Crystallographic orientation dependence of mean etchable fission track length in apatite: An empirical model and experimental observations Available to Purchase

Partially annealed thermal-neutron-induced fission tracks in apatite exhibit mean etch- able lengths that range continuously between a maximum for tracks oriented parallel to the crystallographic c-axis and a minimum for tracks perpendicular to c. A simple empirical model quantifies this crystallographic orientation dependence based on the assumption that the mean etchable fission track length aL ary angle to the c-axis is given by the corresponding radius of an ellipse. The ellipse has semiaxes equal to, respectively, the mean track lengths parallel (l_c) and perpendicular (l_a) to the c-axis.

The elliptical model is tested against 61 isothermal partial annealing experiments per- formed on thermal-neutron-induced fission tracks in Tioga apatite. It is found to adequately characterize etchable fission track length distributions in partially annealed apatites when l_a. ≥ 8.4 μm. The difference between l_c and l_a increases systematically and continu- ously as both mean lengths decrease, and fission-track lengths are distributed normally about their mean with a standard deviation of ca. 0.75 μm, regardless of crystallographic orientation and relative degree of partial annealing. For l_a < 8.4 μm, the elliptical model apparently fails because tracks at relatively high angles to the c-axis experience greatly accelerated etchable length reduction.

Experimentally determined values of l_c and l_a in apatite have greater physical significance than does the arithmetic (conventional) mean fission track length. Application of the pro- posed elliptical annealing model to controlled laboratory experiments may provide valuable insight, heretofore unattainable, into the physical process(es) by which fission tracks anneal in apatite.