Electric Circuits Connected to Detectors

As discussed previously in this chapter, the most important detectors (gas-filled tubes, the photomultipliers of the scintillation, and the semiconductor detectors) produce electric impulses. The electric impulses formed in the gas-filled tubes can be measured, e. g., by a simple electric circuit, as illustrated in Figure 14.8.

In Figure 14.8, the detector is considered to be a resistance that tends to be infinity (the resistance of the detector is much higher than the resistance of R,

Figure 14.8 A simple electric circuit for the measurement of electric impulses in an ionization detector (D—detector, R—resistance, I—current meter, U—voltage meter).

image684which is about GO) when there are no ions produced in the gas-filled tube (i. e., no radiation). In this case, the current tends to be zero; thus, the voltage measured by the U voltage meter also tends to be zero, as expected by Ohm’s law (U = R X I). Under the ionizing effect of radiation, electrons and ions are induced, decreasing the resistance (the resistance of the detector becomes less than the resistance of R). As a result, the current, as well as the voltage on the U voltage meter, increases, and an electric impulse is formed. The voltage is proportional to the amplitude of the electric impulse.

When using scintillation detectors, an anodic resistance and a condenser (RC circle) are used. In the case of semiconductor detectors, a charge-sensitive pream­plifier is required.

The obtained electric impulses can also be amplified or attenuated linearly depending on the voltage required by the signal-processing units. Impulses with different amplitudes can be separated by discriminators in one-channel analyzers or multichannel amplitude analyzers.

In one-channel analyzers, the electric impulses with different amplitudes are separated by differential discriminators. These are filters that permit the signals in the range of VD ± Д VD to pass. The level VD and the width ДVD can be varied. By varying VD, the total gamma spectrum can be scanned step by step. The total gamma spectrum is obtained if the VD is incremented by Д VD in each step.

In multichannel analyzers, the total spectrum is recorded in one measurement. It is very comfortable and saves time, which is especially important when the radio­nuclide has a short half-life or low activity.

The signals produced in the amplifier of the detector transfer to an analog/digital converter (ADC). The condenser in the ADC is charged up, as determined by the amplitude of the electric impulse. Then the condenser partially discharges, and in the meantime, an oscillator emits impulses at a constant speed. The number of the impulses emitted during the discharge of the condenser is proportional to the ampli­tude of the signal input into the ADC, i. e., to the energy of the radiation. The num­ber of the impulses coming out of the oscillator determines the position of the
input signals in the memory. Each position of the memory corresponds to an input signal with a particular energy, and its value always increases by 1 when a signal with the same energy is input into the analog/digital converter.

Previously, the data stored in the memory was recorded by two digital—analog converters (DACs) and an oscilloscope. One of the DACs treated the positions in the memory that define the position of the electron beam along the x-axis of the oscilloscope. The other DAC shows the deviation of the electron beam from the x-axis. As a result, the spectrum is continuously presented on the monitor of the oscilloscope. Nowadays, computers equipped with analyzer cards are used for sig­nal processing. Their operation is not discussed here.

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