Gamma ray spectra following the capture of 0.7, 1.7, and 2.6 MeV neutrons in Tl and Hg, of 0.7 and 2.6 MeV neutrons in Ta, and of 2.6 MeV neutrons in Au were measured with a NaI detector. The spectral distributions were obtained by unfolding the detector response function, and γ-ray strength functions were deduced for Tl, Au, and Ta by a spectrum fitting method. The strength functions in Tl and Au, when compared with the Lorentzian predictions, show a strong decrease below ~5 MeV and in Tl there is a resonance-like structure at ~5.5 MeV. No such structure is found in the strength function for Ta. This behavior is qualitatively interpreted in terms of recent particle–hole calculations.
Accessing the Single-Particle Structure of the Pygmy Dipole Resonance in
Pb208