At accelerator facilities, concrete components around beam lines are exposed to secondary particles during machine operations. Various radionuclides, such as 3H, 22Na, 54Mn, 60Co, and 152Eu, are accumulated in the concrete, and quantitative evaluations of these nuclides are important for radioactive waste management in reconstruction or decommissioning of accelerator facilities. Our previous works have revealed that the radionuclides are mainly produced by thermal neutrons in the concrete of medium-energy accelerators. If we can obtain the thermal neutron fluence during machine operation, it will be useful to estimate the radioactivities in the concrete. In our previous work [1], we have developed a method for AMS of 36Cl in concrete samples and successfully applied to a medium-energy cyclotron. It was found that the AMS measurements of isotope ratios of 36Cl to 35Cl (36Cl/35Cl) is suitable for monitoring the thermal neutron fluence. In this work, we applied the 36Cl AMS to various types of hadron and electron accelerators, of which acceleration energies were mainly in the range of tens to hundreds MeV. The depth profiles of 36Cl/35Cl in concrete walls, floors, and shields were determined and compared with those of radioactivities of γ-emitters. The thermal neutron fluences were calculated from 36Cl/35Cl and specific radioactivities of 60Co and 152Eu. The results estimated from three kinds of reactions agreed well with each other, and it proves the validity of 36Cl AMS to estimate thermal neutron fluences. Long half-life of 36Cl (3.01 × 105 y) is advantageous for cumulative monitor of thermal neutrons because radioactive decay can be neglected. In the case of 60Co (T1/2 : 5.27 y) and 152Eu (T1/2 : 13.5 y), the operational history of the accelerator must be considered in calculating neutron fluences. Thermal neutron fluences at the concrete surface were in the range of 1012 to 1014 n/cm2, and the maxima in thermal neutron fluences were observed at approximately 5 to 10 cm in depth from the surface. These characteristics in depth profiles imply that abundant fast neutrons are slowed down inside concrete, and thermal neutron fluence increases at a depth of 5 to 10 cm. The depth profiles of thermal neutron fluences will be discussed on the basis of the mechanism of neutron emissions from accelerators.
[1] K. Bessho et al., KEK Preprint 2004-96, 171-180 (2005)
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