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At least 847 records · Page 47
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

THE THREE-DIMENSIONAL EVOLUTION TO CORE COLLAPSE OF A MASSIVE STAR

We present the first three-dimensional (3D) simulation of the final minutes of iron core growth in a massive star, up to and including the point of core gravitational instability and collapse. We capture the development of strong convection driven by violent Si burning in the shell surrounding the iron core. This convective burning builds the iron core to its critical mass and collapse ensues, driven by electron capture and photodisintegration. The non-spherical structure and motion generated by 3D convection is substantial at the point of collapse, with convective speeds of several hundreds of km s{sup −1}. We examine the impact of such physically realistic 3D initial conditions on the core-collapse supernova mechanism using 3D simulations including multispecies neutrino leakage and find that the enhanced post-shock turbulence resulting from 3D progenitor structure aids successful explosions. We conclude that non-spherical progenitor structure should not be ignored, and should have a significant and favorable impact on the likelihood for neutrino-driven explosions. In order to make simulating the 3D collapse of an iron core feasible, we were forced to make approximations to the nuclear network making this effort only a first step toward accurate, self-consistent 3D stellar evolution models of the end states of massive stars.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Neutron single-particle strengths at N = 40 , 42: Neutron knockout from Ni 68 , 70 ground and isomeric states

The distribution of single-particle strength in 67,69 Ni was characterized with one-neutron knockout reactions from intermediate-energy 68,70 Ni secondary beams, selectively populating neutron-hole configurations at N = 39 and 41, respectively. The spectroscopic strengths deduced from the measured partial cross sections to the individual final states, as tagged by their γ-ray decays, is used to identify and quantify neutron configurations in the wave functions. While 69 Ni compares well to shell-model predictions, the results for 67 Ni challenge the validity of current effective shell-model Hamiltonians by revealing discrepancies that cannot be explained so far. Furthermore, these results suggest that our understanding of the low-lying states in the neutron-rich, semi-magic Ni isotopes may be incomplete and requires further investigation on both the experimental and theoretical sides.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Determination of the spins and parities for the $0^+_4$ and $0^+_5$ states in $^{100}Zr$

Two 0 + states at 1294.5 and 1774.0 keV, together with three 2 + and one 4 + levels, were identified or unambiguously spin-parity assigned for the first time in 100 Zr utilizing γ-ray spectroscopy and γ-γ angular correlation techniques with the Gammasphere spectrometer, following the β - decay of neutron-rich, mass separated 100,100m Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around ≈1500 keV. According to the measured relative B(E2) relative transition probabilities, the 0+ 5 state exhibits decay properties which more closely align with those predicted for a spherical shape, while the $0^+_4$ level is suggested to be associated with a weakly-deformed shape similar to one related to the $0^+_2$ state.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Determination of the spins and parities for the $0$$^{+}_{4}$ and $0$$^{+}_{5}$ states in 100 Zr

Two 0 + states at 1294.5 and 1774.0 keV, together with three 2 + and one 4 + levels, were identified or unambiguously spin-parity assigned for the first time in 100 Zr utilizing γ-ray spectroscopy and γ-γ angular correlation techniques with the Gammasphere spectrometer, following the β¯ decay of neutron-rich, mass separated 100,100m Y isotopes. Comparisons with recent Monte Carlo Shell-Model (MCSM) calculations indicate that these two states are candidates for the bandhead of a sequence in a shape-coexisting spherical minimum predicted to be located around ≈1500 keV. According to the measured relative B(E2) relative transition probabilities, the $0$$^{+}_{5}$ state exhibits decay properties which more closely align with those predicted for a spherical shape, while the $0$$^{+}_{4}$ level is suggested to be associated with a weakly-deformed shape similar to one related to the $0$$^{+}_{2}$ state.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

PS1-14bj: a hydrogen-poor superluminous supernova with a long rise and slow decay

We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor superluminous supernova (SLSN) at redshift z = 0.5215 discovered in the last months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out because of its extremely slow evolution, with an observed rise of ≳125 rest-frame days, and exponential decline out to ∼250 days past peak at a measured rate of 0.01 mag day{sup −1}, consistent with fully trapped {sup 56}Co decay. This is the longest rise time measured in an SLSN to date, and the first SLSN to show a rise time consistent with pair-instability supernova (PISN) models. Compared to other slowly evolving SLSNe, it is spectroscopically similar to the prototype SN 2007bi at maximum light, although lower in luminosity (L{sub peak}≃4.6×10{sup 43} erg s{sup −1}) and with a flatter peak than previous events. PS1-14bj shows a number of peculiar properties, including a near-constant color temperature for >200 days past peak, and strong emission lines from [O iii] λ5007 and [O iii] λ4363 with a velocity width of ∼3400 km s{sup −1} in its late-time spectra. These both suggest there is a sustained source of heating over very long timescales, and are incompatible with a simple {sup 56}Ni-powered/PISN interpretation. A modified magnetar model including emission leakage at late times can reproduce the light curve, in which case the blue continuum and [O iii] features are interpreted as material heated and ionized by the inner pulsar wind nebula becoming visible at late times. Alternatively, the late-time heating could be due to interaction with a shell of H-poor circumstellar material.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

Microstructural heterogeneity of the buffer layer of TRISO nuclear fuel particles

Tristructural isotropic (TRISO) nuclear fuel particles contain a layered spherical shell designed to retain fission products; however, failure occurs in rare cases—commonly initiated in the porous pyrocarbon buffer layer. Achieving a comprehensive understanding of the buffer-initiated failure mechanisms requires detailed characterization of the buffer porosity and its heterogeneous distribution across multiple length scales. Here we performed FIB-SEM tomography across the buffer layer thickness (~100 µm) to produce 3D reconstructions of the buffer microstructure with 50 nm spatial resolution. We found an average overall porosity of ~14%, which does not solely account for the low density of the buffer (50% of the theoretical density). Additionally, the local porosity and its fluctuation increase from the kernel interface towards the inner pyrocarbon (IPyC) layer, which we attribute to the chemical vapor deposition process conditions during the TRISO particle fabrication. Detailed characterization of the porous microstructure—including analysis of the pore size, distribution, shape, and orientation—provides insight into the process-structure-property-performance relations of TRISO nuclear fuel particles and will inform multiscale models designed to predict the failure of TRISO particles under irradiation.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2020

Clustering and α-capture reaction rate from ab initio 20 Ne

We introduce a new framework for studying clustering and for calculating α partial widths using ab initio wave functions. We demonstrate the formalism for 20 Ne, by calculating the overlap between the 16 O + α cluster configuration and states in 20 Ne computed in the ab initio symmetry-adapted no-core shell model. We present spectroscopic amplitudes and spectroscopic factors, and compare those to no-core symplectic shell-model results in larger model spaces, to gain insight into the underlying physics that drives α clustering. Specifically, we report on the α partial width of the lowest 1 – resonance in 20 Ne, which is found to be in good agreement with experiment.We also present first no-core shell-model estimates for asymptotic normalization coefficients for the ground state, as well as for the first excited 4 + state in 20 Ne that lies in a close proximity to the α + 16 O threshold. This outcome highlights the importance of correlations for developing cluster structures and for describing α widths. The widths can then be used to calculate α-capture reaction rates for narrow resonances of interest to astrophysics. Finally, we explore the reaction rate for the α-capture reaction 16 O(α, γ ) 20 Ne at astrophysically relevant temperatures and determine its impact on simulated x-ray burst abundances.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Experimental study of the S 38 excited level scheme

Information on the 38 S level scheme was expanded through experimental work utilizing a fusion-evaporation reaction and in-beam γ-ray spectroscopy. Prompt γ-ray transitions were detected by the Gamma-Ray Energy Tracking Array (GRETINA) and recoiling 38 S residues were selected by the Fragment Mass Analyzer (FMA). Tools based on machine-learning techniques were developed and deployed for the first time in order to enhance the unique selection of 38S residues and identify any associated γ-ray transitions. The new level information, including the extension of the even-spin yrast sequence through J π =8( + ), was interpreted in terms of a basic single-particle picture as well shell-model calculations which incorporated the empirically derived FSU interaction. A comparison between the properties of the yrast states in the even-Z, N = 22 isotones from Z = 14 to 20, and for 36 Si– 38 S in particular, was also presented with an emphasis on the role and influence of the neutron 1 p 3/2 orbital on the structure in the region.

20 ≤ A ≤ 38↗
DOE OSTI2013

Review Of Rheology Models For Hanford Waste Blending

The area of rheological property prediction was identified as a technology need in the Hanford Tank Waste – waste feed acceptance initiative area during a series of technical meetings among the national laboratories, Department of Energy-Office of River Protection, and Hanford site contractors. Meacham et al. delivered a technical report in June 2012, RPP-RPT-51652 “One System Evaluation of Waste Transferred to the Waste Treatment Plant” that included estimating of single shell tank waste Bingham plastic rheological model constants along with a discussion of the issues inherent in predicting the rheological properties of blended wastes. This report was selected as the basis for moving forward during the technical meetings. The report does not provide an equation for predicting rheological properties of blended waste slurries. The attached technical report gives an independent review of the provided Hanford rheological data, Hanford rheological models for single tank wastes, and Hanford rheology after blending provided in the Meacham report. The attached report also compares Hanford to SRS waste rheology and discusses some SRS rheological model equations for single tank wastes, as well as discussing SRS experience with the blending of waste sludges with aqueous material, other waste sludges, and frit slurries.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2024

Ab initio investigation of the Li 7 ( p , e + e - ) Be 8 process and the X17 boson

Observations of anomalies in the electron-positron angular correlations in high-energy decays in 4 He, 8 Be, and 12 C have been reported recently by the ATOMKI collaboration. These could be explained by the creation and subsequent decay of a new boson with a mass of ≈ 17MeV. Theoretical understanding of pair creation in the proton capture reactions used in these experiments is important for the interpretation of the anomalies. We apply the ab initio no-core shell model with continuum (NCSMC) to the proton capture on 7 Li. The NCSMC describes both bound and unbound states in light nuclei in a unified way with chiral two- and three-nucleon interactions as the only input. We investigate the structure of 8 Be, the p+ 7 Li elastic scattering, the 7 Li(p,y)⁢ 8 Be cross section, and the internal pair creation 7 Li ⁢(p,e + ⁢e - ) 8 Be. Here we discuss the impact of a proper treatment of the initial scattering state on the electron-positron angular correlation spectrum and compare our results to available ATOMKI data sets. Finally, we calculate 7 Li ⁢(p,X)⁢ 8 Be cross sections for several proposed models of the hypothetical X17 particle.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Ab initio no-core Gamow shell model calculations with realistic interactions

No-Core Gamow Shell Model (NCGSM) is applied for the first time to study selected well-bound and unbound states of helium isotopes. This model is formulated on the complex energy plane and, by using a complete Berggren ensemble, treats bound, resonant, and scattering states on equal footing. We use the Density Matrix Renormalization Group method to solve the many-body Schrödinger equation. To test the validity of our approach, we benchmarked the NCGSM results against Faddeev and Faddeev-Yakubovsky exact calculations for 3 H and 4 He nuclei. In this paper, we also performed ab initio NCGSM calculations for the unstable nucleus 5 He and determined the ground state energy and decay width, starting from a realistic N 3 LO chiral interaction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Low irradiance multiphoton imaging with alloyed lanthanide nanocrystals

Abstract Multiphoton imaging techniques that convert low-energy excitation to higher energy emission are widely used to improve signal over background, reduce scatter, and limit photodamage. Lanthanide-doped upconverting nanoparticles (UCNPs) are among the most efficient multiphoton probes, but even UCNPs with optimized lanthanide dopant levels require laser intensities that may be problematic. Here, we develop protein-sized, alloyed UCNPs (aUCNPs) that can be imaged individually at laser intensities >300-fold lower than needed for comparably sized doped UCNPs. Using single UCNP characterization and kinetic modeling, we find that addition of inert shells changes optimal lanthanide content from Yb 3+ , Er 3+ -doped NaYF 4 nanocrystals to fully alloyed compositions. At high levels, emitter Er 3+ ions can adopt a second role to enhance aUCNP absorption cross-section by desaturating sensitizer Yb 3+ or by absorbing photons directly. Core/shell aUCNPs 12 nm in total diameter can be imaged through deep tissue in live mice using a laser intensity of 0.1 W cm −2 .

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2018

Updated β -decay measurement of neutron-rich Cu 74

Here, the β decay of neutron-rich 74 Cu has been studied at the Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory. By using a high-resolution mass separator a purified 74 Cu beam was obtained, thus allowing decay through its isobar chain to stable 74 Ge without any decay chain member dominating. A total of 170γ rays were associated with 74 Cuβ decay with 111 placed in the 74 Zn level scheme. Updated β feeding intensities and estimated log(ft) values are presented, and new J π assignments are proposed using shell model calculations. The progression of simulated total absorption γ-ray spectroscopy (TAGS) based on proposed levels and β-feeding values from previous measurements to this evaluation are presented and demonstrate the need for a TAGS measurements for this and similar decays.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Reexamining Gamow-Teller decays near Ni 78

Decays of neutron-rich nuclei 82,83 Zn and 82,83 Ga produced in proton-induced fission of 238 U were studied at the Holifield Radioactive Ion Beam Facility using on-line mass separation and β–γ spectroscopy techniques. New γ-ray transitions were identified and level schemes, which include states at high excitation energies in the range between 3–7 MeV were constructed. These high-energy levels were identified to be populated through allowed Gamow-Teller β transitions, and their structure was interpreted with new shell-model calculations. Furthermore, a β-delayed neutron branching ratio of 69±7% was deduced for 82 Zn and revised β-decay half-life values of 82 Zn [155(17)(20) ms] and 83 Zn [122(28) ms] were determined.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

CURRENT STATUS OF THE CHARACTERIZATION OF RPV MATERIALS HARVESTED FROM THE DECOMMISSIONED ZION UNIT 1 NUCLEAR POWER PLANT

The decommissioning of Units 1 and 2 of the Zion Nuclear Power Station in Zion, Illinois, after ~ 15 effective full-power years of service presents a unique opportunity to characterize the degradation of in-service reactor pressure vessel (RPV) materials and to assess currently available models for predicting radiation embrittlement of RPV steels [1-3]. Moreover, through-wall thickness attenuation and property distributions are being obtained and the results to be compared with surveillance specimen test data. It is anticipated that these efforts will provide a better understanding of materials degradation associated with extending the lifetime of existing nuclear power plants (NPPs) beyond 60 years of service and subsequent license renewal. In support of extended service and current operations of the US nuclear reactor fleet, the Oak Ridge National Laboratory (ORNL), through the U.S. Department of Energy, Light Water Reactor Sustainability (LWRS) Program, coordinated procurement of materials, components, and other items of interest from the decommissioned Zion NPPs. In this report, harvesting, cutting sample blocks, machining test specimens, test plans, and the current status of materials characterization of the RPV from the decommissioned Zion NPP Unit 1 will be discussed. The primary foci are the circumferential, Linde 80 flux, wire heat 72105 (WF-70) beltline weld and the A533B base metal from the intermediate shell harvested from a region of peak fluence (0.7 X 1019 n/cm2, E > 1.0 MeV) on the internal surface of the Zion Unit 1 vessel. Following the determination of the through-thickness chemical composition, Charpy impact, fracture toughness, tensile, and hardness testing are being performed to characterize the through-thickness mechanical properties of base metal and beltline-weld materials. In addition to mechanical properties, microstructural characterizations are being performed using various microstructural techniques, including Atom Probe Tomography, Small Angle Neutron Scattering, and Positron Annihilation Spectroscopy

Rosseel, T. M.↗
DOE OSTI2016

Low-energy enhancement in the γ -ray strength functions of Ge 73 , 74

Here, t≤he γ-ray strength functions and level densities of 73,74 Ge have been extracted up to the neutron-separation energy S n from particle-γ coincidence data using the Oslo method. Moreover, the γ-ray strength function of 74 Ge above S n has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ ≈ 1–13 MeV for 74 Ge. The obtained data show that both 73,74 Ge display an increase in strength at low γ energies. The experimental γ-ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in 73,74 Ge are adopted in the calculations of the 72,73 Ge(n,γ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2017

Neutron-unbound excited states of N 23

Neutron unbound states in 23 N were populated via proton knockout from an 83.4 MeV/nucleon 24 O beam on a liquid deuterium target. The two-body decay energy displays two peaks at E 1 ~ 100 keV and E 2 ~ 1MeV with respect to the neutron separation energy. The data are consistent with shell model calculations predicting resonances at excitation energies of ~3.6MeV and ~4.5MeV. The selectivity of the reaction implies that these states correspond to the first and second 3/2 - states. Furthermore, the energy of the first state is about 1.3 MeV lower than the first excited 2 + in 24 O. This decrease is largely due to coupling with the πp $-1\atop{3/2}$ hole along with a small reduction of the N = 16 shell gap in 2 3N.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Applying Hanford Tank Mixing Data to Define Pulse Jet Mixer Operation

Pulse jet mixed (PJM) process vessels are being developed for storing, blending, and chemical processing of nuclear waste slurries at the Waste Treatment and Immobilization Plant (WTP) to be built at Hanford, Washington. These waste slurries exhibit variable process feed characteristics including Newtonian to non-Newtonian rheologies over a range of solids loadings. Waste feed to the WTP from the Hanford Tank Farms will be accomplished via the Waste Feed Delivery (WFD) system which includes million-gallon underground storage double-shell tanks (DSTs) with dual-opposed jet mixer pumps. Experience using WFD type jet mixer pumps to mobilize actual Hanford waste in DSTs may be used to establish design threshold criteria of interest to pulse jet mixed process vessel operation. This paper describes a method to evaluate the pulse jet mixed vessel capability to process waste based on information obtained during mobilizing and suspending waste by the WFD system jet mixer pumps in a DST. Calculations of jet velocity and wall shear stress in a specific pulse jet mixed process vessel were performed using a commercial computational fluid dynamics (CFD) code. The CFD-modelled process vessel consists of a 4.9-m- (16-ft-) diameter tank with a 2:1 semi-elliptical head, a single, 10-cm (4-in.) downward facing 60-degree conical nozzle, and a 0.61-m (24-in.) inside diameter PJM. The PJM is located at 70% of the vessel radius with the nozzle stand-off-distance 14 cm (6 in.) above the vessel head. The CFD modeled fluid velocity and wall shear stress can be used to estimate vessel waste-processing performance by comparison to available actual WFD system process data. Test data from the operation of jet mixer pumps in the 23-m (75-ft) diameter DSTs have demonstrated mobilization, solid particles in a sediment matrix were moved from their initial location, and suspension, mobilized solid particles were moved to a higher elevation in the vessel than their initial location, of waste solids. Jet mixer pumps were used in Hanford waste tank 241-AZ-101, and at least 95% of the 0.46-m (18-in.) deep sediment, with a shear strength of 1,500 to 4,200 Pa, was mobilized. Solids with a median particle size of 43 μm, 90th percentile of 94μm, were suspended in tank 241-AZ-101 to at least 5.5 m (216 in.) above the vessel bottom. Analytical calculations for this jet mixer pump test were used to estimate the velocities and wall shear stress that mobilized and suspended the waste. These velocities and wall shear stresses provide design threshold criteria which are metrics for system performance that can be evaluated via testing. If the fluid motion in a specific pulse jet mixed process vessel meets or exceeds the fluid motion of the demonstrated performance in the WFD system, confidence is provided that that vessel will similarly mobilize and suspend those solids if they were within the WTP. The single PJM CFD-calculated jet velocity and wall shear stress compare favorably with the design threshold criterion estimated for the tank 241-AZ-101 process data. Therefore, for both mobilization and suspension, the performance data evaluated from the WFD system testing increases confidence that the performance of the pulse jet mixed process vessels will be sufficient to process that waste even if that waste is not fully characterized.

mixing' slurry↗