Pulse shape selection in very high energy gamma-ray astronomy

Very high energy gamma-ray astronomers using the atmospheric Cerenkov technique must contend with a large background of light pulses due to cosmic ray-initiated cascades. Significant advances have been made in developing techniques to reduce this background: the imaging technique can have a figure of merit, q, in excess of 6 near the zenith. However, q decreases as the zenith angle increases. In this paper we describe a new technique based on the temporal Cerenkov pulse shape and show that the use of rise time and FWHM cuts are effective discriminators at all zenith angles. For flat-spectrum sources at large zenith angles q similar to 2.5. Measured parameter distributions are compared with simulations and the methods of making the cuts are discussed. A sensitivity to cosmic-ray composition is also suggested.