Direct reactions for nuclear astrophysics

¹ Goethe University Frankfurt
² Facility for Antiproton and Ion Research in Europe GmbH

^* e-mail: erbacher@iap.uni-frankfurt.de

Published online: 22 March 2023

Abstract

The neutron activation technique is a well established method to measure neutron capture cross sections relevant for the s-process. The ⁷Li(p,n) reaction at E_p = 1912 keV is often used as a neutron source since the energy distribution of the emitted neutrons closely resembles a Maxwell-Boltzmann spectrum of k_BT = 25 keV, mimicking the ²²Ne(α,n) phase in TP-AGB stars. The weak s-process, which takes place in massive stars, can reach energies up to k_BT = 90 keV. Neutron spectra corresponding to a Maxwell-Boltzmann distribution with k_BT > 25 keV cannot be produced by the ⁷Li(p,n) reaction directly. We developed a method to obtain quasi-Maxwellian neutron capture cross sections over a wide energy range by combining a set of spectrum average cross sections measured at six different proton energies and distances between the lithium target and the sample. The measured spectrum averaged cross section can be used to calculate the Maxwellian-Averaged cross-section (MACS) from k_BT = 25 keV to k_BT = 90 keV. Over the last two years neutron capture cross sections on over 20 isotopes have been measured at Goethe University Frankfurt using this methodology. An overview of the current experimental method, challenges during data analysis and the first results are presented.