Study of highly ionizing particles at mountain altitude

The inspiration for this experiment was the widely acknowledged peculiar nature of the "Centauro" class of ultrahigh-energy interactions discovered at Mt. Chacaltaya about a decade ago by a Brazil-Japan collaboration. The rate of Centauro events is quite low, {~}0.02 m^-2{yr}^{{-}1}$. Among the proposed explanations is the possibility that they might be initiated by a highly charged primary particle. To look for such particles we deployed a series of CR-39 plastic track detectors with large collecting power at the summit of White Mountain, California (603 g/cm²). Two experiments have been completed and analyzed: a single layer with a 10-m²yr exposure, and three layers in coincidence with a 0.025-m²yr exposure. In the first experiment, the single layer was adopted to maximize the collecting area at some sacrifice in velocity information. The results demonstrate the superiority of CR-39 as a detector of highly ionizing particles (Zβ30) and its particular suitability as a collector of very rare particles. The performance of the plastic was evaluated by examining the high density of tracks due to slow, light ions. The low-energy spectra of nuclei with Z≤3 have been measured and are found to be consistent with spectra calculated from a model where the source of ions is atmospheric collisions of energetic hadrons. This is the first time that energetic Li has been identified at mountain altitude and that enough He has been seen to permit measurements of its energy spectrum. A density {~} 10m^-2 of fast particles with Z≥4 found in individual layers is consistent with the exposure received in a 10-hour commercial jet flight. A fast-scanning method was used to examine the entire single-layer array. In the interval 30Zβ100, no events were found, from which we infer an upper limit of 0.4 m^-2{yr}^{{-}1}$ (95% C.L.) on the flux of electrically charged particles with ${Z}{{β}}{~}30 to {~}100$ and on the flux of superheavy magnetic monopoles with ${β}{}0.02$. Two objects were located which, if they are indeed tracks, would correspond to particles with ${Z}{{β}}{}100, with abundance ~0.2{m}^{{-}2}yr^-1. Without sheets in coincidence, it is impossible to distinguish these objects from certain flaws in the plastic. A new detector, with three layers interleaved with absorbers and with an area of 20 m², will correct this shortcoming and be able to measure the spectra of light elements to higher energies as well.