Monte Carlo simulation ofa neutron irradiating system for determining efficiency of thermal neutron detectors

In this work, a neutron irradiating system containing six Am/Be neutron sources in a graphite moderator, which produces a relatively high and uniform thermal neutron flux in an irradiating cavity at the center of the system , has been simulated .The thermal neutron flux in the center of the system has then been calculated and compared with experimental results measured in a cavity at the center of a similar system .Good agreement between calculated and measured values show that true value of thermal neutron flux in this system (or any other irradiating system) can be accurately calculated by simulation method and be used for calibrating neutron detectors and dosimeters. This system has been simulated using the MCNP Monte Carlo computer code. .In the calculations, thermal neutron flux has been first calculated at various distances from a single neutron source in a graphite moderator and then calculations have been performed and results discussed for various cases of 2 , 4 and 6 neutron sources being present in the system. In addition, to investigate the effect of moderator properties on neutron flux distribution in the system, parallel calculations have been done for watermoderator, which show water is not a proper moderator for such a system.