Radiochronometry by Mass Spectrometry: Improving the Precision and Accuracy of Age-Dating for Nuclear Forensics

The model-date of a nuclear material is an important signature in a nuclear forensic investigation. Assuming the material is homogeneous, this parameter is fixed and exact, but it may or may not be the same as the purification date. The decay of a radioactive parent to a radioactive or stable daughter is the basis of the radiochronometers that record this model-date. If upon purification, only the parent isotope is present in the material, then the model-date will be the same as the purification date. Otherwise, if any of the daughter isotope remains upon purification, then the model-date will be further in the past. Regardless, it is a fixed and characteristic signature of the material, and will not vary as long as the system remains closed, i.e., there is no post-purification fractionation of parent and daughter. Measurements of the parent-daughter pairs 234 U230 Th, 235 U231 Pa, 241 Pu241 Am, 137 Cs137 Ba and 90 Sr-( 90 Y)90 Zr can be used to determine the model-dates of a variety of nuclear materials. All of these pairs are measured more precisely by mass spectrometric methods, because, for a given sample, more atoms can be measured by mass spectrometry than decays measured by radiometric methods. State programs have recognized the importance of precise and accurate model-dates as a signature of a nuclear material, and efforts to improve precision and accuracy are being made internationally by national laboratories and institutions charged with developing standards and reference materials. Efforts within the United States include the production of new certified reference materials (U-Th and Cs-Ba radiochronometer standards) and spikes ( 229 Th, 134 Ba, 243 Am, 236 Np, 233 U), and the development of guidance on the interpretation of radiochronometry data. Enhancement of radiochronometric methods and the development of tools needed to improve accuracy and precision are supported collaboratively by the U.S. Department of Homeland Security, U.S. Department of Justice Federal Bureau of Investigation, the U.S. Department of Energy at DOE National Laboratories and U.S. national metrology institutes.