'Atom trap trace analysis' (ATTA) for ultrasensitive isotope analysis of Kr samples

Full text: The cosmogenic radionuclide 81Kr (half-life 2.3 ? 105 years) is a very useful tracer for environmental research, e.g. for dating ancient ice or ground water. However, a quantitative determination is very difficult and even with accelerator mass spectrometry not yet possible routinely. The ATTA method developed recently employs laser manipulation of the atomic species to be analyzed, i.e. slowing down neutral atoms in a thermal beam by a counter-propagating carefully tuned laser beam, trapping these atoms in a magneto-optical trap (MOT), and counting individual trapped atoms by laser-induced fluorescence. The slowing-down process and the successive trapping is extremely isotope selective. Since the first excited state of Kr is about 10 eV above the ground state, metastable atoms have to be formed for tuning to a resonance transition. The new technique was demonstrated in 1999 at Argonne National Laboratory to count individual 81Kr and 85Kr atoms present in a natural (atmospheric) krypton gas sample with isotopic abundances in the range 10-13 and 10-11, respectively, using an ion-laser-pumped titanium-saphire ring laser. Employing a dc discharge source to produce the beam of metastable Kr atoms, the overall efficiency (fraction of sample atoms actually detected) was only about 10-7. An efficiency increase by about an order of magnitude - confirmed by laser spectroscopy - could be achieved using a rf-driven discharge. A further increase of the efficiency with respect to gas consumption is possible by gas recirculation. To make the method more cost-efficient and simpler in handling, the laser system was replaced by narrow-band tunable diode lasers operating around 811 nm. Refs. 2 (author)