Low-level gas proportional counting presents special challenges to the measurement process. Generally, measurement systems for this purpose have unique signal-processing capabilities through specialized electronics and software for maintaining a stable low background, thus achieving extremely low detection and quantification limits for the nuclide of interest. As a result, they are also more sensitive to small signals not germane to the sample which may affect the measurement if not identified. However, a common thread through these systems is often their ability to digitally capture and timestamp each event for retrospective analysis. The NIST low-level counting (llc) system serves as an example which was originally designed to measure atmospheric 37Ar. In response to an event trigger, the system acquires its pulse timestamp, digital waveform, energy, and rise time. A monitor logs each event as either anticoincidence or coincidence in nearly real-time, displays its digital waveform, and presents a 2-dimensional spectroscopic representation of rise time and energy of events as they occur. In post analysis software, other special capabilities include channel-channel interactions, low-level energy discrimination, pulse arrival time and inter-arrival time distribution analysis and the ability to browse waveforms and sort the event record. Thus, through individual pulse acquisition and analysis spurious events can easily be identified. Some examples of these unwanted events are related to ground loops, electrostatic charge, micro-discharges, counting gas purity and laboratory effects such as electromagnetic and mechanical influences. The discussion will briefly describe the capabilities of the NIST llc system, present some observations of events of known and unknown origin, and conclude with an assessment of uncertainty for measurements at the mBq level.
Citation: Journal of Radioanalytical and Nuclear Chemistry
Pub Type: Journals
low-level counting, beta decay, gas proportional counter, atmospheric gas and aerosol samples, digitizer, timestamp, energy, rise time, pulse shape, background count rate, variability