First observation of low-energy gamma-ray enhancement in the rare-earth region

The gamma-ray strength function and level density in the quasi-continuum of 151,153Sm have been measured using bismuth germanate shielded Ge clover detectors of the STARLiTeR system. The Compton shields allow an extraction of the gamma strength down to unprecedentedly low gamma energies of 500 keV. For the first time an enhanced low-energy gamma-ray strength has been observed in the rare-earth region. In addition, for the first time both the upbend and the well-known scissors resonance have been observed simultaneously for the same nucleus. Hauser-Feshbach calculations show that this strength enhancement at low gamma energies could have an impact of 2 - 3 orders of magnitude on the (n,gamma) reaction rates for r -process nucleosynthesis.