Isoscalar monopole and dipole transitions in $^{24}$Mg, $^{26}$Mg and $^{28}$Si

TL;DR

This study investigates isoscalar monopole and dipole excitations in $^{24}$Mg, $^{26}$Mg, and $^{28}$Si using experimental scattering data and theoretical models, revealing the interplay of cluster and mean-field phenomena.

Contribution

The paper combines experimental inelastic alpha scattering with QRPA and AMD+GCM calculations to analyze the nature of low-energy isoscalar excitations in sd-shell nuclei, highlighting cluster and mean-field interactions.

Findings

Low-energy dipole states show cluster and mean-field properties.

Vorticity appears mainly in dipole states with K=1.

A collective state with strong octupole strength exists near the alpha threshold.

Abstract

Nuclei in the $sd$-shell demonstrate a remarkable interplay of cluster and mean-field phenomena. The $N=Z$ nuclei, such as $^{24}$Mg and $^{28}$Si, have been the focus of the theoretical study of both these phenomena in the past. The cluster and vortical mean-field phenomena can be probed by excitation of isoscalar monopole and dipole states in scattering of isoscalar particles such as deuterons or $\alpha$ particles. Inelastically scattered $\alpha$ particles were momentum-analysed in the K600 magnetic spectrometer at iThemba LABS, Cape Town, South Africa. The scattered particles were detected in two multi-wire drift chambers and two plastic scintillators placed at the focal plane of the K600. In the theoretical discussion, the QRPA and AMD+GCM were used. The QRPA calculations lead us to conclude that: i) the mean-field vorticity appears mainly in dipole states with $K=1$, ii) the dipole (monopole) states should have strong deformation-induced octupole (quadrupole) admixtures, and iii) that near the $\alpha$-particle threshold, there should exist a collective state (with $K=0$ for prolate nuclei and $K=1$ for oblate nuclei) with an impressive octupole strength. The results of the AMD+GCM calculations suggest that some observed states may have a mixed (mean-field + cluster) character or correspond to particular cluster configurations. A tentative correspondence between observed states and theoretical states from QRPA and AMD+GCM was established. The QRPA and AMD+GCM analysis shows that low-energy isoscalar dipole states combine cluster and mean-field properties. The QRPA calculations show that the low-energy vorticity is well localized in $^{24}$Mg, fragmented in $^{26}$Mg, and absent in $^{28}$Si.