Nuclear Magnetic Moment of ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$

The magnetic moment 1.876(5)${\mathrm{\mu }}_{\mathit{N}}$ of 4.77-min ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$, the only heavy nucleus with a doubly magic core plus a single $s_{\frac{1}{2}}$ particle or hole, was measured from the hfs by collinear fast-beam laser spectroscopy at ISOLDE (isotope separator at the CERN synchrotron). The result is of theoretical importance as a test case for core polarization since the nuclear structure is relatively simple and the orbital part of the magnetic moment, including strong pion-exchange contribution, is expected to be zero.