Measurement of some EAS properties using new scintillator detectors developed for the GRAPES-3 experiment The GRAPES-3 Collaboration: P.K. Mohanty a , S.R. Dugad a , U.D. Goswami a , S.K. Gupta a, * , Y. Hayashi b , A. Iyer a , N. Ito b , P. Jagadeesan a , A. Jain a , S. Karthikeyan a , S. Kawakami b , M. Minamino b , S.D. Morris a , P.K. Nayak a , T. Nonaka b , A. Oshima b , B.S. Rao a , K.C. Ravindran a , H. Tanaka a , S.C. Tonwar a a Tata Institute of Fundamental Research, Mumbai 400005, India b Graduate School of Science, Osaka City University, Osaka 558-8585, Japan article info Article history: Received 10 September 2008 Received in revised form 29 October 2008 Accepted 9 November 2008 Available online 27 November 2008 PACS: 96.50.S 96.60.sd 95.55.Vj 29.40.Gx Keywords: Cosmic rays Extensive air shower Particle detector Scintillation detector abstract The GRAPES-3 extensive air shower (EAS) array started operation with 256 scintillator detectors at Ooty in India. Each detector is viewed by a fast photomultiplier tube (PMT) mounted at a height of 60 cm above the scintillator. However, for further expansion of the array, an alternative readout of the scintillator using wave-length shifting (WLS) fibers is employed. This resulted in improved performance with a larger photon signal and a more uniform response. With the inclusion of a second PMT, the dynamic range for particle detection has been increased to 5  10 3 particles m 2 . We now use plastic scintillators, devel- oped in-house to cut costs. The measurement of the density spectrum, shows a power law dependence with an index c = 1.57 ± 0.04. Using the zenith angle dependence of the density spectrum, an attenuation length K a = 98–106 g cm 2 for the EAS is obtained. These measurements are found to be consistent with the results reported earlier by other groups. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction The GRAPES-3 (Gamma Ray Astronomy at PeV EnergieS – phase 3) is a high density extensive air shower (EAS) array designed for precision study of the cosmic ray energy spectrum and its nuclear composition using the muon multiplicity distribution [1] in the en- ergy range from 3  10 13 eV to 3  10 16 eV. The experiment was started with 256 plastic scintillator detectors (each 1 m 2 in area) deployed on a hexagonal pattern, at Ooty (2200 m altitude, 11.4°N, 76.7°E) in southern India in 2001. The scintillator detector is viewed by a fast photomultiplier tube (PMT) in a conventional configuration; wherein the PMT is mounted with its photo-cathode face at a height of 60 cm, above the scintillator. The entire assem- bly of scintillator and PMT is housed inside a highly reflecting trap- ezoidal shaped light-tight aluminum container [2]. The GRAPES-3 array also contains a large area (560 m 2 ) tracking muon telescope [3] to detect and measure the muon component in the EAS. The muon telescope is an effective tool, in the study of the nuclear composition of primary cosmic rays. The tracking muon telescope has also proved to be an invaluable tool in the studies of the solar flares, coronal mass ejections and the subsequent Forbush decrease events observed at the Earth [4,5]. The occurrence of the ‘knee’ in the cosmic ray energy spectrum around 3  10 15 eV is believed to be intimately related to the issue of cosmic ray origin. But even after several decades of study, a clear understanding of the origin of the ‘knee’ is yet to emerge. Clearly the data obtained with higher sensitivity and lesser uncertainty in the estimation of the primary energy and the composition are expected to provide a better understanding of this important fea- ture of the high energy astrophysics. An efficient detector system should provide a large separation between the PMT noise and the signal due to the charged particles in the EAS, and should also have a uniform spatial response over the entire area of the scintillator. In the original GRAPES-3 trape- zoidal shaped detectors, the scintillation photons collected at the PMT are, primarily due to diffuse reflection from the inner walls of the container [2]. This is not an efficient process, and results in a sizable loss of the signal. Most of the scintillator detectors had a photon output [10 photo-electrons, even for the 5 cm thick 0927-6505/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.astropartphys.2008.11.004 * Corresponding author. Tel.: +91 9869439435; fax: +91 2222804610. E-mail address: gupta@grapes.tifr.res.in (S.K. Gupta). Astroparticle Physics 31 (2009) 24–36 Contents lists available at ScienceDirect Astroparticle Physics journal homepage: www.elsevier.com/locate/astropart