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Dosimeter

A dosimeter is a pen-like device that measures the cumulative dose of radiation received by the device. It is usually clipped to one's clothing to measure one's actual exposure to radiation. Magnifying lenses (a low-power microscope) and an illumination lens allow one to directly read the dose by aiming the illumination lens at a light source and looking into the device.

For personal use, this is the most useful device to measure radiation, because biological damage from radiation is cumulative.

Dosimeters must be periodically recharged. One's dosimeter is usually read before it is charged, and the dose is logged, to chart one's exposure. In many organizations, possession of the recharger is limited to health physicists to prevent falsification of exposures.

A dosimeter is a very rugged form of a device called an electrometer. It has two electrodes which are charged through an external connection. Since they are the same charge, they repel each other. As ionizing radiation passes between the electrodes and the electrically-conductive case, the charge on the electrodes is neutralized. When the charge reduces, an electrode moves away from the zero calibration. The magnifier displays this motion against a scale.

The charger is a small box, usually powered by a battery. It contains an electronic circuit that steps the battery voltage up to several thousand volts. The box has a fixture that requires one to press the end of the dosimeter on the charging electrode. Some chargers include a light to illuminate the measurement electrode, so that measurement, logging and recharging can occur with one routine motion.

Dosimeters are made in different ranges. Peace-time occupational exposure ranges usually measure up to 500 mrem, which exceeds one's legal yearly dose (360 mrems in the US). War-time fallout meters measure up to 500rem, roughly the lethal dose.

A similar device, used with the same charger, is a "rate meter." This is an inexpensive method for civil defense persons to measure radiation rates. One measures the rate of change of the rate meter for a timed exposure after charging the rate meter. Usually one measures heavy fallout of a thirty-second period, and light fallout over a ten-minute period. The rate meter has two internal scales that read the radiation flus directly in rems for each period.

See also radiation, Geiger counter and scintillometer.



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