Gamma-ray strength functions and their relation to astrophysics

The nuclear γ‐ray strength function is one of the indispensable inputs needed for reaction‐rate calculations, and is particularly important for the neutron‐capture cross section. The nuclear physics group at the Oslo Cyclotron Laboratory has developed a method to extract simultaneously nuclear level density and γ‐ray strength function from particle‐γ coincidence measurements. Data on the strength functions of Sn nuclei as well as for lighter elements are presented. The Sn isotopes all display a resonance‐like structure close to the neutron threshold, that could possibly be due to the neutron‐skin oscillation mode. This so‐called pygmy dipole resonance greatly influences the neutron‐capture rates. In the lighter nuclei, an enhancement of the strength function at low γ energies is observed. The possible impact of this increase on Maxwellian‐averaged reaction rates has been investigated.