Evaluation of resonance ionization mass spectrometry for the determination of 135Cs/137Cs isotope ratios in low-level samples

The ability of Resonance Ionization Mass Spectrometry (RIMS) to reliably determine the ratio of Cs-135/Cs-137 in the presence of large backgrounds has been evaluated. Single-resonance excitation of Cs 6s S-2(1/2)(F = 4) --> 6p P-2(3/2)(F ' = 5) with an extended cavity diode laser was followed by photoionization with the 488 nm line of an argon ion laser. While the principal objective was to avoid isobaric interferences, an optical selectivity of 3 orders of magnitude against stable Cs-133 was attained for both Cs-135 and Cs-137. The overall RIMS efficiency was found to be similar to 2x10(-6) with a combined (laser and mass spectrometer) selectivity of similar to 10(10) for both Cs-135 and Cs-137 against Cs-133. The performance with respect to efficiency, selectivity, and isobar suppression has been compared to conventional thermal ionization mass spectrometry (TIMS). Measurements to determine the chronological age of a burn-up sample have been performed using both methods.