Scintillators are widely used in and studied for particle physics, nuclear physics and astroparticle physics detection. In this study, we estimated the scintillation properties of a CsI:Tl crystal and determined the best method to discriminate $\alpha$-particles, which is an important component of the cosmic-ray background, from $\gamma$-rays. We studied the scintillation properties of the CsI:Tl crystal and the pulse shape discrimination capability by using an $^{241}$Am $\alpha$-particle source and a $^{137}$Cs $\gamma$-ray source. For the pulse shape discrimination study, we used these well-known methods: the mean time method, charge integration method and optimum filter method. To determine which method was best for discrimination, we calculated the FOM value for each method under various conditions. Consequently, the optimum filter method was found to give the best value of the FOM. If we use the optimum filter method when we are observing $\gamma$-rays from the sky, we can distinguish the background of ion particles, such as $\alpha$-particles, from $\gamma$-rays.

Keywords: Scintillator, Gamma-ray observation, Radiation detector, Radiation discrimination, Cosmic ray