Experimental observation of the M1 scissors mode in 254No

Abstract

We present the first experimental evidence of the scissors mode in the superheavy nucleus 254No produced in the 208Pb(48Ca, 2nγ))254No reaction. The spectrum of γrays emitted by the excited 254No nuclei shows an enhanced γ-ray yield for transition energies of ≈2.5MeV. By measuring the linear polarization properties of the emitted γrays, 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 M1component in the γstrength function with total strength B(M1↑)=11.8(19)μ2N. This is in good agreement with the integrated M1strength 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.