Thermal Neutron Filter Design for the Neutron Radiography Facility at the LVR-15 Reactor

In the year 2011, a research project has started focus on building of a neutron radiography facility at the LVR-15 research reactor in Rez, Czech Republic. One of the unused horizontal channels was chosen to be adapted for this purpose. However, the original beam parameters having a high presence on fast neutrons which may damage the neutron detector, and gamma radiation which causes undesired background were unsuitable. The need for an intensive thermal neutron beam with a very low fast neutron ratio led to the decision of installing a thermal neutron filter into the channel tube. As the channel layout is very spatial limiting, a simple solution had to be chosen. Usually large single-crystal ingots of proper material parameters can be used as filters. Single-crystal silicon was chosen as the preferred filter material for its availability in sufficient dimensions and low production costs. Additionally to its ability to significantly reduce the ratio of fast neutrons in the beam, if the filter dimensions are large enough, it provides shielding against the reactor gamma radiation. For the calculation of the required beam dimensions the Monte-Carlo MCNP transport code was used. However, as the code does not include the neutron cross-section libraries for thermal neutron scattering on crystalline structures, the original silicon cross-section libraries had been manually modified using an approximated relation based on thermal neutron scattering theory. Carrying out a series of calculations the filter thickness of 1 m proved good for gaining a beam with desired parameters and a low gamma background. After mounting the filter inside the channel several measurements of the neutron field were realized at the beam exit. The results have justified the calculated values. After the successful filter installing and a series of measurements, first test neutron radiography attempts with chosen samples could been carried out.