Simple empirical E1 and M1 strength functions for practical applications

Abstract

Valuable theoretical predictions of nuclear dipole excitations in the whole chart are of great interest for different nuclear applications, including, in particular, nuclear astrophysics. Here on the basis of experimental and theoretical information on the $E1$ and $M1$ strength functions, inspired both from axially deformed quasiparticle random-phase approximation and shell-model calculations, we derive simple expressions to determine systematically the dipole strength in order to update former prescriptions with a special emphasis on new expressions for the $M1$ spin-flip and scissors modes. We compare our final prediction of the $E1$ and $M1$ strengths with available experimental data at low energies and show that a relatively good agreement is obtained. Its impact on the total radiative width as well as radiative neutron capture cross sections is also discussed. The new expressions are believed to represent an improvement with respect to the analytical systematics proposed in the past and used traditionally in reaction codes, such as the recommended ripl-3 prescriptions.