Interpretation of Quasi-Elastic Scattering of 11-19 MeV Monochromatic Photons by Holmium Using Zero-Point Vibrations in the Hydrodynamic Model

Abstract

A bremsstrahlung monochromator with an energy resolution of 0.6% was used to measure the 135° quasi-elastic scattering of photons by ${\mathrm{Ho}}^{165}$ at 48 energies between 10.92 and 19.06 MeV. No very fine structure was observed but the gross splitting of the giant dipole resonance of this deformed nucleus into two peaks was clearly resolved. The 135° differential scattering cross section has maxima at 12.49 MeV (0.35 mb/sr) and 16.50 MeV (0.45 mb/sr); a minimum occurs at 13.50 MeV (0.28 mb/sr). The observed energy dependence confirms the existence of tensor polarizability (i.e., the absorption cross section depends on the relative orientation of the nuclear spin and the photon polarization). The combination of the observed scattering and photoabsorption results can be used to test photonuclear models; the existing data suggest that it may be necessary to refine the hydrodynamic model by including zero-point vibrations. The absolute magnitude of the scattering implies that the previously reported energy-integrated absorption cross section should be reduced by 21%; this reduces the integrated sum in the giant resonance to 1.07±0.16 times the sum rule prediction without exchange forces.