Thermal annealing of fission tracks in apatite: 1. A qualitative description

Abstract

Confined fission-track lengths have been used to study the degree of annealing of induced fission tracks in samples of a single fluorapatite crystal (Durango apatite), heated for various times (between 20 min. and 500 days) at temperatures between 95° and 400°C. In all annealed samples the mean confined track length is always less than that in unannealed control samples. As annealing progresses, the mean length is reduced and the length distribution broadens, slowly at first, and then more rapidly below a length reduction ( l l 0 ) of ∼ 0.65. In addition, the variation of track length with angle to the crystallographic c-axis becomes progressively more anisotropic. As the mean track length approaches zero, the only tracks left are aligned parallel to the c-acis. In heavily annealed samples ( l l 0 < 0.65 ) observation of individual tracks after sequential etching steps shows the presence of unetchable “gaps” in a small proportion of tracks. The existence of these gaps is borne out by annealing studies of heavy-ion tracks in apatite slices. These studies also show the anisotropy characteristic of annealing in apatite. The annealing of fission tracks in apatite appears to be characterised by two processes. For small degrees of annealing the dominant process brings about a progressive shrinking of the track from each end, with tracks perpendicular to the c-axis shortening more rapidly than those parallel to it. As annealing becomes more severe ( l l 0 < 0.65 ) the tracks begin to break up into discontinuous portions. Track length distributions in annealed apatites reflect the interplay of these processes. The effect of apatite composition on annealing has been studied using spontaneous tracks in apatites from a sample in which considerable geological annealing has occurred. Electron microprobe studies show that apatite grains rich in Cl are more resistant to annealing, while fluorapatite is more readily annealed.