A design study of atmospheric Cerenkov radiation telescope for very high energy gamma-ray astronomy

Detection of cosmic sources of very high energy gamma rays based on the atmospheric Cerenkov technique is discussed. Very high energy gamma-rays initiate, on entering the terrestrial atmosphere, electron-photon cascade showers with in turn produce Cerenkov photons in the air. Parabolic reflectors are used to focus these photons onto fast photomultipliers. Two methods of deployment of parabolic reflectors are in vogue: one in which all the reflectors are located close to each other in a compact array and the other in which the reflectors are spread out farther apart forming a distributed array. In the latter mode, the arrival direction of individual showers can be determined accurately by using the measured relative arrival times between different detectors. Detailed studies with the distributed array helped us to understand the various parameters in the two designs and evaluate their relative merits in reaching the ultimate goals of lowering the energy threshold and improving the signal to background ratio for the detection of gamma-ray sources. It is found that the relative superiority among the two types of arrays is a function of the exponent assumed for the differential power law energy spectrum for the gamma ray source. It is also seen that with the type of reflectors commonly used in atmospheric Cerenkov work, lower energy thresholds can be achieved with use of larger aperture.