Physics and Engineering of Radiation Detection

234 Chapter 4. Liquid Filled Detectors

Table 4.4.1: Properties of liquefied noble gases.

Property Argon Krypton Xenon

Z 18 36 58

A 40 84 131

Radiation Length (cm) 14.2 4.7 2.8

Critical Energy (MeV) 41.7 21.5 14.5

Fano Factor 0.107 0.057 0.041

Normal Boiling Point (K) 87.27 119.8 164.9

Liquid Density at Boiling Point (gcm−^3 ) 1.4 2.4 3.0

Dielectric Constant 1.51 1.66 1.95

Scintillation Light Wavelength (nm) 130 150 175

scintillation detectors. However, as mentioned above, generally liquid xenon is used
in scintillation detectors while liquid argon in ionizing detectors.
The reader might be thinking as to why one does not use regular liquids instead of
the liquefied gases. There are several reasons why liquefied noble gases are preferred
over regular liquids. For example the liquefied noble gases are dielectrics, which
makes them suitable for free charge transport. Also, the large drift lengths of elec-
trons make these liquids suitable for building large area detectors. As we will shortly
see, the larger drift length is a consequence of lower recombination probability in
liquefied noble gases as compared to molecular liquids.
Looking at Table 4.4.1, it becomes clear that xenon is the best choice for scintil-
lating and ionizing detectors. Its stopping power is higher than argon and krypton
due to its higher density. Higher stopping power allows quicker and higher deposi-
tion of energy, which means better timing and energy resolutions. Although liquid
xenon is mostly used as a scintillating medium but it can, in principle, be used to
build ionizing detectors as well. The biggest disadvantage of liquid xenon is its cost,
which is much higher than liquid argon and krypton.

4.5 Sources of Error in Liquid Filled Ionizing Detectors

4.5.A Recombination

Recombination of electrons with the positive ions (or holes) degrades the initial
electron population and therefore has the detrimental effect of signal weakening.