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

Abstract

The ability of Resonance Ionization Mass Spectrometry (RIMS) to reliably determine the ratio of 135 Cs/ 137 Cs in the presence of large backgrounds has been evaluated. Single-resonance excitation of Cs 6s 2 S 1/2 (F=4)→6p 2 P 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 133 Cs was attained for both 135 Cs and 137 Cs. The overall RIMS efficiency was found to be ∼ 2×10 -6 with a combined (laser and mass spectrometer) selectivity of ∼ 10 10 for both 135 Cs and 137 Cs against 133 Cs. 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.