Evidence for the Scissors Mode in 160Tb from the Two-Step Gamma Cascades Measurement

Spectra of two-step γ cascades (TSCs) following the neutron capture in 159Tb were measured by the sum-coincidence technique with a two-HPGe-detector facility [1] installed at a pure sub-thermal neutron beam of the 15 MW light-water research reactor at Rez near Prague. In order to obtain information about the quantities governing the γ decay of 160Tb levels--the photon strength functions (PSFs) and the level density (LD)--experimental TSC spectra were compared with the output of simulations of the γ cascade decay based on the validity of extreme statistical model. These simulations were performed with the aid of the DICEBOX algorithm [2] assuming various combinations of the PSFs and LD. The results obtained lead to an inescapable conclusion that the Scissors Mode (SM) plays a crucial role in the cascade γ decay of the product nucleus 160Tb. This finding represents the first evidence for the existence of the SM in an odd-odd nucleus.