Abstract
Low energy enhancement of radiative strength functions has been deduced from experiments in several mass regions of nuclei. Such an enhancement is believed to impact the calculated neutron capture rates which are crucial input for reaction rates of astrophysical interest. Recently, shell model calculations have been performed to explain the upbend of the γ-strength as due to the M 1 transitions between close-lying states in the quasi-continuum in Fe and Mo nuclei. Beyond mean-↓eld calculations in Mo suggested, however, a non-negligible role of electric dipole in the low energy enhancement. So far, no calculations of both dipole components within the same theoretical framework have been presented in this context. In this work we present newly developed large scale shell model appraoch that allows to treat on the same footing natural and non-natural parity states. The calculations are performed in a large sd − pf − gds model space, allowing for 1p{1h excitations on the top of the full pf -shell con↓guration mixing. We restrict the discussion to the magnetic part of the dipole strength, however, we calculate for the ↓rst time the magnetic dipole strength between states built of excitations going beyond the classical shell model spaces. Our results corroborate previous ↓ndings for the M 1 enhancement for the natural parity states while we observe no enhancement for the 1p{1h contributions. We also discuss in more detail the e↑ects of con↓guration mixing limitations on the enhancement coming out from shell model calculations.
Highlights
Photoneutron cross sections and radiative neutron capture cross sections are fundamental nuclear inputs to stellar model calculations of nucleosynthesis processes
As one can see there is a correlation between the magnitude of the low energy enhancement and the shape of the average B(M1) strength depending on the case
The low energy enhancement of the dipole strength functions was studied in the nuclear shell model for selected A D 44–54 nuclei
Summary
Photoneutron cross sections and radiative neutron capture cross sections are fundamental nuclear inputs to stellar model calculations of nucleosynthesis processes. The γ-SF below the neutron threshold plays a key role in defining the electromagnetic de-excitation taking place after the neutron capture, which, together with the β-decay, drives the s− and r−process nucleosynthesis
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