GAS-FILLED DETECTOR

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A gas filled detector is used to detect incident radiation.

EO 1.4

DESCRIBE the principles of operation of a gas-filled detector to include:

a.         How the electric field affects ion pairs

b.         How gas amplification occurs

The pulsed operation of the gas-filled detector illustrates the principles of basic radiation detection. Gases are used in radiation detectors since their ionized particles can travel more freely than those of a liquid or a solid. Typical gases used in detectors are argon and helium, although boron-triflouride is utilized when the detector is to be used to measure neutrons. Figure 5 shows a schematic diagram of a gas-filled chamber with a central electrode.

Figure 5 Schematic Diagram of a Gas-Filled Detector

The central electrode, or anode, collects negative charges. The anode is insulated from the chamber walls and the cathode, which collects positive charges. A voltage is applied to the anode and the chamber walls. The resistor in the circuit is shunted by a capacitor in parallel, so that the anode is at a positive voltage with respect to the detector wall. As a charged particle passes through the gas-filled chamber, it ionizes some of the gas (air) along its path of travel. The positive anode attracts the electrons, or negative particles. The detector wall, or cathode, attracts the positive charges. The collection of these charges reduces the voltage across the capacitor, causing a pulse across the resistor that is recorded by an electronic circuit. The voltage applied to the anode and cathode determines the electric field and its strength.

As detector voltage is increased, the electric field has more influence upon electrons produced. Sufficient voltage causes a cascade effect that releases more electrons from the cathode. Forces on the electron are greater, and its mean-free path between collisions is reduced at this threshold. Calculating the change in the capacitor's charge yields the height of the resulting pulse. Initial capacitor charge (Q, with an applied voltage (V), and capacitance (C), is given by Equation 6-4.

A change of charge ( Q is proportional to the change in voltage ( V) and equals the height of the pulse, as given by Equation 6-5 or 6-6.

The total number of electrons collected by the anode determines the change in the charge of the capacitor ( Q). The change in charge is directly related to the total ionizing events which occur in the gas. The ion pairs (n) initially formed by the incident radiation attain a great enough velocity to cause secondary ionization of other atoms or molecules in the gas. The resultant electrons cause further ionizations. This multiplication of electrons is termed gas amplification. The gas amplification factor (A) designates the increase in ion pairs when the initial ion pairs create additional ion pairs. Therefore, the height of the pulse is given by Equation 6-7.

where

The pulse height can be computed if the capacitance, detector characteristics, and radiation are known. The capacitance is normally about 10-`` farads. The number of ionizing events may be calculated if the detector size and specific ionization, or range of the charged particle, are known. The only variable is the gas amplification factor that is dependent on applied voltage.

Summary

The operation of gas-filled detectors is summarized below.

Gas-Filled Detectors Summary

The central electrode, or anode, attracts and collects the electron of the ion-pair.

The chamber walls attract and collect the positive ion.

When the applied voltage is high enough, the ion pairs initially formed accelerate to a high enough velocity to cause secondary ionizations. The resultant ions cause further ionizations. This multiplication of electrons is called gas amplification.