Consolidating the concept of low-energy magnetic dipole decay radiation

Abstract

We have made a thorough study of the low-energy behaviour of the $\gamma$-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for $M1$ transitions. There are clear trends in the calculations: From being all but absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find suppport for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with a LEE, and indicate that the slope varies as function of mass number.

arXiv comment11 pages, 13 figures

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• Novel Techniques for Constraining Neutron-Capture Rates Relevant for r-Process Heavy-Element Nucleosynthesis
• Nuclear level densities and gamma-ray strength functions in samarium isotopes
• Evolution of low-lying M1 modes in germanium isotopes
• Role of consistent parameter sets in an assessment of the alpha-particle optical potential below the Coulomb barrier
• Impact of level densities and $\gamma$-strength functions on $r$-process simulations
• Nuclear Level Density and $\gamma$-ray Strength Function of ${}^{67}\mathrm{Ni}$ and the impact on the i-process
• Low-energy enhancement in the magnetic dipole $\gamma$-ray strength functions of heavy nuclei
• Benchmark of many-body approaches for magnetic dipole transition strength
• Decoherence of collective motion in warm nuclei