Measurements of fossil confined fission tracks in geological samples with low track densities

Several natural minerals contain trace amounts of uranium. Spontaneous nuclear fission of U causes the formation of particle (fission) tracks along the trajectories of the ejected nuclear fragments. The lattice damage along a fission track is repaired at elevated temperatures, leading to a gradual reduction of its etchable length that depends foremost on the highest temperature that the track has experienced. Thus fission tracks can be thought of as maximum-reading thermometers that are generated throughout geological time. Apatite occurs in low concentrations in most rocks and is the mineral of choice. The principal limitation of apatite T(t)-modelling is that the number of accessible tracks is low because they are situated within the interior of the grains and only etched via cracks or other tracks intersecting the mineral surface. The present experiments examine the effect of man-made ion irradiations under tilted beam incidence, producing oblique tracks that serve as etchant conduits for revealing the confined fission tracks inside the grains. The studied samples are 80-250 μm sized apatite grains from young rocks from central Myanmar, with fission-track densities between 3×10 and 3×10 cm, corresponding to much less than 10 confined tracks for a typical sample. Polished epoxy mounts of several hundred unoriented apatite grains were irradiated at 15° from normal incidence at the UNILAC with Pb or U ions of ~800 MeV energy (range ~30 μm) and ~2×10 cm fluence. Track etching was carried out with 4M HNO3 at 25 °C in three sequential steps: te (etching time) = 15, 25, and 35s. Fig. 1 shows the number of measurable confined tracks Nc as a function of te. In most samples, the increase of Nc is greater than the increase of te. On average, Nc increases ~3.5 times between te = 15 and 25 s and ~2 times between te = 25 and 35 s. The overall gain of Nc due to sequential etching between te = 15 and 35 s thus comes to a factor of ~7, which is still much lower than the gain from the ion irradiation itself, estimated at ~30 times. The latter firstorder estimate is the average ratio of the ion track and the fission track densities. The maximum gain is achieved for the longest etching time (te = 35 s): Nc is on average ~200 times greater than the estimated number of confined tracks in the unirradiated samples. Increases by a factor of up to >500 were observed for some samples, but there are indications of saturation when the confined tracks start to overlap. The variation of Nc between different samples in Fig. 1 probably results from the fact that the apatite grains in the these samples have different sizes and shapes. The presence of spurious cracks and inclusions that obscure the track end-points can also contribute to variation between the samples. For maximum gain the irradiation conditions should be tailored to the sample characteristics. Although 100 to 200 tracks are considered to be sufficient for T(t)modelling, higher numbers could become important when attempting to model the thermal histories of individual sediment grains. There is no indication that the ion-beam parameters influence the etchable lengths of the confined tracks. The beam orientation does, however, affect the angular distribution of confined tracks in oriented crystals [1], but this is not relevant for mounts of unoriented apatite grains. Sequential etching has no apparent effect on their angular distribution. Its effects on the length of individual tracks, mean confined track length and length distribution are not clear. Overall, the results document an isotropic increase of the mean confined track length between te = 15 and 25 s and an according quasi-uniform shift of the confined track length distribution towards longer lengths but no significant further increase or shift between te = 25 and 35 s [2].