█ LARRY GILMAN
Dosimetry measures the amount of radiation energy absorbed over a given period of time by an object (e.g., human body) or by part of that object (e.g., an organ or tumor). Here, radiation refers not only to ionizing radiation of the sort emitted by radioactive materials—fast particles and gamma rays—but to light, radio waves, or ultrasound. Dosimetry is essential wherever radiation is utilized to treat cancer; the treatment must deliver a sufficient dose to target tissues without delivering too large a dose to other parts of the body. Dosimetry is also needed, wherever radioactive materials are handled in significant quantities, to track the cumulative exposure of individuals and to moniotor for accidental releases of radioactive material.
A device that measures cumulative radiation exposure is a dosimeter . A Geiger counter is a radiation detector, but not a dosimeter, because it gives only a moment-to-moment reading of radiation intensity; a strip of photographic film, however, whose degree of exposure indicates how much radiation it has absorbed (up to its saturation limit), can act as a dosimeter. Filmstrip dosimeters are, in fact, still used to measure exposure to ionizing radiation. By grading the sensitivity of a specially formulated film strip from one end to the other, it can be made to indicate net, cumulative radiation exposure as a bar of darkening that grows from the most sensitive end of the film to the least sensitive end. Such "badge dosimeters" are common in the nuclear weapons and nuclear-power industries. However, they have the disadvantage that they must be developed to be read, and so do not give the bearer immediate knowledge of their exposure level.
Another type of dosimeter is the pen ionization dosimeter. These devices contain a long, narrow chamber filled with a few cubic centimeters of nonconducting gas. A metallic contact touches the interior of the chamber at each end. When the dosimeter is to be used, an initial electric charge is placed on the gas tube; that is, an imbalance of electrons is created between the two ends. Since the gas in the tube is normally nonconducting, electrons cannot travel through it to even out the charge imbalance. However, ionizing radiation passing through the gas forcibly frees electrons from atoms in the gas (i.e., partly ionizes the gas), and these negatively charged electrons are free to flow toward the end of the tube having a positive charge. The more ionizing radiation the pen dosimeter is exposed to, therefore, the more of its initial charge is enabled to leak through the gas tube; the amount of charge lost is a measure of the amount of radiation that has passed through the tube. A pen dosimeter can be read by its bearer at any time, and so gives a current reading of exposure; however, pen dosimeters readings can be affected by mechanical shock or vibration.
A more modern dosimeter design is the thermoluminescent dosimeter (TLD). A TLD contains a tiny crystal of lithium fluoride (sometimes mounted in a finger-ring) that undergoes cumulative structural changes as it is exposed to ionizing radiation. When heated, the crystal glows, giving off an amount of light that is proportional to its radiation exposure. This light is observed by an electronic sensor in a readout unit and recorded digitally. This data can be stored in a central database, a convenient feature if an organization wishes to systematically monitor radiation exposure of a large body of personnel. Databasing of TLD data has been used, for example, by Canada to monitor the exposure of its troops to radiation from depleted-uranium munitions used by NATO in Bosnia. TLDs, unlike film badges, can be re-used; however, they must be inserted in a reader that heats the crystal and records the light emitted, a process that may take 20 to 30 seconds and erases the data in the crystal.
An even more recent entry in the dosimeter field is the optically stimulated luminescence dosimeter (OSLD). In this design, a thin film of crystalline aluminum oxide undergoes cumulative structural changes as it is exposed to ionizing radiation; when an exposure reading is desired, the crystal is exposed to green laser light. The amount of blue light emitted by the film in response is proportional to its radiation exposure. Unlike a TLD, an OSLD can supply an instant readout that can be repeated if necessary.
Solid-state devices that measure radiation by detecting ionization leakage current through a transistor device also exist. Radiation detectors and dosimeters based on such solid-state technology have been available since the 1980s, but have not edged out other dosimeter technologies in terms of cheapness, sensitivity, and accuracy.
Dosimetry for laser light, radio waves, and ultrasound, which is often required in medical contexts, is more difficult than dosimetry of ionizing radiation. One method of measuring dose delivered to a volume of tissue is to measure the temperature increase of the tissue; the more increase, the more radio or sound energy has been absorbed. However, these techniques do not work for tissue embedded in living organisms (where temperature measurement is difficult and where heat is rapidly conducted away) or for whole-body exposure, as biologically tolerable doses of laser, radio, and sound energy produce undetectably slight changes in body temperature. Absorption by the body of radio waves is particularly different from absorption of ionizing radiation; the body acts as a complex antenna whose performance is strongly affected by its posture and orientation and by nearby objects. Dosimetry for radio and ultrasound therefore relies heavily on computational models rather than on direct measurements.
█ FURTHER READING:
"Measuring Occupational Exposures." Health Physics Society. < http://hps.org/publicinformation/ate/faqs/lowmeasure.html > (April 17, 2003).
"Using and Wearing Radiation Dosimeters." Princeton University: Environmental Health and Safety. < http://www.princeton.edu/~ehs/UsingandWearingDosimetry.html > (April 17, 2003).