Giant Resonance in Deformed Nuclei: Photoneutron Cross Sections for Eu153, Gd160, Ho165, and W186

Abstract

Photoneutron cross sections, including $\sigma \left[\right(\gamma , n)+(\gamma , pn\left)\right]$, $\sigma \left[\right(\gamma , 2n)+(\gamma , p2n\left)\right]$, and $\sigma (\gamma , 2n)$, were measured for ${\mathrm{Eu}}^{153}$, ${\mathrm{Gd}}^{160}$, ${\mathrm{Ho}}^{165}$, and ${\mathrm{W}}^{186}$ as a function of photon energy from 8 to 29 MeV. The photon energy resolution varied from less than 300 keV at the lowest to 400 keV at the highest energies, and the data were taken at intervals of 300 keV or less. The source of radiation was the monoenergetic photon beam obtained from the annihilation in flight of fast positrons. The partial cross sections were determined by neutron multiplicity counting, and the average neutron energies for both single- and double-photoneutron events were determined simultaneously with the cross-section data by the ring-ratio technique. Nuclear information extracted from the data includes giant-resonance parameters, integrated cross sections and their moments, nuclear symmetry energies, intrinsic quadrupole moments, and nuclear level density parameters. The data were analyzed to obtain a mean radius parameter $R_0=1.26\mathrm{}\pm 0.02$ F for these nuclei.