Alpha-recoil track densities in mica and radiometric age determination

Recoiling daughters of α-decaying U and Th impurities in mica and other minerals produce localised lattice damage: alpha-recoil tracks. The age of a sample can be calculated from the number of tracks per unit volume (NART). To this end, the mica is etched and the etch pits at the sites of recoil-tracks are counted under the optical microscope. Because the measured track densities increase with etching time, NART is calculated from the fitted regression line. A number of problems inherent in this approach are overcome by the etch-anneal-etch and the mirror-image methods. The track densities determined with these methods are independent of etching time. Although both methods need improvement, they hold the potential of a precise determination of NART from a single measurement in future recoil-track dating.