Year
2013
Abstract
Determining a model-age of nuclear material provides important information for nuclear forensic studies of intercepted materials that may guide agencies in identifying the history of materials and validating declarations. The term “model-age” is used because there are assumptions that should be explicitly defined for any chronometer. What is meant by the real “age,” similarly, should be defined. Analyses of radium-226 (half-life, 1599 years) allow age-dating of nuclear fuel cycle materials using the 226Ra- 230Th chronometer. Precise measurements of the isotopic composition and concentration of radium (Ra) are required in order to determine model-ages using this chronometer. In this study, we present recently developed Ra analysis techniques that include: 1) chemical separation of Ra from nuclear and geologic materials, and 2) technical information for making quantitative Ra analyses by total-evaporation thermal ionization mass spectrometry (TE-TIMS). A 228Ra-enriched spike was prepared at Lawrence Livermore National Laboratory from pure Th-metal (Ames Laboratory) for Ra concentration measurements by isotope dilution. This enriched 228Ra tracer was calibrated with NIST SRM4967A 226Ra standard. Multiple analyses of individualRa samples show that mass fractionation of 226Ra and 228Ra occurs during TIMS measurement; therefore, TE-TIMS, which minimizes mass fractionation effects on measured Ra isotopic compositions, was determined to be the most precise method for TIMS Ra analysis. The calibrated 228Ra-spike was used to conduct isotope dilution Ra concentration measurements on two materials of known age for validation of our spike calibration and analytical procedures: 1) CRM U100 uranium standard that was purified in January 1959, and 2) Table Mountain Latite (TML), a geologic material known to be old enough to have established secular equilibrium in the U-series decay chain (238U- 234U- 230Th- 226Ra). Current procedures allow for accurate and precise Ra measurement of 100 femtograms 226Ra, and continuing research will establish the lower limit of Ra concentration required for accurate TIMS Ra analyses.