Physics and Engineering of Radiation Detection

174 Chapter 3. Gas Filled Detectors

Vp

tn tp

Veff

Vn

N 0 e/C

t

Figure 3.4.4: Pulse shape of an ideal

parallel plate ion chamber. Vnand

Vpare the voltage profiles due mainly

to collection of electrons and ions re-

spectively.

collection time characterizes the shape of the output pulse (see Fig.3.4.5). It is
apparent that the quicker the pulse decays, the easier it will be to distinguish it from
the subsequent pulse. On the other hand very small time constant may amount to
loss of information and even non-linearity. Therefore considerable effort is warranted
to tune the effective time constant according to the requirements. We will learn more
about this in the chapter on signal processing.

τ 1

τ 3

τ 2

τ 4

τ 1 <τ 2 <<τ 3 τ 4

N 0 e/C

Veff

t

Figure 3.4.5: Realistic pulse

shapes of an ion chamber with dif-

ferent time constants. The dif-

ference between the effective time

constant of the detector and its

charge collection time determines

the shape of the pulse.

B.2 CylindricalGeometry

Cylindrical ionization chambers offer a number of advantages over their parallel
plate counterparts. Most notably their charge collection efficiency is much supe-
rior due to the presence of non-uniform electric field strength inside their active
volumes. Such a chamber generally consists of a metallic cylinder and an anode
wire stretched along the axis of the cylinder. The cylinder acts as the gas container
as well as the cathode with the obvious advantage of large ion collection area and
consequent high ion collection efficiency. As the anode is generally very thin, the
electric lines of force around it are extremely dense and concentrated (see Fig.3.4.2).
The electrons, therefore, travel toward the anode at much faster speeds than the ions
moving toward the cathode. This increases the electron drift speed and improves