A mini ACT experiment at low and high altitudes

Abstract

A simple experimental apparatus was used to measure rate of cosmic ray events due to Atmospheric Cerenkov Radiation at two mountain altitude sites with the height difference of 3.5 km. The event rates at Hanle (altitude ~ 4.5 km AMSL) was found to be a factor ~ 2 higher than those at Pachmarhi (altitude ~1 km AMSL). Consideration of the ratio of mean distance of the Cerenkov photon from the axis of the shower for the two altitudes shows a decrease in energy threshold of cosmic rays by a factor of 3.5 ± 0.7 at the higher altitude. A semi Monte Carlo calculation using the lateral distribution curves and the detector response also shows a similar decrease in the energy threshold. However, for a bigger array like the one at Pachmarhi the energy threshold is expected to decrease by a larger factor (~ 6 ± 2) since cosmic rays become inefficient in production of Cerenkov light at lower energies.