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Study of Radionuclides Induced in Low-activation Concrete Using the 12-GeV Proton Synchrotron Accelerator at KEK

Low-activation concrete made of limestone aggregates was irradiated by secondary particles in the beam-line tunnel of the 12-GeV proton synchrotron accelerator at KEK (12GeV-PS), and the radioactive concentrations induced in the concrete were measured using a Ge detector system. Although various radionuclides, such as 22Na, 24Na, 54Mn, 56Mn, 60Co and l52Eu, were induced in the low-activation concrete, those concentrations were below those induced in ordinary concrete made of non-limestone aggregates. Among the induced radionuclides, reducing the 24Na concentration is especially important for preventing the radiation exposures of radiation workers during the maintenance of an accelerator. In order to estimate the 24Na concentration induced in shielding concrete, chemical reagents such as NaHC03, MgO, A1203, Si02 and CaC03 were irradiated at several locations in the beam-line tunnel of the 12GeV-PS, and the 24Na concentrations induced in those chemical reagents were measured. Although the MgO concentration in limestone varies widely from 0.4% to 10% in each mine, the experimental results concluded that low-activation concrete made of limestone aggregates is useful for preventing the radiation exposure of workers as shielding concrete in a particle accelerator facility because the 24Na concentration induced in Mg is below that in Na. It was also confirmed that concentrations of long-lived radionuclides, such as 60Co and 152Eu, induced in the low-activation concrete were lower than those induced in ordinary concrete.