Donald D. Clayton

NUCLEOSYNTHESIS OF HEAVY ELEMENTS BY NEUTRON CAPTURE

Abstract : Nucleosynthesis of elements heavier than the iron group by neutron capture on both slow and fast time scales is evaluated. The s-process calculations of Clayton, Fowler, Hull, and Zimmerman (Ann. Phys. 12:331, 1961) were revised to include more recent experimental results on abundances and neutron capture cross-sections. The solar-system s-process abundances indicate a history of neutron exposure distributions characterized by decreasing probability of high integrated flux; an exponential exposure distribution is extracted. Estimates are made of the s-process contribution to each isotopic abundance; a table gives the amounts of elements produced by each process in the solar-system material. The r-process calculations are carried out using a semi-empirical atomic-mass law to determine neutron-binding energies and beta-decay probabilities. The solar-system r-process material has probably been synthesized in two distinct types of environments. (Author)

Extinct Radioactivities: Trapped Residuals of Presolar Grains

The xenon anomalies trapped in meteorites and the Moon may have first been trapped in circumstellar grains formed in or outside of postexplosive stars. In that case, the initial solar nebula need not have contained most of their radioactive progenitors, and this would necessitate major revision of the history of solar system formation.

Galactic Evolution of Silicon Isotopes: Application to Presolar SiC Grains from Meteorites

We calculate and discuss the chemical evolution of the isotopic silicon abundances in the interstellar medium (ISM) at distances and times appropriate to the birth of the solar system. This has several objectives, some of which are related to anomalous silicon isotope ratios within presolar grains extracted from meteorites; namely: (1) What is the relative importance for silicon isotopic compositions in the bulk ISM of Type II supernovae, Type Ia supernovae, and AGB stars? (2) Are 29Si and 30Si primary or secondary nucleosynthesis products? (3) In what isotopic direction in a three-isotope plot do core-collapse supernovae of different mass move the silicon isotopic composition? (4) Why do present calcu-lations not reproduce the solar ratios for silicon isotopes, and what does that impose upon studies of anomalous Si isotopes in meteoritic silicon carbide grains? (5) Are chemical-evolution features recorded in the anomalous SiC grains? Our answers are formulated on the basis of the Woosley & Weaver super-nova yield survey. Renormalization with the calculated interstellar medium silicon isotopic composition and solar composition is as an important and recurring concept of this paper. Possible interpretations of the silicon isotope anomalies measured in single SiC grains extracted from carbonaceous meteorites are then presented. The calculations suggest that the temporal evolution of the isotopic silicon abundances in the interstellar medium may be recorded in these grains.

Origin of heavy xenon in meteoritic diamonds

The neutrino burst from the collapsed core in Type II supernovae liberates free neutrons by nu, nu-prime(n) reactions which drive isotopic abundances several mass steps heavier. The neutron fluence in the He shell, abetted by another burst 10 s later from alpha, n reactions, is about right for rendering Xe-136, the most abundant Xe isotope. The Xe isotopic composition is a good match to Xe-H, the unshielded neutron-rich Xe component abundant in carbonaceous meteorites. The He shell is implicated because it is the only C-rich shell in massive stars that can be expected to condense solid carbon thermally, a requirement for locating the Xe-H in meteoritic diamonds, which are so common as to require an abundant Galactic source of unoxidized carbon. 47 refs.

COSMORADIOGENIC CHRONOLOGIES OF NUCLEOSYNTHESIS

New methods for using radioactive decays to date the time of nucleosynthesis in out galaxy are presented in this work. The methods depend upon a comparison of the cosmoradiogenic abundances of daughter species with the abundances of their radioactive parents. The decays which can provide useful information of this type are /sup 187/Re-- /sup 187/Os, /sup 235/U-/sup 207/Pb, and /sup 238/U--/sup 206/Pb. It is shown that the amoun t of the enrichment of the daughter products by radioactive decay throughout the history of our galaxy is calculable. This calculation differs from previous radioactive chronologies that have concentrated on the relative abundances of the radioactive species themselves /sup 235/U, /sup 238/U and /sup 232/Th. Definitive numerical calculations at the present time are forestalled by uncertainties in several key measurable quantities, particularly neutron-capture cross sections and solar abundances. Tentative calculations based on probable values of these quantities indicate that galactic nucleosynthesis began at least 5 x 10/sup 9/ years before the formation of the solar system, and perhaps considerably earlier. Measurements capable of reducing the uncertainty of these methods are emphasized. (auth)

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.

Molybdenum and Zirconium Isotopes from a Supernova Neutron Burst

We analyze the nucleosynthesis implications of the recent discovery by M. J. Pellin and collaborators that two odd isotopes of molybdenum, 95Mo and 97Mo, are overabundant in type X SiC grains: X grains condensed within expanding supernova interiors. We find that a rapid release of neutrons (on a timescale of seconds) with fluence τ = 0.07-0.08 neutrons mbarn-1 produces the observed pattern by way of abundant production of progenitor radioactive Zr isotopes. This suggests that the condensing matter was in a supernova shell in which rapid burning was occurring at the time of ejection, probably owing to the passage of the shock wave from the core. Which shell, and the exact source of the neutrons, is still unknown, but we present a model based on the shock of an He shell.

Massive Supernovae, Orion Gamma Rays, and the Formation of the Solar System

We discuss the source of the enhanced carbon and oxygen low-energy cosmic-ray flux in the Orion star-forming region and attribute it to the acceleration of the surface layers of a massive supernova, probably of Type Ib. The gamma rays from Orion are produced by that fast CO ejecta. In this model there would be few Orion-like gamma-ray sources in the Galaxy at any one time. We also postulate that a massive supernova produced the short-lived extinct radioactivities injected into the molecular cloud core that produced the solar system. We find that relative to 26Al the other short-lived extinct radioactivities are excessively produced in massive supernovae but are likely to be more attenuated by postexplosion fallback than 26Al. This is a revival of the supernova trigger hypothesis; to obtain the correct dilutions of the extinct radioactivities, the distance from the supernova to the impacted molecular cloud core must be a few parsecs, and the effective projected collecting area of the cloud must be significantly less than normal core radii.

Gamma-ray lines from novae

An appropriate gamma-ray telescope could detect the gamma-rays associated with radioactive decays. The observable lines would be the annihilation radiation following the positron emission of N-13, O-14, O-15, and Na-22 and the 2.312-MeV line emitted following the O-14 decay and the 1.274-MeV line emitted following the Na-22 decay. The experimental possibility should be borne in mind for the occurrence of novae within a few kiloparsecs.

22Na, Ne–E, extinct radioactive anomalies and unsupported 40Ar

A NEW picture1 of the origin of the known extinct radioactivities (129I and 244Pu) holds that these radioactive species were precipitated in grains forming in the rapidly cooling ejecta of explosive nucleosynthesis, and that their decay occurred in interstellar grains rather than in the meteorites. If so, our interpretation of extinct radioactivities is enlarged. Their detectability is no longer related to the usual criterion that they live long enough for the meteorites to form, but rather that they live long enough for grains to form in the expanding envelope.