An ion cooler-buncher for high-sensitivity collinear laser spectroscopy at ISOLDE

A gas-filled segmented linear Paul trap has been installed at the focal plane of the high-resolution separator (HRS) at CERN-ISOLDE. As well as providing beams with a reduced transverse emittance, this device is also able to accumulate the ions and release the sample in bunches with a well-defined time structure. This has recently permitted collinear laser spectroscopy with stable and radioactive bunched beams to be demonstrated at ISOLDE. Surface-ionized K-39,K-44,K-46 and Rb-85 beams were accelerated to 30 keV, mass separated and injected into the trap for subsequent extraction and delivery to the laser setup. The ions were neutralized in a charge exchange cell and excited with a co-propagating laser. The small ion beam emittance allowed focussing in the ion-laser overlap region, which is essential to achieve the best experimental sensitivity. Fluorescent photons were detected by a photomultiplier tube as a frequency scan was taken. A gate (typically 7-12 mu s wide) was set on the photomultiplier signal to accept the fluorescent photons within the time window defined by the bunch. Thus, using accumulation times of 100 ms, the dominant contribution to background due to continuous laser scattering could be reduced by a factor of up to 4 x 10(4).