Systematics of neutron emission

Abstract

Neutron physics is one of the oldest branches of the experimental nuclear physics,but the investigation of the spontaneous neutron emission from the ground state along the neutron dripline is still at its beginning, in spite of the crucial importance for nuclear astrophysics. The proton dripline is much better investigated and a systematics of spontaneous proton half lives corected by the centrifugal barrier (monopole transitions) is given by the Geiger-Nuttall law $\log_{10}T\sim\chi$, where $\chi\sim ZQ^{-1/2}$ is the Coulomb parameter characterizing the outgoing Coulomb-Hankel wave in terms of the daughter charge $Z$ and Q-value. Our purpose is to propose a similar simple systematics of spontaneous neutron half lives, but in terms of the nuclear reduced radius $\rho=\kappa R\sim A^{1/3}Q^{1/2}$, characterizing the "neutral" outgoing spherical Hankel wave. It turns out that the half life in emission of neutral particles is governed by the scaling law $T\sim\rho^{-2}\sim A^{-2/3}Q^{-1}$ for monopole transitions. We evidence the important role of the angular momentum carried by the emitted neutron. The influence of the neutron wave function generated by a Woods-Saxon nuclear mean field is also analyzed.