Simulation of cold neutron production using ordinary and deuterated cryogenic organic moderators

The objective of this thesis is to investigate the use of deuterated organic moderator to produce cold neutron in a reactor based cold neutron source. This is of interest for two reasons: first, for possible enhancement of the operation of the Texas Cold Neutron Source located at the TRIGA reactor of the Nuclear Engineering Teaching Laboratory at the University of Texas; second, for the design of an Ultracold Neutron Source. Two computer codes were used to achieve this objective: NJOY 97 to generate the necessary thermal scattering cross sections; MCNP 4B to simulate the thermalization and transport of the neutrons in the Texas Cold Neutron Source. The generation of thermal neutron cross sections from neutron scattering laws by NJOY was intensively studied for methane (CH₄) at 22K. The influence of several key physical and numerical parameters of the input file were investigated. The results obtained gave a good understanding on how to generate the cross section in the best capability of the code NJOY. The calculated scattering cross sections were then tested by MCNP to match an experiment performed by Utsuro (1975). Although the experiment was done with a different organic moderator, 1,3,5-trimethylbenzene (mesithylene), the effects associated to the three methyl radicals should be present in the case of methane. The results of this study gave confidence that the cross sections generated by NJOY match rather well the physical cross section of organic materials. Finally, using thermal cross section for methane and deuterated methane generated by NJOY, the performance of the Texas Cold Neutron Source was simulated using MCNP under a variety of conditions. The incident thermal neutron flux coming from the core of the TRIGA reactor at NETL was simulated. Moderation of these neutrons takes place in the TCNS chamber and cold neutrons entering the neutron guide were tallied. The effect of mixing ordinary and deuterated moderators was determined