dosimeter

n.
An instrument that measures and indicates the amount of x-rays or radiation absorbed in a given period.

[DOS(E) + -METER.]

A device used to measure the dose of ionizing radiation received by an individual. There are many types of dosimeters with varying characteristics and capabilities appropriate for different applications. They differ in sensitivity, energy range, and species of radiation to which they respond. Some can be read out directly by the wearer, while others must be sent to a specially equipped facility to determine the dose.

The air ionization chamber dosimeter is usually about the size and shape of a fountain pen with a clip suitable for carrying in a pocket, and is sometimes referred to as a pen meter or pocket dosimeter. The outside housing is usually made of plastic. Inside, a conductive cylinder and an insulated central electrode form an air-filled capacitor which is charged by connecting to a charging voltage supply. X-rays or gamma rays absorbed in the air space ionize the air, discharging the capacitor in proportion to the dose received. The dose is read by connecting the dosimeter to a quartz fiber or other type of electrometer which reads the charge remaining on the capacitor. See also Capacitor; Electrometer; Gamma-ray detectors; Ionization chamber; X-rays.

The housing of the film badge dosimeter is of plastic or metal, typically 50 mm square (2 in.) by 12 mm (0.5 in.) thick and equipped with a clip for attaching to clothing. It contains one or more film packets, usually standard dental x-ray film packets. The film is developed in a standardized procedure, and the optical density is measured and correlated to exposure, greater exposure being darker. The side of the badge housing is equipped with one or more filters of lead, aluminum, copper, silver, or cadmium which cover different parts of the film and leave part uncovered. These filters modify the response of different areas of the film and thereby provide more information to correct the response to approximate that of tissue. See also Radiography.

The physical packaging of a thermoluminescent dosimeter can be similar to a film badge with filters incorporated, but the film is replaced by pieces of thermolumiscent materials. Absorbed radiation energy excites electrons of the thermoluminescent material, creating excited metastable states. These states remain excited until the material is subsequently heated to a sufficient temperature, when they deexcite, emitting visible light. The intensity of the emitted light is proportional to the absorbed radiation energy. The exposed material is placed in an automated instrument which heats it in a controlled manner, and the light is measured with a photomultiplier tube and the resulting signal is converted to a dose. If the exposed material is heated sufficiently to anneal all excited states, the dosimeter can be reused. Lithium fluoride is extensively used for thermoluminescent dosimeters, activated with manganese and titanium. See also Photomultiplier.

Usually rectangular in shape, the electronic dosimeter ranges in size from a transistor radio to a pocket pager, though some devices resemble a fountain pen. It contains as sensing element a Geiger-Müller tube along with high-voltage supply, counting and control electronics, and a digital display, all powered by a battery. A wide variety of features are available. For example, a dose alarm can sound when a preset accumulated dose is reached, or a dose-rate alarm can sound when a preset dose rate is exceeded. An electrical connection can be made with a computer for dose recordkeeping. See also Geiger-Müller counter.

The active element of a track etch dosimeter is a piece of CR-39 plastic (from Columbia resin 39), made by polymerization of allyl diglycol carbonate. When a high-energy charged particle, such as a proton recoiling from a fast neutron, traverses the material, it gives up energy and leaves a trail of chemical damage. The material is subsequently exposed to a suitable etchant such as sodium hydroxide (NaOH), and damage sites are preferentially etched, leaving pits which can be observed and counted under a microscope. This dosimeter is sensitive only to high-energy neutrons, so it is usually used in conjunction with other dosimeters. See also Particle track etching.

The operation of the bubble detector dosimeter is based on the change to gas bubbles of superheated liquid (at a temperature above its boiling point) triggered by fast neutron interactions. A commercial bubble detector personal neutron dosimeter consists of a clear plastic tube with an aluminum fitting at one end, about the size of a large pen. It holds about 8 cm³ (0.5 in.³) of clear, solid elastic polymer containing about 10,000 superheated liquid droplets. On exposure to fast neutrons, small uniform-sized bubbles form. The bubbles are counted visually and a fast neutron dose calculated using a calibration factor determined for that dosimeter. See also Health physics.

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An instrument used to detect and measure exposure to radiation.

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