Physics and Engineering of Radiation Detection

200 Chapter 3. Gas Filled Detectors

coefficient for helium at 0.5atmis approximately 1. 7 × 10 −^7 cm^3 s−^1.

Solution:

Assuming that the initial concentrations of electrons and ions are equal, we

can estimate the required quantity using equation 3.7.8.

n∞ =

√

S

α

=

√

1. 5 × 1011

1. 7 × 10 −^7

=9. 4 × 108 cm−^3

3.7.B EffectsofContaminants

The gases used in radiation detectors are generally not free from contaminants. The
most problematic of these contaminants are the electronegative molecules, which
parasitically absorb electrons and form stable or metastable negative ions. Some of
these impurity atoms are listed in Table 3.7.1. The most commonly found contami-
nants in gaseous detectors are oxygen and water vapors. It is almost impossible to
purify a filling gas completely of oxygen. In fact, generally a few parts per million of
oxygen is present at the filling of any gaseous detector. This concentration increases
with time due to degassing of the chamber and, if the detector windows are very
thin, by diffusion from outside.
The capture of electrons by these contaminants is not only a problem for pro-
portional counters but also for high precision ionization chambers. On the other
hand, for general purpose ionization chambers, capture of a few electrons by the
contaminants is of not much concern as the nonlinearity caused by this generally
falls within the tolerable uncertainty in detector response.

Table 3.7.1: Electro affinities of different molecules and ions (18).

Molecule Electron Affinity (eV) Negative Ion Electron Affinity (eV)

O 2 0.44 O− 1.47

C 2 3.54 C− 1.27

Cl 2 2.38 Cl− 3.61

OH 1.83 H− 0.75

Often one is interested in comparing the probability of electron attachment for
different gases. We can define this quantity with the help of two factors that have
been measured for different gases: the mean lifetimeτefor the electrons and their