Physics and Engineering of Radiation Detection

3.3. Regions of Operation of Gas Filled Detectors 165

of electrons and ions (see, for example (37)). The electrons move much faster than
ions and quickly reach the anode leaving behind a wide tail of positive ions drifting
slowly towards the cathode (see Fig.3.3.2).

+

++

++

+ ++

++ +

+++

++++

++ ++++

++++

++++++++++++++

++++++++++++++

++++++++++

−−−+−++++++

−−−−−−−−−−−−−−

−−

−−−−−

−−−−−−−−−−−

−−−−−−−−−−−−

−

−

+

+

+ +

−

−

−

−

−

+

+

+

+

Ionizing

Radiation

Cathode

Anode

Figure 3.3.2: Typical droplet

shape of avalanche in a gas filled

detector. The incident radiation

(shown by a solid line with an

arrow) produces the charge pairs

along its track. The charges start

moving in opposite directions under

the influence of the applied electric

field. The electrons, being lighter

than positively charged molecules,

move faster and leave behind a

long tail of positive charges drifting

slowly towards cathode. Note that

there is a time lag between the ini-

tial creation of charge pairs and the

formation of droplet.

Example:

Calculate the first townsend coefficient for a helium filled chamber kept under

760 torrwhen an electric field of 10^4 Vcm−^1 is established across the chamber

electrodes. Also estimate the gas gain at a distance of 0.1cmfrom the anode.

Solution:

The first townsend coefficient can be calculated from equation 3.3.7 using the

values ofAandBas given in table 3.3.1.

α = APexp

(

−

BP

E

)

= (3)(760) exp

[

−

(34)(760)

104

]

= 172. 1 cm−^1

The multiplication factor atx=0. 1 cmis then given by

M = eαx

= e(172.1)(0.1)

≈ 3 × 107.