Photoneutron Cross Sections of Astrophysical Significance

Abstract

Presented in this paper are some of the latest measurements of photoneutron cross sections of direct relevance to the p-process nucleosynthesis in the context of the statistical model of compound nuclear reactions. We discuss the p-process origin of the rarest nuclide and the only naturally occurring isomer 180 Ta m , a serious underproduction problem of 138 La, and the nuclear level density of 180 Ta determined from the partial photoneutron cross section for 180 Ta m . As the laser-Compton scattering y ray at AIST has enabled one to directly determine (y,n) cross sections, the blackbody synchrotron radiation to be produced by a ten-Tesla superconducting wiggler at SPring-8 is expected to be a promising tool for exploring (y,a) and (y,p) reactions in the future.Most of heavy nuclides are produced by neutron capture followed by (3~ decay; either s-process or r-process [1]. However, 35 neutron-deficient nuclides (p-nuclei) from Se to Hg with rare solar abundances are bypassed by the s-process flow and shielded from the (3~ decay after the r-process. Photodisintegration of pre-existing seed nuclei of s and/or r-process origin is a primary process of producing the p-nuclei [2]. An extensive reaction network is required to describe the p-process nucleosynthesis, consisting of (y,n), (y,a) (y,p) reactions complemented by n, p, and a-captures, neutrino capture, and weak transformation (electron capture and (3 + decay) [2]. The reaction network involving some 2,000 nuclei and 20,000 reactions effectively produces the p-nuclei in a delicate balance of temperature and time-scale. A narrow temperature window of T = (1.5 -3.5) x 10 9 K is required, on the one hand, to ensure efficient photodisintegrations and prevent the photo-erosion of all the heavy nuclei to ion-peak species. On the other hand, the freeze-out of photodisintegrations on a short time-scale of the order of 1 second is also necessary. These conditions are met in the deep Oxygen and Neon-rich layers of massive stars during their explosions as type II supernovae satisfactorily [3][4][5][6][7]. Other plausible sites include pre-supernova phases of massive stars [3,4,8] or type la supernovae (see [2] for a more complete review).