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Since on-line radiotracer techniques involve most commonly only gamma-ray, the mostcommon gamma-ray scintillator in use is the thallium- activated sodium iodide NaI(Tl) single crystal.Two radiation detectors are needed for simple radiotracer RTD measurement, one at the inlet forrecording the injecting pulse and the other at the outlet for measuring the experimental RTD response.More detectors (4-6) are needed for collecting additional comparative information in particular sites ofthe processing units, and as many as possible (> 10-20) are needed for complex engineering reactors.The most commonly used in field condition is NaI(Tl) detector in waterproof casting. It is verysensitive sensor for gamma radiation, for example a 1’’ × 1’’ NaI (Tl) scintillation detector fordetection of 82 Br in water, in an infinite detection geometry condition, gives 65 cpm/kBq/m 3 .Detection probes are mounted at selected locations at the inlet and outlet of the processing vessel andare shielded by lead collimators to protect them from the natural background and other parasiteradiation may come from around. If needed, detectors are protected from heat (for temperature higherthan 60-70 0 C) by placing aluminum plate between the detector and reactor walls.The data acquisition system (Fig. 22), which collects signals from several radiation detectors, isthe basic equipment for online radiotracer RTD measurements. The data acquisition system ensurescollection, treatment and visualization of the data. Dead time between two measurements is normallyless than 1 µs. The visualization of data is as close as possible to ‘real time’ experiment. Themeasurements are simultaneous and the minimal dwelling time is 1-2 ms. Standard portable dataacquisition systems for industrial radiotracer work are PC based data logger with unlimited possibilityin the number of connected probes. There are several prototypes of data acquisition system(commercial or homemade). The most common probes consist of NaI(Tl) detectors in waterproofcasting, preferably stainless steel. They are very sensitive sensors to gamma radiation, for example a1’’× 1’’ NaI (Tl) scintillation detector for 82 Br measured in water with an infinite detection geometrycondition gives 65 cpm/kBq/m 3 .FIG.22. Data acquisition systems for online radiotracer tests.The probes are placed (Fig. 23):• in selected places at the inlet and outlet of the processing vessel shielded around by collimators toprotect them from the parasite radiation coming from around,• inside or around the tested vessel itself when it is possible,• immerged inside the wastewater treatment units (different places and various depths),• inside a portable detection chamber, where the sewer is pumped.33