Extra γ‐ray strength for $116,117Sn$ arising from pygmy dipole resonance

Photoneutron cross sections were measured for $117Sn$ and $116Sn$ near neutron thresholds with quasi‐monochromatic laser Compton scattering γ‐rays. The measured cross sections for $117Sn$ and $116Sn$ are strongly enhanced from the threshold behavior expected for $L = 1$ neutron emissions after E1 photoexcitation. This suggests the presence of extra γ‐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 $116Sn$ within the framework of the statistical model calculation. It is shown that the extra γ‐ray strength can be interpreted as pygmy E1 resonance which was previously reported in the nuclear resonance fluorescence experiment for $116Sn$ and $124Sn.$