On some factors controlling fission-track etch rates in apatite

Abstract. Fission tracks in apatite must be etched to be counted and measured with an optical microscope. Track etching is described with the aid of two velocities: the track etch rate v_T and the apatite etch rate v_R. On their values depend the efficiencies of track counts, the selection of confined tracks for measurement, and the shapes of etched tracks; v_R is an indicator of the apatite composition and annealing kinetics. In spite of this, direct measurements of v_T and v_R have long been wanting. This work provides numerical data and investigates factors controlling v_T and v_R. The results indicate that v_T is constant over most of the ion- and fission-fragment ranges in apatite (constant core). We observed that, when using a step-etch protocol, the etchant advances at a high, but finite, rate along tracks etched in a preceding step. Thus, step-etching broadens the effective-etch-time distribution, and the summed etch time overestimates the aggregate effect of the procedure. Neither annealing of induced tracks nor ageing of fossil tracks has a measurable effect on v_T. Since the v_T-value along a particular track section does not depend on its radius or on the work expended to create it, it can be considered characteristic of an apatite instead of a track. This could account for the different v_T of the Duluth and Durango apatites. The identical distributions of the host-confined-track intersections, c-axis angles, and track-etch rates across all experiments suggests that confined-track selection is little dependent on the track densities or lengths, but dominated by a condition that is unaffected by annealing. In contrast to v_T, v_R exhibits clear variation related to radiation damage from natural background radiation in the case of fossil tracks and from neutron irradiation in case of induced. Greater v_R shorten the access times to confined tracks, increasing their effective etch times. This implies a need for caution when using Dpar to assess the annealing kinetics.