Proportional counters measure different types of radiation.

EO 2.2

Given a block diagram of a proportional counter circuit, STATE the purpose of the following major blocks:

a. Proportional counter

b. Preamplifier/amplifier

c. Single channel analyzer/discriminator

d. Scaler

e. Timer

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.

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Figure 9 Proportional Counter Circuit

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.

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where

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.

Figure 10 Single Channel Analyzer Operation

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

Figure 11 Single Channel Analyzer Output

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.