Wavelet analysis of monopole strength in highly deformed $^{24}$Mg

TL;DR

This study combines experimental alpha scattering data with QRPA calculations to analyze monopole excitations in $^{24}$Mg, revealing the impact of deformation and low nuclear incompressibility on resonance structures using wavelet analysis.

Contribution

It introduces a comprehensive wavelet analysis of monopole resonance fine structures in $^{24}$Mg, linking experimental data with QRPA models and highlighting the role of deformation and residual interactions.

Findings

Wavelet analysis identifies energy scales of 200-1000 keV in monopole resonances.

Residual interactions are crucial for realistic wavelet power spectra.

Deformation softness significantly affects monopole strength distribution.

Abstract

Experimental data on $\alpha$-particle inelastic scattering for monopole excitations in $^{24}$Mg in the excitation-energy region $E_{\rm x}$$=$$9$$-$$25$ MeV, obtained at the iThemba Laboratory for Accelerator Based Sciences (iThemba LABS), have been analyzed within a fully self-consistent quasiparticle random-phase approximation (QRPA) framework using two Skyrme parametrizations. A good overall agreement with the experimental data is achieved, particularly with the SkP$^{\delta}$ force, which corresponds to a low nuclear incompressibility of $K_{\infty}$$=$$202$ MeV. Extraction of energy scales, by means of wavelet analysis, characterizing the observed fine structure of the isoscalar giant monopole resonance (ISGMR) as well as the low-energy region $10$$-$$18$ MeV of the deformation-induced monopole-quadrupole coupling (MQC) in order to investigate the damping mechanism contributing to their decay widths. Characteristic energy scales are extracted from the fine structure using continuous wavelet transforms. The experimental results are compared to QRPA calculations employing the Skyrme parameterizations SkP$^{\delta}$ and SVbas. A significant, if not decisive, impact of the MQC strength on the wavelet power spectra is observed across the entire excitation-energy range of $10$$-$$24$ MeV. Wavelet features derived from the QRPA and from unperturbed two-quasiparticle (2qp) monopole strengths are compared. The results demonstrate that the residual interaction plays a key role in reproducing realistic wavelet powers and characteristic energy scales. Overall, a continuous range of scales $\delta E$$=$$200$$-$$1000$ keV is obtained rather than distinct isolated scales. The deformation softness of $^{24}$Mg is found to significantly influence both the monopole strength distribution and the wavelet characteristics.