Application of the Triple-to-Double Coincidence Ratio method at national institute of standards and technology for absolute standardization of radionuclides by liquid scintillation counting

A new liquid scintillation spectrometer that uses the Triple-to-Double Coincidence Ratio (TDCR) method to experimentally determine counting efficiencies has been constructed at the NIST for the primary standardization of beta-emitting nuclides. This technique permits the efficiencies of the detector to be determined without the need for standard efficiency tracing and is thus considered a quasi-absolute method. This paper describes the new TDCR system at NIST and presents results of tests aimed at assessing the its operating characteristics. The results indicate that the measured activites for previously calibrated solutions of H-3 (tritiated water) and Ni-63 agree with certified activity values to within 0.04 % and 0.2 % respectively.