Because the geiger counter gives an output pulse each time a particle creates ionisation in its detector tube, it is well suited to the detection of individual particles. If a geiger counter is to detect beta, as well as gamma radiation the detector tube must have walls sufficiently thin for beta particles to penetrate. Such tubes are called thin walled tubes and are often used in portable contamination meters . They may be fitted with a metal sheath which can be slid over the tube to prevent beta particles from entering. Measurements made with and without the sheath will then indicate the presence of beta radiation. These instruments are usually calibrated in counts per minute.
Fig. 6.7
shows a portable contamination meter commonly used
at TRIUMF.
The detector is a flat geiger tube, called a pancake tube
, which has a thin
mica window protected by a wire screen.
Figure: 6.7 Geiger Contamination
Meter
Particle detectors are not designed to measure anything other than the number of ionizing events which occur in the detector per unit time. They are used for monitoring surfaces, floors, hands, clothing, shoes and so on, for contamination. If the activity of the contamination is required the efficiency of the detector for the radionuclide concerned must be taken into account.