Optimization of n_TOF-EAR2 using FLUKA

In 2014 a new vertical flight path was installed at the neutron time-of-flight facility, n_TOF, at CERN. The beam line optimization of this second experimental area (EAR2) was performed by FLUKA simulations with a detailed model of the facility, including the spallation target, neutron beam line, magnet, collimators and beam dump. The beam profile as well as neutron and gamma fluxes were calculated at the measurement position, 150 cm from the exit of the second collimator. The design of the collimator system was optimized taking into account the shape of the beam, the neutron flux and the background. The effect of the background of each studied configuration was evaluated from the response of a C6D6 detector at the measurement position. The design of the beam dump was modified according to each configuration to reduce the backscattered radiation. Among the studied configurations, a final collimator with a conical closing aperture was found to be the best compromise between a high flux and low background at the experimental station. By scoring the neutron flux at the measurement position and taking the air and windows at the nominal sample position into account, the neutron beam has a radius of 2.5 cm and the outside background is three orders of magnitude lower than the maximum flux. The ratio between background and maximum flux is similar to that in the first experimental area (EAR1), but the maximum flux in EAR2 is 30 to 50 times higher than in EAR1 for neutron energies between 100 eV and 30 keV, respectively.