DREB2014 - Direct Reactions with Exotic Beams

Structures of the neutron-rich nuclei around $N~=~20$ nucleus $^{32}$Mg have been studied via the direct reactions induced by the isoscalar probe of $\alpha$ particle. An array of position sensitive segmented Ge detectors, GRAPE, was employed for the in-beam $\gamma$-ray spectroscopy. Special attention is paid to single-particle states populated via the nucleon transfer reactions as well as collective states appeared in the inelastic scattering to clarify the shell evolution in the island-of-inversion nuclei. The $^{32}$Mg is known to have large $B(E2:0_{g.s}^{+}\rightarrow2_{1}^{+})$ from the Coulomb excitation and also the proton inelastic scattering. In the present study, we discuss the differential cross sections of $^{32}$Mg and its neighboring nucleus $^{33}$Al with $\alpha$ particles. We also discuss the systematics of the deformation in this region using the same probe for the cocktail beam of Na, Mg, and Al. The experiment was performed at RIPS in RIBF, RIKEN. A 63-$A$MeV $^{40}$Ar beam from the ring cyclotron bombarded the 1-mm thick carbon target. Its fragmentation produces was separated by RIPS. The secondary beam bombarded the 150-mm/cm$^{2} thick liquid helium target. Incident and outgoing particles were identified in event-by-event basis. For the identification of outgoing particles, a TOF-spectrometer consisting of a superconducting triplet quadruple magnet, a layer of stripped silicon detector and NaI calorimeter. The excited states were identified by the de-excitation $\gamma$ rays measured with the GRAPE. $\gamma-\gamma$ coincidence analysis was performed to reconstruct the level structure. For the low-lying levels, the angular distributions of differential cross sections were obtained. The deformation parameter was deduced by DWBA analysis. In this paper, we will report the excitation structures in island-of-inversion nuclei using the isoscalar probe and discuss their structures from the point of view of deformation.