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

The γ -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 γ strength down to unprecedentedly low γ energies of ≈500 keV. For the first time an enhanced low-energy γ -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 γ energies could have an impact of 2–3 orders of magnitude on the (n,γ ) reaction rates for r-process nucleosynthesis