Large low-energy $M1$ strength for $^{56,57}$Fe within the nuclear shell model

Abstract

A strong enhancement at low $\gamma$-ray energies has recently been discovered in the $\gamma$-ray strength function of $^{56,57}$Fe. In this work, we have for the first time obtained theoretical $\gamma$ decay spectra for states up to $\approx 8$ MeV in excitation for $^{56,57}$Fe. We find large $B(M1)$ values for low $\gamma$-ray energies that provide an explanation for the experimental observations. The role of mixed $E2$ transitions for the low-energy enhancement is addressed theoretically for the first time, and it is found that they contribute a rather small fraction. Our calculations clearly show that the high-$\ell$ ($=f$) diagonal terms are most important for the strong low-energy $M1$ transitions. As such types of $0\hbar\omega$ transitions are expected for all nuclei, our results indicate that a low-energy $M1$ enhancement should be present throughout the nuclear chart. This could have far-reaching consequences for our understanding of the $M1$ strength function at high excitation energies, with profound implications for astrophysical reaction rates.