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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 91 records · Page 5
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Dirac $R$-matrix calculations for the electron-impact excitation of neutral tungsten providing noninvasive diagnostics for magnetic confinement fusion

Neutral tungsten is the primary candidate as a wall material in the divertor region of the International Thermonuclear Experimental Reactor (ITER). The efficient operation of ITER depends heavily on precise atomic physics calculations for the determination of reliable erosion diagnostics, helping to characterize the influx of tungsten impurities into the core plasma. The following paper presents detailed calculations of the atomic structure of neutral tungsten using the multiconfigurational Dirac-Fock method, drawing comparisons with experimental measurements where available, and includes a critical assessment of existing atomic structure data. We investigate the electron-impact excitation of neutral tungsten using the Dirac R-matrix method, and by employing collisional-radiative models, we benchmark our results with recent Compact Toroidal Hybrid measurements. The resulting comparisons highlight alternative diagnostic lines to the widely used 400.88-nm line.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2022

239 Pu R -matrix Analysis and Neutron Multiplicities in the Neutron Energy Region up to a few keVs [Abstract]

The evaluation of 239 Pu neutron resonance parameters coupled to neutron multiplicities $\overline{v}_p$ is of particular importance to investigate the ($\mathcal{n, γf}$) reaction in which a $\mathcal{γ}$-ray emission occurs before the scission of the compound nuclear. This reaction has offered one explanation for the fluctuations in the measured values of $\overline{v}_p$ In this regard, the competition between ($\mathcal{n, γf}$) reaction and the direct fission process can be also included in the R matrix analysis of fission and capture measured data. The goal of this work is the coupled evaluation of the $\mathcal{n}$+ 239 Pu resonance parameters and related neutron multiplicities by ensuring the adoption of thermal neutron constants recently evaluated at the International Atomic Nuclear Energy as well as the recommended (thermal-neutron) induced prompt neutron fission spectrum (PFNS). Moreover, this new set of physical evaluated quantities should also guarantee the agreement for high-leakage solution benchmarks while keeping the good performance of large thermal solution assemblies.

07 ISOTOPE AND RADIATION SOURCES↗
DOE OSTI2019

s -wave scattering lengths for the 7 $\hbox{Be} +$ p system from an R -matrix analysis

The astrophysical S factor for the radiative proton capture reaction on Be 7 ( S 17 ) at low energies is affected by the s -wave scattering lengths. Herein, we report the measurement of elastic and inelastic scattering cross sections for the Be 7 + p system in the center-of-mass energy range 0.474–2.740 MeV and center-of-mass angular range 70 ° – 150 ° . A radioactive Be 7 beam produced at Oak Ridge National Laboratory's (ORNL) Holifield Radioactive Ion Beam Facility was accelerated and bombarded a thin polypropylene ( CH 2 ) n target. Scattered ions were detected in the segmented Silicon Detector Array. Using an R -matrix analysis of ORNL and Louvain-la-Neuve cross-section data, the s -wave scattering lengths for channel spins 1 and 2 were determined to be 17 . 34 – 1.33 + 1.11 and – 3 . 18 – 0.50 + 0.55 fm, respectively. The uncertainty in the s -wave scattering lengths reported in this work is smaller by a factor of 5–8 compared to the previous measurement, which may reduce the overall uncertainty in S 17 at zero energy. The level structure of B 8 is discussed based upon the results from this work. Evidence for the existence of 0 + and 2 + levels in B 8 at 1.9 and 2.21 MeV, respectively, is observed.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2024

Indirect reactions and connection with R-matrix theory

It is often the case that nuclear reactions that are important for societal applications or basic science are difficult to measure directly in existing facilities for accelerated beams. The difficulty might be associated with an exceedingly small cross section, as the ones obtained at beam energies well below the Coulomb barrier, or with the impossibility of devising a short-lived target, as is the case for neutron-induced reactions on unstable isotopes. The fruitful line of experimental research addressing this issue with alternative (indirect) reactions has been developed in parallel to the theory needed to make the connection between the observed data and the desired cross sections of the (direct) reaction under study. Within this context, we present here an explicit connection between the indirect cross section and the R-matrix parameters which describe the energy-dependent cross section of the desired direct reaction.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2020

Complex radius in the R-matrix algorithm for inclusion of direct capture component [Slides]

In the low-energy range there are two possible physical mechanisms for γ-ray emissions. While both mechanisms emit γ-rays, the compound radiative capture (CRC) process is generated by the formation of a compound nucleus state before γ-emission. CRC has a characteristic resonance structure in the cross section, The direct radiative capture (DRC) process goes to the final state by the electromagnetic radiation of the incident neutron. DRC has a smooth behavior in the cross section. For light nuclei, a non-negligible component of the neutron capture process can be often associated to DRC process. Although these two mechanisms should be described by a unified theoretical formalism, in the nuclear data libraries, the CRC and DRC components are evaluated and reported separately. The updated R-matrix algorithm will allow the ability to implicitly include correlations between CRC and DRC mechanisms, to improve the quality of nuclear data evaluations, and to facilitate simple large-scale applicability to current nuclear data evaluations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2022

A Dirac R-matrix calculation for the electron-impact excitation of W +

Abstract Aims : tungsten has been chosen for use as a plasma facing component in the divertor for the ITER experiment, and is currently being used on existing tokamaks such as JET. W + plays an integral role in assessing the impurity influx from plasma facing component of tokamaks and subsequent redeposition. Together with previously calculated a neutral tungsten electron-impact dataset this study allows us to determine neighbouring spectral lines in the same wavelength window of the spectrometer, and detect if there is strong blending of overlapping lines between these two ion stages as well as providing ionisation per photon ratios for both species. The new data is to be used for tungsten erosion/redeposition diagnostics. Methods : a significantly modified version of the GRASP0 atomic structure code in conjunction with DARC (Dirac Atomic R -matrix Code) are used to calculate the Einstein A coefficients and collisional rates used to generate a synthetic W II spectrum. The W II spectrum is compared against tungsten spectral emission experiments. Results : this study is used to model the spectrum of W ii , providing the predictive capability of identifying spectral lines from recent experiments. These results provide an integral part of impurity influx and redeposition determination, as the ionisation rates may be used to calculate S/XB ratios.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2010

Reply to 'Comment on 'Dirac R-matrix method for the calculation of x-ray line polarization''

In response to the Comment written by Zhang et al. [Phys. Rev. A 82, 036701 (2010)] we show that the main ideas and results (about the physics and method) reported in our paper [Phys. Rev. A 79, 062715 (2009)] still appear to be correct. Although none of the conjectures given in the Comment about the potential problems in our code are correct, there was a small error in our code, so the part of our paper with numerical data, presented to support our method, appears to be incorrect. The wealth of resonance features in the x-ray line 3D demonstrated in our paper still appears to be real and is reconfirmed by a revised calculation with the error fixed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2011

R-matrix approach to collision processes: Study of the threshold behavior of {gamma}+He {yields} He{sup +}(2s)+e{sup -} or He{sup +}(2p)+e{sup -}

We report on a study of one-photon single ionization of He(1s{sup 2}) accompanied by excitation to He{sup +}(2s) or He{sup +}(2p) for photon energies at, and up to a few eV above, the excitation threshold. We find, both numerically and analytically, that as the photon energy approaches threshold from above the photoionization cross section turns sharply at about 200 meV and has a cusp at threshold. The cusp is associated with the adiabatic transfer of population between the 2s and 2p bound states of the ion. The photoelectron's electric field induces an energy splitting between the otherwise degenerate 2s and 2p states, and consequently the 2s and 2p populations oscillate with a period defined by this energy splitting. The number of oscillations is almost independent of the photoelectron's asymptotic speed v since over most of its outward journey the photoelectron moves with a speed much larger than v. However, on the last leg of its journey, after it has slowed down sufficiently in the Coulomb field of the ion, the photoelectron does move with an almost constant speed v. On this last leg the energy splitting is so small that there is insufficient time for 2s and 2p populations to undergo a full oscillation; rather, the (equal and opposite) changes in the 2s and 2p populations are proportional to v (i.e., to the square root of the excess photoelectron energy, which gives rise to a cusp).

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗