Simulation and Measurement of Cadmium Ratio Parameter in the Polyethylene Pile

A light element such as polyethylene is one of the neutron moderators. Polyethylene pile can be used for calibration of neutron dosimeters and detectors. The polyethylene pile was designed as a source for slowing down the fast neutrons and used in technology centers.  Dimensions of the pile are 240 cm in length, 200 cm in width and 220 cm in height. 241Am-Be neutron source with intensity of 1.1×107 n/s was placed in center of the pile. Measurement channels are available at any point of the pile by removing the polyethylene blocks. Spatial distribution of thermal and fast neutron flux was measured using BF3 neutron detector and cadmium cover. In order to obtain maximum thermal neutron population, this pile was simulated with MCNPX-2.7.0 code, and the results were in good agreement with experimental results. The quality of thermal neutron field was characterized by the cadmium ratio. Using a NaI(Tl) detector, gamma rays due to neutron source was measured in different directions inside the pile.