The hyperfine splitting of the 1s ground state of hydrogenlike T1 has been
measured for the two stable isotopes using emission spectroscopy in the Sup
erEBIT electron-beam ion trap, giving 3858.22 +/-0.30 Angstrom for Tl-203(8
0+) and 3821.84 +/-0.34 Angstrom for Tl-205(80+) with a wavelength differen
ce Delta lambda =36.38 +/-0.35 Angstrom. This difference is consistent with
estimates based on hyperfine anomaly data for neutral T1 only if finite si
ze effects are included in the calculation. By using previously determined
nuclear magnetic moments, and applying appropriate corrections for the nucl
ear charge distribution and radiative effects, the experimental splittings
can be interpreted in terms of nuclear magnetization radii <r(m)(2)> (1/2)=
5.83(14) fm for Tl-203 and <r(m)(2)> (1/2) = 5.89(14) fm for Tl-205. These
values are 10% larger than derived from single-particle nuclear magnetizati
on models, and are slightly larger than the corresponding charge distributi
ons.