Abstract: A gamma radiation dosimeter comprises a polyester film substrate, of which a central circular region is coated with a gamma-radiation sensitive layer which is covered by an overlayer. The radiation-sensitive layer is formed from styrene-acrylonitrile copolymer and contains 1-bromoadamantane and a leuco dye. The overlayer is formed from an acrylic resin binder containing a UV-absorber. The central region is surround by an annular region which is divided into six equally-sized sections, each of which is printed in a color which corresponds to the color developed in the central region when it has been exposed to successive increasing amounts of gamma radiation. After exposure of the device, the amount of radiation to which the device has been exposed can be determined by comparing the color generated in the region with the colors of the six sections.