Minimizing the background radiation in the new neutron time-of-flight facility at CERN: FLUKA Monte Carlo simulations for the optimization of the n_TOF second experimental line

At the particle physics laboratory CERN in Geneva, Switzerland, the Neutron Time-of-Flight facility has recently started the construction of a second experimental line. The new neutron beam line will unavoidably induce radiation in both the experimental area and in nearby accessible areas. Computer simulations for the minimization of the background were carried out using the FLUKA Monte Carlo simulation package. The background radiation in the new experimental area needs to be kept to a minimum during measurements. This was studied with focus on the contributions from backscattering in the beam dump. The beam dump was originally designed for shielding the outside area using a block of iron covered in concrete. However, the backscattering was never studied in detail. In this thesis, the fluences (i.e. the flux integrated over time) of neutrons and photons were studied in the experimental area while the beam dump design was modified. An optimized design was obtained by stopping the fast neutrons in a high Z material and by adding a hydrogenous material for neutron moderation. In addition, a material with high neutron absorption cross-section was used to stop the slow neutrons. The neutron and photon backscattering were decreased between one and two orders of magnitude depending on the energy range of the neutron beam. Furthermore, the neutron-induced radiation dose levels in the nearby ISR tunnel were estimated. Being a permanent workplace puts strict limits on the acceptable equivalent dose rate. The aim of this study was to reinforce the existing shielding in the ISR tunnel in order to reach below 0.5 $\mu$Sv/h. A problem in this study was the crane operating in the tunnel which puts restrictions on the height of the shielding wall. As a consequence, two shielding scenarios were explored. In the scenario where the crane could be blocked, the optimal design was to build a 160 cm thick concrete shielding wall reaching up to the roof. In the scenario where the crane was kept in operation the wall could only be made 4 m high and the optimal design was to keep the wall 160 cm thick and add iron plates to slow down fast neutrons. In both scenarios the average equivalent dose was below or on the limit of 0.5 $\mu$Sv/h.