By using a discriminator,the scaler can be set to read only the larger pulses produced by the neutron. A discriminator is basically a single channel analyzer with only one setting. Figure 12 illustrates the operation of a discriminator.

If the discriminator is set at 2 volts, then any input pulse 2 volts causes an output pulse.

Figure 13 shows a typical circuit used to measure neutrons with a BF₃ proportional counter.

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Figure 12 Discriminator

Figure 13 BF₃, Proportional Counter Circuit

The BF₃ proportional counter is used to monitor low power levels in a nuclear reactor. It is used in the "startup" or "source range" channels. Proportional counters cannot be used at high power levels because they are pulse-type detectors. Typically, it takes 10 to 20 microseconds for each pulse to go from 10% of its peak, to its peak, and back to 10%. If another neutron interacts in the chamber during this time, the two pulses are superimposed. The voltage output would never drop to zero between the two pulses, and the chamber would draw a steady current as electrons are being produced.

Summary

Proportional counter circuitry is summarized below.

Proportional Counter Circuitry Summary

The proportional counter measures the charge produced by each particle of radiation.

The preamplifier/amplifier amplifies the voltage pulse to a usable size.

The single channel analyzer/discriminator produces an output only when the input is a certain pulse size.

The scaler counts the number of pulses received during a predetermined length of time.

The timer provides the gating signal to the scaler.