Nuclear astrophysics with neutrons

Abstract

Neutrons play a crucial role in astrophysics during the heavy element nucleosynthesis. The largest fraction of isotopes heavier than iron is produced by neutron capture processes on short (r process) and long timescales (s process). During the "slow neutron capture process" (s process) heavier elements are produced by successive captures of in-situ produced neutrons from the reactions 13 C(α,n) 16 O and 22 Ne(α,n) 25 Mg (with densities of 10 6 − 10 10 cm −3 ) in the interior of stars and following β -decays. With this scenario the reaction path runs along the valley of stability up to 209 Bi and produces about 50% of the solar abundances of the heavy elements. Important nuclear physics parameters for s-process nucleosynthesis are neutron capture cross sections (for E n = 0.3-300 keV, corresponding to stellar temperatures between kT = 8 and 90 keV) and β -decay half-lives. Neutron capture measurements can be performed via activation in a quasi-stellar neutron spectrum utilizing several (p, n) reactions, or by the time-of-flight technique.The "rapid neutron capture process" (r process) is responsible for the remaining 50% of the solar abundances. Here neutrons with densities of 10 20 − 10 30 cm −3 are captured on a very fast timescale (ms) during a Core Collapse Supernova in a region close to the forming neutron star. The r-process nuclei are thus very short-lived, neutron-rich isotopes up to the actinides, which can only be produced and investigated at large-scale radioactive-beam facilities. Here the most important nuclear physics parameters are masses, half-lives, and at later stages also β -delayed neutrons. This paper will summarize the role of neutrons in nuclear astrophysics and give a short overview about the related astrophysics programs at the GSI Helmholtz research center and the FRANZ facility in Germany.

Research opportunities with compact accelerator-driven neutron sources

“…It is scheduled to attempt measurements of a series of extremely small neutron-capture cross sections and unstable branch-point nuclei which are essential to further the understanding of s-process in nucleosynthesis. [154,155].…”

“…In recent years accelerator-based neutron sources have been under development for BNCT in Argentina, Belgium, Italy, Japan, Russia, UK, and other countries [155]. Results from first clinical tests are expected around 2016 from studies carried out at a CANS based on 30-MeV protons from a cyclotron and a Be target for neutron production, jointly developed by Sumitomo Heavy Industries, Ltd. And KURRI in Japan [166].…”

The AMS technique as an important tool for the measurement of astrophysical cross sections

“…Underground accelerators, compact accelerator-based neutron sources and indirect techniques are the main solutions to develop an important amount of interesting reactions. Several studies have been carried out using these kind of methods [1][2][3][4][5][6][7][8][9]. Nevertheless it is always important to find alternative solutions, considering the big importance of the understanding of certain reactions with difficult access and essential for a better description of stellar and primordial nucleosynthesis.…”