Ultra Heavy Cosmic Rays

Abstract

The existence in the cosmic rays of ultra-heavy (UH) nuclei with Z ≥ 30 was established by two separate experiments in 1966. Fleischer et al. ⁽¹⁾ first demonstrated the fossil tracks of such nuclei in certain meteoritic crystals and shortly afterwards Fowler⁽²⁾established their existence in present-day cosmic rays with the detection of their tracks in photographic emulsion which had been exposed during a high altitude balloon flight. The fluxes of such nuclei are very low, only ∼ 10⁻⁴ of that of iron, and the most suitable method of detection to date has been the analysis of the tracks formed by these particles in very large(several m²) arrays of plastic detector material, notably Lexan polycarbonate. Such exposures on balloons and on Skylab⁽³⁾ have provided practically all present knowledge of the UH cosmic rays. Unfortunately, the charge resolution obtained was disappointing, even though scrupulous care was taken in the handling and etching of the material, and the charge scale itself of necessity had to be based on a considerable extrapolation from the iron peak and could not be used with great confidence. The situation now, however, is in the process of being transformed. We have two satellite experiments devoted to the study of UH cosmic rays and in operation at the moment. These are the Bristol University experiment on Ariel 6 launched on 3rd June 1979 and the joint group under Israel, Waddington and Stone on HEAO-C launched in September 1979. It is therefore appropriate, I believe, if I devote this review to the new preliminary results and a comparison of this material with the published data.