E0 strength inC12fromLi

Abstract

The ( Li, Li') reaction was studied on "C at EL; =156 MeV at extreme forward angles including 6I=O. Spectra were taken in the excitation energy region E 30 MeV. EO strength was deduced at E"=7. 65 MeV (9.5% energy weighted sum rule) around E =10.2 MeV (5%+1% energy weighted sum rule) and in the region 19 MeV & E & 21.5 MeV (5%+2% energy weighted sum rule). The isoscalar electric giant monopole resonance (GMR) is now well established in medium and heavy nuclei, mainly from small angle a scattering. ' In some cases, information about decay properties is also available from coincidence experiments. In light nuclei, there exists considerable less information on the GMR. Especially in ' C the situation with regard to the EO strength distribution is not yet clear. Besides the well known 0+ state at E =7.65 MeV, exhausting about 10% of the usually used energy weighted sum rule (EWSR, cf. , e.g. , Ref. 7), there is only a few and, to some extent, confusing data about additional EO strength. In nuclear data compilations, a strength concentration around E"=10. 3 MeV has been tentatively assigned to be 0+. In a more recent He small angle scattering experiment, monopole strength of about 2.1% EWSR was reported at a significant lower energy of E =9.15 MeV with a width of I =1.8 MeV. This could not be confirmed in an scattering experiment by Youngblood et al. and in an earlier experiment of our group, ' also using a scattering at extreme forward angles. In Ref. 2, however, a broad bump around E = 10.3 MeV ( I = 2. 5 MeV) was reported, consistent with a 0+ assignment. Evidence for EO strength at higher excitation energies was only observed in the above-mentioned He-scattering experiment (about 2.5% EWSR around 20.5 MeV) and in a recent (e, e'p) coincidence experiment, " where ) 1% EO EWSR was reported in the same energy region. In order to obtain a conclusive and quantitative picture of the EO strength distribution in ' C, we performed a Li-scattering experiment at extreme forward angles, where the monopole assignment should be unambiguous. Li scattering has the advantage of a very favorable resonance-to-background ratio, especially at higher excitation energies, due to the low binding energy of the Li projectiles. ' The experiment was carried out at the 156 MeV Li + beam of the Karlsruhe cyclotron, using the new magnetic spectrograph "Little John. "' We measured the angular distributions for 0 &OL;~,b&2. 5 in 0.5 steps in the excitation energy region 0 & E 5 30 MeV. In the upper part of Fig. 1, the most interesting region of an excitation spectrum taken at O~,b --2. 5' is shown. At this angle, mono-pole strength is only weakly excited, as can be seen from the 7.65 MeV state. In contrast, the well known 3 state at 9.64 MeV and the 1 strength at 10.85 MeV are relative strongly excited. At 1. 5' (middle part), the relative intensities of the 0+ and 3 states have already changed. In the 0' spectrum (lower part) the EO strength has achieved its maximum.In addition to the 0+ state at 7.65 MeV, a broad bump around 10.2 MeV with a wi...

Excitation of giant monopole resonance inMg24usingLi

“…Therefore the loss of excited strength is not important. Confirming this decision, earlier 0' measurements from our group on the nucleus ' C [7] show that the high portion of the background at g, b=0' renders the analyses more difficult.…”

Isoscalar monopole and dipole excitations of cluster states and giant resonances inC12

“…The separation of the low-energy ISM strengths from the ISGMR was also found in 12 C [5,9]. The ISM strength distributions in 12 C observed by (α, α ′ ) and ( 6 Li, 6 Li ′ ) scatterings [5,9,17] show that the low-energy ISM strengths in E 12 MeV exhaust the significant percentage of the EWSR comparable to the high-energy strengths in E > 12 MeV of the ISGMR. In 12 C, a couple of excited 0 + states have been experimentally observed near the 3α threshold energy (see, for instance, Refs.…”

“…6(a) and Table II). The data evaluated by (α, α ′ ) [9] and ( 6 Li, 6 Li ′ ) [17]scatterings is 7.6±0.9% and 9.5%, which is about a half of the values measured (e, e ′ ) scatterings. This inconsistency of the ISM strength for the 0 + state might come from the reaction model dependence in the DWBA analysis of nuclear scatterings.…”

Giant resonances inSn116from 240 MeVLi6scattering

“…ISGMR studies in 12 C and 24 Mg with 156 MeV 6 Li inelastic scattering have been reported respectively by Eyrich et al [16] and Dennert et al [17] However, the low bombarding energy limited the useful excitation energy range to E x 30 MeV.…”