M. A. Gardner

Theoretical determination of bound–free absorption cross sections in Ar+2

Ab initio calculations have been carried out for the potential energy curves and transition moments of the 2Σ+u, 2Πg, 2Πu, and 2Σ+g states of Ar+2 which arise from the 2P+1S ion–atom asymptote. These data have been used in a theoretical calculation of the dissociative absorption cross sections from the bound 2Σ+u state to the repulsive 2Πg and 2Σ+g states. The 2Σ+u→2Πg transition, which is dominated by spin–orbit effects, has a maximum absorption cross section of 2.6×10−19 cm2 centered at 716 nm with a full width at half‐maximum of 185 nm at room temperature. The 2Σ+u→2Σ+g transition is found to be much stronger with a maximum cross section of 0.5×10−16 cm2 centered at 300 nm with a full width at half‐maximum of 75 nm at room temperature.

Mechanism for negative‐ion production in the surface‐plasma negative‐hydrogen‐ion source

The system parameters and surface adsorption conditions of the surface‐plasma negative‐hydrogen‐ion source are reviewed. A mechanism is developed for the production of negative ions by incident energetic hydrogen atoms backscattering from a cesiated tungsten surface. The active electronic level during the course of the collision is approximated as the sum of the negative electron affinity of the cesium hydride negative molecular ion and the image potential. The model predicts negative‐ion formation for atomic collisions with all alkali‐coated surfaces for alkalis from sodium through cesium. In the case of lithium, the model does not apply due to the breakdown of the image potential approximation close to the surface. Negative‐ion formation is also expected to occur for incident positive ions which backscatter from the substrate tungsten as neutrals. The energy thresholds for incident atoms are established.

Abinitio MC–SCF ground‐state potential energy curves for LiH−, NaH−, and CsH−

We have evaluated the 1Σ+ ground states of LiH, NaH, and CsH and the 2Σ+ ground states of LiH−, NaH−, and CsH−, as well as the 2Σ+ ground state of CsH+, over a wide range of internuclear distances. Multiconfiguration‐self‐consistent‐field wavefunctions were obtained with the optimized valence configuration approach to the description of chemical bonding. Four configurations for the neutral molecules and seven for the negative ions provided satisfactory descriptions. All of the negative‐molecular‐ion ground states are attractive and have lower potential energies than the neutral‐parent‐hydride molecules over the internuclear distances studied. Molecular electron affinities at the equilibrium internuclear separations were found to be 0.283 eV (LiH), 0.278 eV (NaH), and 0.357 eV (CsH).

The impact of calculated photon-induced isomer production in 176Lu on its use as a stellar chronometer and/or thermometer

The authors have calculated the production and destruction of the 3.7-h, 123-keV isomeric state of 176Lu, by photons as well as by neutrons. They used absolute photon strength functions and discrete nuclear level sets for 175-177Lu. Their results for the photon-induced production of 176mLu are remarkably different from the experimental measurements in the current literature. They calculate much larger cross sections for this reaction, and a lower threshold than had previously been assumed. These two effects combine to indicate an enormous correction, a factor of 107, needs be applied to shorten the current estimate of the half-life against photoexcitation of 176mLu as a function of temperature.

Structure in the total 27Al(n, α)24Na cross section around 14 MeV☆

Abstract An excitation function for the 27 Al(n, α) reaction has been measured in steps of about 15 keV throughout most of the energy range between 13.45 and 14.91 MeV in an effort to observe structure in the cross section. The insulated core transformer (ICT) neutron facility was used as the neutron source; a rotating, tritiated titanium target, capable of handling ion beams with currents up to 10 mA and with energies up to 450 keV, was employed. Using the activation method, a total of about 500 relative cross-section measurements were made at 87 different neutron energies. Fluctuations with magnitudes of about 1.5 % were observed in the cross section after all of the data had been correlated and corrected for the anisotropy and scattering effects that had been determined experimentally. A statistical treatment of the data yielded a compound nucleus level width of about 30 keV, which is reasonable for a 28 Al nucleus excited to 20–22 MeV. The observed structure can be explained in terms of Ericson fluctuation theory.

Isomer ratio calculations using modeled discrete levels

We have calculated isomer ratios for the 175Lu(n,γ), 175Lu(n,2n), 237Np(n,2n), 241Am(n,γ), and 243Am(n,γ) reactions using modeled level structures in the deformed, odd‐odd product nuclei. We find: that the hundreds of discrete levels and their gamma‐ray branching ratios provided by the modeling are necessary to achieve agreement with experiment, that many rotational bands must be included in order to obtain a sufficiently reprsentative selection of K quantum numbers, and that the levels of each band must be extended to apropriately high values of angular momentum.

Intrinsic excitations in 192Ir

The structure of the odd‐odd 192Ir nucleus presents an interesting and challenging problem for both experimentalists and theorists. As a result of the common efforts of nine laboratories, it is possible, for the first time, to propose an extended scheme with 34 levels. The experiments included observation of γ‐ray transitions and of conversion electrons emitted after thermal and resonance neutron capture, of direct (d,p) and (d,t) neutron transfer reactions and of the angular distribution of γ‐rays from aligned 192Ir nuclei. The results are interpreted in the framework of the rotor‐plus‐particle and IBFFM models. The nuclear states appear to be strongly mixed. The complex and interesting ground‐state configuration is discussed.

Calculational tools for the evaluation of nuclear cross-section and spectra data

Abstract Continued improvements in the calculational methods for obtaining nuclear reaction cross sections and spectra provide indispensable tools for nuclear data evaluations.

Absolute dipole gamma‐ray strength functions for 176Lu

We have derived absolute dipole strength‐function information for 176Lu from an average resonance capture study of 175Lu with 2‐keV neutrons, and from neutron capture cross‐section measurements with neutrons from 30 keV to about 1 MeV. We found that we needed to increase our previous estimate of the relative m1/E1 strengths near 5 MeV by a factor of 3, and to revise downward the absolute magnitude of our E1 strength function. We accomplished the latter, while still maintaining continuity with the photonuclear data, by adjusting the one free parameter in our line shape. The present E1 and M1 strengths now seem correct both near the neutron separation energy and also around 1 MeV.