Relativistic ²³⁸U ion tracks in olivine and cosmic-ray track studies

Abstract

An energetic ion slowing down in an insulating crystal produces radiation damage which can be preferentially etched, making what is called a track¹. Such nuclear tracks have numerous applications in many disciplines, including cosmochemistry and astrophysics¹. Fleischer et al.², for example, identified cosmic ray nuclei heavier than Fe by means of fossil tracks recorded in meteoritic crystals. Attempts to use tracks in meteorites to study the chemical composition of ultraheavy cosmic rays, however, have led to ambiguous results (see ref. 3). One possible reason for this lack of success has been the absence of calibrations for ions heavier than krypton, because no particle accelerator could accelerate them to sufficiently high energies. This barrier has been removed by the recent acceleration, at the Bevalac of the Lawrence Berkeley Laboratory, of uranium ions to relativistic energies^4–6. The results on tracks of ²³⁸U ions of 190 MeV per nucleon in olivine that we now report, are the first of this kind obtained with a mineral nuclear track detector. They show that the etchable part of a U ion trail is surprisingly long (nearly 3 mm). These results have an important bearing both on the interpretation of cosmic-ray tracks in lunar and meteoritic crystals, and on models of track formation in minerals.