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Si : a bubble nucleus to probe the spin-orbit interaction in atomic nuclei

Olivier Sorlin (GANIL - Caen)

The ‘so-called’ spin-orbit (SO) interaction was postulated in 1949 by Mayer, Haxel, Suess and Jensen in order to account for the large shell gaps observed in the spectra of certain stable atomic nuclei, renamed ‘‘magic nuclei’’. These nuclei display remarkable properties : to quote a few they are spherical, hard to excite and more bound than their neighbours. Consequently they are (when stable) the most abundant on earth, or (when unstable) the main survivors in explosive stellar environments.

It is now well admitted that this SO interaction comes from the fundamental properties of the nuclear force. However, its density and isospin (proton to neutron effect) dependences are not known since they have never been tested experimentally. Due to this lack of constraint, the modelling of a possible island of superheavy nuclei as well as of the rapid neutron capture process (that is synthesizing about half of the elements above Fe) are very uncertain.

We have discovered that the unstable 34Si is an ideal nucleus to test the properties of the spin orbit interaction. This nucleus is a doubly magic nucleus and very likely displays a significant central depletion in its interior, forming a bubble nucleus. I will present the many motivations for studying such a nucleus, explain why it is thought to be a bubble nucleus, and will show recent experimental results obtained at the Grand Accélérateur National d’Ions Lourds (GANIL) related to the understanding of the SO interaction. Fascinating perspectives and preliminary consequences of this discovery will be addressed at the end of the seminar.