Structures of Ground and Excited States of 11BE and 12BE

The low lying particle and rotational excitations of ¹¹Be and ¹²Be are here investigated by employing the Isomorphic Shell Model, which is a hybridization of the conventional Shell Model and the Liquid Drop Model taking into account the nucleon finite size and using no adjustable parameters. Seven particle excitation levels for ¹¹Be and six for ¹²Be together with one rotational band for each isotope is studied and the numerical results are in very good agreements with the available experimental data. Through the present work a different explanation is given for the very interesting new phenomena observed in exotic nuclei [extended radius (halo), small separation energy of the last one (¹¹Be) or two (¹²Be) valence neutrons, the huge moments of inertia, the inversion of the expected order of levels, the breakdown of the magic numbers 8 and 20 for very neutron rich nuclei, etc.]. Through the present work is supported that exotic nuclei have a structure similar to that of ordinary nuclei and what is really different in these nuclei is an extra degree of freedom responsible for almost all aforementioned new phenomena. This degree of freedom is an internal collective rotation of some nucleons mixed with the usual internal motion (of shell model type) and which is indispensable part of the nucleon motion even in the nuclear ground state. This mixing is inevitable due to the adiabatic approximation invalidity in very light nuclei.