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 radiation 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.