High-Resolution Laser Spectroscopy on the Hyperfine Structure of Fm 255 ( Z = 100 )

We report on high-resolution laser spectroscopy of Fm 255 ( T 1 / 2 = 20 h ), one of the heaviest nuclides available from reactor breeding. The hyperfine structures in two different atomic ground-state transitions at 398.4 nm and 398.2 nm were probed by in-source laser spectroscopy at the RISIKO mass separator in Mainz, using the perpendicularly illuminated laser ion source and trap (PI-LIST) high-resolution ion source. Experimental results were combined with hyperfine fields from various atomic calculations, in particular using multiconfiguration Dirac-Hartree-Fock theory, as implemented in 18. In this manner, the nuclear magnetic dipole and electric quadrupole moments were derived to be μ = − 0.75 ( 5 ) μ N and Q s = + 5.84 ( 13 ) eb , respectively. The magnetic moment indicates occupation of the ν 7 / 2 [ 613 ] Nilsson orbital, while the large quadrupole moment confirms strong, stable prolate deformation consistent with systematics in the heavy actinides. Comparisons with available expectation values from nuclear theory show good agreement, providing a stringent benchmark for the used theoretical models. These results revise earlier data and establish Fm 255 as a reference isotope for future high-resolution studies.