Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

A Reference Material for Evaluation of Cs-137 Radiochronometric Measurements



Kevin B. Lavelle, Richard Essex, Jeffrey T. Cessna, Kevin P. Carney, Cole R. Hexel


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, with a reference date of July 7, 2011, for the attributes needed to use the 137Cs/137Ba chronometer: the molality (nmol g-1) of 137Cs and of the radiogenic portion of 137Ba in the material (hereafter referred to as 137Ba*). The 137Cs molality is (0.7915 ± 0.0073) nmol g-1; this value was calculated from the massic activity of 137Cs, (348 ± 3) 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) a. The 137Ba* molality of (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 purification date of June 22, 1964 with an expanded uncertainty of 200 days is calculated from the certified 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.
Journal of Radioanalytical and Nuclear Chemistry


Radiochronometry, Cesium-137, Nuclear Forensics, Reference Materials, Mass Spectrometry, Radiological Forensics


Lavelle, K. , Essex, R. , , J. , Carney, K. and Hexel, C. (2018), A Reference Material for Evaluation of Cs-137 Radiochronometric Measurements, Journal of Radioanalytical and Nuclear Chemistry, [online],, (Accessed June 19, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created August 15, 2018, Updated June 24, 2021