Cross-shell excitations near the “island of inversion”: Structure ofMg30

Abstract

Excited states in 30 Mg have been populated to ∼6h and 5 MeV excitation energy with the 14 C( 18 O,2p) reaction. Firm spin assignments for states with J > 2h have been made in this nucleus. The level scheme is compared to shell-model calculations using the Universal sd effective interaction and the Monte Carlo shell model method. Calculations employing a full sd model space fail to reproduce the observed levels. The results indicate that excitations across the N = 20 gap are required at relatively low excitation energy to achieve a description of the data. The incorporation of the f 7/2 and p 3/2 orbitals into the model space gives improved results but indicate the need for further refinement of the models to reproduce the observed spectra.

Re-examining the transition into the N = 20 island of inversion: Structure of 30Mg

“…Its ground state is characterised by a strongly prolate deformed intruder structure with an almost pure neutron 2p − 2h configuration [16,17]. In contrast, 30 Mg is firmly placed outside the IoI and its structure interpreted as a spherical 0p − 0h ground state [18] coexisting with a neutron 2p − 2h intruderdominated deformed 0 + 2 isomeric state at 1.788 MeV [19,20] and with negative parity levels expected to appear, according to shell model calculations, at a relatively high excitation energy (>3.5 MeV [21]).…”

“…1, after Doppler and add-back corrections were applied. Nine known transitions [28,21,29] were observed. The energies and intensities are listed in Table 1 and the deduced decay scheme shown in Fig.…”

Shape fluctuations in the ground and excited0+states of30,32,34Mg

“…In neutron-rich Mg isotopes, the increase of the excitation energy ratio E(4 1 + )/E(2 1 + ) [1][2][3] and the enhancement of B(E2; 2 1 + → 0 1 + ) from 30 Mg to 34 Mg [4-6] indicate a kind of quantum phase transition from spherical to deformed shapes taking place around 32 Mg. These experiments stimulate microscopic investigations on quadrupole collective dynamics unique to this region of the nuclear chart with various theoretical approaches: the shell model [7][8][9][10], the Hartree-Fock-Bogoliubov (HFB) method [11,12], the parityprojected Hartree-Fock (HF) [13], the quasiparticle random phase approximation (QRPA) [14,15], the angular-momentum projected generator coordinate method (GCM) with [16] and without [17,18] restriction to the axial symmetry, and the antisymmetrized molecular dynamics [19].…”

“…2. (Color online) Comparison of calculated excitation energies of the 2 1 + and 4 1 + states (upper panel) and B(E2; 2 1 + → 0 1 + ) values (lower panel) in 30−36 Mg with experimental data[1][2][3][4][5][6].…”

Probing the N= 14 subshell closure: g factor of the Mg26(21+McCormick1, Stuchbery2, Kibédi3 et al. 2018Physics Letters B

“…The case of 31 Mg illustrates this point: The measured 1/2 + ground-state moment [49] is reasonably well described by USDB shell-model calculations, but the predicted 1/2 + state is at an excitation energy above 2 MeV and is not the ground state [25]. Studies of the excited-state spectroscopy of 30 Mg have shown that the sd-shell model fails at moderate spin, and cross-shell (pf ) excitations are needed at rather low excitation energy [50]. Certainly, the 2 + 1 states must be expected to contain more pf admixtures than the ground states, and g(2 + 1 ) values may show a smoother transition to the island of inversion than the ground-state moments of the odd-A isotopes.…”