Thermoluminescent dosimeters (TLDs) and films badges are wearable devices that measure ionizing radiation exposure levels. These instruments are often worn by personnel near the torso as this represents the primary location of body mass and organs, but they may also be attached to objects. These devices typically remain in place for extended intervals to assess cumulative exposure. They are considered 'delayed read' dosimeters as the instruments must be processed post-exposure to obtain dosage measurements.
Safety: It should be stressed that dosimeters offer no active protection or mitigation from harmful radiation, do not alert individuals when exposed to high dosages, and are not radioactive themselves.
TLDs are composed of phosphor crystals that measure ionizing radiation primarily by trapping propagated gamma and neutron exposure. Some TLDs can measure beta particles if the instrument is of the correct construction. The crystal chips are contained behind one or more radiation shields or filters to determine the exposure levels for different depths of tissue. The crystal and filters are contained in a badge with identifying labels.
Incident energy is absorbed by some of the crystal's atoms when exposed to ionizing radiation, thereby producing free electrons and corresponding electron holes. Free electrons are trapped in the band gap by the imperfect lattice structure of the crystal that is created due to doping impurities.
The crystal is heated by an electric current, heating element, RF frequency, hot air, or heating lamp and the crystal vibrates to release the free electron back to its ground state. Trapped ionization is released as light, which is measured by photomultiplier tubes. This value is in ratio with the ionizing radiation captured by the phosphor, and represents the dosage administered to a person, provided the equipment was mounted properly.
The results of a TLD measurement are interpreted by a glow curve. As electrons in the bad gap are released at varying intervals, light output intensity varies. When the light intensity is graphed, the highest peaks are used to interpret radiation dosage. Only a fraction of incident energy is absorbed by a TLD. The ratio of the thermoluminescent light emitted per unit mass over the absorbed dose is known as intrinsic efficiency.
Modern TLDs measure the skin dose, eye dose and deep dose as outlined by the United States Nuclear Regulatory Commission's regulation Part 20 guidelines, which specify that different measurements be accounted for certain biological tissue depths. Many TLDs contain various filters to discriminate photon energies, their penetration depth, and the nature of incident radiation.
TLDs do not provide measurements while accumulating energy or a permanent record. They can be immediately analyzed once removed, but reading the results resets the TLD to zero. Trapped ionization begins to face once removed from a radiation source, so results should be interpreted promptly. Potential residual signals can be completely removed via annealing. They can be reused until the phosphor expires or is compromised from excessive heat or inadequate processing during the annealing process.
Calibration is an important aspect of TLD reading. TLDs can identify doses as low as one millirem, but they have the same radiation detection capabilities as film badges for most applications. Higher dosages improve measurement accuracy, as low dosages can have up to 15 percent tolerance. This tolerance can be reduced to as low as one percent if calibrated. The measurement of a field TLD is divided by the average measurement for several calibrating TLDS and provides a correction factor to improve the efficiency of each field TLD.
Phosphor crystals are most often comprised of the following materials. Tissue equivalent materials, those compounds recommended for personnel monitoring, include: lithium fluoride (LiF), lithium borate (Li2B4O7), beryllium oxide (BeO), and magnesium borate (MgB4O7). The materials used for environmental monitoring are
calcium sulphate (CaSO4), calcium fluoride (CaF2), aluminum oxide(Al2O3), and magnesium orthosilicate (Mg2SiO4).
Emission spectra: the type and intensity of electromagnetic wavelengths emitted by the phosphor.
Energy response: the value of the energy absorbed by the phosphor versus the energy absorbed by a reference material when irradiated equally.
Measurement range: the minimum and maximum irradiation values detectable by a TLD.
Sensitivity: the inverse of the calibration factor (see Measurement, above).
Films badges are effective at measuring gamma rays, x-rays, beta particles, and neutrons. It consists of two components: a film with an emulsion coating and a housing to contain that film. Often, multiple films with different emulsion sensitivities, or a single film with multiple layers of emulsion, are used to discern various thresholds of radiation dosage. The film holder or badge is typically metal when the film badge is monitoring for gamma or x-rays, or plastic if monitoring for beta particles. The badge prevents light, liquids, and vapors from entering the protective envelope and compromising results.
To determine dosage, the badge uses up to five radiation-filtering materials to attenuate radiation at different sections of the film. These materials include aluminum, copper, lead/tin, plastic, as well as an open window that offers no shielding. The level of radiation is measured by comparing the results after ionizing radiation has passed through the different filters and darkened the film at different rates.
Film badges remain an economical dosimetry instrument, but are have been largely replaced by TLDs.
The film emulsion contains silver bromide, therefore resulting in a higher atomic number on the film than what is received by biological tissues. An algorithm accounts for the different rates of film exposure and correlates it to actual exposure dosages. Typically these computations are processed by a commercial laboratory.
Film badges are highly accurate for exposures more than 100 millirem. They also create a permanent exposure record, can determine the energy levels of photons, and can measure multiple types of ionizing radiation. Film badges require processing for measurement and are ineffective for doses less than 100 millirem unless proper filtering techniques are applied.
Energy response: the value of the energy absorbed by the film badge versus the energy absorbed by a reference material when irradiated equally.
Measurement range: the minimum and maximum detection values for the film badge.
TLDs and film badges are commonly available as whole-body dosimeters, meaning they are worn on the torso, near the waist or neck, to best represent the exposure level of the entire body. Finger and wristband personal dosimeters also exist, which only represent the exposure levels of extremities.
TLDs and film badges are typically employed where personnel may be exposed to ionizing radiation, such as in biomedical diagnostic processes and nuclear facilities and research centers. They are also used to monitor exposure for devices or environments, such as electronics subjected to radiation hardness testing.
In the United States, occupational exposure to ionizing radiation is governed by OSHA standard 1910.1096 which states, "Every employer shall supply appropriate personnel monitoring equipment, such as film badges, pocket chambers, pocket dosimeters, or film rings, and shall require the use of such equipment." Other countries and jurisdictions have similar regulations pertaining to employees, as applicable.
Industrial standards for TLDs and films badges include:
ASTM E668 Dosages of radiation hardness-tested electronics
ASTM ISO/ASTM 51956 TLD standard practices
ASTM E2450 TLDs in mixed neutron-photon environments