A reference material for evaluation of Cs radiochronometric measurements.

A new nuclear forensic reference material has been characterized as a standard for radiochronometric determination of the model purification date for Cs 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 Cs/Ba chronometer: the molality (reported here as nmol g) of Cs and of the radiogenic portion of Ba in the material (hereafter referred to as Ba*). All measurement results were decay-corrected to represent the composition of the material on the reference date of July 7, 2011. The molality of Cs is (0.7915 ± 0.0073) nmol g; this value was calculated from the massic activity of Cs, (348.4 ± 3.0) kBq g, as measured in the NIST 4- secondary standard ionization chamber (previously calibrated by 4-(e+)--coincidence efficiency extrapolation counting) and the evaluated half-life of Cs, (30.05 ± 0.08) years. The molality of Ba*, (1.546 ± 0.024) nmol g, was measured by isotope dilution mass spectrometry using the measured relative proportion of Ba in the material to apply a correction for the Ba 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 Cs 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.