Experimental observation of the M1 scissors mode in 254No.

We present the first experimental evidence of the scissors mode in the superheavy nucleus 254No produced in the 208Pb(48Ca, 2 n γ))254No reaction. The spectrum of γ rays emitted by the excited 254No nuclei shows an enhanced γ -ray yield for transition energies of ≈2.5 MeV. By measuring the linear polarization properties of the emitted γ rays, we confirm that the transitions in the enhancement region are predominantly of magnetic-dipole character, characteristic for the scissors mode. To further characterize the enhanced γ -ray yield, simulations of the electromagnetic decay of 254No were performed. The observed enhancement is reproduced by including an M 1 component in the γ strength function with total strength B (M 1 ↑) = 11.8 (19) μ N 2. This is in good agreement with the integrated M 1 strength from sum-rule estimates and new calculations within the quasi-particle random-phase approximation presented here. Our results provide a stringent test of phenomenological formulae for the scissors mode currently used in stellar nucleosynthesis calculations. We find that those formulae are not satisfactory, and we recommend using sum-rule estimates assuming a rigid-body moment of inertia instead for describing the scissors mode in superheavy nuclei. [ABSTRACT FROM AUTHOR]