Cross section and analyzing power measurements for the giant resonance region in 208Pb with 200-MeV protons.

Abstract

The giant resonance region in $^{208}\mathrm{Pb}$ has been studied using inelastic scattering of 200-MeV polarized and unpolarized protons. Both differential cross sections \ensuremath{\sigma}(theta) and analyzing power ${A}_{y}$(theta) measurements were made. The isoscalar giant quadrupole resonance at 10.6\ifmmode\pm\else\textpm\fi{}0.5 MeV, and the combined isovector giant dipole resonance and isoscalar giant monopole resonance at 14.0\ifmmode\pm\else\textpm\fi{}0.6 MeV are clearly observed. Within uncertainties all three giant resonances are in accord with full energy-weighted sum rule depletions, based on comparisons with macroscopic distorted wave Born approximation calculations. Data for a peak at 20.9\ifmmode\pm\else\textpm\fi{}1.0 MeV are found to be consistent with an isoscalar giant octupole resonance exhausting 36\ifmmode\pm\else\textpm\fi{}12 % of the energy-weighted sum rule strength. A peak located at an excitation energy of 12.0\ifmmode\pm\else\textpm\fi{}0.7 MeV is shown to be a 2\ensuremath{\Elzxh}\ensuremath{\omega}, L=4 transition depleting 8\ifmmode\pm\else\textpm\fi{}3 % of the hexadecapole energy-weighted sum rule strength. The measured reduced transition probability for the ${3}^{\mathrm{\ensuremath{-}}}$ state at 2.614 MeV is consistent both with the accepted value and with results measured at 334 and 800 MeV. Our results for the ${3}^{\mathrm{\ensuremath{-}}}$ state at 2.614 MeV and for the giant quadrupole resonance do not support the anomalous behavior found in earlier studies at 104, 156, and 201 MeV. This shows that the macroscopic distorted wave Born approximation can be used to extract deformation lengths providing meaningful comparison with electromagnetic energy-weighted sum rules for proton inelastic scattering to at least 800 MeV incident proton energy.