Laser spectroscopic observations of nuclear reaction products produced with intensities of less than 104 atoms/second are now possible with several different methods. We describe the recoil into gas method which has recently been successful. This method is not Dopplerfree, but can give reasonable spectra if the resolution requirements of the spectra are not too high. It has the great advantage that it very efficiently uses the atoms, and spectra have been observed with primary production rates of less than 103 atoms/sec. Our recent work has concentrated on developing the recoil into gas method for the refractory element Hf. In order that the atoms could be cycled to produce many fluorescence photons, nitrogen and hydrogen impurity gases were added to the argon buffer gas to quench metastable levels to the ground state. In this way spectra could be obtained with fluxes of 104 atoms/second. Future prospects for trapping radioactive atoms in a magneto-optic trap will be discussed.

An apparatus has been developed for on-line studies of the optical spectra of radioactive atoms formed from heavy-ion fusion evaporation reaction products. The unstable nuclei recoil into cell containing a noble gas in which they are neutralized and thermalized by the gas. A laser beam passing through the cell is tuned through an optical resonance of the diffusing atoms, and the resulting fluorescence is detected. The apparatus provides a relatively long laser-atom interaction time and allows the study of atoms too short lived or produced in amounts too small or from elements too refractory to be studied by other means. Isotope shifts of 152,154,156,158Yb have been measured, as well as the shifts between the stable isotopes of the refractory element Hf.

Optical isotope shifts were measured on the 618-nm, ground-state transition of Hf i for the isotopes $^{174\mathrm{\ensuremath{-}}180}\mathrm{Hf}$. For the isotope pair 178-180, a specific mass shift of approximately -8 times the normal mass shift was deduced for this transition. The large size of this shift is consistent with other specific mass shifts that result from a broken d-shell electron pair. A transtion electronic factor of ${\mathit{E}}_{618}$=0.158\ifmmode\pm\else\textpm\fi{}0.012 was extracted from the data. Excited-state hyperfine coefficients for the odd isotopes $^{179}\mathrm{Hf}$ and $^{177}\mathrm{Hf}$ were also deduced. The values of mean-square-charge radii obtained for the isotopic chain agree well with previously published results.

Fluorescence spectra from atoms of radioactive nuclides produced by heavy-ion fusion reactions have been measured on line with a gas-filled, liquid-nitrogen-cooled resonance cell. This device yields useful spectra from Yb nuclides produced at rates lower than 1000 s−1. Spectra have now been obtained for Yb isotopes more than 18 neutrons away from the line of β stability. These data show a large isotope shift surprisingly near the N = 82 shell closure. The apparatus has also been used to measure isotope shifts of the refractory element Hf.

Isotope shifts have been measured for the neutron deficient even Yb isotopes up to the neutron shell closure at N=82. The isotope shifts were measured using the 556-nm atomic resonance transition from the1 S 0 ground state to the3 P 1 level. The heavier isotopes of Yb have been investigated by Buchinger et al./1/. The change in (r 2) observed for Yb isotopes with N=82–90 has considerably different behavior than for the lighter rare earths.

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 ${10}^{3}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$. 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.