4.1. Properties of Liquids 227
Figure 4.1.6: Variation of electron drift velocity with electric
field strength for liquefied noble gases (in pure state and
with addition of nitrogen). Liquid temperatures for argon,
krypton, and xenon are 87K,12K, and 165Krespectively
(32).fluctuations in temperature change the mobility, which changes the drift properties
of electrons. This is a serious drawback of using liquefied noble gases in radia-
tion detectors as it requires very careful monitoring and control of the temperature.
Keeping the liquid at such low temperatures is generally accomplished by a liquid
nitrogen flow system, which is costly as well as maintenance intensive. Even with
these difficulties the liquefied noble gases are still the choice for most radiation de-
tector developers. A number of room temperature liquids are now gaining exposure
and although they have not yet been very successful but it looks like a matter of
time for them to succeed their low temperature counterparts.
B.2 DriftofIons...........................
Unfortunately the transport of ions in liquids, that are commonly used in radiation
detectors, has not been studied as rigorously as the transport of electrons. Since
such studies are mostly need-related therefore we might be tempted to assume that
for liquids the behavior of ions in liquids is not as important as in gases. This,
however, is not a true statement because the output signal of the detector depends
on movement of both negative and positive charges. The reader would recall our
argument in the chapter on gaseous detectors that the output pulse is actually
generated due to the change in the effective potential inside the chamber, which
depends on how the positive and negative charges move under the influence of the
applied electric field. The situation is not different for the liquid filled detectors
either, even though here the output is almost always measured from the anode.