Biomedical Engineering Reference
In-Depth Information
tion of the gas results in a pulse, the size being independent of the number of initial
ion pairs. With still further increase of the voltage, the field eventually becomes so
strong that it ionizes gas atoms directly and the tube continually discharges.
Compared with an alpha particle, the pulse-height curve for a beta particle is
similar, but lower, as Fig. 10.4 shows. The two curves merge in the GM region.
Gas-Filled Detectors
Most ionization chambers for radiation monitoring are air-filled and unsealed, al-
though sealed types that employ air or other gases are common. Used principally
to monitor beta, gamma, and X radiation, their sensitivity depends on the vol-
ume and pressure of the gas and on the associated electronic readout components.
The chamber walls are usually air equivalent or tissue equivalent in terms of the
secondary-electron spectra they produce in response to the radiation.
Ionization chambers are available both as active and as passive detectors. An
active detector, such as that illustrated by Fig. 10.1, gives an immediate reading in
a radiation field through direct processing of the ionization current in an external
circuit coupled to the chamber. Examples of this type of device include the free-air
ionization chamber (Section 12.3) and the traditionally popular cutie pie (Fig. 10.5),
a portable beta-gamma survey rate meter still in use today.
Passive pocket ionization chambers were used a great deal in the past. Basically
a plastic condenser of known capacitance
C
, the unit is given a charge
Q
CV
at
a fixed potential difference
V
before use. Exposure to radiation produces ions in
the chamber volume. These partially neutralize the charge on the chamber and
=
Fig. 10.5
Portable ionization-chamber survey meter (cutie pie). (Courtesy Victoreen, Inc.)
