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 In recent years, we have been migrating to more flexible digital systems based on field programmable gate array (FPGA)technology. NIST has developed a FPGA-based TDCR acquisition system using the LabView programming environment.
The current FPGA-based system has been 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, have shown 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 flexibility afforded by FPGA technology is being leveraged to implement new coincidence and anticoincidence schemes for activity standardizations and acquisition of nuclear data.
Most recently, the TDCR system has been deployed in primary standardizations of the pure beta-emitting nuclides 3H and 14C. In the 14C case, the measurements were part of a bilateral comparison of liquid scintillation counting with NRC Canada. Both the 3H and the 14C studies supported the standard reference materials (SRM) program.
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.