Level density and radiative strength of $^{116,117}$Sn

We have determined the level density and radiative strength function for energies less than the neutron separation energy for the isotopes $^{116,117}$Sn using the Oslo method. The excited nuclei were produced by the $(^3$He,$\alpha)$ and $(^3$He,$^3$He$^\prime)$ reactions, respectively, from a 38-MeV-$^3$He beam bombarding a highly enriched $^{117}$Sn target. The level densities show the characteristic near-exponential increase with energy and a factor 5 difference in magnitude due to the odd-even effect. Step structures which indicate successive pair breaking are superimposed the general trend. The radiative strength function of $^{117}$Sn shows a dramatic increase in slope above 4.5 MeV $\gamma$ energy. Connecting our data, from below the neutron separation energy to literature data above this value (obtained from the $(\gamma,n)$ reactions) suggests the presence of a pygmy resonance with a centroid just above the neutron separation energy of 7 MeV and about 5--10 times the strength of the M1 spin-flip resonance or 1--2\% of the TRK sum rule.