Alpha-decay fine structure in even-even nuclei

Abstract

The aim of this work has been to study the systematics of -decay fine structure in those cases, where the daughter nucleus is doubly even and has a 2+ state as the lowest excited state. Restriction to these decays is a practical one, as doubly even nuclei have a simpler excitation level structure than other nuclei. Also the lowest 2+ state corresponds to either a one phonon excitation or the first rotational state, and that helps us to keep our microscopic and macroscopic models simple. We have concentrated on the main features reproducible with simple models, trying to find fundamental connections, thumb-rules and order-of-magnitude estimates instead of complex fit formulas to reproduce exactly all given numbers. This trend is reflected by our choice of units. We deal mostly with logarithmic units of time and intensity ratios, because the time scales and intensity scales vary hugely over the nuclide chart. This kind of systematic analysis about -decay fine structure did not exist previously, even though there were lots of measured data. Although the general impression about -decay at the time this work was started was, that everything important about this decay mode was already known from the 1950 s-1960 s, the fine structure and even the preformation part of ground-state-to-ground-state - decay is still worth investigating even at a rather basic level. This work is divided into macroscopic and microscopic approach. The first is a collective model using coupled channels formalism and double folding integration over the matter density in both the -particle and daughter nucleus to create an effective potential for the alpha particle to escape from. The second is a microscopic quasi-particle model where both the daughter and parent nucleus are constructed from an inert core and some active nucleons that occupy some single-particle levels. Here the probability to decay to a certain final configuration is calculated as an overlap integral between the starting configuration and the final configuration. The macroscopic part is further divided to a rotational case and a vibrational case. Rotational nuclei exhibit clearly more collective characteristics than vibrational ones, so this was the first and more straightforward application of the model. Later, with some modifications, we have successfully implemented a similar model also for the vibrational cases.