3.2. Diffusion and Drift of Charges in Gases 159
Figure 3.2.4: Variation of drift velocity of electrons in mixtures of (krypton
+CO 2 ) and (krypton +CH 4 ) with respect to the ratio of the electric field
strength and the gas pressure (10).Using this, we can defineλcas the distance traveled by electrons such that about
63% of them get captured. The capture mean free path depends on the electron
attachment coefficientη, which characterizes the probability of electron capture in
any one scattering event. Ifσis the total electron scattering cross section of the
electronegative gas, thenησwill represent the attachment cross section. IfNmis
the number density of the gas molecules (number of molecules per unit volume) and
fis the fraction of its electronegative component, then the capture coefficient and