Low-energy enhancement of the magnetic dipole radiation in actinide nuclei

A low-energy enhancement (LEE) has been observed experimentally in the gamma-ray decay of several compound nuclei. If this LEE persists in heavy neutron-rich nuclei it would have profound effects on r-process nucleosynthesis through the enhancement of radiative neutron capture rates. Conventional configuration-interaction shell model calculations found a LEE in the magnetic dipole gamma strength function of medium-mass nuclei, but such calculations are prohibited in heavy open-shell nuclei. Recently the shell model Monte Carlo (SMMC) method, in combination with other many-body methods, was used to identify a LEE in lanthanide nuclei. Here we present SMMC calculations for the magnetic dipole strength function of several actinide nuclei (²³²Th, ²³⁸U, and ²⁴⁰Pu). These calculations show the existence of a LEE, serving as the first theoretical prediction of such enhancement in the actinides.