note3 - Med Phys 4R06/6R03 Radioisotopes and Radiation...

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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology 3-1 Chapter 3 Gas Filled Detectors 3.1. Ionization chamber A. Ionization process and charge collection The interactions of charged particles (either direct charged particles or secondary particles produced by interactions with photons or neutrons) with a gas lead to ionized and excited molecules along the path. The important information is the total number of electron-ion pairs created along the track of the radiation. The W-value is defined as the average energy lost by the incident particle per ion pair formed. Due to the competing mechanism of the energy loss, i.e. excitation, W-value is always greater than the ionization energy. The table below shows W-values for common gases. Ionization potential and W-values for different gases. After being created in the gas, electron-ion pairs are in the random thermal motion and diffuse away from the high density region. Different types of collisions happen between free electrons, ions and neutral gas molecules as shown in Fig. 3.1. + + Charge transfer - Electron attachment e- Recombination e- + + - Fig. 3.1. Collisions between free electrons, ions and neutral gas molecules.
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Med Phys 4R06/6R03 Radioisotopes and Radiation Methodology 3-2 Charge transfer occurs when a positive ion encounters a neutral molecule. In this case, an electron is transferred from the neutral molecule to the ion. When a free electron meets a gas molecule that attaches electrons easily, a negative ion is formed. Oxygen is a good example. Thus, free electrons diffusing in air are rapidly converted to negative ions. In contrast, nitrogen, hydrogen and noble gases have low electron attachment coefficients, and make electrons continuously migrate. To collect electron-ion pairs produced in a gas-filled detector, an electric field must be applied. If the electric field inside the detector is strong enough that recombination becomes negligible, all the charges are efficiently collected without loss. The steady state current measured at this condition is called ionization current , which is the basic principle of the DC ion chamber. I V Fig. 3.2. Basic component of an ion chamber and the voltage-current characteristics. B. Operation of DC ion chambers When an ion chamber is operated in direct current mode, the negative charges can be collected either as free electrons or as negative ions. Thus, any filling gas could be used. Air is the most common filling gas. Typical ionization currents in most applications are very small (1 pA or less). Thus, the leakage current through the insulator can perturb measuring the ionization current. Fig. 3.3. Ion chamber employing guard ring. For example, if we want to keep the leakage current at 1 % level of an ionization current of 1 pA for an applied voltage of 100 V, the insulator should have a resistance of 10 16 ohms. Moreover, moisture or surface contaminants can make additional troubles. Practically, a
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This note was uploaded on 03/03/2011 for the course ECON 3 taught by Professor Costescu during the Spring '11 term at Faculty of English Commerce Ain Shams University.

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note3 - Med Phys 4R06/6R03 Radioisotopes and Radiation...

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