J. B. Blake

A possible alternative to the r-process

It is shown that it is possible to produce those nuclei commonly attributed to the r-process in a process wherein n,..gamma.. reactions compete with ..beta..-decay, as opposed to the r-process in which a n,..gamma.. arrow-right-left..gamma..,n equilibrium exists and the beta rates are much slower. This neutron-capture ..beta..-decay competition process (called the n-process) can reproduce solar-system r-process abundances under conditions very different and less extreme from those required for an r-process. In particular some current nova and supernova shock models may qualify. Thus it might be easier to find an astrophysical site for the n-process than for the r-process. (AIP)

Effects of convective overshoot on lithium depletion in main-sequence stars

A current problem in stellar evolution is to understand the lithium depletion in low-mass main-sequence stars. Standard stellar models do not produce temperatures in the outer convective zone high enough to allow lithium burning to occur. Convective overshoot could extend the mixing region deep enough to allow lithium burning. However, the strong temperature dependence of the relevant reaction rates initially seems to imply either total destruction or no destruction. Nevertheless, observations of main-sequence stars indicate a smooth variation of lithium abundance with stellar mass between 0.8 and 1.1 M/sub sun/ as well as dependence on stellar age. A gradual dependence of the degree of convective overshoot with stellar mass can be obtained if one uses a dynamical approach for describing the convective process. Thus both the mass and age dependence of lithium depletion might be understood. Specific examples of the Hyades and Pleiades clusters as well as the Sun are discussed. (AIP)

Cosmic-ray spallative origin of the rare odd-odd nuclei, consistent with light-element production

Reeves, Fowler, and Hoyle have suggested that the light nuclei could be synthesized by mutual spallation of the cosmic rays and the interstellar medium. Preliminary work by Audouze showed that the rare odd-odd nuclides /sup 50/V and /sup 180/Ta might also be produced by this process. An integral is performed over all cosmic-ray energies, utilize the newly avilable more accurate cross sections for peripheral reactions are utilized, and consider the effect of large fluxes of low-energy (5--25 MeV) particles in the region where (p,n) cross sections are very high is considered, to show that one might be able in some specific models to produce not only additional /sup 7/Li and /sup 11/B, which are not fitted well in standard models, but also some additional /sup 138/La, thus possibly filling in a gap in those models by adding that nuclide to the other rare odd-odd nuclides /sup 50/V and /sup 180/Ta produced by spallation at higher energies. (AIP)

Isotopic anomalies from neutron reactions during explosive carbon burning

The possibility that the newly discovered correlated isotopic anomalies for heavy elements in the Allende meteorite were synthesized in the secondary neutron capture episode during the explosive carbon burning, the possible source of the O-16 and Al-26 anomalies, is examined. Explosive carbon burning calculations under typical conditions were first performed to generate time profiles of temperature, density, and free particle concentrations. These quantities were inputted into a general neutron capture code which calculates the resulting isotopic pattern from exposing the preexisting heavy seed nuclei to these free particles during the explosive carbon burning conditions. The interpretation avoids the problem of the Sr isotopic data and may resolve the conflict between the time scales inferred from 1-129, Pu-244, and Al-26.

Ultraheavy cosmic rays - Theoretical implications of recent observations

The recent extreme ultraheavy cosmic-ray observations (Z greater than or equal to 70) are compared with r-process models. A detailed cosmic ray propagation calculation is used to transform the calculated source distributions to those observed at the earth. The r-process production abundances are calculated using different mass formulae and beta-rate formulae; an empirical estimate based on the observed solar-system abundances is also used. There is the continued strong indication of an r-process dominance in the extreme ultraheavy cosmic rays. It is shown that the observed high actinide/Pt ratio in the cosmic rays cannot be fitted with the same r-process calculation which also fits the solar-system material. This result suggests that the cosmic rays probably undergo some preferential acceleration in addition to the apparent general enrichment in heavy (r-process) material. An estimate is also made of the expected relative abundance of superheavy elements in the cosmic rays if the anomalous heavy xenon in carbonaceous chondrites is due to a fissioning superheavy element.

Post Event Neutron Exposure of r-Process Material,

Abstract : The effects of neutron fluxes on r-process material following the r-process event are considered. The calculations examine the case of neutron exposure occurring after the r-process material has decayed back to or near the valley of beta stability. Such exposure may occur during the r-process freeze-out or in some other subsequent neturon exposure. Limitations on these neutron fluxes are inferred from odd-even effects in the A about 130 and A about 195 r-process peaks and from the abundances of certain s-only nuclei. The effects of these neutron fluxes on the abundances of lead, bismuth and the nucleochronologically important actinides are examined. (Author)