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Volume 23: Pages 28-43, 2010

A simulation study of the improvement in trigger efficiency and angular resolution of a surface array

Sonali Bhatnagar 1, GRAPES3 Collaboration1

1Department of Physics and Computer Science, Dayalbagh Educational Institute, Agra 282 005, India

A recently published article in Science [A. Cho, Science, 318(5852), 896 (2007)] by the Pierre Auger Laboratory has helped pinpoint the source of ultrahigh energy cosmic rays, which are the most energetic particles known in the universe. The sources found indicate support for the “active galactic nuclei” theory, which is that the ultrahigh energy cosmic rays come from supermassive black holes found at the centers of galaxies. The Pierre Auger Observatory (website: AUGER EXPERIMENT, www.auger.org) records the extensive air showers through an array of 1600 particle detectors placed 1.5 km apart in a grid spread across 3000 km2. The window of TeV gamma-ray astrophysics was opened less than 2 decades ago, when the Crab Nebula was detected for the first time. After several years of development, the technique used by imaging atmospheric Cherenkov telescopes such as HESS and MAGIC is now allowing to conduct sensitive observations in the TeV regime. Water Cherenkov instruments such as Milagro are also providing results. Different types of extragalactic and galactic sources have been detected, showing a variety of interesting phenomena that are boosting theory in very high energy gamma-ray astrophysics. Here I review basic cosmic rays, the detection methods, and then present a simulation study of the gamma-ray astronomy at PeV energies—phase 3 (GRAPES3) experiment. GRAPES-3 is a scintillator detector array equipped with a muon detector tracker at Ooty, India. The capabilities of the array can be enhanced by changing the detector separation, etc. This can reduce the energy threshold (Eth), improve the angular resolution of the array, and hence improve the significance that the array can get from a gamma-ray source. This makes the array suitable to be used for gamma-ray source detection and studies. In the present article a study of the effect of reduction in detector separation, on the performance of the array, using simulation techniques has been presented. Simulation shows that reducing the detector separation by a factor of 2 results in a factor of 2 reduction in Eth. It also gives direction of study for further improvement in the angular resolution and significance, specifically from a Crab Nebula-like spectrum.

Keywords: TeV Gamma-Ray Astronomy, Cascade Showers, Hadronic Showers, Cosmic Rays, High Energy Physics, Cherenkov Detectors, Fluorescence, Ground Arrays, Electromagnetic Showers, Angular Resolution, Trigger Efficiency

Received: January 1, 2008; Accepted: November 19, 2009; Published Online: February 11, 2010