Theoretical neutron-capture cross sections for r-process nucleosynthesis in the $^{48}$Ca region

TL;DR

This paper computes neutron capture cross sections for isotopes near $^{48}$Ca, including both compound nucleus and direct capture mechanisms, revealing enhanced rates and shifted r-process path points due to direct capture effects.

Contribution

It introduces the inclusion of direct capture in neutron cross section calculations for r-process isotopes, extending beyond traditional compound nucleus models.

Findings

Direct capture increases neutron capture rates.

Shift of r-process path to higher mass numbers.

Sensitivity of cross sections to nuclear deformation.

Abstract

We calculate neutron capture cross sections for r-process nucleosynthesis in the $^{48}$Ca region, namely for the isotopes $^{40-44}$S, $^{46-50}$Ar, $^{56-66}$Ti, $^{62-68}$Cr, and $^{72-76}$Fe. While previously only cross sections resulting from the compound nucleus reaction mechanism (Hauser-Feshbach) have been considered, we recalculate not only that contribution to the cross section but also include direct capture on even-even nuclei. The level schemes, which are of utmost importance in the direct capture calculations, are taken from quasi-particle states obtained with a folded-Yukawa potential and Lipkin-Nogami pairing. Most recent deformation values derived from experimental data on $\beta$-decay half lives are used where available. Due to the consideration of direct capture, the capture rates are enhanced and the "turning points" in the r-process path are shifted to slightly higher mass numbers. We also discuss the sensitivity of the direct capture cross sections on the assumed deformation.