Estimation of thermal neutron fluences in the concrete of proton accelerator facilities from 36Cl production

Abstract The thermal neutron fluence that poured into the shielding concrete of proton accelerator facilities was estimated from the in situ production of 36 Cl. The thermal neutron fluences at concrete surfaces during 10–30 years of operation were in the range of 10 12 –10 14  n/cm 2 . The maxima in thermal neutron fluences were observed at ≈5–15 cm in the depths analyzed for 36 Cl/ 35 Cl by AMS. These characteristics imply that thermalization of neutrons occurred inside the concrete. Compared to the several tens of MeV cyclotrons, secondary neutrons penetrate deeper into the concrete at the high-energy accelerators possessing acceleration energies of 400 MeV and 12 GeV. The attenuation length of neutrons reflects the energy spectra of secondary neutrons emitted by the nuclear reaction at the beam-loss points. Increasing the energy of secondary neutrons shifts the maximum in the thermal neutron fluences to deeper positions. The data obtained in this study will be useful for the radioactive waste management at accelerator facilities.