Physics and Engineering of Radiation Detection

3.7. Sources of Error in Gaseous Detectors 201

collision frequencyνe.Intermsofτeandνethe probability of capture in a single
collision can be written as (37)

p=

1

τeνe

. (3.7.9)

The values of these parameters for some common gases are shown in Table 3.7.2. It
is apparent that the extremely small capture lifetimes of these gases can be a serious
problem for at least high precision systems. Even small quantities of contaminants
such as oxygen and water can produce undesirable non-linearity in detector response.

Table 3.7.2: Mean capture timeτe, collision frequencyνe, and probability of electron
capture in a single collisionpfor some common contaminants and filling gases for
radiation detectors (37). All the values correspond to electrons at thermal energies
and gases under standard atmospheric conditions.

Gas τe(s) νe(s−^1 ) p

O 2 7. 1 × 10 −^42. 2 × 1011 6. 4 × 10 −^9

CO 2 1. 9 × 10 −^72. 1 × 1011 2. 5 × 10 −^5

H 2 O 1. 4 × 10 −^72. 8 × 1011 2. 5 × 10 −^5

Cl 2 4. 7 × 10 −^94. 5 × 1011 4. 7 × 10 −^4

Let us now have a look at different mechanisms by which the contaminants cap-
ture electrons.

B.1 RadiativeCapture

In this kind of capture, the capture of electron leaves the molecule in such an excited
state that leads to the emission of a photon. Radiative capture can be symbolically
represented as

e+X → X−∗

X−∗ → X−+γ, (3.7.10)

where (∗) represents the excited state of moleculeX. The radiative capture occurs
in molecules that have positive electron affinity. Fortunately enough, for the con-
taminants generally found in filling gases of radiation detectors the cross section for
this reaction is not significant (39).

B.2 DissociativeCapture......................

In this process the molecule that has captured an electron dissociates into simpler
molecules. The dissociation can simply be the emission of an electron with an energy