Efficient isobar suppression by photodetachment in a RF quadrupole ion cooler

A highly efficient method for suppressing isobar contaminants in negative radioactive ion beams by photodetachment is demonstrated. A laser beam having the appropriate photon energy is used to selectively neutralize the contaminant if the electron affinity of the contaminant is lower than the electron affinity of the desired radioactive ions. The photodetachment efficiency can be dramatically increased when the laser-ion interaction is made inside a radio frequency quadrupole ion beam cooler where the ion residence time can be a few milliseconds. In off-line experiments with ion beams of stable isotopes, more than 99.9% suppression of S^- and O^- ions by photodetachment has been obtained. These ions would be the contaminants of desired beams of CI^- and F^-, respectively. Under similar conditions no reduction in CI^- and F^- ions was observed. The off-line results demonstrate the potential of this technique for on-line purification of a number of interesting radioactive beams and possible applications in accelerator mass spectrometry.