Abstract
The β− decay of Mg34 was used to study the Al34 nucleus through γ spectroscopy at the Isotope Separator On-Line facility of CERN. Previous studies identified two β-decaying states in Al34 having spin-parity assignments Jπ=4− dominated by the normal configuration π(d5/2)−1⊗ν(f7/2) and Jπ=1+ by the intruder configuration π(d5/2)−1⊗ν(d3/2)−1(f7/2)2. Their unknown ordering and relative energy have been the subject of debate for the placement of Al34 inside or outside the N=20 “island of inversion.” We report here that the 1+ intruder lies only 46.6 keV above the 4− ground state. In addition, a new half-life of T1/2=44.9(4)ms, that is twice as long as the previously measured 20(10) ms, has been determined for Mg34. Large-scale shell-model calculations with the recently developed sdpf-u-mix interaction are compared with the new data and used to interpret the mechanisms at play at the very border of the N=20 island of inversion.
Highlights
The existence of special numbers of constituents that give rise to enhanced stability in atoms and atomic nuclei can be explained using very simplified meanfield descriptions [1,2]
This term can be decomposed into central, spin-orbit, and tensor components [7,8], each of which modifies the amplitude of spherical shell gaps
The rest of the nucleon-nucleon interaction is contained in the multipole terms of the Hamiltonian dominated by the pairing and the quadrupole-quadrupole interactions [9]. The latter acts more efficiently between nucleons located in orbits whose angular momenta differ by two units ofh [10]. It plays a decisive role in the onset of deformation and in the disappearance of magicity far from stability, in particular, when the spherical shell gap is reduced by monopole-driven effects
Summary
The existence of special numbers of constituents (called magic numbers) that give rise to enhanced stability in atoms and atomic nuclei can be explained using very simplified meanfield descriptions [1,2]. Nuclei near the magic neutron number N = 20 in which the ground-state (g.s.) configuration corresponds to np-nh excitations belong to the so-called “island of inversion” (IoI) [11,12,13,14].
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