Magnetic dipole -ray strength functions of heavy nuclei in the configuration-interaction shell model

A low-energy enhancement (LEE) has been observed in the deexcitation gamma-ray strength function (gamma SF) of compound nuclei. The LEE has been a subject of intense experimental and theoretical interest since its discovery, and, if the LEE persists in heavy neutron-rich nuclei, it would have significant effects on calculations of r-process nucleosynthesis. Standard configuration-interaction (CI) shell-model calculations in medium-mass nuclei have attributed the LEE to the magnetic dipole gamma SF but such calculations are computationally intractable in heavy nuclei. We review a combination of beyond-mean-field many-body methods within the framework of the CI shell model that enables the calculation of gamma SF in heavy nuclei, and discuss the recent theoretical identification of a LEE in the magnetic dipole gamma SF of lanthanide isotopes.