Answer to Question #10252 Submitted to "Ask the Experts"

Category: Instrumentation and Measurements — Personnel Monitoring (PM)

The following question was answered by an expert in the appropriate field:

Q

Do we have to use just crystalline materials in dosimetric investigations? Is it possible to use an element for dosimetry? I wonder why methods like thermoluminescent dosimetry and optically stimulated luminescence rely on crystals, and no simple element is used in dosimetry.

A

For passive dosimetric devices such as thermoluminescent dosimeters (TLDs) and dosimeters that use optically stimulated luminescence (OSL), the energy delivery and storage process relies on the crystalline nature of the materials. The crystal exhibits a significant energy gap between the electronic valence band and the conduction band. Energy-trapping sites that typically lie between the valence and conduction band are provided by impurities introduced into the crystals (e.g., Mg and Ti in common LiF TLD-100). Electrons elevated out of the valence band by ionizing radiation interactions may travel in the conduction band and be trapped in metastable energy states provided by the impurities and held there until they are released by elevating them out of those states by heat or light input. The entire process could not function in a noncrystalline material or in crystalline materials that did not have a sufficient energy gap between the valence and conduction bands.

There are other types of radiation detectors that have been used for passive dosimetry that do not rely upon crystalline materials in this same way. The oldest of these is film, which consists of a gelatinous matrix in which small crystals of AgBr are dispersed. It is true that the AgBr is crystalline, but it functions quite differently from crystals used in TLDs and OSL devices. The film system works by way of the chemical reduction of Ag+1 to elemental silver as a consequence of electrons set free by ionizing radiation interactions in the film emulsion being captured in the AgBr. The increased reduced silver content is measured by the increased darkening of the film. There are a variety of other systems that have been used in radiation dosimetry.

These include such devices as radiophotoluminescent glasses, usually phosphate-based glasses with small amounts of Ag+1, that respond to ionizing radiation by yielding photoluminescent optically active centers that can be stimulated to emit visible light when irradiated with ultraviolet light; radiochromic dye systems, either liquid, gel, or plastic, that use color changes induced by ionizing radiation exposure as a consequence of electron transfers within organic dye compounds; a number of plastics that respond to ionizing radiation by undergoing structural changes that exhibit as color changes; and several liquid chemical dosimeters that rely on oxidation or reduction of certain chemical species to yield measurable effects (the most well-known of  these is the Fricke dosimeter that uses an acidic solution of Fe++ which, when exposed to oxidizing species produced as radiolytic products of irradiation, yield Fe+++, which can be measured spectrophotometrically). Many of these dosimetric systems are not suited to low-dose measurements that are of interest in personnel dosimetry, but they have applications in high-dose measurements for such categories as industrial radiation processing and medical therapeutic applications of ionizing radiation.

There are many more systems that have been used in active radiation measurements, especially in radiation-detection instrumentation. Some use multielement crystals such as NaI(Tl) that also rely on crystal energy gap structure and added impurities to yield fluorescent photon emission in response to excitation by ionizing radiation. Others may use a single element, such as crystalline germanium detectors, used most notably for gamma spectrometry, and silicon diodes used in electronic personal dosimeters. Gas-filled detectors such as Geiger Mueller detectors, ionization chambers, and gas proportional detectors rely on gas ionization by ionizing radiation to produce measurable pulses (or currents) and have had a wide variety of applications.

In summary, there are a number of detectors/sensors used in dosimetry that rely on certain characteristics of crystals, but there are many more that use other materials and/or other operating principles to provide a response to ionizing radiation. If you are unfamiliar with many of these, you might want to consult a good reference text on radiation detection. One of the most useful and popular such texts is Glenn F. Knoll's Radiation Detection and Measurements, 4th ed., Wiley, 2010, available through many book stores.

George Chabot, PhD
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