Effect of Nuclear Polarization on the Giant Dipole Resonance ofHo165

Abstract

The ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections of polarized ${\mathrm{Ho}}^{165}$ were measured between 10 and 21 MeV using the nearly monoenergetic photon beam from the annihilation in flight of fast positrons. Two independent cross sections were obtained by orienting the polarization axis of the target nuclei perpendicular to and parallel to the photon beam. From these, the cross sections corresponding to the intrinsic modes of the giant resonance were computed. The target consisted of nine single crystals of holmium metal having a total mass of 50 g, and was polarized by cooling to approximately 0.13\ifmmode^\circ\else\textdegree\fi{}K in the presence of a 15-kOe magnetic field. The alignment parameter ${f}_{2}$ was determined to be 0.43\ifmmode\pm\else\textpm\fi{}0.05 by measuring the anisotropy of the $\ensuremath{\gamma}$ rays emitted by ${\mathrm{Ho}}^{166m}$ (created in the target by neutron activation). Both major peaks of the total photoneutron cross section, ${\ensuremath{\sigma}}_{t}=\ensuremath{\sigma}[(\ensuremath{\gamma}, n)+(\ensuremath{\gamma}, pn)+(\ensuremath{\gamma}, 2n)]$, exhibited a dependence upon target orientation that agrees qualitatively with the collective model for deformed nuclei. The observed anisotropy was (74\ifmmode\pm\else\textpm\fi{}13)% of that predicted by hydrodynamic models.