Activity metrology goes digital
The purpose of radionuclide activity metrology is to accurately measure radioactive decay within a given time. This means that two variables must be accurately managed: the number of pulses detected in the measuring system, and the period of measurement. The actual measuring period during which the system used is considered able to detect a pulse is called the “active time”. Each measuring system also has its own sources of dead time, ranging from the detector’s finite reaction time to the electronic instrumentation used. As a result, it is preferable to impose a known dead time after each pulse is detected.
LIST’s LNHB has been using this analog measuring method in several systems (germanium detectors, liquid scintillation, crystal well, coincidental measurements, etc.) for several years, providing greater accurary and robustness than the statistical corrections generally used.
The performance levels now offered by digital technologies (high-speed digitizers and programmable components processing digitized signals in real time), however, mean that migration to a digital acquisition system is now feasible while still ensuring the continuity of existing facilities, portability of measuring systems, and openness to new data processing methods. The signal is then digitized as near as possible to the detector. The functions are applied directly to the signal’s digitized samples as algorithms. If a DSP is used, the algorithm is implemented in the component itself. If a FPGA is used, which logical gates must be applied to the component in order to perform the required functions must firstly be defined in a simulation and summary phase.
LIST has developed a first prototype version of a PCI data acquisition board and tested it on a crystal well, for which dead time processing is essential. Comparison tests between the conventional data acquisition chain and the new digital chain, performed for various radionuclides, have shown a difference of less than 1 in 1,000 and no count is lost throughout the entire range defined in the initial specifications, thus validating the proof of concept. This prototype shows the new possibilities offered by switching to digital processing. In particular, the fact that the data can be processed offline means that several analyses can be performed on the same acquisition data, an essential factor in measuring the short-lived radionuclides used increasingly frequently in biomedical applications. It also means that special analytical methods can be applied (such as correlation methods for radionuclides having a metastable state with a long lifetime, measuring of the count rate through time interval statistics, etc.). A patent is pending.
A second prototype, for the coincidence measurement instrumentation, is currently being tested and has already precisely defined the role of experimental parameters previously inaccessible to the equipment’s operator. As we have now obtained proof of concept, the next phases will consist in perfecting the prototypes’ performance and user-friendliness (dedicated input electronics, signal optimization though digital filtering, switching to a USB/Ethernet connector, etc.) and expanding their use to other measuring techniques.
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