What Is An Ionization Chamber ?

Oct 27

Ionization Chambers

Ever wonder what ionization chambers do?The number of ion pairs generated within a gas as a result of incident radiation is detected and measured using an ionization chamber. A voltage is applied to sustain an electrical field in a gas-filled chamber with two electrodes: anode and cathode. In most situations, the anode is positively charged in relation to the cathode, and the cathode is the outer chamber.

When incident ionizing radiation ionizes the gas between the electrodes, the electric field induces the formation of positive ions and electrons. These particles move to electrodes of the opposite polarity, generating ionization current to be measured by an electrometer circuit. The overall ionizing dosage entering the chamber is represented by the ionization current. Each ion produced by the interaction of incident radiation with the gas generates an electric charge, which is proportional to the number of ion pairs produced, and hence the radiation dose.

The electric field allows the ionization chamber to operate continuously by cleaning up electrons that may otherwise induce ion pair recombination and a decrease in ion current. In contrast to the Geiger-Muller tube, which provides pulse output, the ionization chamber's output signal is continuous current.

Ion chambers are available in a variety of shapes and sizes

A compartment with no walls.
Chamber with a ventilation system
Chamber with high pressure
Low-pressure compartment that is completely sealed.

Ion chambers respond well to radiation over a wide range of energies, making them useful in research laboratories, radiography, nuclear power industries, radio-biology, and environmental monitoring.

Charged particles interact with a gas to produce ionized and excited molecules (direct charged particles or secondary particles produced by interactions with photons or neutrons). The total number of Electron-ion pairs formed along the radiation's path is the most essential piece of information. The average energy lost by the incident particle per produced ion pair is defined as the W-value. The W-value is always greater than the ionization energy due to the competing mode of energy loss, namely excitation.

For routine use, ion chambers range from 0.1cc to 1.0cc.

For example, if we wish to keep the leakage current at 1% of an ionization current of 1 pA at 100 V, the insulator should have a resistance of 1016 ohms. Moisture or pollutants on the surface might also cause complications. In low-current applications of ion chambers, a guard ring is used to limit insulator leakage. The insulator is divided into two pieces, as shown in Fig. 3.3. The majority of the applied voltage passes through the outer insulator, with no leakage current contributing to the measured current I. The ionization current is too tiny to be measured with conventional galvanometers. As shown in Fig. 3.4, an electrometer, also known as an extremely sensitive electronic voltmeter, measures current indirectly by sensing the voltage drop across a series resistance (109 1012 ohms). The steady state value of the ion current is given by I = VR/R if the time constant RC does not vary.