Monte Carlo simulations and benchmarks for the radiological characterization of the LHC experiments

With Run 3 and the High Luminosity LHC project, the Large Hadron Collider (LHC) at the European Organization for Nuclear Research (CERN) is expected to almost tenfold the integrated luminosity by the early 2040s. With increasingly harsher radiation conditions, the computation of induced radioactivity in detector and infrastructure components will soon become a pressing need to fully ensure a smooth and safe operation of the LHC and its experiments over an extended period of time. The calculation of induced radioactivity is essential in various steps of the lifecycle of any component to be installed in accelerators or high-energy physics detectors. It is for instance required during the design phase for the selection of appropriate materials, and it is of paramount importance for the estimation of residual dose rates for planned exposure situations during shutdown periods to establish the appropriate protection measures needed to keep doses to levels as low as reasonably achievable (ALARA principle). At the same time, the knowledge of the radionuclide inventory is fundamental for the decommissioning of the accelerators and detectors themselves to determine the appropriate pathways for the disposal of each component and the related costs. As soon as one deviates from simple textbook cases, the study of the generation and the time evolution of the induced radioactivity quickly becomes very challenging. The main focus of this work has been the development of the novel fluence conversion coefficients method for the calculation of induced radioactivity with Monte Carlo transport codes which, in virtue of its fast convergence and good visualization capabilities, can be applied in complex radiological studies even when traditional methods would be inapplicable. The method was benchmarked with activation data from experimental campaigns planned and executed at the CERN High energy AcceleRator Mixed field (CHARM) facility in 2022, and with results from a full-scale activation campaign at the LHC experiments. Very good agreement was generally found between measurements and Monte Carlo simulations. The fluence conversion coefficients method immediately found important practical applications, particularly for radiological studies for the LHC experiments. These studies include the reinforcement of the CMS forward shielding, the assessment of the activation of the steel of the absorber plates of the future CMS HGCal, preliminary radiological zoning calculations for ALICE, and studies in preparation for the LHC pilot beam conducted in 2021.