Pergamon Radiation Measurements, Vol. 25, Nos 1--4, pp. 137-140, 1995 Copyright© 1995 Elsevier ScienceLtd Prin~l in ~m Britain. All rights rese~v~l 1350-4487/95 $9.50 + .00 13~4,187(95)00098-4 MODELING THE FISSION TRACK ETCHING PROCESS IN APATITE: SEGMENTATION OR CRYSTALLOGRAPHY INFLUENCE F. VILLA, M. GRIVET, M. REBETEZ, C. DUBOIS and A. CHAMBAUDET Laboratoire de Microanalyses Nucldaires, UFR des Sciences et Techniques, 16 route de Gray, 25030 Besangon cedex, France ABSTRACT In this study, two factors which can influence fission track etching in apatite are considered: track segmentation (induced by thermal annealing) and variable radial etching speed (due to the reagent diffusion during the etching process). During the latent track annealing, two distinguishable steps can be identified by measuring track lengths or diameters. A length reduction is firstly observed, followed by a segmentation process which leads to the emergence of disrupted regions (gaps). At present time, electron microscopy studies on fission tracks in apatite show profiles which lead to hypotheses of a variable radial etching speed versus depth. These variations can be interpreted in terms of acid diffusion along the track. Moreover, the existence of several bulk etching speeds related to crystallographic orientation is approached. Taking into account these different points, a software program, integrating parameters as original track orientation and depth, number of gaps, etc., is developed in order to model the track profile evolution during the etching process. Comparison with experiments in Durango apatite (Mexico) are also undertaken. KEYWORDS Fission track; etching; segmentation; simulation; crystallography; annealing. INTRODUCTION The latent fission track registration in Solid-State Nuclear Track Detectors (SSNTD) has many applications, especially with apatite for oil prospecting: the analysis of track densities in a mineral may allow to determine its age, and the employment of the track length distributions may allow the reconstitution of its thermochronological history (with confined and surface tracks). However, the reduced dimensions of the latent tracks prevent a direct measure of their lengths. Their observation with a microscope is possible only after etching by an acid solution. The parameters then measured and used in the models depend on the characteristics of the reagent. Indeed, etching generates a difference between the measured etched track lengths and the latent ones, because of the under- or over-etching phenomena. In the goal of establishing the optimal etching conditions, i.e. to get a minimal deviation between latent and etched track lengths, we realised a simulation of etching in apatite. Two parts will be separately treated in this paper: first, the influence of track segmentation on the etching process and second, the introduction of crystallographic parameters. INFLUENCE OF TRACK SEGMENTATION ON THE ETCHING PROCESS Track segmentation The fission tracks are very sensitive to a thermal effect: it produces an annealing, resulting in a reduction of their initial lengths. Carlson (1990) has established that a very high annealing may generate gaps and may result in a random segmentation. This segmentation do have some effects on the length distribution, and consequently it is important to understand the etching mechanism of a segmented track. 2s:l/4-t 13V