Experiments on improving the track technique for the determination of Th content in natural samples have been performed. For Th concentration measurements, γ-rays with energies of 18 and 20 MeV were used. For γ-rays in the energy range 6–23.5 MeV, the ratio of fission fragment track densities N U/ N Th of these elements for thick targets has been obtained. The ratio ranges from 1.7 to 3.2 with the maximum at a γ-ray energy of 15 MeV. The determined Th concentrations in the calibrated Th standards and iron manganese nodules agree, within experimental error, with the known values. For 9.1 MeV/n α-particles, the track density ratio N Th/ N U has been found to be equal to (1.06 ± 0.07), but the Th content was lowered, possibly due to the vertical inhomogeneity of the α-particle beam and also due to partial thermal fading of the tracks in mica. The investigation was performed at the Laboratory of Nuclear Reactions, JINR.