Highly ionizing cosmic rays at mountain altitude

Large arrays of CR-39 plastic track detectors are being used at various depths in the atmosphere to study energetic, highly ionizing particles. We have made the first measurements of the energy spectra of He and Li nuclei at mountain altitude (603 g/cm2) and the first observations of relativistic heavy nuclei (Z/β>=12; 8<=Z<=14) at subsonic aircraft altitude (186 g/cm2). We report a 95% Cl upper limit of 0.4 m-2yr-1 for the flux at 603 g/cm2 of particles with Z/β>=30 and of monopoles with velocity as low as 10-2c