Improved fission-track length measurements using ion irradiation

Apatite [Ca5(PO4)3(F,Cl)] is a common mineral in rocks that contains ~10 ppm U. U disintegrates by spontaneous fission and the fission fragments produce linear damage trails of ~20 μm length and 5-10 nm width. The lattice damage along such a fission track is repaired at elevated temperatures, leading to a gradual decrease of its etchable length. Since fresh tracks are formed as existing tracks are annealed, the track-length distribution provides a record of temperature variations in the rock [1]. A temperature-time path, T(t), is reconstructed on the basis of measurements of the lengths of horizontally confined tracks in the interior of the grains [2,3]. The resolution of the modelled T(t)-path depends on the number of measured confined tracks. This number is often small because confined tracks are only etched via cracks or fission tracks that intersect the surface. The goal of 100 length measurements is often not reached and there are indications that even this number is not sufficient to constrain the T(t)-function. In the present investigation, we examine the effect of different ion irradiation conditions on the number of confined tracks in apatite. The irradiations were performed at the UNILAC, using 330-MeV Xe and 800-MeV U ions of fluence ~2.5×10 cm. For geometrical reasons, the confined-track densities (ρC) are proportional to the ion-track densities (ρIT) and lengths (lIT), and to the fission-track densities (ρFT) and lengths (lFT), i.e. with the combined fissionand ion-track lengths per unit area: ρC ~ (ρIT lIT) (ρFT lFT). However, there exist important deviations from the geometrical model (Fig. 1). These are correlated with the orientation of the ion beam relative to the crystal symmetry, as indicated by the higher normalised confined-track densities for samples irradiated at 15° from the surface normal (DUR 5/6), compared to those irradiated under normal incidence (DUR 3/4). This, and the fact that higher confined-track densities are recorded when the azimuth direction of the ion tracks is parallel to the c-axis (DUR 6) than when it is perpendicular to the c-axis (DUR 5), indicates that the effect is, in part, related to the geometries of the etched ion tracks and fission tracks, and thus to the anisotropic etching properties of apatite. In contrast, the difference between the samples with fossil (DUR 3/4) and induced fission tracks (DUR 1/2) is not related to the track geometries, and assumed to result from a fundamental difference between "aged" fossil and "fresh" fission tracks or a fundamental difference in the structure of tracks produced by 330-MeV Xe (DUR 1/2) and 800-MeV U ions (DUR 3/4). Figure 2 shows the increase of the normalised confinedtrack densities as a function of etching time (te). A geometrical model predicts a quadratic (or higher) increase if a modest increase of the ion-track length is taken into account. This is the case for DUR 1-4 up to step 3 (te = 35 s) and for DUR5-6 up to step 2 (te = 25 s). The deviations past these points result from track overlap.