Radiation DetectorsPROPORTIONAL COUNTER CIRCUITRYThe BF3proportional counter is used to monitor low power levels in a nuclear reactor. It is usedin the "startup" or "source range" channels. Proportional counters cannot be used at high powerlevels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for eachpulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts inthe chamber during this time, the two pulses are superimposed. The voltage output would neverdrop to zero between the two pulses, and the chamber would draw a steady current as electronsare being produced.SummaryProportional counter circuitry is summarized below.Proportional Counter Circuitry SummaryThe proportional counter measures the charge produced by each particle ofradiation.The preamplifier/amplifier amplifies the voltage pulse to a usable size.The single channel analyzer/discriminator produces an output only when the inputis a certain pulse size.The scaler counts the number of pulses received during a predetermined lengthof time.The timer provides the gating signal to the scaler.Rev. 0 Page 27 IC-06
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