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.

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Figure 31 Single and Two-Stage Amplifier Circuits

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 1.0volt 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.

Discriminator Circuit

A discriminator circuit selects the minimum pulse height. When the input pulse exceeds the discriminator preset level, the discriminator generates an output pulse. The discriminator input is normally an amplified and shaped detector signal. This signal is an analog signal because the amplitude is proportional to the energy of the incident particle.

The biased diode circuit is the simplest form of discriminator. Figure 32 shows a biased diode discriminator circuit with its associated input and output signals.

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Figure 32 Biased Diode Discriminator

Diode D₁ is shown with its cathode connected to a positive voltage source +V. A diode cannot conduct unless the voltage across the anode is positive with respect to the cathode. As long as the voltage at the anode is less than that of the cathode, diode D₁ does not conduct, and there is no output. At some point, anode voltage exceeds the bias value +V, and the diode conducts. The input signal is allowed to pass to the output.

Figure 32 illustrates input and output signals and how the discriminator acts to eliminate all pulses that are below the preset level. The output pulses of this circuit have the same relative amplitudes as the input pulses.