Radiation protection for the CNGS facility : design versus reality

Full text: In the CERN Neutrino to Gran Sasso (CNGS) facility an intense proton beam with an energy of 400 GeV interacts with a graphite target. The produced charged pions and kaons are focused with a set of magnetic lenses and eventually decay in a 1 km long evacuated pipe into neutrinos which are sent through 732 km of Earth to the Gran Sasso laboratory. Due to the low probability of the investigated neutrino oscillation processes the facility has to operate with the highest power possible which can be produced in the preceding accelerator chain of about 500 k W. The facility was commissioned during the past two years and will reach nominal performance in 2008. Due to the high beam power and associated strong activation of components and environment radiation protection aspects have been considered since day one of the design. This has included detailed Monte Carlo simulations of stray radiation, residual dose rates, of air and water activation as well as of damage to components. Results allowed a careful optimization of the design in all aspects. During each step of the commissioning of CNGS radiological quantities were closely monitored and compared to the estimations. Good agreement was found in many aspects including residual dose rates and activation of cooling water. However, during the commissioning also unforeseen technical problems were encountered and solutions had to be implemented. The paper summarizes the radiation studies performed during the design and demonstrates how they lead to a detailed optimization. Furthermore, it discusses the radiation protection precautions taken during commissioning which allowed a controlled, fast and efficient repair work and at the same time minimization of the doses to the workers. (author)