[Determination of uranium contents in fossil bones using fission track method--new counting method of fission tracks with high density].

Determination of the average uranium contents in several fossil bones was carried out using an improved fission track method. Four standard samples were prepared from silica-gel powder containing known amounts of natural uranium. The powdered fossil bones was fixed on a muscovite detector as well as the standard samples and followed by the neutron irradiation. After cooling, the muscovites were chemically etched with HF-solution to develop the fission tracks. By using standard samples, it was found that a reliable track counting is impossible through the transmission optical microscope in the region over track density of 5 X 10(5) cm-2 because of overlap of fission tracks in each other. In order to improve this situation bearing most of the present muscovites, the observation using the reflection microscope was applied after coating the muscovite surface with the vacuum-evaporated gold film and verified to be extremely efficient to the counting of high track density up to 5 X 10(7) cm-2, which could be no longer to distinguish intrinsic tracks through ordinary microscope. All of the fossil samples were subjected to the track counting through the reflection microscope and after correcting the estimated difference of track ranges between the standard and fossil bone samples the experimental uranium contents were evaluated. The uranium contents in the fossil bones were in the range of 30-460 ppm.