Generation of Gigavolt potential in thunderclouds was predicted by Nobel Laureate C.T.R. Wilson in 1929. The unexpected discovery of the terrestrial gamma-ray flashes in 1994 by the BATSE instrument on board Compton Gamma Ray Observatory provided the signature of large electric potential in thunderclouds. More recently, the AGILE satellite detected gamma-rays extending up to 100 MeV energy which would require bremsstrahlung of very high energy electrons and presence of potentials of hundreds of Megavolt. Due to the tropical location, Ooty observes a large number of thunderstorms during the month of April-June and September-November. Recently, the GRAPES-3 muon telescope discovered a record break 1.3 Gigavolt potential in a thundercloud. Simultaneous observations through muons and gamma-rays could provide better understanding about the atmospheric accelerator which motivated us to set up a gamma-ray detector at the GRAPES-3 experiment. In this talk, I will discuss my involvement in the design, development, installation and operation of a NaI detector array with plastic scintillator as a veto detector for the rejection of charged particle background. In second part of my talk, I shall give an overview on the development of an analysis framework to study multi-TeV gamma-ray sources with the GRAPES-3 air shower data.
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