Radiation protection measurements during the commissioning of the third-generation n_TOF neutron spallation target at CERN

The neutron Time-Of-Flight (n_TOF) facility, operational at CERN since 2000, is a pulsed neutron spallation source producing neutrons between a few meV and several GeV for two experimental stations located 185 m horizontally from the target and 20 m above the target. The neutron production target, made of pure lead, is impacted by the 20 GeV/c proton beam of the Proton Synchrotron. In 2018, the second-generation target reached the end of its lifetime, and the facility underwent a major upgrade during CERN’s Long Shutdown 2 (2019-2021), which included the installation of the new third-generation spallation target. The previous targets were based on water-cooled massive lead blocks, whereas the new target is cooled by nitrogen gas slightly above atmospheric pressure to limit erosion-corrosion and contamination of the cooling circuit with radioactive lead spallation products. A radiation protection study, performed with the FLUKA Monte Carlo code, contributed to the optimisation of the target design and to assess the safe operation of the facility. In 2021, during the commissioning of the new target, the simulation results were complemented by dedicated radiation measurements, which are described in this work: • Monitoring of stray radiation (neutrons and photons) in accessible areas by using fixed and mobile instrumentation (high-pressure ionisation chambers and the LUPIN extended range rem-counter, specifically conceived to measure pulsed neutrons) to ensure the compliance with the radiological area classification. • Monitoring of residual dose rate to provide useful information for planning and optimising accesses and interventions. • Monitoring of the nitrogen activation by means of a flow-through differential ionization chamber, for which a field calibration factor was calculated combining FLUKA simulations and experimental measurements. • Activation studies for zoning purposes by installing in specific locations of the n_TOF facility samples of materials typically used in particle accelerators (aluminium, copper, stainless steel); during the Year End Technical Stop 2021-2022, their specific activity were measured by γ-spectrometry and compared against the applicable CERN clearance limits.