Measurement of 135Cs/137Cs Isotopic Ratio in a Standard Radioactivity Sample Using Resonance Ionization Mass Spectrometry

L. Pibida, B.A. Bushaw* , W. Nörtershäuser* , J.M.R. Hutchinson, L. Karam

National Institute of Standards and Technology, Gaithersburg, MD 20899

* Pacific Northwest National Laboratory, Richland, WA 99352

We measured the isotopic ratio of 135Cs/137Cs using resonance ionization mass spectrometry (RIMS) as a demonstration of the capabilities of this method for future measurements requiring extreme background suppression. The results are compared with values measured by thermal ionization mass spectrometry (TIMS) and the chronological age of a burn-up sample was determined. A single-step resonance ionization scheme of Cs for the 6s 2S1/2(F=4)® 6p 2P3/2(F=3,4,5) transition, together with a resonance depletion technique, was used to increase the isotopic selectivity in the RIMS measurements. A depletion factor of approximately 103 was obtained for 133Cs and no isobaric interference from Ba was observed. The overall RIMS efficiency was found to be 510-6 with a combined (mass spectrometer plus laser) selectivity of approximately 11010. For TIMS the overall efficiency was 7.410-3, however, isobaric selectivity against Ba isotopes was only 3104.

This work is funded by NIST and DOE Office of Science.