In GRAPES-3 air shower array, consisting of about 400 scintillator detectors, the arrival direction of the shower is obtained from the relative arrival times of shower particles at various detectors. Since the angular resolution of the array is a critical parameter in the search for point sources of cosmic gamma rays, the relative arrival times and shape of the shower front have to be determined accurately. The arrival direction is primarily dependent on the fixed arrival time, referred to as time offset, of the signal from the detector to the measuring device. The time offsets vary with temperature due to changes in propagation delay in signal cables. Hence a technique was devised to calculate the time offset of each detector using shower data. In addition, the particles far away from the core are delayed due to the additional path length traversed due to lateral spread of particles, due to transverse momenta imparted during secondary particle production, and multiple Coulomb scattering. Therefore, the initial calculation of arrival direction obtained by fitting the showerfront with a plane is inadequate. For this reason the exact shape of the showerfront is determined by calculating the mean of the time residuals with respect to planar fit. A study was also done to observe the variation of shower curvature with the energy of primary cosmic rays. Details of the work will be presented during seminar. Some techniques used, to improve the measurement of energy of primary cosmic rays will also be presented, wherein a new technique was developed to measure the gain of scintillator detectors on an hourly basis .
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