Abstract
The nuclear g factor and quadrupole moment (Qs) have been measured for the low-lying isomeric 1+ state of 34Al, using the β-detected nuclear magnetic resonance (β-NMR) and quadrupole resonance (β-NQR) methods. Spin-polarized 34Al isotopes were produced in a one-neutron pickup reaction induced by a 36S beam on a 9Be target at a kinetic energy of 77.5 MeV/nucleon, and were collected with the LISE fragment separator at GANIL. The measured g factor (|g| in a range of 1.757±0.014) and quadrupole moment (|Qs|=38(5) mb) are in good agreement with shell-model calculations using effective interactions. The similarity between the g factor of 34mAl and that of 32Al confirms a dominant neutron 2p1h configuration π(d5/2)−1⊗ν(d3/2)−1(f7/2)2 of 34mAl. In addition, the enhanced quadrupole moment of 34mAl relative to that of 32Al suggests that the neutron 1p1h excitation across the N=20 shell gap introduces extra nucleon–nucleon correlation and deformation.
Highlights
Nuclear moments of the low-lying isomeric 1+ state of 34Al: Investigation on the neutron 1p1h excitation across N = 20 in the island of inversion
Theoretical works using large-scale shellmodel calculations associated this deformation with 2p2h excitations across N = 20, in which two neutrons are excited from the sd shell to the p f shell [4,8]
For isotopes belonging to the island of inversion, the energy of those 2p2h intruder configurations is lower than that of the normal configurations, giving rise to the deformed ground states
Summary
It has been revealed that even though the shell structure in stable nuclei can be well interpreted using simplified mean fields plus spin-orbit interactions [1,2], it may vary drastically in exotic nuclei due to the residual nucleon– nucleon interaction which produces different effective single particle energies depending on the proton-to-neutron ( Z /N) ratio [3] One of such typical examples is the N = 20 island of inversion [4], where unexpected ground-state deformation has been observed in the neutron-rich isotopes in the vicinity of 32Mg ( Z = 12). For a spin-polarized ensemble, the experimental β-decay asymmetry A is defined as
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