Proportional counters measure the charge produced by each particle of radiation. To make full use of the counter’s capabilities, it is necessary to measure the number of pulses and the charge in each pulse. Figure 9 shows a typical circuit used to make such measurements.
The capacitor converts the charge pulse to a voltage pulse. The voltage is equal to the amount of charge divided by the capacitance of the capacitor, as given in Equation 6-8.
where
V = voltage pulse (volts)
Q = charge (coulombs)
C = capacitance (farads)
The preamplifier amplifies the voltage pulse. Further amplification is obtained by sending the signal through an amplifier circuit (typically about 10 volts maximum). The pulse size is then determined by a single channel analyzer. Figure 10 shows the operation of a single channel analyzer.
The single channel analyzer has two dial settings: a LEVEL dial and a WINDOW dial. For example, when the level is set at 2 volts, and the window at 0.2 volts, the analyzer will give an output pulse only when the input pulse is between 2 and 2.2 volts. The output pulse is usually a standardized height and width logic pulse, as shown in Figure 11.
Since the single channel analyzer can be set so that an output is only produced by a certain pulse size, it provides for the counting of one specific radiation in a mixed radiation field. This output is fed to a scaler which counts the number of pulses it receives. A timer gates the scaler so that the scaler counts the pulses for a predetermined length of time. Knowing the number of counts per a given time interval allows calculation of the count rate (number of counts per unit time).
Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays.
Gamma rays can be eliminated because the neutron-induced alpha particles produce more ionizations than gamma rays produce. This is due mainly to the fact that gamma ray-induced electrons have a much longer range than the dimensions of the chamber; the alpha particle energy is, in most cases, greater than gamma rays produced in a reactor. Therefore, neutron pulses are much larger than gamma ray-produced pulses.
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.
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.
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