IOP PUBLISHING JOURNAL OF PHYSICS G: NUCLEAR AND PARTICLE PHYSICS J. Phys. G: Nucl. Part. Phys. 36 (2009) 115201 (9pp) doi:10.1088/0954-3899/36/11/115201 Instrumentation development for an array of water Cherenkov detectors for extensive air shower experiments F Sheidaei, M Bahmanabadi, A Keivani and J Samimi Department of Physics, Sharif University of Technology, PO Box 11155-9161, Tehran, Iran E-mail: bahmanabadi@sharif.edu Received 5 April 2009 Published 25 September 2009 Online at stacks.iop.org/JPhysG/36/115201 Abstract A new small array of Cherenkov detectors has been deployed in Tehran, 1200 m above sea level. This array contains four tanks of distilled water with a diameter of 64 cm and a height of 130 cm. The effective area of each tank is about 1382 cm 2 . They are used to detect air showers and to record the arrival time of the secondary particles. We have collected about 640000 extensive air showers (EAS) in 8298 h of observation time from November 2006 to October 2007. The distribution of air showers in zenith and azimuth angles has been studied and a cos n θ distribution with n = 6.02 ± 0.01 was obtained for the zenith angle distribution. An asymmetry has been observed in the azimuthal distribution of EAS of cosmic rays due to geomagnetic field. The first and second amplitudes of the asymmetry are A I = 0.183 ± 0.001 and A II = 0.038 ± 0.001. Since the recent results are in good agreement with our previous results of scintillation detectors, and tanks of distilled water are cheaper, we prefer to use them instead of scintillators in a future larger array. By simulation, we have improved the size of the detectors to yield the highest efficiency. The best dimensions for each tank with a photomultiplier tube in the center of its lid are 40 cm in diameter and 60 cm in height. 1. Introduction Extensive air showers (EASs) are initiated by very high energy (E > 10 13 eV) radiation in the form of cosmic rays or gamma rays entering into the earth’s atmosphere. They are recorded by the detection of a fraction of the charged secondary particles reaching the ground level. Recently, observational studies have focused on two aspects of ultra-high energy (UHE) cosmic ray astronomy, namely, on sources of gamma rays [1, 2] and on the nuclear composition of UHE cosmic rays [3–5]. Meanwhile, EAS experiments have played an important role in the study of the UHE cosmic gamma rays astronomy, especially in trying to observe TeV gamma 0954-3899/09/115201+09$30.00 © 2009 IOP Publishing Ltd Printed in the UK 1