Identification of Very Heavy Cosmic-Ray Tracks in Meteorites

Ions of Fe, Br, and I from the Oak Ridge tandem van de Graaff accelerator have been used to form charged-particle tracks in crystals of the minerals hypersthene and diopside, two of the most common minerals in meteorites. The results establish that the thresholds for track formation in these minerals, like those previously measured in more sensitive track-storing materials, are consistent with a primary specific ionization criterion. The direct calibration obtained for Fe ions confirms that the most abundant cosmic-ray tracks found in meteorites are due to nuclei of the Fe group ($Z=20\mathrm{} \mathrm{to} 30$). Together with the use of the primary ionization criterion. this calibration provides improved relations between observed track length and atomic number of slowing-down cosmic-ray ions in hypersthene and diopside. The minimum atomic numbers for track formation are $Z=25\mathrm{}$ for hypersthene and $Z=24\mathrm{}$ for diopside. Fe nuclei should leave tracks up to ∼8 μ long in hypersthene and up to ∼15 μ long in diopside. From the present work and a recent study of cosmic-ray tracks in meteorites, we calculate that the abundance of cosmic rays with $Z>\mathrm{}30\mathrm{}$, relative to the abundance of Fe-group nuclei, is $\frac{1}{4550}$.