γ-ray strength function forSn116,117with the pygmy dipole resonance balanced in the photoneutron and neutron capture channels

Abstract

Photoneutron cross sections were measured for $^{117}\mathrm{Sn}$ and $^{116}\mathrm{Sn}$ near the neutron thresholds at $6.94$ and $9.56$ MeV, respectively, with quasi-monochromatic laser-Compton scattering $\ensuremath{\gamma}$ rays. The $^{117}\mathrm{Sn}$ cross section, which is strongly enhanced near the low threshold, provides evidence for the presence of extra $\ensuremath{\gamma}$ strength in the low-energy tail of the giant dipole resonance. A coherent analysis of the photoneutron data for $^{117}\mathrm{Sn}$ together with the neutron capture on $^{116}\mathrm{Sn}$ shows that the $\ensuremath{\gamma}$-ray strength function is balanced in the photoneutron and neutron capture channels in terms of the microscopic Hartree-Fock-Bogoliubov plus quasiparticle random-phase approximation model of $E1$ strength combined with a pygmy $E1$ resonance at $8.5$ MeV. The high-energy part of the pygmy resonance is also suggested in the photoneutron cross section for $^{116}\mathrm{Sn}$.