Isoscalar giant resonances inPb208—in particular2ℏωL=0, 2, 4, and 6excitations—studied in small angleαscattering

Abstract

Inelastic excitation of giant resonances has been studied in 172 MeV $\ensuremath{\alpha}$ scattering from $^{208}\mathrm{Pb}$ at scattering angles of 1.5\ifmmode^\circ\else\textdegree\fi{}-8\ifmmode^\circ\else\textdegree\fi{}. Fine structures in the region of the giant monopole and quadrupole resonances indicate multipolarities also different from $L=0 \mathrm{and} 2$. The isoscalar $L=0 \mathrm{and} 2$ strengths in the giant resonance region yield (90\ifmmode\pm\else\textpm\fi{}20)% and (70\ifmmode\pm\else\textpm\fi{}20)% of the corresponding energy weighted sum rule limit. This is consistent with other hadron scattering results, with higher energy electron scattering, and with microscopic random phase approximation results. However, the strength for $L=2$ excitations is significantly larger than extracted from lower energy electron scattering. Fine structures observed are compared with high resolution (p,p\ensuremath{'}) and (e,e\ensuremath{'}) results. At scattering angles 5\ifmmode^\circ\else\textdegree\fi{}-8\ifmmode^\circ\else\textdegree\fi{} new structures have been observed at ${E}_{x}=12.5 \mathrm{and} 16.0$ MeV consistent with multipolarities $L=4 \mathrm{and} 6$. Together with the contribution of higher multipolarity extracted at lower excitation energies from the angular distributions of giant monopole and quadrupole excitations, this indicates rather broad distributions of $2\ensuremath{\hbar}\ensuremath{\omega}$ $L=4 \mathrm{and} 6$ excitations with centroid energies of 12.2 and 13.7 MeV and widths of 4.5 and 7.8 MeV, respectively. The extracted sum rule strengths for $L=4 \mathrm{and} 6$ are in the order of 30% in each case. In the region of the giant monopole resonance evidence for additional $L=2$ excitation was found (\ensuremath{\le}14% energy weighted sum rule strength). The fact that strong contributions of $L>0$ are obtained solves the problem of a former too large monopole cross section in $\ensuremath{\alpha}$ scattering which was inconsistent with microscopic descriptions. A recently suggested "antiscaling" mode for the giant monopole resonance, in which the nuclear surface vibrates opposite in phase to the "scaling" mode, can be ruled out by our experimental data. The existence of two odd parity giant resonances at ${E}_{x}=18.7$ MeV ($L=3$) MeV ($L=1$) with widths of 5.0 and 5.9 MeV, respectively, are confirmed by our small angle scattering experiments. Our description of the dipole compressional mode is in excellent agreement with 200 MeV proton scattering data.NUCLEAR REACTIONS $^{208}\mathrm{Pb}(\ensuremath{\alpha}, {\ensuremath{\alpha}}^{\ensuremath{'}})$; ${E}_{\ensuremath{\alpha}}=172$ MeV. Small angle scattering, $\ensuremath{\theta}=1.5\ifmmode^\circ\else\textdegree\fi{}\ensuremath{-}8\ifmmode^\circ\else\textdegree\fi{}$; measured $\ensuremath{\sigma}(\ensuremath{\theta})$. Deduced excitation strength distributions for giant resonances of multipolarities $L=0\ensuremath{-}6$.