Test of gamma-ray strength functions in nuclear reaction model calculations

The impact of models for E1 and M1 gamma-ray strength functions on the results of nuclear model calculations of total average radiation widths, radiative capture cross sections, and gamma-ray spectra has been studied. We considered strength functions that reproduce photoabsorption and/or average resonance data but significantly differ from each other at low gamma-ray energies. As the calculated quantities critically depend on the strength functions in this energy region, model calculations can be used to test the low-energy behavior of strength functions. By analyzing the $^{197}\mathrm{Au}$, $^{143}\mathrm{Nd}$, $^{105}\mathrm{Pd}$, and $^{93}\mathrm{Nd}$ neutron capture reactions we found strong evidence for a model of the E1 strength function, which is characterized by the following properties: (i) an energy dependent spreading width of the underlying Lorentzian for the photoabsorption cross section and (ii) a nonzero, temperature dependent, limit as the transition energy tends to zero. This model is founded in theoretical work by Zaretskij, Sirotkin, and Kadmenskij and represents a partial breakdown of Brink's hypothesis.