  A Cosmic Muon Veto (CMV) detector using extruded scintillators is being
designed around the mini-Iron Calorimeter detector at the transit campus of the
India-based Neutrino Observatory at Madurai for measuring its efficiency at
shallow depth underground experiments. The scintillation signal is transmitted
through a Wavelength Shifting (WLS) fibre and readout by Hamamatsu
Silicon-Photomultipliers (SiPMs). A Light Emitting Diode (LED) system is
included on the front-end readout for in-situ calibration of the gain of each
SiPM. A characterization system was developed for the measurement of gain and
choice of the overvoltage (Vov) of SiPMs using LED as well as a cosmic muon
telescope. The Vov is obtained by studying the noise rate, the gain of the
SiPM, and the muon detection efficiency. In case of any malfunction of the LED
system during the operation, the SiPM can also be calibrated with the noise
data as well as using radioactive sources. This paper describes the basic
characteristics of the SiPM and the comparison of the calibration results using
all three methods, as well as the Vov of the SiPMs and muon selection criteria
for the veto detector.
