Extra Gamma-Ray Strength For Sn-116,Sn-117 Arising From Pygmy Dipole Resonance

Photoneutron cross sections were measured for Sn-117 and Sn-116 near neutron thresholds with quasi-monochromatic laser Compton scattering gamma-rays. The measured cross sections for Sn-117 and Sn-116 are strongly enhanced from the threshold behavior expected for L=1 neutron emissions after El photoexcitation. This suggests the presence of extra gamma-ray strength in the low-energy tail of the giant dipole resonance. The present cross sections were analyzed together with radiative neutron capture cross sections for Sn-116 within the framework of the statistical model calculation. It is shown that the extra gamma-ray strength can be interpreted as pygmy El resonance which was previously reported in the nuclear resonance fluorescence experiment for Sn-116 and Sn-124.