The triple-to-double coincidence ratio (TDCR) method of liquid scintillation counting employs three detectors to facilitate the experimental determination of counting efficiencies so that activity can be measured independent of calibration standards. Together with the 3H efficiency tracing liquid scintillation counting method and live-timed anticoincidence counting, TDCR counting is at the heart of many "primary" activity standardizations performed at NIST.
Implementation of the TDCR method at NIST has, until recently, relied upon the analog MAC3 logic unit first described by Bouchard and Cassette.1 The unavailability of new and/or replacement units and the promise of increased flexibility in data acquisition parameters has spurred several laboratories to migrate to digital systems based on field programmable gate array (FPGA) technology. NIST has developed a FPGA-based TDCR acquisition system using the LabView programming environment. Code for coincidence and livetime logic as well as for counting is compiled and uploaded to the FPGA, while code on the host computer provides the user interface, allowing for a wide range of acquisition parameters. The FPGA-based system was benchmarked to the MAC3 unit via a splitting of the amplified photomultiplier tube pulse signals for simultaneous acquisition of TDCR counting data on both systems. Tests with numerous radionuclides, including Am-241, Tc-99, F-18, Ni-63, H-3, and Pu-241, showed that typical agreement between the systems is better than ≈ 0.4 %. The FPGA-based system offers several advantages over its predecessor, including the ease of adjusting the coincidence resolving time and the extending deadtime. In addition, the adoption of the FPGA-based system will facilitate future modifications such as the addition of a gamma channel for beta-gamma coincidence counting or the implementation of Compton efficiency tracing. The further development of diagnostic and spectroscopic tools is underway. Finally, a version of the NIST software is being adopted and tested by our colleagues at the National Research Council (Canada).
1Bouchard J, Cassette P. 2000. MAC3: an electronic module for the processing of pulses delivered by a three photomultiplier liquid scintillation counting system. Applied Radiation and Isotopes 52(3):669