Physics and Engineering of Radiation Detection

3.4. Ionization Chambers 171

E

E

+V

−V

+V −V

+V −V

anode cathode

(a)

−V +V

anode

cathode

(b)

Figure 3.4.2: (a) Parallel plate ion chamber and

a two dimensional view of electric field inside its

active volume. The curved electric field at the

sides may induce nonlinearity in the response.

(b) Cylindrical ion chamber and a two dimen-

sional view of radial electric field in its active vol-

ume. The increased flux of electric lines of force

near the positively charged anode wire greatly

enhances the electron collection efficiency.

lines of force at the edges of such a detector can potentially cause nonlinearity in
the response, but with proper designing this problem can be overcome. In fact,
very high precision parallel plate ionization chambers have been developed (see, for
example (1)).
Let us see how the output voltage pulse from such a chamber looks. For this we
note that the voltage pulse is actually the result of the perturbation in the electric
potential caused by the movement of charge pairs towards opposite electrodes. This
is because the electrons and ions generated inside the chamber decrease the effective
electric field. The strength of this effective field varies as the charges move toward
opposite electrodes, generating a voltage pulse at the output. The effective voltage
at any timetinside the chamber can be written as

Vef f(t)=V 0 −Vnp(t). (3.4.1)

whereV 0 is the static applied potential andVnp(t) is the potential difference at time
tcaused by the electrons and ions inside the chamber.