A reference material for evaluation of ¹³⁷ Cs radiochronometric measurements

A new nuclear forensic reference material has been characterized as a standard for radiochronometric determination of the model purification date for ¹³⁷Cs sources. The purification date of a radioactive source is a potentially diagnostic nuclear forensic signature for determining the provenance of a radioactive material. Reference values have been measured for the attributes needed to use the ¹³⁷Cs/¹³⁷Ba chronometer: the molality (reported here as nmol g^-1) of ¹³⁷Cs and of the radiogenic portion of ¹³⁷Ba in the material (hereafter referred to as ¹³⁷Ba*). All measurement results were decay-corrected to represent the composition of the material on the reference date of July 7, 2011. The molality of ¹³⁷Cs is (0.7915 ± 0.0073) nmol g^-1; this value was calculated from the massic activity of ¹³⁷Cs, (348.4 ± 3.0) kBq g^-1, as measured in the NIST 4π-γ secondary standard ionization chamber (previously calibrated by 4π-(e+x)-γ-coincidence efficiency extrapolation counting) and the evaluated half-life of ¹³⁷Cs, (30.05 ± 0.08) years. The molality of ¹³⁷Ba*, (1.546 ± 0.024) nmol g^-1, was measured by isotope dilution mass spectrometry using the measured relative proportion of ¹³⁸Ba in the material to apply a correction for the ¹³⁷Ba contribution from natural Ba. A model age of (47.04 ± 0.56) years, corresponding to a model purification date of June 22, 1964 with an expanded uncertainty of 200 days is calculated from the reference material values. This age is consistent with the date engraved on the capsule that contained the ¹³⁷Cs starting material and with a prior independent determination of the model purification date. A full discussion of the uncertainties of the reference material values is included.