Physics and Engineering of Radiation Detection

3.3. Regions of Operation of Gas Filled Detectors 161

whereNAis the Avogadro’s number,ρis the weight density, andAis the

atomic mass. Hence for argon we have

Nm =

(6. 022 × 1023 )(1. 78 × 10 −^3 )

18

=5. 97 × 1019 atoms/cm^3.

The percent loss of electronsnabs, according to equation 3.2.14 is then given

by

nabs =

N 0 −N

N 0

×100 =

[

1 −e−fNmησx

]

× 100.

=

[

1 −exp{−(0.01)(5. 97 × 1019 )(10−^5 )(5× 10 −^14 )(0.5)}

]

× 100

=13.8%

This example clearly shows how problematic small amounts of contaminants

can be in a detector. For precision measurements, parasitic absorption of about

14% of the electrons could be enough to deteriorate the signal to unacceptable

levels.

3.3 Regions of Operation of Gas Filled Detectors

Fig. 3.3.1 shows different regions of operation of a gas filled detector. Based on the
applied bias voltage, a detector can be operated in a number of modes, which differ
from one another by the amount of charges produced and their movement inside
the detector volume. Choice of a particular mode depends on the application and
generally detectors are optimized to work in the range of the applied voltage that
is typical of that particular mode only. These operation regions are discussed here
briefly.

3.3.A RecombinationRegion......................

In the absence of an electric field the charges produced by the passage of radiation
quickly recombine to form neutral molecules. At the application of the bias voltage
some of the charges begin to drift towards the opposite electrodes. As this voltage is
raised the recombination rate decreases and the current flowing through the detector
increases. The recombination region depicted in Fig.3.3.1 refers to the range of
applied voltage up to the value when the recombination is negligibly small. Because
of appreciable recombination in this region, the current measured at the output of
the detector does not honestly reflect the energy deposited by the incoming radiation.
Consequently in terms of measuring the properties of radiation, it is useless to
operate the detector in this region.

3.3.B IonChamberRegion.......................

The collection efficiency of electron-ion pairs in the recombination region increases
with applied voltage until all the charges that are being produced get collected. This