Physics and Engineering of Radiation Detection

240 Chapter 4. Liquid Filled Detectors

The reaction rate constants have also been determined for different impurities
by several researchers. The data varies considerably from one specie to the next.
for example for oxygen the value has been found to be 6× 1011 M−^1 s−^1 while for
sulfur hexafluoride and carbon tetrachloride the constant assumes a value about
three orders of magnitude higher than that for oxygen (see, for example (9)).
Because of the inverse relationship between the electron lifetime and the rate
constant, the lower the rate constant the better for the detector since it would imply
that more electrons survive to induce the potential change necessary to produce the
output signal.

Example:

Determine the lifetime of electrons in a liquid argon filled detector having 2

ppmof oxygen as impurity.

Solution:

2 parts per million (ppm) of oxygen would be equivalent to a molar concen-

tration of

Coxy=2× 10 −^6 M. (4.5.13)

According to equation 4.5.9 the mean lifetime of electrons is given by

τ =

1

kCoxy

=

1

(6× 1011 )(2× 10 −^6 )

≈ 0. 8 μs.

Such a small lifetime can be detrimental to the performance of the detector

specially in low radiation environments.

4.6 CherenkovDetectors

In chapter 2 we discussed the process of production of Cherenkov radiation in trans-
parent media. The detectors based on the exploitation of this phenomenon to detect
particles are called Cherenkov detectors. Before we go on to the discussion of such
detectors, let us first summarize the basic points related to the emission process of
Cherenkov radiation.

Whenever a charged particle in a medium moves faster than speed of lightin

that medium, it emits Cherenkov radiation.

Cherenkov radiation is composed of photons with wavelengths mostly in the

visible region of the electromagnetic spectrum.

The radiation is emitted in the shape of a cone with the path of the particle

as its axis. The angleθcof the cone is related to the velocityvof the particle