Nuclear structure of light thallium isotopes as deduced from laser spectroscopy on a fast atom beam.

After optimizing the system by experiments on ^201,203,205Tl the neutron-deficient isotopes ^189-193Tl have been studied using the col linear fast atom beam laser spectroscopy system at UNISOR on-line to the Holifield Heavy Ion Research Facility. A sensitive system for the measurements was developed since the light isotopes were available in mass-separated beams of only 7 x 10⁴ to 4 x 10⁵ atoms per second. By laser excitation of the 535 nm atomic transitions of atoms in the beam, the 6s²7s ²S_1/2 and 6s²6s ²P_3/2 hyperfine structures were measured, as were the isotope shifts of the 535 nm transitions. From these, the magnetic dipole moments, spectroscopic quadrupole moments and isotopic changes in mean-square charge radius were deduced. The magnetic dipole moments are consistent with previous data. The ^190,192Tl isotopes show a considerable difference in quadrupole deformations as well as an anomalous isotope shift with respect to ¹⁹⁴Tl. A large isomer shift in ¹⁹³Tl is observed implying a larger deformation in the 9/2^- isomer than in the ½⁺ ground state. The ^189,191,193Tl isomers show increasing deformation away from stability. A deformed shell model calculation indicates that this increase in deformation can account for the dropping of the 9/2^- band in these isotopes while an increase in neutron pairing correlations, having opposite and compensating effects on the rotational moment of inertia, maintains the 9/2^- strong-coupled band structure.