An ultrasensitive laser spectroscopic method has been developed to perform hyperfine spectroscopy of heavy-ion-induced reaction products. It is based on resonance ionization in a buffer gas cell combined with radioactive decay detection. The feasibility has been demonstrated using the β-active ${\mathrm{isotope}}^{208}$ Tl. A sensitivity of 1.3×${10}^{\mathrm{-}3}$ of the two-step resonance-ionization process via the n=17 Rydberg level has been determined, utilizing a pulsed excimer-dye-laser combination. The previously unknown nuclear magnetic moment μ${(}^{208}$Tl) = 0.292(13)${\mathrm{\mu }}_{\mathit{n}}$ (${\mathrm{\mu }}_{\mathit{n}}$ is the nuclear magneton) and a rms radius difference between ${}_{}{}^{208}{}_{}{}^{}\mathrm{Tl}$ and ${}_{}{}^{207}{}_{}{}^{}\mathrm{Tl}$ of δ〈${\mathit{r}}^2$〉=0.099(15) ${\mathrm{fm}}^2$ were deduced from the 6p ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{1/2}$–7s ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$ transition.

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