Optimisation and characterisation of neutron imaging beam at the TRIXIE instrument

The neutron imaging instrument TRIXIE is located at one of the horizontal beamlines of the research reactor LVR-15 in the Czech Republic. Before the construction of the TRIXIE instrument, the beamline was not equipped with a collimator; therefore, a new collimator had to be designed and implemented. The beamline was initially equipped with a meter-long single-crystal silicon filter. But, because of the lack of space in the beam (the size of the original filter limited the space for the collimator) and because the beamline needs to be filtered (the spectrum of an empty beamline roughly copies the reactor spectrum, with a high ratio of fast neutrons and gamma rays), a new filter had to be designed as well. The optimisation process of calculations of the collimator and filters of different materials and thicknesses was performed using the Monte Carlo calculation code MCNP. After the installation of the collimator and filters into the beamline, a series of measurements was performed to confirm the calculated results and to characterise the neutron beam.