High-efficiency ultra trace detection of⁸⁵Kr

Abstract

We have developed a novel version of collinear fast beam laser spectroscopy based on particle detection. The sample to be analyzed is loaded into an ion source and a mass separated fast ion beam is produced. In it we can detect with high efficiency the various krypton isotopes, and in particular the long lived rare isotope 85j by observing the optical hyperfine structure spectrum. The technique utilizes cascade two-step excitation to pump metastable krypton atoms to a high-lying Rydberg level. The metastable atom level is effectively populated from the Kr ion beam by a near resonance charge exchange reaction with rubidium vapor. Thereupon the Rydberg atoms are field-ionized and the resulting signal ions, which have been at resonance with two laser frequencies, are detected. The following transitions are used in the two-step excitation process: 5s2 [3/2] 2 -."5p2 [5/2]3 -29d[7/2J4. . The technique has been successfully applied to all the stable krypton isotopes, and to 85j< but not yet to 81j< Afterthe Kr ions are emitted from the ion source the overall detection efficiency is to date 8%. The selectivity is at the one part in 1010 level and the sensitivity at a few hundred ions/s. Obvious further improvements are possible and are outlined. A similar excitation scheme can also be applied to the other noble gases including radon and to their rare isotopes.