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CR-39, or allyl diglycol carbonate, is a plastic polymer commonly used in the manufacture of eyeglass lenses. An alternative use includes a purified version that is used to measure neutron radiation, a type of ionizing radiation, in neutron dosimetry. The CR stands for Columbia Resin.
CR-39 is transparent in visible spectrum and almost completely opaque in infrared and ultraviolet range, it has high abrasion resistance, about half the weight of glass and index of refraction only slightly lower than that of crown glass, making it an advantageous material for eyeglasses and sunglasses lenses. A wide range of colors can be achieved by dyeing of the surface or the bulk of the material. CR-39 is also resistant to most of solvents and other chemicals, to gamma radiation, to aging, and to material fatigue. It can withstand the small hot sparks from eg. welding. It can be used continuously in temperatures up to 100 °C and up to one hour in 130 °C.
In the radiation detection application, raw CR-39 material is exposed to proton recoils caused by incident neutrons. The proton recoils cause tracks, which are enlarged by an etching process in a caustic solution of sodium hydroxide.
The enlarged tracks are counted under a microscope (commonly 200x), and the number of tracks is proportional to the amount of incident neutron radiation.
CR-39 is made by polymerization of diethyleneglycol bis allylcarbonate (ADC) in presence of diisopropyl peroxydicarbonate (IPP) catalyst. The presence of the allyl groups allows the polymer to form cross-links; thus, it is a thermoset resin. The monomer structure is