R. A. Ward

s-Process Studies In the Light of New Experimental Cross Sections: Distribution of Neutron Fluences and r-Process Residuals

A best set of neutron-capture cross section has been evaluated for the most important s-process isotopes. With this data base, s-process studies have been carried out using the traditional model which assumes a steady neutron flux and an exponeatial distribution of neturon irradiations. The calculated sigmaN curve is in excellent agreement with the empricial sigmaN-values of pure s-process nuclei. Simultaneously, good agreement is found between the difference of solar and s-process abundances and the abundance of pure r-process nuclei. We also discuss the abundance pattern of the iron group elements where our s-process results complement the abundances obtained from explosive nuclear burning. The results obrtained from the tradiational s-process model such as seed abundances, mean neutron irradiations, or neutron densities are compared to recent stellar model calculations which assume the He-burning shells of red giant stars an the site for the s-process.

Quantitative measurement of mid-z opacities

Abstract Results of recent experiments measuring x-ray absorption by a hot, dense, germanium plasma are presented. A general discussion of the experimental technique is given showing the requirements that must be met in order to extract quantitative transmission data. The resulting spectrally resolved absorption measurements can then be used to test the capabilities of LTE opacity codes. Meaningful comparisons require that the sample be in LTE, and that the temperature and density of the sample be uniform and accurately measured. Comparisons between the experiment and calculations are shown.

s-process studies - Xenon and krypton isotopic abundances

We propose an analysis of the s-process contributions to the isotopes of xenon and krypton. The object is to aid studies of the possibility that meteorites may contain gas that was carried in presolar grains that were grown in stellar ejecta and that were not degassed prior to incorporation into parent bodies. That model suggests routine interstellar fractionation of s-isotopes from r-isotopes owing to differential incoporation into dust. We show that a deficiency of s-process nuclei cannot yield details of Xe-X, but the gross similarities are strong enough to lead one to think that such a deficiency may play a role in a more complicated explanation. We predict the existence of an s-rich complement somewhere if fractional separation of this type has played a role in Xe-X. We show that the analogous decomposition of krypton is more uncertain, and we call for measurements of neutron-capture cross sections to alleviate these uncertainties.

s-Process Studies: Branching and the Time Scale

The theory of S-process heavy-element formation is reformulated to allow for competition between beta decay and neutron capture at various nuclei along the path. Solutions to the resulting branching network equations are presented (under the assumption of constant temperature and neutron flux) that do not require steady flow for the neutron current. Using the exponential exposure distribution rho (tau) =G exp(-tau/tau/sub 0/) and recently calculated temperature-dependent beta-decay rates, comparison of several key branches yields the following average conditions for the solar-system S-process environment: Tapprox. =3.1 x 10/sup 8/ K, n/subn/approx. =1.6 x 10/sup 7/ neutrons cm/sup -3/. For tau/sub 0/=0.25 n mb/sup -1/ we find that about 4.8 neutron captures per exposed iron seed are required over a time of the order of a few thousand years for synthesis of the bulk of the solarsystem S-process material, with an average neutron capture time approx.10 years (for sigmaapprox.500 mb). (AIP)

S-process studies - The effects of a pulsed neutron flux

Recent investigations of the astrophysical site of the s-process strongly suggest the helium-burning shell of the helium-shell-flashing stars of intermediate mass as the most likely site for the s-process event which produced the solar-system abundances. Previous analyses of branching in the s-process have been in terms of a constant temperature and density environment. The occurrence of periodic thermal instabilities seems to make this assumption inappropriate. The current effort is to reformulate the mathematics of the branched s-process to allow for the influence of thermal pulses. The effect of pulses on the unique isobars between branches is generally small; the largest effect of pulses is at those branch points where the branching ratio for ..beta..-decay during a pulse is small, but the ..beta..-decay lifetime during the interpulse period is shorter than the time between pulses. It is shown that the decay of the unstable branch nuclei between neutron irradiations allows the mean s-process neutron flux responsible for the solar-system abundances in such a pulsed environment to be considerably larger than that for a single continuous-exposure event.

On emission lines in the cosmic gamma-ray background

The composite spectrum of gamma-rays is calculated which results from the decay of Ni-56 to Co-56 to Fe-56 throughout the history of the universe. The results for several cosmological models are presented and compared with the Apollo 15 measurements at low resolution of the cosmic background. The radioactivity background is a significant fraction of the total, and several of its features may be detectable.-

EXPERIMENTAL TECHNIQUES TO MEASURE THERMAL RADIATION HEAT TRANSFER

Abstract We discuss a spectroscopic technique for measuring the time-dependent transfer of heat by x-rays. Both 1D burnthrough foils and more complicated 2D geometries are discussed. The initial x-ray drive flux has been well characterized in terms of its time, angular, and frequency dependence. A few planned extensions of these initial experiments are also outlined.

Opacity Measurements in a Hot Dense Medium

Measurements of the opacity of aluminum in a well characterized, hot, dense, laser produced plasma are reported. Measurements of the absorption of X-rays by 1 to 2 transitions in Al XII through Al VIII have been made in a laser-heated slab plasma at the measured temperature and density of 58 ± 4 eV and 0.020 ± 0.007 g cm-3. Separate measurements of the temperature and density were made. The conditions in the plasma were determined to be reproducible, spatially uniform, and in nearly complete local thermodynamic equilibrium. The absorption spectra and the temperature-density data obtained provide an improved means for comparison with detailed atomic physics and opacity calculations.

PRODUCTION AND SURVIVAL OF 99 Tc IN He-SHELL RECURRENT THERMAL PULSES

After a brief introduction to the present state of art of nuclear beta-decay studies in astrophysics, we report our recent work on the long-standing 99Tc problem. Having combined a detailed study of the recurrent He-shell thermal-pulse, third dredge-up episodes in a 2.25 M⊙ star and an s-process network calculation, we show that a substantial amount of 99Tc can be produced by the s-process and can survive to be dredged up to the stellar surface. We stress that the factual observation of 99Tc the surface of certain stars does not necessarily preclude the 22Ne(α,n)25Mg reaction from remaining as the neutron source for the s-process. The calculated surface abundances of 99Tc and elements with neighboring atomic numbers are compared with observations.

Some nuclear data needs in astrophysics

In this paper we discuss a number of astrophysical environments and how improved nuclear data could facilitate a better understanding of them. One area of interest includes proton and alpha-particle reactions with unstable nuclei which are necessary for understanding the nucleosynthesis and energy generation in hot hydrogen-burning environments. Efforts underway at LLNL and elsewhere to develop the technology for the measurement of these reaction rates are discussed. Heavy-element nucleosynthesis in the late stages of red-giant stars and supernovae requires a complete network of neutron capture rates and beta-decay rates for nuclei near and far from stability. Experimental and theoretical efforts at LLNL to supply the input data and to model the nucleosynthetic environments will be outlined. Suggestions are made as to which nuclear data are most critical for the various scenarios. 42 refs., 11 figs., 1 tab.