Deformation effects on isoscalar giant resonances inMg24

Abstract

Strength distributions for isoscalar giant resonances with multipolarity L ≤2 have been determined in 24 Mg from "instrumental background-free" inelastic scattering of 386-MeV α particles at extremely forward angles, including 0• . The isoscalar E0, E1, and E2 strengths are observed to be 57±7%, 111.1 +10.9 −7.2 %, and 148.6±7.3%, respectively, of their energy-weighted sum rules in the excitation energy range of 6 to 35 MeV. The isoscalar giant monopole (ISGMR) and quadrupole (ISGQR) resonances exhibit a prominent K-splitting which is consistent with microscopic theory for a prolate-deformed ground state of 24 Mg. For the ISGQR it is due to splitting of the three K components, whereas for the ISGMR it is due to its coupling to the K=0 component of the ISGQR. Deformation effects on the isoscalar giant dipole resonance are less pronounced, however.

The compression-mode giant resonances and nuclear incompressibility

“…They observed a dramatic variation in the extracted ISGMR strength distributions in these nuclei. In particular, the A=92 nuclei, 92 Zr and 92 Mo, emerged quite disparate from the others: the ISGMR energies (E ISGMR ) for 92 Zr and 92 Mo were observed to be, respectively, 1.22 and 2.80 MeV higher than that of 90 Zr. Consequently, the K A values determined for 92 Zr and 92 Mo were, respectively, ∼27 MeV and ∼56 MeV higher than the K A for 90 Zr (see Fig.…”

“…However, because of the aforementioned LE component of the ISGDR even in the spherical nuclei, this coupling is not as clearly evident as in case of the ISGMR. However, two effects are discerned in going from 144 Sm to 154 Sm, both The most interesting result on the effect of deformation on the ISGMR strength was presented recently in the nucleus 24 Mg [91,92]. Generally, the ISGMR strength (indeed, all multipole strengths) in the lighter-mass nuclei (A<58) is fragmented over a wide excitation energy range and does not form a nice "peak" as in the higher-A nuclei (see, for example, Refs.…”

“…The data are from Refs. [85,8,86,16,27,87,88,89,90,91,92]. multipoles (hence the inclusion of multipoles up to L=7, typically, in the MDA).…”

“…An experiment was subsequently performed at RCNP to further investigate and elucidate this unusual, and unexpected, effect. In "background-free" inelastic α-scattering experiments on 90,92 Zr, and 92 Mo, no evidence was found for this anomalous effect in the A=92 nuclei [87,174]. This was clear already in the 0 • spectra (where the ISGMR is excited maximally) and in the "difference spectra" (which contain primarily the ISGMR strength; see the discussion in the section on Experimental Techniques): these spectra are virtually identical for the three nuclei.…”

“…ISGMR strength distribution in 24 Mg (solid circles). The dash-dotted (blue) and solid (red) lines show microscopic calculations for spherical and prolate ground-state deformation, respectively.Figure fromRef [92]…”