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• Check the batteries (most RADIACs are battery powered). This is normally done
by either pushing a button labeled BAT(T), or turning the switch to BAT and
watching the meter. If it moves to within the designated area, it is satisfactory. On
the newer digital meters, a self-diagnostic may run as soon as the meter is
switched on, and the battery level will be displayed.
• In all cases, follow the procedures approved at your facility for use of these
instruments.
There are two main classes of RADIAC instruments used to detect and survey for
ionizing radiation: gas-filled and scintillation detectors.
Gas-filled detectors. The first and oldest type of RADIAC is the gas-filled detector.
These are constructed by filling an electrically conductive chamber with an inert gas. The
system contains two electrodes, one through the center of the chamber and the chamber
itself, which also functions as an electrode. When a voltage is applied to the system, the
center wire becomes positively charged (anode) and the outer (chamber) electrode takes
on a negative charge (cathode). The fill gas, usually a noble gas such as argon, helium, or
neon, is ionized by the radiation that strikes and penetrates into the chamber. This
produces a pair of ions (a positive base ion and a free electron), which are attracted to the
electrode of the opposite charge. The free electrons that are collected along the anode
then create an electrical impulse. The electrical impulse (or current) is measured and
converted to a meter reading, generally in counts/min or mR/hour.
As the amount of voltage applied to the tube increases, the attractive forces of the anode
and cathode get stronger and the production of secondary ionizations increases.
Secondary ionizations are those not caused by the incident radiation, but by
multiplication within the inert gas (gas amplification). There are six distinct regions
corresponding to the amount of voltage applied.
Detectors are described according to the radiation regions they detect. The regions most
commonly used for detection are the Ion Chamber Region, the Proportional Region, and
the Geiger-Mueller Region.
• Ion chambers operate at low voltages, which do not produce any gas
multiplication. The applied voltage is just enough that the ions migrate to their
opposite poles; without secondary ionization, they travel through the gas. This
means that only the primary ions created by the initial radiation event are
collected, so that one radiation interaction produces one count. The ion chamber is
very good for exposure measurements but not for detection. It is therefore used
mainly as a dosimeter (see below).
• Proportional counters rely on gas multiplication to amplify the charge created by
the original ionization in the gas. The pulses they produce are therefore
considerably larger than those from ion chambers used under the same conditions.
This allows proportional counters to be used when the number of ion pairs is too
low for effective detection by an ion chamber. Proportional counters are widely
used in detection of low energy x-rays and neutrons. Because the current signal in
a proportional counter is “proportional” to the energy of the radiation, these
counters can also be used for spectroscopy of low energy x-rays.
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