226 Chapter 4. Liquid Filled Detectors
Let us now turn our attention to the liquids that can be used as active detection
media in detectors at room temperature. In Table 4.1.1 we mentioned four such
compounds: tetramethylsilane, tetramethylgermanium, tetramethyltin, and hexam-
ethyldisilane. One thing that can be readily observed is that the number of charge
pairs created in these liquids is less than the liquefied noble gases. This implies that
the signal obtained by such detectors will be weaker as compared to the conventional
liquid filled detectors used in the same radiation field. Another problem that may
be of concern is the creation of charge pairs through thermal agitation. However,
since the difference between the valence and the conduction bands of liquids is higher
than the thermal energy of molecules at room temperature, this effect can be safely
ignored. In solid state detectors, such as silicon based detectors, the band gap is
shorter and therefore creation of thermally agitated electron hole pairs can not be
ignored. We will discuss this in detail in the Chapter on solid state detectors.
4.1.B DriftofCharges
B.1 DriftofElectrons........................
While discussing the drift of charges in gases we noted that the drift velocity of
electrons, to a very good approximation, is proportional to the applied electric field.
In liquids the situation is not that simple, as can be deduced from Fig.4.1.6, which
shows the variation of drift velocity with respect to the electric field strength for
liquefied noble gases as well as their mixtures with nitrogen. It can be seen that at
low electric field strengths the drift velocity can be fairly accurately described by
the relation
vd=μeE, (4.1.2)
whereμeis the mobility of electrons. However at higher field strengths, the electron
drift velocity becomes less and less proportional to the field strength and at very
high fields it becomes essentially independent of the field strength. The drift velocity
of electrons at this stage is generally referred to as thesaturation velocity.This
nonlinear behavior of drift velocity is mainly due to the underlying nonlinearity in
the energy gained by electrons through multiple collisions with increasing electric
field strength.
An important thing that can be observed in Fig.4.1.6 is the independence of drift
velocity on small amounts of nitrogen in the main liquid at low to moderate electric
field strengths. The effect at high fields is an increase in the saturation velocity.
However this behavior is not typical of all impurities or contaminants. In fact, some
impurities have been seen to change the drift velocity even at very low electric field
strengths (cf. Fig.4.1.7). In most cases the impurities change the drift properties of
electrons significantly since their molecules act as scattering centers for the electrons.
The electrons loose their energy through inelastic collisions and consequently their
velocity distribution changes. In most cases, this effect is more pronounced at high
field strengths when the energy gained by an electron becomes equal to or greater
than the excitation energy of the impurity molecule.
Let us now turn our attention to the mobility of electrons in liquefied gases. Ta-
ble.4.1.2 shows the mobilities and saturation velocities of commonly used liquefied
noble gases at different temperatures. The values shown clearly demonstrate the
sensitivity of electron transport on the temperature of the liquid. The drift velocity
plots we saw earlier correspond to values obtained at specific temperatures. Small