3.2. Diffusion and Drift of Charges in Gases 157
Figure 3.2.2: Variation of drift velocity of electrons in a mixture of
argon, propane, and isobutane with respect to electric field strength.
The curves have been drawn for different gas pressure values (23).3.2.C EffectsofImpuritiesonChargeTransport...........
In most applications, gaseous detectors are filled with a mixture of gases instead
of a single gas. The ratio of the gases in the mixture depends on the type of
detector and the application. In addition to these, the detector also haspol lutantsor
impurities, which degrade its performance. Most of these pollutants are polyatomic
gases, such as oxygen and air. Since they have several vibrational energy levels
therefore they are able to absorb electrons in a wide energy range. Such agents are
calledelectronegativeand their electron attachment coefficients are generally high
enough to be of concern. The main effect of these impurities is that they absorb
electrons and result in degradation of the signal.
There are two methods by which electron capture occurs in gaseous detectors:
resonance capture and dissociative capture. Resonance capture can be written as
e+X→X−∗, (3.2.11)whereXrepresents the electronegative molecule in the gas and∗denotes its excited
state. To de-excite, the molecule can either transfer the energy to another molecule
X−∗+S→X−+S∗, (3.2.12)or emit an electron
X−∗→X+e. (3.2.13)