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Full calculations of six-nucleon reactions with a three-body final state have been elusive and a long-standing issue. We present neutron spectra from the T(t,2n)α (TT) reaction measured in inertial confinement fusion experiments at the OMEGA laser facility at ion temperatures from 4 to 18 keV, corresponding to center-of-mass energies (E_{c.m.}) from 16 to 50 keV. A clear difference in the shape of the TT-neutron spectrum is observed between the two E_{c.m.}, with the ^{5}He ground state resonant peak at 8.6 MeV being significantly stronger at the higher than at the lower energy. The data provide the first conclusive evidence of a variant TT-neutron spectrum in this E_{c.m.} range. In contrast to earlier available data, this indicates a reaction mechanism that must involve resonances and/or higher angular momenta than L=0. This finding provides an important experimental constraint on theoretical efforts that explore this and complementary six-nucleon systems, such as the solar ^{3}He(^{3}He,2p)α reaction.

A new technique that uses inertial confinement implosions for measuring low-energy nuclear reactions important to nuclear astrophysics is described. Simultaneous measurements of n–D and n–T elastic scattering at 14.1 MeV using deuterium–tritium gas-filled capsules provide a proof of principle for this technique. Measurements have been made of D(d,p)T (dd) and T(t,2n)4He (tt) reaction yields relative to the D(t,n)4He (dt) reaction yield for deuterium–tritium mixtures with f T /f D between 0.62 and 0.75 and for a wide range of ion temperatures to test our understanding of the implosion processes. Measurements of the shape of the neutron spectrum from the T(t,2n)4He reaction have been made for each of these target configurations.

A new method has been applied to the determination of neutron nonelastic cross sections for iron 56 Fe and lead 208 Pb for energies between 5 and 26 MeV. These data have estimated errors of only a few percent and do not suffer from the ambiguities encountered in earlier nonelastic data. We attempt to fit these high-precision data using both a semiclassical single phase shift model (nuclear Ramsauer model) as well as a recent global optical model that well reproduces a wide body of neutron scattering observables. At the 5% uncertainty level, both models produce satisfactory fits. However, neither model gives satisfactory fits to these new precise data. We conclude that fitting precise data, i.e., data with errors of ∼2% or less, may require a nuclear mass dependence of radii that reflects structure effects such as shell closures.

We describe the next generation general purpose Evaluated Nuclear Data File, ENDF/B-VII.0, of recommended nuclear data for advanced nuclear science and technology applications. The library, released by the U.S. Cross Section Evaluation Working Group (CSEWG) in December 2006, contains data primarily for reactions with incident neutrons, protons, and photons on almost 400 isotopes, based on experimental data and theory predictions. The principal advances over the previous ENDF/B-VI library are the following: (1) New cross sections for U, Pu, Th, Np and Am actinide isotopes, with improved performance in integral validation criticality and neutron transmission benchmark tests; (2) More precise standard cross sections for neutron reactions on H, 6 Li, 10 B, Au and for 235,238 U fission, developed by a collaboration with the IAEA and the OECD/NEA Working Party on Evaluation Cooperation (WPEC); (3) Improved thermal neutron scattering; (4) An extensive set of neutron cross sections on fission products developed through a WPEC collaboration; (5) A large suite of photonuclear reactions; (6) Extension of many neutron- and proton-induced evaluations up to 150 MeV; (7) Many new light nucleus neutron and proton reactions; (8) Post-fission beta-delayed photon decay spectra; (9) New radioactive decay data; (10) New methods for uncertainties and covariances, together with covariance evaluations for some sample cases; and (11) New actinide fission energy deposition. The paper provides an overview of this library, consisting of 14 sublibraries in the same ENDF-6 format as the earlier ENDF/B-VI library. We describe each of the 14 sublibraries, focusing on neutron reactions. Extensive validation, using radiation transport codes to simulate measured critical assemblies, show major improvements: (a) The long-standing underprediction of low enriched uranium thermal assemblies is removed; (b) The 238 U and 208 Pb reflector biases in fast systems are largely removed; (c) ENDF/B-VI.8 good agreement for simulations of thermal high-enriched uranium assemblies is preserved; (d) The underprediction of fast criticality of 233,235 U and 239 Pu assemblies is removed; and (e) The intermediate spectrum critical assemblies are predicted more accurately. We anticipate that the new library will play an important role in nuclear technology applications, including transport simulations supporting national security, nonproliferation, advanced reactor and fuel cycle concepts, criticality safety, fusion, medicine, space applications, nuclear astrophysics, and nuclear physics facility design. The ENDF/B-VII.0 library is archived at the National Nuclear Data Center, BNL, and can be retrieved from www.nndc.bnl.gov .

The nuclear Ramsauer model is a semi‐classical, analytic approximation to nucleon‐nucleus scattering that reproduces total cross‐section data at the 1% level for A > 40, En = 5–60 MeV with 7–10 parameters. A quick overview of the model is given, demonstrating the model’s utility in nuclear data evaluation. The Ramsauer model predictions for reaction cross section, elastic cross section, and elastic scattering angular distributions are considered. In a recent paper it has been shown that the nuclear Ramsauer model does not do well in predicting details of the angular distribution of neutron elastic scattering for incident energies of less than 60 MeV for 208Pb. However, in this contribution it is demonstrated that the default angular bin dispersion most widely used in Monte Carlo transport codes is such that the observed differences in angular shapes are on too fine a scale to affect transport calculations. Simple studies indicate that 512–2048 bins are necessary to achieve the dispersion required for calc...

Absolute partial γ-ray cross sections for discrete γ-ray production in the 239Pu(n,2nγi)238Pu reaction have been measured. The experiments were performed at LANSCE/WNR on the 60R neutron flight path. Reaction γ-rays were measured using the large-scale Compton-suppressed array of Ge detectors, GEANIE. The motivation for this experiment, an overview of the partial γ-ray cross-section measurement, and an introduction to the main experimental issues will be presented. The energy resolution of the Ge detectors allowed identification of reaction γ rays above the background of sample radioactivity and fission γ rays. The use of planar Ge detectors with their reduced sensitivity to neutron interactions and improved line shape was also important to the success of this experiment. Absolute partial γ-ray cross sections are presented for the 61+ → 41+ member of the ground state rotational band in 238Pu, together with miscellaneous other γ-ray partial cross sections. The n,2n reaction cross section shape and magnitude...

Efforts to model and measure absolute neutron-induced reaction cross sections have for the most part been limited by experimental techniques to neutron energies found in reactors and light-ion fusion reactions (i.e., t(d,n){alpha} etc.). This can in large part be attributed to the difficulty involved in making high-flux mono-energetic neutron beams with E{sub n} > 14 MeV. The result has been a lack of guidance for (n,xnypz{alpha}) reaction modeling for E{sub n} > 1-2 MeV. These limitations become particularly exacerbated in nuclei where structure effects (i.e., near shell closures, high deformation etc.) or fission complicates the models. The GEANIE spectrometer at LANSCE/WNR fills this gap in experimental technique by allowing for efficient measurement of neutron-induced {gamma}-ray partial cross sections over a wide range of incident neutron energies (1 < E{sub n} (MeV) < 250). GEANIE consists of 20 Compton-suppressed and 6 unsuppressed hpGe detectors located at the 60{sup o} right beam line at the LANSCE/WNR spallation source. Eleven of the GEANIE detectors are Low Energy Planar Spectrometers (LEPS) with excellent timing and energy resolution. LANSCE/WNR provides a white source of neutrons through the spallation of a tungsten target with an 800 MeV 2-5 {micro}amp beam of protons. Neutron energy is determinedmore » from the time-of-flight technique. Early results from GEANIE [Ber98] highlighted the deficiencies in the modeling of neutron-induced reaction modeling on a {sup 196}Pt target. More recently, measurements on a {sup 92}Mo target have identified {gamma}-rays in over 2 dozen reaction products [Gar00a]. In this paper we will present an overview of several recent results from GEANIE, including data on a {sup 92}Mo and two Actinide ({sup 235}U and {sup 239}Pu) targets. The implications of these results for reaction modeling from low to high (E{sub n} < 250 MeV) energy will be presented and plans for future experiments discussed.« less

The average g factors of high spin, high-excitation energy, quasicontinuum structures in {sup 194,193}Hg were measured by observing the precessions of the angular distributions of {gamma}-ray transitions in several normal-deformation bands that coalesce in the decay of the entry distribution of states. The average g factors of the states leading to the three main bands in the {sup 193,194}Hg isotopes were: {l_angle}g({sup 193}Hg){r_angle}=+0.19(1) and {l_angle}g({sup 194}Hg){r_angle}=+0.26(1), respectively. These average g factors are smaller than the average of the g factors of the high energy states in the three superdeformed bands of {sup 194}Hg, {l_angle}g(SD; {sup 194}Hg){r_angle}=+0.41(8). While the nucleus in the superdeformed well behaves like a rigid rotor, the present results demonstrate the important role played by multiple, quasiparticle neutron configurations in the structure of normal-deformation, highly-excited nuclear states.

The average g factors of the high-energy states of the three superdeformed bands in ${}^{194}\mathrm{Hg}$ were determined directly in a transient field experiment. The reaction ${}^{150}\mathrm{Nd}{(}^{48}\mathrm{Ca}{,4n)}^{194}\mathrm{Hg}$ at a beam energy of 203 MeV was used to provide recoiling reaction product nuclei with sufficient velocity to traverse a gadolinium ferromagnetic layer. The resulting g factors $g(\mathrm{SD}1)=0.36(10),$ $g(\mathrm{SD}2)=0.41(20),$ and $g(\mathrm{SD}3)=0.71(26)$ are in agreement with cranked Hartree-Fock calculations as well as with the picture of a rigid rotation for which $g=Z/A.$

Excited states in the nucleus ${}^{191}\mathrm{Pb}$ have been investigated by in-beam \ensuremath{\gamma}-ray spectroscopic techniques using the Gammasphere array. The reaction ${}^{173}\mathrm{Yb}{(}^{24}\mathrm{Mg},6n)$ at a beam energy of 134.5 MeV was used to populate states of ${}^{191}\mathrm{Pb}.$ The level scheme of ${}^{191}\mathrm{Pb}$ has been extended up to 5.1 MeV excitation energy and spin $45/2.$ The lower part (below $\ensuremath{\approx}2.3\mathrm{MeV}$) presents the general features common in all odd-mass Pb isotopes, i.e., spherical states built on the ${13/2}^{+}$ isomer, with bandlike structures that follow weak-coupling expectations. In the upper part of the level scheme two dipole bands are observed with bandhead energies of $\ensuremath{\sim}2.512$ and 2.291 MeV. A comparison of dipole band 1 with the strongest negative-parity dipole bands in the heavier odd-mass Pb isotopes reveals striking similarities. The spherical states are interpreted as the coupling of the ${i}_{13/2}$ neutron hole to the yrast states in the even-mass ${}^{192}\mathrm{Pb}$ core. The dipole bands are interpreted as $M1$ cascades which arise from the coupling of two protons in high-$j$ orbitals above the $Z=82$ gap to neutron holes in the ${i}_{13/2}$ orbital.

CIELO (Collaborative International Evaluated Library Organization) provides a new working paradigm to facilitate evaluated nuclear reaction data advances. It brings together experts from across the international nuclear reaction data community to identify and document discrepancies among existing evaluated data libraries, measured data, and model calculation interpretations, and aims to make rapid progress in reconciling these discrepancies to create more accurate ENDF-formatted files. Our goal is for this first step to be achieved within three years. The focus will initially be on a small number of the highest-priority isotopes, namely 1H, 16O, 56Fe, 235,238U, and 239Pu. The present paper is a first step to identify discrepancies between various evaluations of the highest priority isotopes, which was commissioned by the OECD's Nuclear Energy Agency WPEC (Working Party on International Nuclear Data Evaluation Co-operation) during a meeting held in May 2012. The evaluated data for these materials in the existing nuclear data libraries - ENDF/B-VII.1, JEFF-3.1, JENDL-4.0, CENDL-3.1, ROSFOND, IRDFF 1.0 - are reviewed, disrepancies are identified, and some integral properties are given. The paper summarizes a program of nuclear science and computational work needed to create the new CIELO nuclear data evaluations., JRC.D.4-Standards for Nuclear Safety, Security and Safeguards

Measurements of the D(d,p)T (dd) and T(t,2n)(4)He (tt) reaction yields have been compared with those of the D(t,n)(4)He (dt) reaction yield, using deuterium-tritium gas-filled inertial confinement fusion capsule implosions. In these experiments, carried out on the OMEGA laser, absolute spectral measurements of dd protons and tt neutrons were obtained. From these measurements, it was concluded that the dd yield is anomalously low and the tt yield is anomalously high relative to the dt yield, an observation that we conjecture to be caused by a stratification of the fuel in the implosion core. This effect may be present in ignition experiments planned on the National Ignition Facility.

For the first time the differential cross section for the elastic neutron-triton (n-{sup 3}H) and neutron-deuteron (n-{sup 2}H) scattering at 14.1 MeV has been measured by using an inertial confinement fusion facility. In these experiments, which were carried out by simultaneously measuring elastically scattered {sup 3}H and {sup 2}H ions from a deuterium-tritium gas-filled inertial confinement fusion capsule implosion, the differential cross section for the elastic n-{sup 3}H scattering was obtained with significantly higher accuracy than achieved in previous accelerator experiments. The results compare well with calculations that combine the resonating-group method with an ab initio no-core shell model, which demonstrate that recent advances in ab initio theory can provide an accurate description of light-ion reactions.

In a recent report Navratil et al. evaluated the angle-integrated cross section and the angular distribution for 14-MeV n+T elastic scattering by inferring these cross sections from accurately measured p+3He angular distributions. This evaluation used a combination of two theoretical treatments, based on the no-core shell model and resonating-group method (NCSM/RGM) and on the R-matrix formalism, to connect the two charge-symmetric reactions n+T and p+{sup 3}He. In this report we extend this treatment to cover the neutron incident energy range 6-14 MeV. To do this, we evaluate angle-dependent correction factors for the NCSM/RGM calculations so that they agree with the p+{sup 3}He data near 6 MeV, and using the results found earlier near 14 MeV we interpolate these correction factors to obtain correction factors throughout the 6-14 MeV energy range. The agreement between the corrected NCSM/RGM and R-Matrix values for the integral elastic cross sections is excellent ({+-}1%), and these are in very good agreement with total cross section experiments. This result can be attributed to the nearly constant correction factors at forward angles, and to the evidently satisfactory physics content of the two calculations. The difference in angular shape, obtained by comparing values of the scattering probability distribution P({mu}) vs. {mu}(the cosine of the c.m. scattering angle), is about {+-}4% and appears to be related to differences in the two theoretical calculations. Averaging the calculations yields P({mu}) values with errors of {+-}2 1/2 % or less. These averaged values, along with the corresponding quantities for the differential cross sections, will form the basis of a new evaluation of n+T elastic scattering. Computer files of the results discussed in this report will be supplied upon request.

Precise thermal neutron capture \ensuremath{\gamma}-ray cross sections ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ were measured for all elements with $Z=1\text{\ensuremath{-}}83,90$, and 92, except for He and Pm, at the Budapest Reactor. These data were evaluated with additional information from the literature to generate the Evaluated Gamma-ray Activation File (EGAF). Isotopic radiative neutron cross sections can be deduced from the total transition cross section feeding the ground state, ${\ensuremath{\sigma}}_{0}=\ensuremath{\Sigma}{\ensuremath{\sigma}}_{\ensuremath{\gamma}}(\mathrm{GS})$ if the decay scheme is complete. The EGAF file contains partial \ensuremath{\gamma}-ray cross sections for all stable palladium isotopes. None of these decay schemes are complete, although in each case transitions de-exciting low-lying levels are known. We have performed Monte Carlo simulations of the palladium thermal neutron capture decay schemes using the computer code DICEBOX. The simulated populations of low low-lying levels are normalized to the measured ${\ensuremath{\sigma}}_{\ensuremath{\gamma}}$ values from EGAF and the total radiative neutron cross section ${\ensuremath{\sigma}}_{0}$ is obtained. The ${\ensuremath{\sigma}}_{0}$ values derived for the palladium isotopes agree well with previous measurements and were in several cases more precise. Complementary use of \ensuremath{\gamma}-ray cross-section data and Monte Carlo calculations has proven effective in determining both the palladium total radiative cross sections and new nuclear structure information.

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

Experimental values for r.m.s. charge density radii show deviations from a simple A 1/3 dependence of several percent for statically deformed nuclei and around shell closures. These data, for both stable nuclei and nuclei away from the valley of stability, are well described by Hartree-Fock-Bogolyubov calculations carried out at Bruy` eres- le-ChBy using the neutron and proton r.m.s. radii from these same HFB calculations in conjunction with a folding model to produce an optical potential, we can then predict the expected results from neutron scattering experiments. We have made a detailed study of this procedure in the A = 40-70 mass range. The calculated 14-MeV neutron nonelastic and total cross sections show deviations of several percent from a smooth dependence on A. We conclude that using mean field - folding model calculations is useful for estimating neutron cross sections on nuclei both on and away from the valley of stability.

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

The level structures of the N=50 {sup 85}Br and {sup 87}Rb isotopes have been studied in the fission of the compound system formed in three different heavy-ion-induced reactions. In {sup 87}Rb many states above the previously known 9/2{sup +} isomer have been established. The coupling of the odd proton occupying the g{sub 9/2} orbital to the yrast states in the {sup 86}Kr core can account for the first excited states above the isomer in {sup 87}Rb. A comparison with the first excited states above the 9/2{sup +} state in {sup 85}Kr and {sup 89}Y reveals similarities in the coupling. At higher excitations similar behavior to {sup 89}Y is observed, indicating a possible presence of neutron-core excitations. In {sup 85}Br several states up to {approx}5 MeV excitation energy have been established. Two states at {approx}2 MeV excitation energy are candidates for the 9/2{sup +} state originating from the odd proton occupying the g{sub 9/2} orbital. The experimental results for both isotopes are compared with predictions of the shell model.

Superdeformed excitations in $^{195}\mathrm{Pb}$ were populated via the $^{174}\mathrm{Yb}$($^{26}\mathrm{Mg}$,$5n$) reaction, and \ensuremath{\gamma}-ray spectroscopy measurements were made with Gammasphere. Analysis of the intensities of the discrete transitions between superdeformed states indicates that the decay out occurs from the favored band of the lowest signature pair. The quasicontinuous spectra associated with the decay of superdeformed bands in $^{195}\mathrm{Pb}$ have been extracted for both signatures. The onset of \ensuremath{\gamma}-ray intensity and the normalized \ensuremath{\gamma}-ray intensity of the quasicontinuum decay spectrum are used to set a lower bound of 2.5 MeV at $\ensuremath{\sim}11\ensuremath{\hbar}$ for superdeformed excitation energy relative to normal deformed excitations in $^{195}\mathrm{Pb}$.

The authors present an evaluation of cross sections describing reactions with neutrons incident on the arsenic isotopes with mass numbers 75 and 74. Particular attention is paid to (n,2n) reactions. The evaluation for {sup 75}As, the only stable As isotope, is guided largely by experimental data. Evaluation for {sup 74}As is made through calculations with the EMPIRE statistical-model reaction code. Cross sections describing the production and destruction of the 26.8 ns isomer in {sup 74}As are explicitly considered. Uncertainties and covariances in some evaluated cross sections are also estimated.

Superdeformed excitations in {sup 198}Po were studied using the {sup 174}Yb({sup 29}Si,5n) reaction and {gamma}-ray spectroscopy with Gammasphere. Energy-spin entry distributions for populating normal (ND) and superdeformed (SD) excitations were extracted by gating on discrete ND and SD transitions, respectively. Information on the SD excitation energy, SD well depth, and fission barrier in {sup 198}Po was deduced.

The Evaluated Gamma‐ray Activation File (EGAF), a new database of prompt and delayed neutron‐capture γ‐ray cross sections, has been prepared as part of an International Atomic Energy Agency (IAEA) Coordinated Research Project to develop a “Database of Prompt Gamma‐rays from Slow Neutron Capture for Elemental Analysis.” Recent elemental γ‐ray cross‐section measurements performed with the guided neutron beam at the Budapest Reactor have been combined with data from the literature to produce the EGAF database. EGAF contains thermal cross sections for ≈35,000 prompt and delayed γ rays from 262 isotopes. New precise total thermal radiative cross sections have been derived for many isotopes from the primary and secondary gamma‐ray cross sections and additional level scheme data. An IAEA TECDOC describing the EGAF evaluation and tabulating the most prominent γ rays will be published in 2004. The TECDOC will include a CD‐ROM containing the EGAF database in both ENSDF and tabular formats with an interactive viewer for searching and displaying the data. The Isotopes Project, Lawrence Berkeley National Laboratory, continues to maintain and update the EGAF file. These data are available on the Internet from both the IAEA and Isotopes Project websites.

The Livermore Computational Nuclear Physics group is charged with producing updated neutron incident cross‐section evaluations for all the actinides in the coming year, concentrating on neutron‐induced fission, neutron capture, and (n,2n) cross sections. We attack this daunting task either by adopting other recent evaluations or by performing our own. Owing to the large number of nuclei involved, we seek to automate this process as much as possible. For this purpose, we have developed a series of computer codes: x4i, an interface to the EXFOR database, fete, a code that translates ENDF/B formatted evaluations into a computationally convenient form, and da_fit, a fitting code that takes all relevant EXFOR data for a reaction or set of reactions and performs a generalized least‐squares fit to them, subject to various constraints and other prior information.The Livermore Computational Nuclear Physics group is charged with producing updated neutron incident cross‐section evaluations for all the actinides in the coming year, concentrating on neutron‐induced fission, neutron capture, and (n,2n) cross sections. We attack this daunting task either by adopting other recent evaluations or by performing our own. Owing to the large number of nuclei involved, we seek to automate this process as much as possible. For this purpose, we have developed a series of computer codes: x4i, an interface to the EXFOR database, fete, a code that translates ENDF/B formatted evaluations into a computationally convenient form, and da_fit, a fitting code that takes all relevant EXFOR data for a reaction or set of reactions and performs a generalized least‐squares fit to them, subject to various constraints and other prior information.

Rotational bands in {sup 175}Lu have been extended through investigation with the (n,n{prime}{gamma}) reaction. Spallation neutrons bombarded Lu samples, and the resulting {gamma} rays were detected in a large-scale Compton-suppressed Ge detector array. Prompt- and delayed-{gamma}{gamma} coincidences have been used to extend most of the existing known bands, and to tentatively assign a new band, based on the 7/2{sup -}[523] configuration, from its band head to spin 13/2. The 3-quasiparticle K{sup {pi}} = 19/2{sup +} isomer is confirmed and its half life determined to be 984 {+-} 13(stat.) {+-} 30(sys.) {micro}s, in agreement with previous results.

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.

We have measured prompt {gamma} rays after thermal and resonant neutron capture in {sup 171}Yb. The {gamma} rays were measured with three high resolution ({approx}3 keV) and high efficiency (two {approx}80% Ge(HP) and one {approx}200% segmented Clover) detectors. We have obtained singles and two-fold coincidence spectra as function of neutron energy using the time-of-flight technique. Two-fold coincidences where the summed energy adds up to the neutron binding energy B{sub n} or to B{sub n} minus the energy of the first excited stated ({approx}79 keV) will be used to determine the multipolarity of the pygmy resonance in {sup 172}Yb. This pygmy resonance is a resonant structure in the radiative strength function around {approx}3 MeV in deformed rare earth nuclei [1]. The following goals were met: (1) We have shown the feasibility of prompt, high-resolution {gamma} spectroscopy at FP14 after thermal as well as resonant neutron capture. (2) Online analysis shows that sufficient statistics ({approx}7000 counts) are expected in each of the two relevant peaks in the summed-energy spectrum. Analysis of the experiment is in progress. It still remains to show that the multipolarity of the pygmy resonance can be determined experimentally from two-step cascade intensities.

Prompt γ rays from the neutron-induced fission of 235U have been studied using the GEANIE spectrometer situated at the LANSCE/WNR “white” neutron facility. Gamma-ray production cross sections for 29 ground-state-band transitions in 18 even-even fission fragments were obtained as a function of incident neutron energy, using the time-of-flight technique. Independent yields were deduced from these cross sections and fitted with standard formulations of the fragment charge and mass distributions to study the transition from asymmetric to symmetric fission. The results are interpreted in the context of the disappearance of shell structure at high excitation energies.

The ${}^{239}\mathrm{Pu}{(n,2n)}^{238}\mathrm{Pu}$ cross section has been deduced using a combination of measured partial $\ensuremath{\gamma}$-ray cross sections and enhanced Hauser-Feshbach reaction modeling from threshold to ${E}_{n}l~20 \mathrm{MeV}.$ Eight ${}^{239}\mathrm{Pu}(n,2n\ensuremath{\gamma}{)}^{238}\mathrm{Pu}$ partial $\ensuremath{\gamma}$-ray cross sections were measured using the GEANIE spectrometer, the time-of-flight technique, and energetic neutrons produced at the LANSCE/WNR facility. The $(n,2n)$ cross section was obtained by multiplying the sum of observed noncoincident partial $\ensuremath{\gamma}$-ray cross sections by the calculated ratio of the $(n,2n)$ cross section divided by the sum of the same partial $\ensuremath{\gamma}$-ray cross sections. A comparison between the data and model calculations indicates potential deficiencies in the modeling of the $\ensuremath{\gamma}$-ray cascade.

High‐spin states in the neutron‐rich 106,108,110,111,112Rh isotopes have been investigated in the fission of the compound system formed in three heavy‐ion induced reactions. Four bands were assigned to 106,108,110,112Rh, respectively. Comparison with the lighter odd‐odd Rh isotopes supports the assignment of the bands to the πg9/2 ⊗ vh11/2 configuration. In 111Rh the level scheme consists of two rotational bands. In the ground‐state band the odd‐proton occupies the πg9/2 orbital.

Lee A. BERNSTEIN, Darren L. BLEUEL, Joseph A. CAGGIANO, Charles CERJAN, Richard J. FORTNER, Julie GOSTIC, Patrick M. GRANT, Narek GHARIBYAN, Christian HAGMANN, Robert HATARIK, Eugene A. HENRY, Daniel SAYRE, Dieter H. G. SCHNEIDER, Wolfgang STOEFFL, Dawn A. SHAUGHNESSY, Dennis P. McNABB, Charles B. YEAMANS, Natalia P. ZAITSEVA, Joshua A. BROWN1), Brian H. DAUB1), Nicholas M. BRICKNER1), Paul F. DAVIS1), Bethany L. GOLDBLUM1), Karl A. VAN BIBBER1), Jasmina VUJIC1), Mohammad S. BASUNIA2), Richard B. FIRESTONE2), Aaron M. HURST2) and Andrew M. ROGERS2)

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.

Recently, new cross section measurements by the GEANIE collaboration have been published for {sup 239}Pu(n, 2n{gamma}) [Ber00] and {sup 235}U(n, 2n{gamma}) [You00] from threshold to 20 MeV. When combined with nuclear reaction calculations [Che00, Cha99, Cha01], these measurements provide the most accurate information available on the shape and magnitude of the {sup 239}Pu(n, 2n) cross section for incident neutron energies, E{sub n} {approx}< 14 MeV. This new data has prompted a re-evaluation of the {sup 239}Pu(n, 2n) {sup 238}Pu reaction cross section considering all available experimental data. The data prior to the measurement of Bernstein et al. [Ber00] is illustrated in Figure 1a. These data sets were considered by previous evaluations [ENDL] of the {sup 239}Pu(n, 2n) cross section. The most precise experiment was an activation measurement done by Lougheed et al. [Lou00] for incident neutron energies, E{sub n}, between 13-15 MeV. In addition, there were two neutron-counting experiments, one by Mather et al. [Mat72] and one by Frehaut et al. [Fre85]. These two measurements cover a wide incident neutron range, with data points from threshold to E{sub n} {approx} 14 MeV. The available data sets are in poor agreement with each other and in some cases do not meetmore » basic expectations. These experiments will be reviewed in detail. The new contributions to this evaluation are (1) the GEANIE data coupled with reaction modeling, illustrated in Figure 1b, and (2) the systematic use of other nuclear data in order to put constraints on the shape and magnitude of the {sup 239}Pu(n, 2n) cross section. The approach of this evaluation has been to use consistency arguments supported by nuclear data to resolve the measurement differences, with the goal of providing: (1) A comprehensive picture of our knowledge on the {sup 239}Pu(n, 2n) cross section; and (2) A new evaluation including the best possible estimate of the cross section and a one-sigma estimate the uncertainties. This paper is organized in the following fashion: Section 2: Evaluation of {sup 239}Pu(n, 2n) measurements--Measurements of the {sup 239}Pu(n, 2n) cross section are evaluated for reliability (systematic uncertainties) and for renormalization possibilities. Section 3: Consistency with other experimental data--Experimental data and nuclear data evaluations for non-elastic, inelastic, fission, (n, 2n), and neutron production cross sections are reviewed in the context of unitarity and other constraints. Conclusions with regard to the shape and magnitude of the {sup 239}Pu(n, 2n) cross section are drawn. Section 4: Recommended {sup 239}Pu(n, 2n) cross section and uncertainties--The results of Section 2 and 3 are combined to provide an overall ''best'' data set with evaluated uncertainties. The recommended {sup 239}Pu(n, 2n) cross sections and relative uncertainties are given in tabular format and compared with other evaluations.« less

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.

Previous experimental efforts to measure the {sup 239}Pu(n,2n) reaction cross section have relied on the detection of evaporated neutrons [Mat72,Fre85]. These efforts were hampered by the presence of the large ({sigma} {approx} 2 barns) neutron-induced fission channel which produces on average 3-4 neutrons [How71]. This paper is one of three manuscripts that document an effort to determine the {sup 239}Pu(n,2n) channel cross section using the GErmanium Array for Neutron Induced Excitations (GEANIE) spectrometer [Bec97] at the Los Alamos Neutron Science Center/Weapons Neutron Research (LANSCE/WNR) [Lis90] facility. In this document we report the measurement of several {sup 239}Pu(n,xn{gamma}){sup 240-x}Pu partial cross sections. The other two papers report results for a parallel proof-of-principle experiment using a {sup 235}U target as a surrogate for {sup 239}Pu [Youn00] and a detailed calculation and measurement of the efficiency of the GEANIE spectrometer [McN00]. Results from these two works are frequently referred to in this paper. A later report from Becker et al. will use the partial (n2n{gamma}) cross sections reported here, together with the predictions of the GNASH [Cha99,Cha00] and IDA [Ros99] reaction models, to extract a total cross section for the (n,2n) channel. The results of three experiments are reported here. Two were carriedmore » out in 1998 using a 0.010 inch (referred to as ''thin'') and 0.020 inch (referred to as ''thick'') {sup 239}Pu target. The remaining one was run in 1999 using the same thin target. This work is being carried out in parallel with a similar effort to measure (n,xn{gamma}) cross sections on a {sup 235}U target being analyzed by Younes et al., [You00]. This experiment was performed using the same {gamma}-ray spectrometer and a similar analysis approach, thereby allowing a detailed comparison between the two data sets and a check on the techniques used. The report is comprised of three portions. In the first part the experimental techniques used for all three runs will be presented. The next section will describe how the partial (n,xn{gamma}) cross sections were extracted from a combination of all three data sets, including a detailed discussion of the level spectroscopy of levels in {sup 238}Pu populated in this experiment. The last portion of the report will focus on a comparison between the observed partial cross sections and the predictions of the GNASH reaction model [Cha99,Cha00]. Several methods will be suggested in which the model predictions may be coupled with the measured partial cross sections to extract a (n,2n) channel cross section. A separate appendix will present the results from the three separate experiments individually.« less

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.

States in {sup 92}Mo have been observed with the (n,n{sup '}{gamma}) reaction with spallation neutrons produced at the LANSCE-WNR facility. Gamma rays were detected with the GEANIE spectrometer, and the neutron energies were determined by the time-of-flight method. Combining excitation function and {gamma}{gamma} coincidence analysis, 13 new levels have been deduced. The level scheme has been extended considerably for low- to medium-spin levels below 5 MeV in excitation energy. (c) 2000 The American Physical Society.

The {sup 239}Pu(n,2n{gamma}){sup 238}Pu partial reaction cross-section, {sigma}{sub (n,2n{gamma})}, has been measured as a function of neutron energy for several transitions in {sup 238}Pu. Partial {gamma}-ray cross sections for yrast, ''collector'' transitions, can provide especially valuable constraints on the magnitude and shape of the total (n,2n) reaction cross-section. In essence, nuclear reaction models will be used to infer the shape and magnitude of the total (n,2n) reaction cross-section from the measured partial {gamma}-ray cross-sections. The reason for undertaking this somewhat indirect approach is that previous measurements of the {sup 239}Pu(n,2n{gamma}) have been hampered by a variety of constraints. Activation measurements have several hurdles: (1) intense flux and long counting times are required to overcome the relatively long half-life of {sup 238}Pu (87 years) and (2) isotopically pure samples of {sup 239}Pu in an environment free of {sup 238}Pu contamination are difficult to come by. Neutron counting experiments are subject to significant uncertainties because (1) large background statistics from fission neutrons and (2) the experimental fission neutron multiplicity spectrum is subject to systematic errors because the flux of low-energy neutrons which induce fissions in thermally-fissile {sup 239}Pu is very difficult to characterize. In this measurement, 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 one {sup 235}U fission chamber and one {sup 238}U fission chamber. The {sup 238}U is not sensitive to background from low-energy neutrons, whereas the {sup 235}U fission chamber has better statistics. Hence, in essence the partial {gamma}-ray cross sections are normalized to the evaluated fission cross sections of {sup 235}U and {sup 238}U. As a check of our normalization to provide additional constraints to the nuclear reaction modeling, benchmark measurements of {sup nat}Fe(n, n{prime}{gamma}) and {sup 235}U(n,2n{gamma}) have also been undertaken. The secondary {gamma}-rays are measured with the GEANIE array. GEANIE consists of eleven Compton-suppressed planar detectors, nine suppressed and six unsuppressed co-axial detectors. Any absolute cross section measurement requires a complete understanding of array performance, flux normalization, and target effects. Important items to consider in this experiment include intrinsic detector efficiency, beam and detector geometry corrections, target attenuation, and deadtime. Radioactive targets give rise to significant counting rates in the GEANIE array resulting a large deadtime. The magnitude, energy dependence, and uncertainties of these effects and other corrections are the subject of this paper.

GEANIE at LANSCE/WNR combines the precision energy resolution of germanium detectors with the advantages of a white source providing neutrons with energies 1 < E{sub n}(MeV) < 250 to address a variety of topics in nuclear physics. The authors present the analysis of two data sets, n+{sup 235}U and n+{sup 92}Mo, acquired at GEANIE during the 1998 beam cycle. These data showcase the breadth of subjects under study at this facility, including the spectroscopy of stable and near-stable nuclei, reaction dynamics, fission studies, and the relative population of isomer and ground states in neutron-induced reactions.

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 potential for using deep inelastic reactions to populate high-spin states in neutron-rich nuclei is studied in a series of experiments using GAMMASPHERE for gamma-ray detection and a silicon st ...