Using Cherenkov pulse shapes to estimate mass composition

Composition measurements at TeV energies are traditionally made with satellite or balloon borne detectors which benefit from good discrimination but are severely limited by their collecting area and so yield poor statistics, particularly at higher energies. We have made ground based measurements with the atmospheric Cherenkov telescope BIGRAT, discriminating between different species of cosmic ray primary through characteristics of the Cherenkov pulse shape. By collecting data at low elevations, an increasing energy threshold for the detector allows us to probe a large energy range while maintaining reasonable statistics. A comparison of our data with Monte Carlo simulation is presented here.