Answer to Question #887 Submitted to "Ask the Experts"

The types of detectors you are referring to (photochromic radiation damage detectors) are radiation-sensitive devices that change color in response to physical and chemical changes brought about by exposure to ionizing radiation, such as x rays or gamma rays. The most common materials in this category are thin plastic films, sometimes impregnated with dyes. The ionizing radiation causes removal of electrons from molecules that make up the material, or dye, often causes breaking of chemical bonds between atoms, and frequently induces migration of electrons within molecules, resulting in changes in chemical configuration that are accompanied by production of color centers, different from the original material. One common plastic material that has been used for many years is polymethyl methacrylate (PMMA), sometimes sold under the name of Perspex. Various formulations, some with added dyes, are available to cover a rather wide range of radiation doses, roughly from 300 gray to 50,000 gray (the gray is a unit of radiation dose and represents an energy absorption of 1 joule per kilogram of material). Another rather common type uses nylon plastic film impregnated with selected dyes that undergo color changes when irradiated. Depending on sensitivity, which can vary with the type and concentration of the dye, as well as the thickness of the film and the wavelength at which the light transmission is read after irradiation, these nylon film dosimeters can cover the dose range from approximately 500 gray to 200,000 gray. There are also liquid dye systems that are used in a similar fashion to monitor doses; these usually contain a dye dissolved in a small volume of appropriate solvent and sealed in a small tube. In all cases, sufficient radiation dose produces a change in color (usually enhanced or darker color with increasing dose) of the photochromic dosimeter The extent of color change is measured with a spectrophotometer that measures the transmission of light of selected wavelengths through the film. The devices, used as small pieces of film, often with area dimensions on the order of a square centimeter or so, are placed on or in the package to be irradiated; after the irradiation the films are read in the spectrophotometer to determine the delivered doses. Typical detectors of this type are used primarily to measure relatively large doses and have common applications in measuring doses delivered during radiation processing. Examples of radiation processing would be the treatment of medical products or foods for sterilization purposes or the treatment of certain polymeric coatings to increase cross-linking to alter certain physical characteristics. The photochromic detectors are generally not sensitive enough for routine use as dosimeters for personnel monitoring in radiation protection where allowed doses are much lower than could be measured with the color-sensitive devices. I hope this information is adequate for your needs.

George Chabot, CHP, PhD