Direct reactions for nuclear astrophysics

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 Ep = 1912 keV is often used as a neutron source since the energy distribution of the emitted neutrons closely resembles a Maxwell-Boltzmann spectrum of kBT = 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 kBT = 90 keV. Neutron spectra corresponding to a Maxwell-Boltzmann distribution with kBT > 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 kBT = 25 keV to kBT = 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.