Many body study of the Bohr-Weisskopf effect in the thallium atom

We report the relativistic coupled cluster study of the hyperfine structure and effect of the nuclear magnetization distribution (Bohr-Weisskopf effect) in the $6P_{1/2}$, $6P_{3/2}$ and $7S_{1/2}$ states of several Tl isotopes. It is shown that the Gaussian basis set can be used in such electronic structure calculations and provide a good accuracy. For the ground electronic state of the neutral Tl atom achieved uncertainty for the hyperfine structure constant is smaller than 1\%. A strong Bohr-Weisskopf correction (about 16\%) was found for the $6P_{3/2}$ state which can be of interest for the nuclear structure theory. Basing on the theoretical treatment as well as the available experimental data nuclear magnetic moments of short lived ${}^{191}$Tl and ${}^{193}$Tl isotopes were also predicted.