Possible Gamma-Ray Signatures of an r-Process Event

Abstract

The r-(or rapid) process of neutron capture nucleosynthesis is responsible for the formation of roughly half of the abundances of the nuclei with mass greater than A ≈ 80 and for all of the actinides. It is known that the r-process occurs in a hot (T > 108 K), high neutron number density n n > 1020cm–3) environment where neutron captures typically occur more rapidly than β-decays. The nuclei thus tend to follow a path in the neutron number-proton number plane many neutrons rich of the β-stability line. At closed neutron shells (N = 50, 82, and 126), the β-decay rates are especially slow so that the nuclear abundances build up at these points along the r-process path. Once the r-process has stopped, the nuclei decay from the r-process path to the stability line, thereby giving the final abundances of the r-process. The large abundances built up at the closed neutron shells then yield the three well-known abundance peaks in the r-process abundance distribution at A = 80, 130, and 195. For a general review of the r-process, see SCHRAMM [l] and MATHEWS and WARD [2]-