Studying the Transition to the Island of Inversion

The existence of deformed (2p2h) intruder ground states in A$\sim $30 N$\sim $20 nuclei (``island of inversion'') signals a modification of conventional shell structure in neutron-rich nuclei. While intruder ground states have been identified in e.g. $^{30}$Ne and $^{31}$Na, questions remain as to where the normal-to-deformed transition occurs and the nature of their collectivity: data on excited states will help answer these questions. An experiment was conducted at MSU to study N$\sim $20 Ne and Na nuclei. A 140 MeV/A $^{48}$Ca primary beam produced secondary-beam ``cocktails'' ($^{29}$Na/$^{30}$Mg/$^{32}$Al, $^{32}$Mg/$^{33}$Al/$^{35}$Si) which underwent secondary reactions to produce Ne and Na; $\gamma $-ray decays were detected by the segmented Ge array, SeGA, and $\gamma -\gamma $ coincidences were critical to establish a correct level scheme in e.g. $^{28}$Ne. The data provide information on the transition to the island of inversion and a test of recent shell-model calculations.