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

The ability of Resonance Ionization Mass Spectrometry (RIMS) to reliably de termine 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) --> 6 p P-2(3/2)(F ' = 5) with an extended cavity diode laser was followed by pho toionization with the 488 nm line of an argon ion laser. While the principa l objective was to avoid isobaric interferences, an optical selectivity of 3 orders of magnitude against stable Cs-133 was attained for both Cs-135 an d Cs-137. The overall RIMS efficiency was found to be similar to 2x10(-6) w ith a combined (laser and mass spectrometer) selectivity of similar to 10(1 0) for both Cs-135 and Cs-137 against Cs-133. The performance with respect to efficiency, selectivity, and isobar suppression has been compared to con ventional thermal ionization mass spectrometry (TIMS). Measurements to dete rmine the chronological age of a burn-up sample have been performed using b oth methods.