Tracks of heavy primary cosmic rays in meteorites

Tracks of primary cosmic rays with atomic number ≳20 have been identified in 23 out of 42 meteorites of various types. They satisfy the criteria of rapid attenuation with depth, directional anisotropy, proper length distribution, proper absolute number, and equal numbers in adjacent minerals with widely different uranium contents. From measurements of track densities in samples of meteorites with known cosmic-ray age, the original depths of the samples below the preatmospheric surfaces of the meteoroids were obtained. These ranged from ∼3 to 50 cm. Minimum values of the preatmospheric radii ranging from 10 to 55 cm were also obtained. Using a computed relationship between track lengths and particle mass, the observed distribution of track lengths was used to infer the existence of cosmic rays that are considerably heavier than iron. The relative abundance of cosmic rays with Z ≳ 32 was estimated to be ∼1:4000 that of the iron group (20 ≤ Z ≤ 30).