Thesis (M.Sc.)--Chulalongkorn University, 2004

Neutron Monitors are devices for detecting cosmic ray neutrons. The detection of neutrons allows us to study the astrophysical phenomena in the outer space. The Neutron counting rates are used to predict the arrival of high energy primary cosmic rays, which cause many serious effects to our technologies. In this thesis, a study has been carried out to investigate the physics phenomena in the neutron monitor using geometry modeled from the Princess Sirindhorn Neutron Monitor. The functions of the main neutron monitor's components have been determined by computer simulation using GEANT4 program. The simulations provided results which are in agreement with the theoretical considerations. The effects of scattered neutrons from water on the ground have been determined. The simulations also indicated the contribution of charged particles arriving into the detector besides neutrons.