158 Chapter 3. Gas Filled Detectors
Figure 3.2.3: Variation of drift velocity of electrons in a mixture of xenon and
C 2 H 6 with respect to electric field strength. The curves have been drawn for
different values of externally applied magnetic fields. The variation of drift ve-
locity with magnetic field is very small and therefore except for very high field
strengths it can be neglected for most practical purposes (31).HereScan be any gas molecule in the gas but is generally an added impurity called
quench gas. We will learn more about this later in the chapter. The process of
electron emission is favorable for radiation detectors because the only effect it has is
the introduction of a very small time delay between capture and re-emission of the
electron. It does not have any deteriorating effect on the overall signal height.
If a constant electric field is applied between two electrodes, the number of elec-
trons surviving the capture by electronegative impurities after traveling a distance
xis given by
N=N 0 e−μcx, (3.2.14)
whereN 0 is the number of electrons atx=0andμcis the electron capture coef-
ficient, which represents the probability of capture of an electron. It is related to
electron’s capture mean free pathλcby
μc=1
λc