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Basic Metrology: Advancing Triple-to-Double Coincidence Ratio Liquid Scintillation Counting

Summary

The triple-to-double coincidence ratio (TDCR) method of liquid scintillation (LS) counting employs three detectors to determine experimental counting efficiencies so that activity can be measured independent of calibration standards. TDCR counting is one of several LS-based methods at the heart of NIST primary activity standards. We are exploring innovative approaches to data acquisition and analysis.

Description

NIST triple-to-double coincidence ratio (TDCR) liquid scintillation counter
The NIST triple-to-double coincidence ratio (TDCR) liquid scintillation counter.
Credit: Denis Bergeron

The NIST triple-to-double coincidence ratio (TDCR) liquid scintillation counter.

NIST implements the TDCR method with custom-built instrumentation. The system used routinely for activity standardizations is based on an optical chamber designed for 20 mL scintillation vials and uses field programmable gate array (FPGA) technology to apply coincidence logic in real-time. For more than a decade, this robust configuration has supported precision activity measurements for radionuclides decaying by beta emission or electron capture. Advances in computing power, data storage, and digitizer speed have begun to demonstrate the limitations of the FPGA-based approach.

While our custom system allows for the fine-tuning of various experimental parameters (photomultiplier tube thresholds, coincidence resolving times, extending deadtimes, etc.), these must be optimized and selected in advance of a measurement. For measurements of short-lived radionuclides especially, time spent acquiring data under sub-optimal conditions means the frustration of time (and data) lost. This is one powerful motivation for an alternative measurement approach referred to as “list mode” or “real-time” data acquisition.

List mode acquisition allows the real-time logging of events with timestamps. Coincidence logic, extending deadtime, and more can be applied in post-processing; which means a dataset can be analyzed with varying imposed experimental conditions with no losses.

At NIST, we are developing new data acquisition systems for our TDCR systems. We are also adding gamma-ray detectors that let us implement various digital coincidence counting schemes for independent activity determinations with reduced model-dependence. Finally, we are exploring new hardware to enable the deployment of more portable versions of our TDCR system.

Created February 15, 2011, Updated March 22, 2021