Session LM: Nuclear Structure A=20-30

The MoNA-LISA research program at NSCL and FRIB

The Modular Neutron Array/Large multi-Institutional Scintillator Array (MoNA-LISA) has been used in nuclear physics experiments to investigate the nuclear structure of neutron-unbound states at the National Superconducting Cyclotron Laboratory of Michigan State University since 2003. While most experiments study neutron-unbound states, other types of experiments have been performed and are being planned as well. Part of this presentation will give an overview of the types of measurements that the MoNA Collaboration has done, in terms of physics and instrumentation. The other part will look into the future setup and possible upgrades of MoNA-LISA at NSCL, where this neutron detector array will move to a different vault, and contribute to the base equipment for the High Rigidity Spectrometer of the upcoming Facility for Rare Isotope Beams.   

The Structure of $^{28}$Mg from the $^{26}$Mg(t,p)$^{28}$Mg reaction

We have studied the neutron rich isotope $^{28}$Mg with the $^{26}$Mg(t,p)$^{28}$Mg reaction using HELIOS at Argonne National Laboratory. This nucleus, located outside the ``Island of Inversion'', can be used as a bridge to understanding the rapidly changing nuclear properties in the region. A beam of $^{26}$Mg produced by the ATLAS accelerator facility at ANL bombarded a tritiated titanium foil at an energy of 6MeV/u. Here, low-lying 0$^+$ states populated with the (t,p) reaction are sensitive to shape coexistence and configuration mixing. The reaction was conducted in inverse kinematics with heavy recoil coincidence provided by an array of E-$\Delta$E detectors, while protons were detected with HELIOS. The data were acquired with a digital acquisition system and pulse shape analysis algorithms were used to refine the timing resolution. Several calculations of the properties of magnesium isotopes with A=24 to 34, including those done with state of the art shell model, Relativistic Mean Field and Beyond Mean Field frameworks, show the evolution from normal to inverted deformed hierarchy. Analysis is in progress and preliminary results will be presented.
  

Influence of the neutron f7/2 subshell on the structure of 28Mg

Excited states in ²⁸Mg were studied in an experiment at the ISAC-II facility at TRIUMF to investigate the evolution of the f_7/2 orbital as a function of neutron excess near the N=20 island of inversion. For the first time ²⁸Mg was investigated using ¹²C(¹⁸O,2p)²⁸Mg fusion-evaporation, leading to preferential population of states at high spin and excitation energy where the influence of the f_7/2 subshell is expected.

Data corresponding to ²⁸Mg was extracted by gating on protons and gamma rays. Proton gating was achieved utilizing a new CsI(Tl) scintillator array, part of the TIGRESS Integrated Plunger (TIP) infrastructure. TIP was coupled with the TIGRESS array of HPGe clovers for gamma-ray detection. Coincidence spectroscopy was performed from Doppler corrected gamma-ray spectra taken using a thin target. Lifetime measurements were performed via the Doppler shift attenuation method using a second target with a thick gold backing.

Angular distribution measurements and comparisons to levels in neighboring isotopes indicate that new candidates for the yrast 6⁺ state and negative parity states are observed. A high precision measurement of the lifetime of the yrast 4⁺ state and new lifetime limits for higher lying states were also obtained. Detailed results will be presented.