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

Abstract

The neutron-deficient isotopes $^{189\mathrm{\ensuremath{-}}194}\mathrm{Tl}$ have been studied using collinear fast atom beam laser spectroscopy with mass-separated beams of 7\ifmmode\times\else\texttimes\fi{}${10}^{4}$ to 4\ifmmode\times\else\texttimes\fi{}${10}^{5}$ atoms per second. By laser excitation of the 535 nm atomic transitions of atoms in the beam, the 6${s}^{2}$7s $^{2}S_{1/2}$ and 6${s}^{2}$6p $^{2}P_{3/2}$ 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 radii were deduced. A large isomer shift in $^{193}\mathrm{Tl}$ was observed, implying a larger deformation in the ${(9/2}^{\mathrm{\ensuremath{-}}}$ isomer than in the ${(1/2}^{+}$ ground state. The $^{189}$,191,193${\mathrm{Tl}}^{\mathrm{m}}$ isotopes have deformations that increase as the mass decreases. A deformed shell model calculation indicates that this increase in deformation can account for the drop in energy of the ${(9/2}^{\mathrm{\ensuremath{-}}}$ bandhead in these isotopes. An increase in neutron pairing correlations, having opposite and compensating effects on the rotational moment of inertia, maintains the spacing of the levels in the ${(9/2}^{\mathrm{\ensuremath{-}}}$ strong-coupled band. Results for $^{194}\mathrm{Tl}^{\mathrm{m}}$ differ from previously published values, but are consistent with the $^{190}$,192${\mathrm{Tl}}^{\mathrm{m}}$ data.