Shape and Configuration Coexistence -- The Case From N=20 to N=28

Figure 1: (a) Simulated ground-state rotational band in 32Mg, based on calculated energy levels [Pov13] and populated in multi-nucleon removal from 46Ar at 197 MeV/u at FRIB, using GRETA and the proposed high-rigidity spectrometer (HRS).  (b) SImulated spectrum gated on the transitions from the 2+ and 4+ states in 32Mg, showing identification up to the 10+ state in 32Mg, a 0.05% branch.
Recent spectroscopic work has shown that, at least in the Mg isotopes, deformation in fact extends all the way from the island of inversion near N=20 to 40Mg [Doo13, Cra14]
,which sits at or near to the dripline. Calculations [Now09] that predict 40Mg to be a well-deformed prolate rotor also indicate the last bound neutron orbital to be the low-l p3/2 state, leading to the possibility that weak binding effects could play a role. How collective modes develop near the dripline is an open and challenging question.Quantifying the extent of deformation and the underlying single-particle states requires beam intensities that will only be reached at FRIB. Proton and neutron knockout experiments will provide the detailed information to fully map out the changing proton and neutron single-particle energies and occupancies in this region, while relativistic Coulomb excitation and lifetime measurements will provide a quantification of the degree of collectivity.  As for the Ca isotopes, and indeed, many other regions of the nuclear chart, these measurements will be feasible only with the resolving power of GRETA.  Complex level schemes will need to be understood, requiring the efficiency in γγ coincidence detection afforded by GRETA, while the possibility for spin assignments based on polarization and angular-distribution data supplements the available information.   

Closer to stability, near N=20, FRIB will provide reaccelerated beams of neutron-rich Si, Mg, Ne and Na isotopes with rates sufficient to perform experiments aimed at identifying signatures of collective modes, such as rotational bands.  Light- and heavy-ion induced transfer, and Coulomb excitation reactions will allow detailed studies of such nuclei, including the odd-mass systems characterized by higher level densities (and thus higher γ-ray multiplicity) than their even-A neighbors.  GRETA will be crucial for these measurements, with the resolution to disentangle complex level schemes, the angular coverage to allow for angular correlation measurements, and the polarization sensitivity to allow for multipolarity and spin assignments. 

GRETA’s resolving power is illustrated Figure 1 for the simulated case of a rotational cascade in 32Mg produced using a fast-beam multi-nucleon knockout reaction at FRIB.  The increased efficiency and energy resolution will be essential to use γγ coincidences (perhaps higher fold in some cases) that are needed to establish that the sequence is indeed a rotational cascade. 



[Pov13]  A. Poves, private communication.