  Photomultiplier tube (PMT) and Silicon Photo Multiplier (SiPM) are often used
for detecting small number of photons or very weak radiations. A light guide
usually connects these photodetectors to the test space. In this article, we
investigate the effect of background signals caused by cosmic muons
scintillation or interactions with PMT and SiPM, their light guide or input
window materials. We study such interactions by making simulations using GATE
software package and undertaking experiments using detector circuits developed
as in~\cite{Raki:2022lwn}. The background cosmic muons can generate photons
which will lead to errors in low radiation and single photon detection
experiments especially if standard scintillators are not used. For such
experiments, we conclude that the most useful method for cutting down the
cosmic muons background should be by conducting the experiments deep
underground or inside tunnels with several tens of meters of materials above
it.
