Fast Fourier transform to measure pressure coefficient of muons in the GRAPES-3 experiment

The GRAPES-3 large area (560 m(2)) tracking muon telescope is operating at Ooty in India since 2001. It records 4 x 10(9) muons of energy >= 1 GeV every day. These high statistics data have enabled extremely sensitive measurements of solar phenomena, including the solar anisotropies, Forbush decreases, coronal mass ejections etc. to be made. However, prior to such studies, the variation in observed muon rate caused by changes in atmospheric pressure needs to be corrected. Traditionally, the pressure coefficient (beta) for the muon rate was derived from the observed data. But the influence of various solar effects makes the measurement of beta somewhat difficult. In the present work, a different approach to circumvent this difficulty was used to measure beta, almost independent of the solar activity. This approach exploits a small amplitude (similar to 1 hPa) periodic (12 h) variation of atmospheric pressure at Ooty that introduces a synchronous variation in the muon rate. By using the fast Fourier transform technique the spectral power distributions at 12 h from the atmospheric pressure, and muon rate were used to measure beta. The value of pressure coefficient was found to be beta = (-0.128 +/- 0.005)% hPa(-1). (C) 2016 Elsevier B.V. All rights reserved.