  A Cosmic Muon Veto (CMV) detector using extruded plastic scintillators is
being designed around the mini-Iron Calorimeter (mini-ICAL) detector at the
transit campus of the India based Neutrino Observatory, Madurai for the
feasibility study of shallow depth underground experiments. The scintillation
signals that are produced in the plastic due to muon trajectories are absorbed
by wavelength shifting (WLS) fibres. The WLS fibres re-emit photons of longer
wavelengths and propagate those to silicon photo-multipliers (SiPMs). The SiPMs
detect these photons, producing electronic signals. The CMV detector will use
more than 700 scintillators to cover the mini-ICAL detector and will require
around 3000 SiPMs. The design goal for the cosmic muon veto efficiency of the
CMV is >99.99%. Hence, every SiPM used in the detector needs to be tested and
characterised to satisfy the design goal of CMV. A mass testing system was
developed for the measurement of gain and choice of the overvoltage ($V_{ov}$)
of each SiPMs using an LED driver. The $V_{ov}$ is obtained by studying the
noise rate, the gain of the SiPM. This paper describes the experimental setup
used to test the SiPMs characteristics along with detailed studies of those
characteristics as a function of temperature.
