In-beam γ -ray spectroscopy of Mg32 via direct reactions

Abstract

Background: The nucleus 32 Mg (N = 20 and Z = 12) plays a central role in the so-called "island of inversion" where in the ground states sd-shell neutrons are promoted to the f p-shell orbitals across the shell gap, resulting in the disappearance of the canonical neutron magic number N = 20.Purpose: The primary goals of this work are to extend the level scheme of 32 Mg, provide spin-parity assignments to excited states, and discuss the microscopic structure of each state through comparisons with theoretical calculations.Method: In-beam γ-ray spectroscopy of 32 Mg was performed using two direct-reaction probes, one-neutron (twoproton) knockout reactions on 33 Mg ( 34 Si). Final-state exclusive cross sections and parallel momentum distributions were extracted from the experimental data and compared with eikonal-based reaction model calculations combined with shell-model overlap functions.Results: Owing to the remarkable selectivity of the one-neutron and two-proton knockout reactions, a significantly updated level scheme for 32 Mg, which exhibits negative-parity intruder and positive-parity normal states, was constructed. The experimental results were confronted with four different nuclear structure models. Conclusions:In some of these models, different aspects of 32 Mg and the transition into the island of inversion are well described. However, unexplained discrepancies remain, and even with the help of these state-of-the-art theoretical approaches, the structure of this key nucleus is not yet fully captured.

Intruder configurations in 29Ne at the transition into the island of inversion: Detailed structure study of 28Ne

“…Thus, the decrease of the excitation energies of negativeparity states could be understood as a reduced gap between the 1d 3/2 and 2p 3/2 orbitals, thereby supporting the breakdown of the N = 20 shell gap at Z = 10. Such a lowering of the negative-parity states is similar to that for the 1 − state in 12 Be along the N = 8 isotones [62].…”

“…Even assuming the 3055-keV state is the 4 + 1 state, the present data still suggests weak f -wave strength because of its small cross section of 0.1(1) mb. This experimental finding in Ne is different from those in the Mg isotopes, where considerable f -wave strength to the bound states has been identified via oneneutron removal reactions [7, 9,10,11,12]. Such a difference in the f -wave occupancies between Ne and Mg might be related to the gap size between the 2p 3/2 and 1f 7/2 orbitals.…”

“…As mentioned above, the present direct neutron-removal reaction of 29 Ne on a proton target has also been studied with the eikonal dynamical model [48]. This eikonal approach, widely used for reactions on light-nucleus targets [10,11,12,15], was also recently applied to the protoninduced 30 Ne(p, pn) [16], 29 Ne(p, 2p) and 29 F(p, pn) [59] reactions. In the case of a proton target, the nucleon re-moval is due to nucleon-nucleon collisions, described in the model by their elastic S-matrix.…”

“…Many experimental efforts have been made to explore the mechanism driving the emergence of the island of inversion. While extensive studies have been performed for the magnesium isotopic chain [7,8,9,10,11,12], such detailed structure information remains more limited for the Ne isotopes. Along the Ne isotopic chain, the first onset of a pf -shell intruder configuration was identified in 28 Ne [13], while 29 Ne [14,15,16,17] borders, and 30−32 Ne [18,19,20,21] lie within the island of inversion.…”

The Nuclear Shell Model towards the Drip Lines

“…The discovery of "magic numbers" lead M. Goeppert-Mayer, and independently J. Hans D. Jensen in Germany, one year later, in 1949, to the construction of the shell model with strong spin-orbit coupling, and to the Nobel Prize they shared with Wigner in 1963. In addition to the data from [8], recent data for 40 Mn [9], 62 Ti [10], 66 Cr [11] and 70,72 Fe [11] are added.…”