Etchable length reduction of induced fission tracks in apatite at room temperature (∼23oC); crystallographic orientation effects and 'initial' mean lengths

Etchable length reduction of thermal-neutron induced fission tracks is observed in four apatites that experienced temperatures no greater than approximately 230C subsequent to track formation. In each apatite, the mean etchable fission track lengths parallel (lc) and perpendicular (la) to the crystallographic c-axis as well as the arithmetic (conventional) mean (1m) decrease by approximately 0.5 μm during the time interval ∼10 min-∼3 weeks after irradiation. Two crystallographic orientation dependencies of mean etchable fission track length at room temperature are identified: (a) 1c>1a in all cases (tracks etched for 25 s at 230C in 5 M HNO3) and (b) la decreases approximately twice as fast as lc. These measurements strongly support previous suggestions that natural charged-particle tracks in crystalline materials, including fission tracks, undergo slow annealing at low ambient temperatures. The observed sensitivity of induced fission tracks in apatite to low-temperature annealing requires that care be taken to avoid heating apatites before and during track etching for so-called ''initial'' mean track length measurements. (author)