Detection of relativistic cosmic ray iron nuclei in the plastic track detector CR-39

We report here the first successful detection of highly relativistic cosmic ray iron group nuclei in the new type of solid state track detector, CR-39 (Allyl diglycol carbonate), in a stack composed of these plastics, nuclear emulsions and other types of plastic detectors, which was exposed in a balloon flight from Hyderabad, India, at 6 mb residual atmosphere for 8 h. The high value of the vertical geomagnetic cut-off rigidity of 16.7 GV ensured the relativistic nature of the particles and these were identified as Fe-group nuclei from the measurements in adjacent sheets of nuclear emulsion. Very well-defined etched cones were observed in the CR-39 plastics after 48 h etch in 6.25 N NaOH at 70 °C, at the locations corresponding to Fe-group nuclei tracks in the nuclear emulsions. The track etch rate (Vt) for relativistic (≥ GeV/nucleon) Fe-group nuclei was measured as 3.5 ± 1.3 μm/h and the bulk etch rate (Vg) as 1.5 ± 0.6 μm/h. The normalised track etch rate, (Vt/Vg) - 1, is compared with the data obtained at lower energies by other workers and the results are found to be consistent. The charge detection threshold for Cr-39 is obtained as Z≈20 for |β {reversed tilde equals} 1. © 1979