Soft Radiative Strength in Warm Nuclei

摘要

We present data on the soft (E(sub (gamma)) < 3-4 MeV) radiative strength function (RSF) for electromagnetic transitions between warm states (i.e. states several MeV above the yrast line) from two different types of experiments. The Oslo method provides data on the total level density and the sum (over all multipolarities) of all RSFs by sequential extraction from primary-(gamma) spectra. Measurements of two-step-decay spectra following neutron capture yields two-step-cascade (TSC) intensities which are roughly proportional to the product of two RSFs. Investigations on (sup 172)Yb and (sup 57)Fe have produced unexpected results. In the first case, a strong (B(M1 (up-arrow)) = 6.5 (mu)(sub N)(sup 2)) resonance at E = 3.3 MeV was identified. In the second case, a large (more than a factor of 10) enhancement compared to theoretical estimates of the very soft (E(sub (gamma)) (le) 3 MeV), summed RSF for transitions between warm states was observed. A somewhat weaker (factor (approx) 3) enhancement of the RSF in Mo isotopes observed within the Oslo method still awaits confirmation from TSC experiments.