Optics CR-39 is transparent in the
visible spectrum and is almost completely opaque in the
ultraviolet range. It has the highest
abrasion/scratch resistance of any uncoated optical plastic. CR-39 is about half the weight of
glass with an index of refraction only slightly lower than that of
crown glass, and its high
Abbe number yields low
chromatic aberration, altogether making it an advantageous material for eyeglasses and
sunglasses. 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
solvents and other chemicals,
gamma radiation, aging, and to
material fatigue. It can withstand the small
hot sparks from
welding, something glass cannot do. It can be used continuously in temperatures up to 100 °C and up to one hour at 130 °C.
Radiation detection tracks in CR-39 In the radiation detection application, CR-39 is used as a
solid-state nuclear track detector (SSNTD) to detect the presence of
ionising radiation. Energetic particles colliding with the polymer structure leave a trail of broken chemical bonds within the CR-39. When immersed in a concentrated
alkali solution (typically
sodium hydroxide) hydroxide ions attack and break the polymer structure, etching away the bulk of the plastic at a nominally fixed rate. However, along the paths of damage left by charged particle interaction the concentration of radiation damage allows the chemical agent to attack the polymer more rapidly than it does in the bulk, revealing the paths of the charged particle
ion tracks. The resulting etched plastic therefore contains a permanent record of not only the location of the radiation on the plastic but also gives spectroscopic information about the source. Principally used for the detection of
alpha-emitting
radionuclides (especially
radon gas), the radiation-sensitivity properties of CR-39 are also used for proton and neutron
dosimetry and historically
cosmic ray investigations. The ability of CR-39 to record the location of a radiation source, even at extremely low concentrations is exploited in
autoradiography studies with alpha particles, and for (comparatively cheap) detection of alpha-emitters like uranium. Typically, a thin section of a biological material is fixed against CR-39 and kept frozen for a timescale of months to years in an environment that is shielded as much as possible from possible radiological contaminants. Before etching, photographs are taken of the biological sample with the affixed CR-39 detector, with care taken to ensure that prescribed location marks on the detector are noted. After the etching process, automated or manual 'scanning' of the CR-39 is used to physically locate the ionising radiation recorded, which can then be mapped to the position of the radionuclide within the biological sample. There is no other non-destructive method for accurately identifying the location of trace quantities of radionuclides in biological samples at such low emission levels. ==See also==