222 Chapter 4. Liquid Filled Detectors
Unfortunately, unlike gases, in liquids the energy needed to create a charge pair does
depend on the type of liquid. In the Chapter on gaseous detectors we introduced
atermW-valueto signify the energy needed to create an electron ion pair. The
same terminology is used for liquids as well even though the values in liquid state
are quite different from the ones in gaseous state. For liquid, another term that
is extensively used is theG-value, which is defined as the yield of electrons for an
energy of 100eV. That is, the number of electrons (or the charge pairs) generated
when the incident radiation deposits an energy of 100eV. The reader should note
that this is simply a conventional terminology and has nothing to do with the physics
of pair generation in liquids. Certainly theWand theGvalues can be derived from
one another through the relation
W=100
G
, (4.1.1)
whereWwill be ineV.
Table.4.1.1 gives theWandGvalues for various liquids that have been found to
be suitable for use in radiation detectors. The reader would readily note that the
energy needed to produce a charge pair in the liquefied noble gases are lower than
the usual 30eV for gases. This is encouraging for their use in radiation detectors
since it would imply that more charge pairs are produced in liquids as compared to
gases with the deposition of the same amount of energy. Another positive factor for
liquids is their higher molecular density, because of which the total deposited energy
per unit path length traversed by the radiation is also higher. The higher density
in liquids implies spatial proximity of molecules, which increases the recombination
probability of charges. This is a negative effect as far as radiation detectors are
concerned since it introduces some uncertainty in the proportionality of measured
pulse height with the deposited energy.
Liquefied argon is the most commonly used detection medium in large area de-
tectors, such as liquid calorimeters for high energy physics experiments. Liquid
xenon is generally used as a scintillation medium, that is, it produces light when its
molecules are excited by the incident radiation. Liquid xenon filled detectors will
be discussed in the Chapter on scintillators.
The basic principle of creation of a charge pair in a liquid is the same as in a
gas. However since the energy states in liquid state are quite different than those in
gaseous state therefore the process is a bit more complicated for the case of liquids.
To understand this the reader is referred to Fig.4.1.1, which shows idealized energy
level sketches of an element in gaseous and liquid states. The first point to note here
is that the energy levels in a gas are discrete while in a liquid they are so closely
spaced that they are said to form valence and conduction bands. In a liquid, the
difference between the bottom of the conduction band to the top of the valence band
is the band gap, which determines the energy required by an electron in the valence
band to jump to the conduction band and become free to move around. In a gas
this gap is much larger and therefore more energy is needed to force an electron in
one of the valence energy levels to become free.
Up until now we have deliberately avoided to use the term electron ion pairs for
liquids. The reason can be inferred from the energy level structure of liquids as shown
in Fig.4.1.1. In a gaseous state, at least to a good approximation, each molecule can
be regarded as an individual entity with its own discrete energy levels. In liquids
the situation is not that simple since the spatial proximity of molecules makes them