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, 2nγ))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 M1 component in the γ strength function with total strength B(M1↑)=11.8(19)μ_N². This is in good agreement with the integrated M1 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.