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• Geiger-Mueller or G-M counters operate at a significantly higher voltage than
either ion chambers or proportional counters. They produce a large signal due to
the “avalanche effect,” in which one initial event can cause millions of subsequent
secondary ionizations. Due to this effect, the signal is not proportional to energy,
and the G-M detector cannot discriminate between different types and energies of
radiation. Additionally, at high radiation levels, G-M counters will experience
dead time, in which an ionization event is lost because it occurs too quickly after
the preceding event. At high count rates, these dead time losses can become
severe, requiring a correction to ensure accurate measurements. The G-M
instrument is very good for detection but not so good for measurement of
exposure. That is to say, it can accurately confirm contamination on a patient, but
it cannot quantify it accurately. The probe is held approximately 0.5 inches off the
surface and moved at a rate of 1–2 inches/second.
Scintillation detectors. The other common type of detector is the scintillation detector,
which uses a liquid or a solid phosphor crystal. When radiation strikes the crystal, the
energy of the incident radiation is expended to produce ionization and excitation. The ion
that was formed eventually recombines, or the excitation decays, both of which result in
the production of light. On average, the number of light photons emitted is proportional
to the amount of energy originally deposited. A photomultiplier tube is then used to
convert the scintillation photons (light) into an electrical pulse, which is counted or
otherwise analyzed electronically. Some common scintillators use zinc sulfate, sodium
iodide, or plastics. To detect alpha particles, scintillation detectors must be held 1/16 th –
1/8 th of an inch from the surface being monitored because of the short range of alpha
radiation.
The so-called multi-function RADIACs represent a modern innovation in radiation
monitoring. These instruments use one readout unit, which can be connected to various
probes so that most types of radiation and contamination can be detected. Most of these
units use what is called a “smart box,” which can detect the type of probe being used.
These “smart” units automatically show the correct units for the monitor being used and
calibrate themselves accordingly. Many of these units come with everything in one case,
eliminating the need to carry many separate detectors.
Dosimeters
Dosimeters are a special type of instrument used to determine the total dose of radiation
that a person receives. The simplest dosimeters are made of film (like camera or x-ray
film), tiny wafers of plastic, or other specially formulated materials that respond to
radiation in ways that can be assessed and matched to a particular radiation dose. Simple
dosimeters require special processing to determine dose but are very reliable and
accurate.
Slightly more complex are pocket ion chambers, which use the ionization caused by
radiation to move a very tiny fiber that tracks the dose. Pocket chambers are inexpensive
and can be read directly, but they are not very accurate and have to be protected from
bumps that might cause the needle to move.
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