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

A strong enhancement at low $γ$-ray energies has recently been discovered in the $γ$-ray strength function of $^{56,57}$Fe. In this work, we have for the first time obtained theoretical $γ$ decay spectra for states up to $\approx 8$ MeV in excitation for $^{56,57}$Fe. We find large $B(M1)$ values for low $γ$-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 $0\hbarω$ 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.

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