit from the development of high resolution neutron resonance absorption imaging (NRAI), enabled by recently developed spatially-resolved neutron time-of-flight detectors. In this technique the neutron transmission of the sample is measured as a function of spatial location and of neutron energy. In the region of the spectra that borders the resonance energy for a particular isotope, the reduction in transmission can be used to acquire an image revealing the 2-dimensional distribution of that isotope within the sample. Provided that the energy of each transmitted neutron is measured by the neutron detector used and the irradiated sample possesses neutron absorption resonances, then isotope-specific location maps can be acquired simultaneously for several isotopes. This can be done even in the case where samples are opaque or have very similar transmission for thermal neutrons and X-rays or where only low concentrations of particular isotopes are present ( In this paper we present the proof-of-principle of NRAI and transmission Bragg edge imaging performed at Flight Path 5 (FP5) at the LANSCE pulsed, moderated neutron source of Los Alamos National Laboratory. A set of urania mockup fuel assemblies with intentionally introduced defects was investigated. The maps of elemental composition of pellets containing urania and tungsten were obtained simultaneously by resonance absorption imaging with spatial resolution better than ∼200 μm, while the voids and cracks were revealed by the transmission images obtained with thermal and cold neutrons. Our proof-of-principle experiments demonstrate that simultaneous acquisition of resonance and Bragg edge spectra enables concurrent mapping of isotope distributions, imaging of cracks and voids as well as measurements of some crystallographic parameters of fuel assemblies and their cladding. A detailed study of energy-dependent neutron statistics achievable at FP5 with our present detection system is also presented for a wide range of neutron energies.

The outgoing neutron energy spectra from neutron-induced fission of various actinides are important for basic understanding of the fission process near the scission point as well as playing a large role in neutron transport codes, which are heavily relied upon in the design of advanced nuclear reactors and simulations of critical assemblies. The reliability of the results of neutron transport models is a strong function of the quality of the nuclear data used as input. Currently, the world's experimental database of fission neutron spectra is severely incomplete (especially for higher incident neutron energies) with large uncertainties in key portions of the outgoing energy spectra. Many transport codes use evaluated data libraries, which are based on the approach of the Los Alamos model. Other theoretical models have been developed, but the available data cannot distinguish the results of different models (as is the case for 239Pu). Better measurements are needed for all incident and outgoing neutron energies, but most urgently in the low-energy (below 1 MeV) and high-energy (above 6 MeV) portions of the outgoing spectra where theoretical model results differ greatly. We present the design considerations (and some characterization results) of the two Chi-Nu neutron detector arrays: one array of 6Li-glass detectors and one array of liquid-scintillator detectors. These detector arrays are being constructed to meet the challenge of measuring the prompt fission neutron spectra (for a few common actinides) to a higher accuracy and precision than achieved previously and over a larger incident energy range than has been covered by previous experimenters. We see a significant reduction in neutron-scattering backgrounds with our new array designs.

Cross-section measurements were made of prompt discrete γ -ray production as a function of incident neutron energy ( E n = 1 – 135 MeV ) on a 150Sm sample of 1550 mg / cm 2 of Sm 2 O 3 enriched to 95.6% in 150Sm. Results are compared with enhanced Hauser–Feshbach model calculations including the pre-equilibrium reactions. Energetic neutrons were delivered by the Los Alamos Neutron Science Center facility. The prompt-reaction γ -rays were detected with the compton-suppressed germanium array for neutron-induced excitations (GEANIE). Incident neutron energies were determined by the time-of-flight technique. Excitation functions for thirteen individual γ -rays up to E x = 0.8 MeV in 149Sm and one γ -ray transition between the first excited and ground state in 148Sm were measured. Partial γ -ray cross-sections were calculated using GNASH, an enhanced Hauser–Feshbach statistical nuclear reaction model code, and compared with the experimental results. The particle transmission coefficients were calculated with new systematic “global” optical model potential parameters. A coupled-channels optical model based on a soft rotor model was employed to calculate the particle transmission coefficients. The pre-equilibrium part of the spin-distribution in 150Sm was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK) and incorporated into the GNASH reaction model code. The partial cross-sections for discrete γ -ray cascade paths leading to the ground state in 149Sm and 148Sm have been summed (without double counting) to estimate lower limits for reaction cross-sections. These lower limits are combined with Hauser–Feshbach model calculations to deduce the reaction channel cross-sections. These reaction channel cross-sections agree with previously measured experimental and ENDF/B-VII evaluations.

Cross-section measurements were made of prompt gamma-ray production as a function of incident neutron energy (E{sub n} = 1 to 35 MeV) on an enriched (95.6%) {sup 150}Sm 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 gamma 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 {gamma}-ray excitation functions were converted to partial {gamma}-ray cross sections taking into account the dead-time correction, target thickness, detector efficiency and neutron flux (monitored with an in-line fission chamber). Partial {gamma}-ray cross sections were predicted using the Hauser-Feshbach statistical reaction code GNASH. Above E{sub n} {approx} 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 pre-equilibrium spin distributions were incorporated into a new version of GNASH and the {gamma}-ray production cross sections were calculated and compared with experimental data. The difference in the partial {gamma}-ray cross sections using spin distributions with and without pre-equilibrium effects is discussed.

The cross sections for populating the residual nucleus in the reaction AZX(n,x)A-4Z-2Y exhibit peaks as a function of incident neutron energy corresponding to the (n,n'α) reaction and, at higher energy, to the (n,2p3n) reaction. In addition, the relative magnitudes of these peaks vary with the Z of the target nucleus.

Two-step-cascade spectra of the 171Yb(n, γγ)172Yb reaction have been measured using thermal neutrons. They are compared to calculations based on experimental values of the level density and radiative strength function obtained from the 173Yb(3He, aγ)172Yb reaction. The multipolarity of a 6.5(15) μN2 resonance at 3.3(1) MeV in the strength function is determined to be M1 by this comparison.

Discrete {gamma}-ray spectra have been measured as a function of incident neutron energy for nuclei produced in the {sup 196}Pt(n,xn{gamma}) reactions. Spectroscopy was done using the large-scale Compton suppressed Ge {gamma}-ray spectrometer GEANIE. The {open_quotes}white{close_quotes} source neutron beam was produced at the Los Alamos Neutron Science WNR facility. Reaction neutron energy was determined using the time-of-flight technique. Reaction-channel yields were inferred from the measured intensity sum of the 2{sub 1}{sup +}{r_arrow}0{sub 1}{sup +} and the 2{sub 2}{sup +}{r_arrow}0{sub 1}{sup +} transitions for the {sup 196}Pt(n,xn) reactions for x{le}15. Weisskopf-Ewing calculations (including precompound) done with the HMS-ALICE code correctly predict the bulk of the (n,xn) reaction products for low multiplicity. However, they do not accurately predict yield ratios of the different (n,xn) reactions for x{ge}9. In addition, there is no consistent experimental indication of charged-particle reaction channels (n,pxn) for incident neutron energies above 60 MeV where they are predicted to account for approximately 1/3 of the total reaction cross section. Several possible causes are discussed for these discrepancies. Finally, the region of E-J phase space populated in this reaction is probed for several of the strongest reaction channels through the observation of relative yields for different yrast and off-yrast states. {copyright}more » {ital 1998} {ital The American Physical Society}« less

Prompt fission neutron spectra in the neutron-induced fission of ${}^{239}$Pu have been measured for incident neutron energies from 1 to 200 MeV at the Los Alamos Neutron Science Center. Mean energies deduced from the prompt fission neutron spectra (PFNS) lead to the observation of the opening of the second chance fission at 7 MeV and to indications for the openings of fission channels of third and fourth chances. Moreover, the general trend of the measured PFNS is well reproduced by the different models. The comparison between data and models presents, however, two discrepancies. First, the prompt neutron mean energy seems constant for neutron energy, at least up to 7 MeV, whereas in the theoretical calculations it is continuously increasing. Second, data disagree with models on the shape of the high energy part of the PFNS, where our data suggest a softer spectrum than the predictions.

Prompt K X-ray emission yields in the fission induced by neutrons on 238U have been measured for the first time for incident energies ranging from below 1MeV up to 400MeV. Results are used to investigate the evolution with incident neutron energy of the fragment elemental distribution and the X-ray emission probability per element. The progressive increase of the symmetric fission probability with neutron energy is observed in qualitative agreement with Wahl systematics for the primary fission fragment charge yields.

High-resolution \ensuremath{\gamma}-ray spectra from the interaction of neutrons in the energy range from 3 to 200 MeV with $^{207,208}\mathrm{Pb}$ were measured with the white neutron source at the weapons neutron research (WNR) facility at Los Alamos National Laboratory. From these data, excitation functions for prominent \ensuremath{\gamma} transitions in $^{200,202,204,206,207,208}\mathrm{Pb}$ were derived from threshold to 200 MeV incident neutron energy. These \ensuremath{\gamma}-production cross sections reflect the excitation cross sections for the respective residual nuclei. The results are compared with the predictions of nuclear reaction calculations based on the exciton model for precompound emission, the Hauser-Feshbach theory for compound nucleus decay, and coupled channels calculations to account for direct excitation of collective levels. Good agreement was obtained over the entire energy range covered in the experiment with reasonable model parameters. The results of this work clearly demonstrate that multiple preequilibrium emission has to be taken into account above about 40 MeV, and that the level density model of Ignatyuk, which accounts for the gradual disappearance of shell effects with increasing excitation energy, should be used instead of the Gilbert-Cameron and backshifted Fermi-gas models if excitation energies exceed about 30 MeV. No indication for a reduction of the nuclear moment of inertia below the rigid body value was found.

Prompt fission neutron spectra from $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ were measured for incident neutron energies from 1 to 200 MeV at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center, and the experimental data were analyzed with the Los Alamos model for the incident neutron energies of 1--8 MeV. A CEA multiple-foil fission chamber containing deposits of 100 mg $^{235}\mathrm{U}$ and 90 mg $^{239}\mathrm{Pu}$ detected fission events. Outgoing neutrons were detected by the Fast Neutron-Induced $\ensuremath{\gamma}$-Ray Observer array of 20 liquid organic scintillators. A double time-of-flight technique was used to deduce the neutron incident energies from the spallation target and the outgoing energies from the fission chamber. These data were used for testing the Los Alamos model, and the total kinetic energy parameters were optimized to obtain a best fit to the data. The prompt fission neutron spectra were also compared with the Evaluated Nuclear Data File (ENDF/B-VII.0). We calculate average energies from both experimental and calculated fission neutron spectra.

Higher quality measurements of outgoing prompt neutron spectra from neutron-induced fission as a function of the incoming neutron energy are needed. These data can be used in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security applications. As part of the program to measure the prompt fission neutron spectra (PFNS) from the fission of 239 Pu at the Los Alamos Neutron Science Center, we are developing a new array of liquid-scintillator detectors. This array will be used to measure the PFNS over a range of outgoing neutron energies from approximately 600 keV to 12 MeV and incident neutron energies from 0.5 to 30 MeV. A complete characterization of the detectors and the array as a whole will be carried out, targeted at understanding the light-output curves, efficiencies, and the neutron multiple-scattering backgrounds.

The {sup 205}Tl(n,2n{gamma}) reaction was used to populate excited states in {sup 204}Tl. The {gamma}-ray detection was accomplished with the GEANIE spectrometer, a Compton suppressed array of 26 Ge detectors. An energetic beam of neutrons was provided by the pulsed neutron source of the Los Alamos Neutron Science Center's WNR facility. The time-of-flight technique was used to determine the incident neutron energies. {gamma}-ray excitation functions were determined from incident neutron energy of 1 MeV up to E{sub n}=25 MeV. The level scheme of {sup 204}Tl was enriched and the partial level scheme and nuclear structure above the previously known 7{sup +} isomer at 1104-keV excitation energy were established for the first time up to E{sub x}{approx}2.3 MeV. The high-spin part of the level scheme exhibits striking similarities to that of the neighboring {sup 202}Tl isotope, suggesting similarities in the underlying nuclear structure. The half-life of the 7{sup +} isomer was measured with a more precise result (T{sub 1/2}=60.7{+-}1.2 {mu}s), in agreement with literature values. A lower limit for the excitation energy of the {pi}h{sub 11/2}{nu}i{sub 13/2} structure with J{sup {pi}}=12{sup -} is proposed.

Cross section measurements were made of prompt gamma‐ray production as a function of neutron energy using the germanium array for neutron induced excitations (GEANIE) at LANSCE. Measuring the prompt reaction gamma rays as a function of incident neutron energy provides more precise understanding of the spins populated by the pre‐equilibrium reaction. The effect of the spin distribution in pre‐equilibrium reactions has been investigated using the GNASH reaction code. Widely used classical theories such as the exciton model usually assume that the spin distribution of the pre‐equilibrium reaction is the same as the spin distribution of the compound nucleus reaction mechanism. In the present approach, the pre‐equilibrium reaction spin distribution was calculated using the quantum mechanical theory of Feshbach, Kerman, and Koonin (FKK). This pre‐equilibrium spin distribution was incorporated into the GNASH code and the gamma‐ray production cross sections were calculated and compared with experimental data. Spin distributions peak at lower spin when calculated with the FKK formulation than with the Compound Nuclear theory. The measured partial gamma‐ray cross sections reflect this spin difference. Realistic treatment of the spin distribution improves the accuracy of calculations of gamma‐ray production cross sections.

Nuclear model calculations of discrete \ensuremath{\gamma}-ray production cross sections produced in $^{48}\mathrm{Ti}$($n,{n}^{'}{\ensuremath{\gamma}}_{i}$)$^{48}\mathrm{Ti}$ and $^{48}\mathrm{Ti}$($n,2n{\ensuremath{\gamma}}_{i}$)$^{47}\mathrm{Ti}$ reactions were made as a function of incident neutron energy from ${E}_{n}=1$ MeV to 35 MeV and compared with new experimental results using the large-scale Compton-suppressed germanium array for neutron induced excitations (GEANIE) at LANSCE. The Hauser-Feshbach reaction code GNASH, incorporating the spin distribution for the preequilibrium process calculated with the Feshbach-Kerman-Koonin (FKK) quantum-mechanical preequilibrium theory, was used to calculate partial \ensuremath{\gamma}-ray transition cross sections. The comparisons of calculated and experimental data demonstrate that, the FKK model for preequilibrium leads a better overall reproduction of the experimental data above ${E}_{n}=10$ MeV, where preequilibrium processes are important. The FKK calculation predicts a strong reduction in the high-spin state population in $^{48}\mathrm{Ti}$ by inelastic scattering. Population of low-spin states was also affected, however the change in the low-lying 983.5-keV (${2}^{+}$) state production is small because almost all \ensuremath{\gamma}-ray decay cascades feed this transition. In addition, the FKK calculation has a significant impact on the partial \ensuremath{\gamma}-ray transition cross sections for the ($n,2n$) reaction above ${E}_{n}=15$ MeV. The calculated cross sections for high-spin states in $^{47}\mathrm{Ti}$ are reduced, and those from the low-spin states are enhanced, in agreement with the experimental cross section data.

Taking advantage of the neutron source of the LANCSE, it has been possible to obtain a measure of the velocity distribution and the number of prompt-neutrons emitted in the neutron-induced fission of {sup 238}U and {sup 237}Np over a broad incident neutron energy range. The mean kinetic energy was extracted and is shown as the function of the incident-neutron energy. We confirm here the observation, for both reactions, of a dip around the second chance fission which is explained by the lower kinetic energy of the pre-fission neutrons. Such a observation is reproduced by Los Alamos model as implemented at Bruyeres le Chatel and by the Maslov model. As far as the neutron multiplicity is concerned, a similar dip is observed. However, such a behavior is not present in data measured by other groups.

A large number of partial γ‐ray cross sections produced in neutron‐induced reactions with neutrons in the energy range 1 < En (MeV) < 200 have been measured over the past eight years. Partial γ‐ray cross sections are measured as a function of incident neutron energy using the time‐of‐flight technique. Reaction channel cross sections were deduced from these measurements with the aid of nuclear modeling. Enabling facilities are the intense “white” source of neutrons at the LANSCE/WNR 60R 20‐meter flight path, and the precision γ‐ray spectrometry of the Compton‐suppressed Ge detector array GEANIE. The first focus of the measurements was on the 239Pu(n,2n) cross section, followed by a series of other experiments on nuclei throughout the periodic table, with an emphasis on neutron‐fluence activation detectors (or “RadChem detectors”). Representative measurements will be presented, along with the techniques. Experiments in progress and future plans are mentioned.

Discrete γ‐ray spectra have been measured for nuclei populated in 191Ir(n,xnγ) reactions, with x ⩽ 11, as a function of incident neutron energy using neutrons from the “white” neutron source at the Los Alamos Neutron Science Center’s WNR facility. The energy of the neutrons was determined using the time‐of‐flight technique. The data were taken using the GEANIE spectrometer. The cross sections for emission of 202 γ rays of 181–191Ir were determined for neutron energies 0.2 MeV < En < 300 MeV. Comparison with model calculations, using the GNASH reaction model, and with GEANIE results from the similar 193Ir(n,xnγ) reactions is made.

The {sup 75}As(n, 2n) cross section for the population of the T{sub 1/2} = 26.8-ns isomer at E{sub x} = 259.3 keV in {sup 74}As has been measured as a function of incident neutron energy, from threshold to E{sub n} = 20 MeV. The cross section was measured using the GEANIE spectrometer at LANSCE/WNR. For convenience, the {sup 75}As(n, 2n) population cross section for the {sup 74}As ground state has been deduced as the difference between the previously-known (n, 2n) reaction cross section and the newly measured {sup 75}As(n, 2n){sup 74}As{sup m} cross section. The (n, 2n) reaction, ground-state, and isomer population cross sections are tabulated in this paper.

Nuclear data collected in an experiment performed at LANSCE/WNR with the GEANIE spectrometer to measure the cross sections for n + {sup 89}Y for E{sub n} < 20 MeV are reported. The estimated (n,2n) cross sections to the ground and isomeric states of {sup 88}Y differ significantly from those listed in the RADCHEM data base. Folded with a fast neutron spectrum, these results lead to integral differences of +15%, +2%, and -70% vs. the RADCHEM evaluation.

The population cross section of the T{sub 1/2} = 25-minute, E{sub x} = 76.8-keV isomer in {sup 235}U via the {sup 235}U(n,n{prime}{gamma}) reaction has been estimated in the E{sub n} = 2.1-20-MeV range. Gamma rays populating both isomer and ground states were detected using the GEANIE spectrometer at the LANSCE/WNR ''white-source'' neutron facility. Partial {gamma}-ray cross sections were obtained as a function of incident neutron energy, using {gamma}-ray spectroscopy and the time-of-flight technique. A correction for unobserved transitions was applied to the measured partial cross sections using the Hauser-Feshbach code GNASH to produce population cross sections for the isomer- and ground-state levels. The deduced isomer population cross section at E{sub n} = 2.1 MeV is 1.1(1) b, and the isomer-to-ground state population ratio decreases from 0.9 to 0.06 over the E{sub n} = 2.1-20-MeV range. The details of the measurement and recommendations to improve the current results are discussed.

Timepix pixel detectors have been used to study the response of silicon hybrid pixel detectors to fast neutrons from a pulsed neutron beam at WNR FP30R, a 14 m long flight path, in the Los Alamos Neutron Science Center. Neutrons with kinetic energies up to 600 MeV were available. In order to enhance the conversion of neutrons to energetic charged particles, several converter foils and filters were attached to the 300 μm thick silicon sensor, i.e. polyethylene, polyethylene with aluminum, 6LiF, 6LiF with aluminum, aluminum. The Time-of-Arrival mode of the Timepix detectors has permitted the application of the Time-of-Flight (TOF) technique for the assignment of the detected interactions in the form of clusters (groups of adjacent pixels) in the pixel matrix, to the kinetic energies of the incident neutrons. It was found that, for lower neutron energies ( ~ MeV range) the cluster rates below the polyethylene and the polyethylene and aluminum region, produced by recoil protons, are a good measure for the mean kinetic energies of neutrons. For energies above 50 MeV nuclear reactions in the silicon dominate the detector response. In this energy range the shape of the clusters indicates the neutron kinetic energy.

Prompt $\ensuremath{\gamma}$ rays from the neutron-induced fission of ${}^{235}\mathrm{U}$ have been studied using the GEANIE spectrometer situated at the LANSCE/WNR ``white'' neutron facility. Transitions characteristic of specific fission fragments were identified using a combination of single-fold and coincidence $\ensuremath{\gamma}$-ray analysis. Gamma-ray production cross sections were obtained as a function of incident neutron energy using the time-of-flight technique. Yields were extracted for 206 $\ensuremath{\gamma}$ rays produced by 56 distinct fission fragments for $1l~{E}_{n}(\mathrm{MeV})l~250.$ Fission-channel cross sections were deduced for a subset of 22 even-even fragments, where the measured $\ensuremath{\gamma}$-ray yields represent a large fraction of the full de-excitation strength of the fragment. Independent yields were deduced from these cross sections and fitted with standard formulations of the fragment charge and mass distributions. The results were used to study the transition from asymmetric to symmetric fission as a function of internal excitation energy of the fissioning system. This transition is interpreted in the context of the disappearance of shell structure at high excitation energies.

Excitation functions from the interaction of fast neutrons (up to an energy of 250 MeV) on a target of {sup 92}Mo have been measured. Yields have been determined for reaction {gamma} rays from {sup 88-92}Mo, {sup 88-92}Nb, {sup 86-91}Zr, {sup 84-86,88}Y, and {sup 80,82-86}Sr. These results are compared with model calculations using the GNASH code which takes into account compound nucleus, preequilibrium emission, multiple preequilibrium emission, and direct reaction contributions. The model calculations are in good agreement overall, but significant discrepancies emerge for some high-multiplicity, charged-particle-exit channels.

Measurements of partial {sup 235}U(n,2n{gamma}) {gamma}-ray cross sections have been carried out as a function of incident neutron energy using the GEANIE spectrometer at LANSCE/WNR. The yields of {gamma} rays resulting from the population of discrete levels in the residual nucleus {sup 234}U have been measured at incident neutron energies in the 1-20-MeV range. These data provide, with the aid of nuclear reaction modeling, a measurement of the {sup 235}U(n,2n) reaction cross section and serve as a proof of principle of the y-ray technique for the parallel 23gPu(n,2n) measurement [l]. This paper presents the analysis of the {gamma}-ray data and the extraction of partial {gamma}-ray cross sections as a function of incident neutron energy. Uncertainties associated with the spectroscopic analysis of the data and validation of the results are discussed in detail.

GEANIE is the first large-scale Ge detector array used in conjunction with a high-energy neutron spallation source. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Spallation neutrons are provided by the LANSCE/WNR facility, and reaction neutron energies are determined via time-of-flight. neutron flux is monitored in-beam with a fission chamber. GEANIE at LANSCE/WNR currently emphasizes the measurement of partial gamma-ray cross sections as a function of neutron energy. Absolute cross section measurements require a complete understanding of array performance. Important effects include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Measurements and calculations of these effects will be presented for the specific cases of iron and actinide targets. The use of radioactive targets incurs a large deadtime penalty. In order to increase data throughput they are making plans to move to a triggerless data acquisition system. These modifications and other improvements to the electronics for better timing will be discussed.

The neutron spectrum from neutron-induced fission needs to be known in designing new fast reactors, predicting criticality for safety analyses, and developing techniques for global security application. The experimental data base of fission neutron spectra is very incomplete and most present evaluated libraries are based on the approach of the Los Alamos Model. To validate these models and to provide improved data for applications, a program is underway to measure the fission neutron spectrum for a wide range of incident neutron energies using the spallation source of fast neutrons at the Weapons Neutron Research (WNR) facility at the Los Alamos Neutron Science Center (LANSCE). In a double time-of-flight experiment, fission neutrons are detected by arrays of neutron detectors to increase the solid angle and also to investigate possible angular dependence of the fission neutrons. The challenge is to measure the spectrum from low energies, down to 100 keV or so, to energies over 10 MeV, where the evaporation-like spectrum decreases by 3 orders of magnitude from its peak around 1 MeV. For these measurements, we are developing two arrays of neutron detectors, one based on liquid organic scintillators and the other on 6Li-glass detectors. The range of fission neutrons detected by organic liquid scintillators extends from about 600 keV to well over 10 MeV, with the lower limit being defined by the limit of pulse-shape discrimination. The 6Li-glass detectors have a range from very low energies to about 1 MeV, where their efficiency then becomes small. Various considerations and tests are in progress to understand important contributing factors in designing these two arrays and they include selection and characterization of photomultiplier tubes (PM), the performance of relatively thin (1.8 cm) 6Li-glass scintillators on 12.5 cm diameter PM tubes, use of 17.5 cm diameter liquid scintillators with 12.5 cm PM tubes, measurements of detector efficiencies with tagged neutrons from the WNR/LANSCE neutron beam, and efficiency calibration with 252Cf spontaneous fission neutrons. Design considerations and test results are presented.

High-resolution {gamma}-ray spectra from the interaction of neutrons in the energy range from 3 to 200 MeV with {sup 207,208}Pb were measured with the white neutron source at the WNR facility at Los Alamos National Laboratory. From these data, excitation functions for prominent {gamma} transitions in {sup 200,202,204,206,207,208}Pb were derived from threshold to 200 MeV incident neutron energy. These {gamma}-production cross sections represent formation cross sections for excited states of the residual nuclei. The results are compared with the predictions of nuclear reaction calculations based on the exciton model for precompound emission, the Hauser-Feshbach theory for compound nuclear decay, and coupled channels calculations to account for direct excitation of collective levels. Good agreement was obtained over the entire energy range covered in the experiment with reasonable model parameters. The results demonstrate that multiple preequilibrium emission has to be taken into account above about 40 MeV, and that the level density model of Ignatyuk should be used instead of the Gilbert-Cameron and back-shifted Fermi-gas models if excitation energies exceed about 30 MeV.

High resolution Ge detectors coupled with the WNR high‐intensity, high‐energy, pulsed neutron source at LAMPF recently have been used to measure a variety of reactions including (n,xnγ) for 1≤x≤11, (n,nαγ), (n,npγ), etc. The reactions are identified by the known gamma‐ray energies of prompt transitions between the low‐lying states in the final nuclei. With our spallation neutron source cross section data are obtained at all neutron energies from a ℏω MeV to over 200 MeV. Applications of the data range from assisting the interpretation of the planned Mars Observer mission to map the elemental composition of the martian surface, to providing data for nuclear model verification and understanding reaction mechanisms. For example, a study of the Pb(n,xnγ) reactions, for 2≤x≤11, that populate the first excited sates of the even Pb isotopes is underway. These data will be used to test preequilibrium and other reaction models.

Energy distributions of prompt neutrons in coincidence with fission induced on 238 U were measured for incident neutron energies up to 200 MeV. The double time-of-flight technique was used to deduce incident and emitted neutron energies. The experimental average and standard deviations of the fission neutron spectra (FNS), for emitted neutron energies from 0.65 to 7.5 MeV, are reported. The results compare well to predictive calculations with the improved Los Alamos model below 20 MeV incident neutron energy. The observed dip at the opening of the second chance fission channel at 6 MeV is confirmed and analyzed. Above 20 MeV, the experimental results of the FNS are smaller than the calculated ones. At 50 MeV and higher, the data suggest a slight increase of the temperature and the kinetic energy of the fission fragments.