tool in the laboratory for research and calibration purposes. Historical chambers Condenser chamber The condenser chamber has a secondary cavity within the stem which acts as a capacitor. When this capacitor is fully charged, any ionization within the thimble counteracts this charge, and the change in charge can be measured. They are only practical for beams with energy of 2 MeV or less, and high stem leakage makes them unsuited to precise dosimetry. Extrapolation chamber Similar in design to a parallel plate chamber, the upper plate of an extrapolation chamber can be lower using micrometer screws. Measurements can be taken with different plate spacing and extrapolated to a plate spacing of zero, i.e. the dose without the chamber. Instrument types Hand-held Ion chambers are widely used in hand held radiation survey meters to measure beta and gamma radiation. They are particularly preferred for high dose rate measurements and for gamma radiation they give good accuracy for energies above 50-100 keV. There are two basic configurations; the "integral" unit with the chamber and electronics in the same case, and the "two-piece" instrument which has a separate ion chamber probe attached to the electronics module by a flexible cable. The chamber of the integral instrument is generally at the front of the case facing downwards, and for beta/gamma instruments there is a window in the bottom of the casing. This usually has a sliding shield which enables discrimination between gamma and beta radiation. The operator closes the shield to exclude beta, and can thereby calculate the rate of each radiation type. Some hand held instruments generate audible clicks similar to that produced by a G-M counter to assist operators, who use the audio feedback in radiation survey and contamination checks. As the ion chamber works in current mode, not pulse mode, this is synthesised from the radiation rate. Installed For industrial process measurements and interlocks with sustained high radiation levels, the ion chamber is the preferred detector. In these applications only the chamber is situated in the measurement area, and the electronics are remotely situated to protect them from radiation and connected by a cable. Installed instruments can be used for measuring ambient gamma for personnel protection and normally sound an alarm above a preset rate, though the Geiger–Müller tube instrument is generally preferred where high levels of accuracy are not required. General precautions in use Moisture is the main problem that