Etchable length reduction of induced fission tracks in apatite at room temperature (≈ 23°C): Crystallographic orientation effects and “initial” mean lengths

Abstract

Etchable length reduction of thermal-neutron induced fission tracks is observed in four apatites that experienced temperatures no greater than approximately 23°C subsequent to track formation. In each apatite, the mean etchable fission track lengths parallel ( l c ) and perpendicular ( l a ) to the crystallographic c -axis as well as the arithmetic (conventional) mean ( l m ) 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) l c > l a in all cases (tracks etched for 25 s at 23°C in 5 M HNO 3 ) and (b) l a decreases approximately twice as fast as l c . 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.