Physics and Engineering of Radiation Detection

178 Chapter 3. Gas Filled Detectors

3.4.D SpecialTypesofIonChambers.................

Ionization chambers are perhaps the most widely used radiation detectors. Because
of their heavy usage and applicability in diverse applications, we will now visit some
of the most commonly used variants of the standard ionization chamber geometry.

D.1 ParallelPlateFrischGridChamber

The simple parallel plate geometry we discussed earlier has a major flaw: the pulse
amplitude depends on the position of charge pair production (see equation 3.4.4).
A cylindrical chamber, on the other hand, does not have such a dependence. This
problem can be solved in parallel plate geometry as well by using the so calledFrisch
gridbetween the two electrodes. Fig.3.4.8 shows such a chamber. The grid (shown
as dashed line) is simply another electrode kept at a potential that is intermediate to
that of anode and cathode. Mechanically, the grid is made porous so that electrons
could pass through it easily on their way to the anode. The chamber is designed in
such a way that the incident radiation interacts only within the region enclosed by
the grid and the cathode.

Vout

x d

R

IncidentRadiation

Shield

C

−

Frisch Grid

Figure 3.4.8: Parallel plate ioniza-

tion chamber with a Frisch grid. The

incident radiation is directed with

the help of shield to interact only

within the volume bounded by the

grid and the cathode. In such a

chamber the pulse shape does not de-

pend on the position of charge pair

production.

The output voltage in such a chamber is measured across a resistor between the
anode and the cathode and is exclusively due to the motion of electrons. This implies
that as long as there are no electrons moving between the grid and the anode, the
output voltage remains constant. The point of interaction, therefore, does not have
any effect on the shape of the signal. Using similar arguments as in the case of simple
parallel plate ion chamber, we can deduce the expression for the output voltage. In
this case, since the signal starts developing as soon as electrons pass through the
grid, therefore it can be considered as a simple ionization chamber in which the