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.