Three ranges are used to monitor the power level of a reactor throughout the full range of reactor operation: source range, intermediate range, and power range. The source range normally uses a proportional counter, while the intermediate and power ranges use ionization chambers. A compensated ion chamber is used for the intermediate range. The power range uses an ncompensated ion chamber. Each of the three different ranges makes use of some or all of the following types of components.
Preamplifiers and Amplifiers
Radiation detector output signals are usually weak and require amplification before they can be used. In radiation detection circuits, the nature of the input pulse and discriminator determines the characteristics that the preamplifier and amplifier must have. Two stages of amplification are used in most detection circuits to increase the signal-tonoise
ratio.
Figure 31 shows how a two-stage amplifier increases the signal-to-noise ratio.
The radiation detector is located some distance from the readout. A shielded coaxial cable transmits the detector output to the amplifier. The output signal of the detector may be as low as 0.01 volts. A total gain of 1000 is needed to increase this signal to 10 volts, which is a usable output pulse voltage. There is always a pickup of noise in the long cable run; this noise can amount to 0.001 volts. If all amplification were done at the remote amplifier, the 0.01-volt pulse signal would be 10 volts, and the 0.001 noise signal would be 1 volt. This is a signal-to-noise ratio of 10 and could be significantly reduced by dividing the total gain between two stages of amplification. A preamplifier located near the detector and a remote amplifier could be used. The preamplifier virtually eliminates cable noise because of the short cable length. If, for a total gain of 1000, the preamplifier has a gain of 100 and the amplifier has a gain of 10, the output signal from the preamplifier is 1 volt. The signal transmitted via the long cable run still picks up the 0.001-volt noise. The amplifier amplifies the 1.0- volt pulse signal and the 0.001-volt noise signal by a factor of 10. The result is a 10-volt pulse signal and a 0.01-volt noise signal. This gives a signal-to-noise ratio of 1000.
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