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Nuclear reactions taking place in stars are not straightforward to study in laboratories on Earth. Now, inertial-confinement fusion implosion experiments are reported that mimic the conditions for the hydrogen-burning phase in main-sequence stars.

The OECD Nuclear Energy Agency (NEA) Working Party on International Nuclear Data Evaluation Cooperation (WPEC) was established in 1989 to facilitate collaboration in nuclear data activities. Over its thirty year history, different Subgroups have been created to address topics in nearly every aspect of nuclear data, including: experimental measurements, evaluation, validation, model development, quality assurance of databases and the development of software tools.WPEC has recently completed activities on fission yield evaluation, the general nuclear database structure (GNDS) to replace the ENDF-6 format, methods to provide feedback to evaluation, studies of specific capture cross sections, new methods in thermal scattering kernel evaluation and the Collaborative International Evaluated Library Organisation (CIELO) Pilot Project. Ongoing activities in GNDS application programming interface (API) development, methods for covariance evaluation and quality assurance in nuclear data validation using the International Criticality Safety Benchmark Evaluation Project (ICSBEP) database are complemented by the work of two Expert Groups that oversee the High-Priority Request List (HPRL) for Nuclear Data and the continuous development of the GNDS. New activities on the use of integral experiments for nuclear data validation and adjustment, as well as the use of the Shielding Integral Benchmark Archive and Database (SINBAD) for validation have begun and will be coordinated alongside future Subgroups.After three decades we will review the status of WPEC, how it integrates other collections and activities organised by the NEA and how it dovetails with the initiatives of the IAEA and other bodies to effectively coordinate international activities in nuclear data.

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.

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.

Few-body nuclear physics often relies upon phenomenological models, with new efforts at the ab initio theory reported recently; both need high-quality benchmark data, particularly at low center-of-mass energies. We use high-energy-density plasmas to measure the proton spectra from ^{3}He+T and ^{3}He+^{3}He fusion. The data disagree with R-matrix predictions constrained by neutron spectra from T+T fusion. We present a new analysis of the ^{3}He+^{3}He proton spectrum; these benchmarked spectral shapes should be used for interpreting low-resolution data, such as solar fusion cross-section measurements.

The OECD Nuclear Energy Agency (NEA) organizes cooperation between the major nuclear data evaluation projects in the world. The NEA Working Party on International Nuclear Data Evaluation Cooperation (WPEC) was established to promote the exchange of information on nuclear data evaluation, measurement, nuclear model calculation, validation, and related topics, and to provide a framework for cooperative activities between the participating projects. The working party assesses nuclear data improvement needs and addresses these needs by initiating joint activities in the framework of dedicated WPEC subgroups. Studies recently completed comprise a number of works related to nuclear data covariance and associated processing issues, as well as more specific studies related to the resonance parameter representation in the unresolved resonance region, the gamma production from fission product capture reactions, the 235U capture cross section, the EXFOR database, and the improvement of nuclear data for advanced reactor systems. Ongoing activities focus on the evaluation of 239Pu in the resonance region, scattering angular distribution in the fast energy range, and reporting/usage of experimental data for evaluation in the resolved resonance region. New activities include two subgroups on improved fission product yield evaluation methodologies and on modern nuclear database structures. Future activities under discussion include a pilot project for a Collaborative International Evaluated Library Organization (CIELO) and methods to provide feedback from nuclear and covariance data adjustment for improvement of nuclear data. In addition to the above mentioned short-term task-oriented subgroups, WPEC also hosts a longer-term subgroup charged with reviewing and compiling the most important nuclear data requirements in a high priority request list (HPRL)., JRC.D.4-Standards for Nuclear Safety, Security and Safeguards

We estimated statistical uncertainties of a nondestructive assay system using nuclear resonance fluorescence (NRF) for spent nuclear fuel including low-concentrations of actinide nuclei with an intense, mono-energetic photon beam. Background counts from radioactive materials inside the spent fuel were calculated with the ORIGEN2.2-UPJ burn-up computer code. Coherent scattering contribution associated with Rayleigh, nuclear Thomson, and Delbruck scattering was also considered. The energy of the coherent scattering overlaps with that of NRF transitions to the ground state. Here, we propose to measure NRF transitions to the first excited state to avoid the coherent scattering contribution. Assuming that the total NRF cross-sections are in the range of 3–100 eV b at excitation energies of 2.25, 3.5, and 5 MeV, statistical uncertainties of the NRF measurement were estimated. We concluded that it is possible to assay 1% actinide content in the spent fuel with 2.2–3.2% statistical precision during 4000 s measurement time for the total integrated cross-section of 30 eV b at excitation energies of 3.5–5 MeV by using a photon beam with an intensity of 10 6 photons/s/eV. We also examined both the experimental and theoretical NRF cross-sections for actinide nuclei. The calculation based on the quasi-particle random phase approximation suggests the existence of strong magnetic dipole resonances at excitation energies ranging from 2 to 6 MeV with the scattering cross-sections of tens eV b around 5 MeV in 238 U.

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.

Light nuclei were created during big-bang nucleosynthesis (BBN). Standard BBN theory, using rates inferred from accelerator-beam data, cannot explain high levels of 6Li in low-metallicity stars. Using high energy-density plasmas we measure the T(3He,γ)6Li reaction rate, a candidate for anomalously high 6Li production; we find that the rate is too low to explain the observations, and different than values used in common BBN models. In conclusion, this is the first data directly relevant to BBN, and also the first use of laboratory plasmas, at comparable conditions to astrophysical systems, to address a problem in nuclear astrophysics.

We report the results of an experimental study of the sensitivity of two distinct classes of systems that exploit nuclear resonance fluorescence (NRF) to search for illicit materials in containers. One class of systems is based on the direct detection of NRF photons emitted from isotopes of interest. The other class infers the presence of a particular isotope by observing the preferential attenuation of resonant photons in the incident beam. We developed a detailed analytical model for both approaches. We performed experiments to test the model using depleted uranium as a surrogate for illicit material and used tungsten as a random choice for shielding. We performed the experiments at Duke University’s High Intensity Gamma Source (HIGS). Using the methodology we detail in this paper one can use this model to estimate the performance of potential inspection systems in certifying containers as free of illicit materials and for detecting the presence of those same materials.

Studies of nuclear resonance fluorescence based applications are presented. Important for these applications are data for isotopes such as {sup 239}Pu. Nuclear resonance fluorescence measurements of {sup 239}Pu were performed at the free electron laser facility at UC Santa Barbara using photons from a bremsstrahlung beam with an endpoint energies between 4.0 MeV and 5.5 MeV. Though no discrete states with significant confidence level were measured, we have excluded the region above 27(3) eV-barns, or 4-sigma, where we would expect only a small chance of false positives. Details of the measurements and the results are presented here.

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 .

This paper considers an approach for representing nuclear data that is qualitatively different from the approach currently adopted by the nuclear science community. Specifically, they examine a representation in which complicated data is described through collections of distinct and self contained simple data structures. This structure-based representation is compared with the ENDF and ENDL formats, which can be roughly characterized as dictionary-based representations. A pilot data representation for replacing the format currently used at LLNL is presented. Examples are given as is a discussion of promises and shortcomings associated with moving from traditional dictionary-based formats to a structure-rich or class-like representation.

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...

This paper describes the development of a platform to study astrophysically relevant nuclear reactions using inertial-confinement fusion implosions on the OMEGA and National Ignition Facility laser facilities, with a particular focus on optimizing the implosions to study charged-particle-producing reactions. Primary requirements on the platform are high yield, for high statistics in the fusion product measurements, combined with low areal density, to allow the charged fusion products to escape. This is optimally achieved with direct-drive exploding pusher implosions using thin-glass-shell capsules. Mitigation strategies to eliminate a possible target sheath potential which would accelerate the emitted ions are discussed. The potential impact of kinetic effects on the implosions is also considered. The platform is initially employed to study the complementary T(t,2n)α, T(3He,np)α and 3He(3He,2p)α reactions. Proof-of-principle results from the first experiments demonstrating the ability to accurately measur...

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 time evolution of the decay of high-spin, normal deformed structures in 193,194 Hg was studied by measuring, in the Gammasphere array, the Doppler shift attenuation of selected transitions depopulating various bands. The data suggest that a fraction of states feeding selective yrast levels is non-collective in nature and show resemblance to A = 150 nuclei where shape polarizing effects have been linked to particles occupying high- j orbitals.

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 $g=Z/A.$

The level structures of several Ru isotopes near $A=104$ have been studied in the fission of the compound nucleus ${}^{197}\mathrm{Pb},$ formed in the ${}^{24}{\mathrm{Mg}\mathrm{}+}^{173}\mathrm{}\mathrm{Yb}$ reaction at 134.5 MeV. A sequence of five transitions, observed in coincidence with known transitions in the ${}^{86}\mathrm{Sr}$ and ${}^{87}\mathrm{Sr}$ complementary fragments, has been assigned to ${}^{105}\mathrm{Ru}.$ Comparison with the neighboring odd-mass Ru isotopes supports assignment of this sequence as built on the ${h}_{11/2}$ excitation in ${}^{105}\mathrm{Ru}.$ The positive-parity band built on the ${7/2}^{+}$ state of ${}^{103}\mathrm{Ru}$ has been extended above the backbending induced by the alignment of two ${h}_{11/2}$ neutrons. In addition, the ground-state bands of ${}^{104}\mathrm{Ru}$ and ${}^{106}\mathrm{Ru}$ have been extended to higher spins.

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.

The {sup 173}Yb({sup 24}Mg,5n) reaction at E({sup 24}Mg) = 134.5 MeV with a gold-backed target was used to populate high-angular momentum states in {sup 192}Pb. Resulting {gamma} rays were detected with Gammasphere located at the 88-Inch Cyclotron. A discrete transition of energy E{sub {gamma}} = 2057.7(6) keV has been identified in the depopulation the yrast superdeformed (SD) band in {sup 192}Pb. This transition has been tentatively placed in the level scheme based on coincidence relationships. Thus, the excitation energy of the SD level populated by the 262-keV in-band transition is tentatively assigned E{sub x} = 4.572(1) MeV. Confidence limits are discussed. Other transitions in coincidence with the SD band and general features of the band and its decay are also presented. In particular, we confirm that the J{sup (2)} of the SD band decreases by {approx}13{percent} at the lowest frequency. Also, by comparing the masses of the SD ground states in {sup 192,194}Pb to liquid drop model expectations and to each other we have been able to confirm enhanced stability for N=112 at superdeformation. {copyright} {ital 1997} {ital The American Physical Society}

The yrast superdeformed band in {sup 194}Pb has been populated using the {sup 174}Yb({sup 25}Mg,5n){sup 194}Pb reaction at E{sub beam}=130 MeV. Decay {gamma} rays were detected using the GAMMASPHERE array at the 88-Inch Cyclotron. Twelve {gamma}-ray transitions have been observed directly linking three members of the {sup 194}Pb yrast superdeformed band to low-lying normal deformed levels. Anisotropy measurements indicate that these linking decays include E1, M1, and mixed M1/E2 multipolarities. The radiative widths deduced are very inhibited, typically B(E1){approximately}10{sup {minus}8} Weisskopf units (W.u.) and B(M1){approximately}10{sup {minus}5} W.u. Without recourse to {ital a priori} assumptions J{sup {pi}}=6{sup +} and E{sub x}=4878.4(3) keV have been unambiguously assigned to the lowest-lying observed superdeformed state (the state populated by the 170-keV intraband transition). The intensity of the observed primaries accounts for 21(2){percent} of the superdeformed band population. {copyright} {ital 1997} {ital The American Physical Society}

Excitations in {sup 192}Po have been studied via in-beam {gamma}-ray spectroscopy following the {sup 164}Er + 164 MeV {sup 32}S reaction. The experiment was performed at the ATLAS facility using ten Compton-suppressed Ge detectors and the Fragment Mass Analyzer. The {alpha} decay of {sup 192}Po has been used to identify prompt {gamma}-ray transitions by means of the recoil decay tagging technique. The first three excited states of {sup 192}Po were established. The particle-core model has been successfully used to model these excitations and the quasiparticle random phase approximation has been applied to extract microscopic wave functions of the 2{sub 1}{sup +} states. {copyright} {ital 1997} {ital The American Physical Society}

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

Four bands of enhanced dipole transitions, with weak crossovers, have been observed in Pb-195. Three of these bands are connected to the spherical levels. in addition, the spherical level scheme ha ...

Prompt, in-beam γ rays following the reaction Yb170 + 142 MeV Si28 were measured at the ATLAS facility using 10 Compton-suppressed Ge detectors and the Fragment Mass Analyzer. Transitions in Po194 were identified and placed using γ-ray singles and coincidence data gated on the mass of the evaporation residues. A level spectrum up to J10 was established. The structure of Po194 is more collective than that observed in the heavier polonium isotopes and indicates that the structure has started to evolve toward the more collective nature expected for deformed nuclei. © 1995 The American Physical Society.

Measurements of the neutron spectrum from the T(t,2n)4He (tt) reaction have been conducted using inertial confinement fusion implosions at the OMEGA laser facility. In these experiments, deuterium-tritium (DT) gas-filled capsules were imploded to study the tt reaction in thermonuclear plasmas at low reactant center-of-mass (c.m.) energies. In contrast to accelerator experiments at higher c.m. energies (above 100 keV), these results indicate a negligible n + 5He reaction channel at a c.m. energy of 23 keV.

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.

For the first time the differential cross section for the elastic neutron-triton (n-(3)H) and neutron-deuteron (n-(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 (3)H and (2)H ions from a deuterium-tritium gas-filled inertial confinement fusion capsule implosion, the differential cross section for the elastic n-(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.

We have studied the efficacy of using nuclear resonance fluorescence (NRF)-based techniques to assay spent nuclear fuel for Pu content using quasi-monoenergetic sources. We have developed two techniques to precisely determine the Pu content in a fuel rod/pin. One of our approaches is virtually free of systematic uncertainties. Using analytical models, we have determined the amount of time required to measure the Pu content in spent nuclear fuel rods and spent fuel assemblies to within 1% precision. We note that Pu content can be determined in a fuel assembly about as fast as in a single fuel pin. The performance of NRF-based assay techniques with improved photon sources, which are currently under development, will also estimated. For follow-on research we propose to: (1) Construct research prototype detection systems for both of the NRF-based assay systems proposed in this paper and measure their calibration curves; (2) Determine the systematic errors associated with both assay methods, explore ways to reduce the errors and fold the results into future performance calculations; (3) Develop an algorithm to assay a fuel assembly; (4) Perform validation measurements using a single pin and scaled assemblies; (5) Research and develop current-mode detection and/or threshold detection techniques to improve assay times; (6) Characterize the flux of newly constructed sources and fold the results into the calculations presented here to determine the feasibility of a variety of proposed sources; and (7) Collaborate with others in the safeguards community to build a prototype system and perform an NRF-based assay demonstration on spent fuel.

A monoenergetic gamma-ray (MEGa-ray) source based on Compton scattering, targeting nuclear physics applications such as nuclear resonance fluorescence, has been constructed and commissioned at Lawrence Livermore National Laboratory. In this paper, the overall architecture of the system, as well as some of the design decisions (such as laser pulse lengths and interaction geometry) made in the development of the source, are discussed. The performances of the two laser systems (one for electron production, one for scattering), the electron photoinjector, and the linear accelerator are also detailed, and initial $\ensuremath{\gamma}$-ray results are presented.

A high peak brilliance, laser-based Compton-scattering $\ensuremath{\gamma}$-ray source, capable of producing quasimonoenergetic photons with energies ranging from 0.1 to 0.9 MeV has been recently developed and used to perform nuclear resonance fluorescence (NRF) experiments. Techniques for characterization of $\ensuremath{\gamma}$-ray beam parameters are presented. The key source parameters are the size ($0.01\text{ }\text{ }{\mathrm{mm}}^{2}$), horizontal and vertical divergence ($6\ifmmode\times\else\texttimes\fi{}10\text{ }\text{ }{\mathrm{mrad}}^{2}$), duration (16 ps), and spectrum and intensity (${10}^{5}\text{ }\text{ }\mathrm{photons}/\mathrm{shot}$). These parameters are summarized by the peak brilliance, $1.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\text{ }\text{ }\mathrm{photons}/{\mathrm{mm}}^{2}/{\mathrm{mrad}}^{2}/\mathrm{s}/0.1%$ bandwidth, measured at 478 keV. Additional measurements of the flux as a function of the timing difference between the drive laser pulse and the relativistic photoelectron bunch, $\ensuremath{\gamma}$-ray beam profile, and background evaluations are presented. These results are systematically compared to theoretical models and computer simulations. NRF measurements performed on $^{7}\mathrm{Li}$ in LiH demonstrate the potential of Compton-scattering photon sources to accurately detect isotopes in situ.

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.

What we believe to be the first demonstration of isotope-specific detection of a low-Z and low density object shielded by a high-Z and high-density material using monoenergetic gamma rays is reported. The isotope-specific detection of LiH shielded by Pb and Al is accomplished using the nuclear resonance fluorescence line of L7i at 478 keV. Resonant photons are produced via laser-based Compton scattering. The detection techniques are general, and the confidence level obtained is shown to be superior to that yielded by conventional x-ray and gamma-ray techniques in these situations.

Stewardship of this nation's nuclear weapons is predicated on developing a fundamental scientific understanding of the physics and chemistry required to describe weapon performance without the need to resort to underground nuclear testing and to predict expected future performance as a result of intended or unintended modifications. In order to construct more reliable models, underground nuclear test data is being reanalyzed in novel ways. To improve the interpretation of these experiments with quantified uncertainties, improved nuclear data is required. As an example, the thermonuclear yield of a device was often inferred through the use of radiochemical detectors. Conversion of the detector activations observed to thermonuclear yield was accomplished through explosion code calculations (models) and a good set of nuclear reaction cross-sections. Because of the unique high-fluence environment of an exploding nuclear weapon, many reactions occurred on radioactive nuclides, for which only theoretically calculated cross-sections are available. Surrogate nuclear reactions at STARS/LIBERACE offer the opportunity to measure cross-sections on unstable nuclei and thus improve the quality of the nuclear reaction cross-section sets. One radiochemical detector that was loaded in devices was mono-isotopic yttrium ({sup 89}Y). Nuclear reactions produced {sup 87}Y and {sup 88}Y which could be quantified post-shot as a ratio more » of {sup 87}Y/{sup 88}Y. The yttrium cross-section set from 1988 is shown in Figure 1(a) and contains approximately 62 cross-sections interconnecting the yttrium nuclides. The 6 experimentally measured cross-sections are shown in Figure 1(b). Any measurement of cross-sections on {sup 87}Y or {sup 88}Y would improve the quality of the cross-section set. A recent re-evaluation of the yttrium cross-section set was performed with many more calculated reaction cross-sections included. « less

The first demonstration of isotope-specific detection of a low-Z, low density object, shielded by a high-Z and high density material using mono-energetic gamma-rays is reported. Isotope-specific detection of LiH shielded by Pb and Al is accomplished using the nuclear resonance fluorescence line of {sup 7}Li at 0.478 MeV. Resonant photons are produced via laser-based Compton scattering. The detection techniques are general and the confidence level obtained is shown to be superior to that yielded by conventional x-ray/{gamma}-ray techniques in these situations.

A Compton scattering {gamma}-ray source, capable of producing photons with energies ranging from 0.1 MeV to 0.9 MeV has been commissioned and characterized, and then used to perform nuclear resonance fluorescence (NRF) experiments. The performances of the two laser systems (one for electron production, one for scattering), the electron photoinjector, and the linear accelerator are also detailed, and {gamma}-ray results are presented. The key source parameters are the size (0.01 mm{sup 2}), horizontal and vertical divergence (6 x 10 mrad{sup 2}), duration (10 ps), spectrum and intensity (10{sup 5} photons/shot). These parameters are summarized by the peak brightness, 1.5 x 10{sup 15} photons/mm{sup 2}/mrad{sup 2}/s/0.1% bandwidth, measured at 478 keV. Additional measurements of the flux as a function of the timing difference between the drive laser pulse and the relativistic photo-electron bunch, {gamma}-ray beam profile, and background evaluations are presented. These results are systematically compared to theoretical models and computer simulations. NRF measurements performed on {sup 7}Li in LiH demonstrate the potential of Compton scattering photon sources to accurately detect isotopes in situ.

Nuclear Resonance Fluorescence measurements were performed at the free electron laser facility at UC Santa Barbara using a bremsstrahlung beam. Three endpoint energies were chosen for the bremsstrahlung to cover as much area above 2.5 MeV as possible. We were able to set an upper limit of NRF state strengths between 2.5 and 3.8 MeV at roughly 38(5) eV barns at the 4-sigma level and 9(2) eV barns at the 1-sigma level. Published results on states near 2.4 MeV indicate strengths about 10(2) eV barns. Details of the results are presented in this report.

We describe a novel laser system used in a Compton-scattering based monoenergetic gamma-ray source, commissioned at LLNL. This system consists of a UV laser for electron production and a high power CPA based scattering laser.

Homeland Security programs are developing systems that use nuclear resonance fluorescence (NRF) to isotopically map a container. One such system being developed at LLNL is FINDER (Fluorescence Imaging in the Nuclear Domain with Extreme Radiation). The proposed FINDER system works by impinging a tunable monoenergetic gamma ray beam onto a container under investigation. The photons pass through the container and a fraction of them scatter off of the interior components through various electromagnetic processes. One of these processes is NRF. At specific resonance energies, incident photons interact directly with the nuclei of special nuclear material (SNM) or other materials in the container. The incident beam is absorbed and scattered into all directions, depleting the spectrum at the resonant energy. The transmitted gamma ray beam accrues a notch a few eV in width after passing through the material of interest. This notched spectrum will impinge on a witness foil placed on the opposite side of the container relative to the gamma ray source. The witness foil will be made of material identical to the one being sought after. If there is a notch in the spectrum then there will be no NRF photons scattered from the witness foil. The corollary ismore » that if there is no notch in the transmitted spectrum then there will be NRF photons scattered from the witness foil. A simple arrangement of gamma-ray detectors focused on the witness foil, are used to measure the NRF photons. If the detectors see NRF scatter then there was no NRF scatter within the container; therefore, no material of the nature being sought after was in that container. Conversely, if there was no NRF scatter from the witness foil, then the NRF scatter took place from within the container; therefore, the material of interest is inside of the container. Recently, initial feasibility tests of FINDER was performed at the HIgS (High-Intensity Gamma Source) located at Duke University [1]. The preliminary results of these tests are discussed in this report. Our goals for these measurements were to demonstrate the concept of transmission detection and perform some initial validation of models of the FINDER concept. In particular, our models [2] indicated that backgrounds and nuisances are too small to obscure the high signal to noise of the FINDER technique. Therefore, FINDER offers extremely clear positive and negative signals for detecting SNM when measuring the high contrast attenuation of on-resonance gamma rays in transmission. Our initial demonstration of transmission detection provides a first check on our models--e.g. is there a physical process that we have forgotten to include? Also, previous work indicated that the notch could be obscured by small-angle scattering refilling the notch if the interrogating photon source is too broad in energy. While we did not expect notch refilling to be significant at HIgS which provides beam of gamma rays with {Delta}E/E {approx} 3%, we were able to set an experimental upper limit on the amount of notch refilling in our experimental set up. This report first describes the experimental setup for these measurements, gives a summary of the important results, and then concludes by discussing the extent to which we were able to validate our models [2] of the FINDER technique.« less