A gas-filled, liquid-nitrogen-cooled cell on-line with an accelerator permits fluorescence spectroscopy of atoms of heavy-ion reaction products generated at rates less than ${10}^3$ ${\mathrm{s}}^{\mathrm{-}1}$. This low required rate of production allows the optical spectroscopy of radioactive atoms to be extended further from stability. With the cell we have measured isotope shifts of ${}_{}{}^{152,154,156,158,166}{}_{}{}^{}\mathrm{Yb}$, reaching N=82, four nuclides further than previously reached with other techniques. The change in rms charge radius near the shell closure is surprisingly large.

• Received 7 September 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.1463