In addition to the critical role in reactor physics and nuclear technology, neutrons are used in the study of nuclear structures. In these studies, being electrically neutral and thus having no Coulomb interaction with atomic nuclei, is the main advantage of neutrons over the other particles ,but this property could make neutron detection and spectroscopy difficult. Due to the high efficiency, good energy resolution and low sensitivity to gamma radiations, organic scintillations either solid or liquid are widely used in neutron detection and spectroscopy. Light in organic scintillation are released due to direct interaction of charged particles with the environment or due to the secondary charged particle produced by neutral particles such as neutrons. In an organic scintillation detector which is exposed to neutrons, energy is transferred to protons, carbon nuclei or alpha particles produced in neutron traort. Light emitted from scintillation materials has essentially two components: fast component ranged 2-3 ns and slower component around 300-600 ns. For an appropriate simulation, response function required for organic scintillation can be achieved by empirical calculations (Verbinski et al.1968) or Monte Carlo simulation such as .In this study, BC501A organic liquid scintillation detector response function، considering the simulation of scintillation lights, has been investigated and different surface effects and dimensions of light guide have been considered. GEANT4 Monte Carlo simulation tool in C + + programming language is used for simulation. Response function of organic scintillation cylindrical shape with dimensions 4.6 cm diameter and 4.65 cm length at different energies is simulated and the results are compared with experimental data (Verbinski 1968), the results are consistent.Moreover, the effects of surface, the size of scintillator and the size of light guide and also effect of reflective color on surface of light guide either in continuous form or in tape form on response function have been investigated. Key Words: GEANT4, BC501A، NE213, scintillator detector, Response function, Light traort
