Photoneutron reactions on $^{165}$Ho and $^{169}$Tm in the giant dipole resonance region

Abstract

Photoneutron reactions were investigated for the deformed $^{165}$Ho and $^{169}$Tm nuclei from the vicinity of the neutron emission threshold up to $\sim$40~MeV, well above the giant dipole resonance (GDR) region, using quasimonochromatic laser Compton scattering $γ$-ray beams provided at the NewSUBARU facility, Japan. A high-and-flat efficiency moderated array of $^3$He counters was used for the neutron detection and an associated neutron multiplicity sorting method for extracting the $(γ,\,1nX)$, $(γ,\,2nX)$, $(γ,\,3nX)$ and $(γ,\,4nX)$ reaction cross sections and average neutron emission energies. The present $^{165}$Ho cross sections were compared to existing data, revealing discrepancies with the Saclay multiplicity sorting results and an overall 10$\%$ strength difference with the Livermore ones. There are no other data for $^{169}$Tm. GDR parameters based on phenomenological Lorentzian models were extracted by fitting the present $σ(γ,\,Sn)$ data with adjustments for the missing contribution of charged-particle-only reactions not observed experimentally. For both nuclei we observed high energy structures at 20-25~MeV, matching giant quadrupole resonance KMFR predictions. Based on the present centroid energies of the first and second GDR peaks, hydrodynamic model predictions gave intrinsic electric quadrupole moments of +7.00(34)~b and +7.38(28)~b for the ground states of $^{165}$Ho and $^{169}$Tm, respectively. The present experimental excitation functions and photoneutron energies were compared to statistical model calculations. Using the EMPIRE code, we performed a sensitivity test to phenomenological models of photon strength functions and nuclear level densities. The TALYS code was used to reproduce the present experimental $(γ,\,inX)$ cross sections and average neutron energies using microscopic nuclear level density models.