Structure of28Mg studied byβ-decay spectroscopy of spin-polarized28Na: The first step of systematic studies on neutron-rich Mg isotopes

Abstract

As the first step of systematic studies on neutron-rich Mg isotopes, the structure of 28 Mg has been investigated by β-decay spectroscopy of spin-polarized 28 Na at TRIUMF. Spin-parity assignments have been successfully performed for levels in 28 Mg by taking advantage of the anisotropic β decay. Previous assignments for three levels were reconfirmed. New assignments were performed for four levels including a newly found 2 + level at 7.461 MeV. The experimental results were compared, on a level-by-level basis, with the shell-model calculation in the model space restricted to the sd shell. Overall good agreement between the experimental and theoretical results indicates dominance of the normal configurations in most of the 28 Mg levels. High performance of the present method promises successful application to heavier Mg isotopes to elucidate the shell evolution in the region of the N = 20 island of inversion.

Structure of Mg28 and influence of the neutron pf shell

“…The level energy is consistent with that of a level at 7200.3(6) keV previously observed via beta decay of 28 Na [9]. However the placement of gamma rays differs compared to the previous work, which does not show this level decaying to the 4 + 1 or 2 + 1 levels, indicating that the gamma rays observed in this work correspond to depopulation of a new level and that there are most likely two levels near 7.2 MeV in 28 Mg.…”

“…E1 transitions in 28 Mg have previously been shown to be hindered by a factor of 10 −3 − 10 −4 with respect to the Weisskopf estimate in the case of the 5172.9(7) keV level, as discussed in Ref. [9] and as indicated by the measured lifetime of this level in Table III. This hindrance, combined with the reduced energy of a hypothetical E1 transition compared to the observed 4664.9(12) keV (M2) transition, may explain the non-observation of competing transitions from the 6139.0(12) keV level.…”

“…The possibility of the 3181(3) keV gamma ray being a single escape peak for the gamma ray at 3698(3) keV was ruled out by its observation in coincidence with the 4 + 1 → 2 + 1 transition. In order to accurately measure the branching of transitions depopulating the 7203(3) keV level, the contamination of a single escape peak was taken into account when determining the intensity of the 3181(3) keV gamma ray by measuring the intensity relative to that of the 1473.59 (9) keV gamma ray when observed in coincidence with the 4 + 1 → 2 + 1 transition. After performing this correction, the intensity of the 5723(5) keV gamma ray was determined to be 27(13)% of the 3181(3) keV gamma ray.…”

“…Level energies include a correction for the recoil energy of the 28 Mg nucleus following gamma-ray emission. Intensities of gamma rays I γ,rel are listed relative to the intensity of the 1473.59 (9) keV gamma ray. Where appropriate, 1σ statistical and systematic uncertainties δτ stat and δτ sys are listed for lifetime values.…”

“…In the case of 28 Mg, which lies halfway between N = Z 24 Mg and the IoI at 32 Mg, shell model calculations in the sd-shell are able to reproduce the level energies of low lying states with reasonable accuracy [9]. Recently, developments in ab initio models such as the symmetry adapted no-core shell model (SA-NCSM) [10] allow for a detailed probe of the wavefunctions of sd shell nuclei without the use of effective charges -an approach which is useful for describing low-lying states in 28 Mg where nucleons are expected to primarily occupy the sd shell.…”