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. Howeve r, q decreases as the zenith angle increases. In this paper we describ e a new technique based on the temporal Cerenkov pulse shape and show that the use of rise time and FWHM cuts are effective discriminators a t all zenith angles. For flat-spectrum sources at large zenith angles q similar to 2.5. Measured parameter distributions are compared with s imulations and the methods of making the cuts are discussed. A sensiti vity to cosmic-ray composition is also suggested.