Measurements of 36Cl production rates from Cl, K, and Ca in concrete at the 500-MeV neutron irradiation facility of KENS

In high-energy accelerator facilities, concrete components around beam lines are exposed to secondary neutrons having various energies during machine operation. The neutrons produce the various long half-life radionuclides, such as 3H, 36Cl, 60Co, and 152Eu, in the concrete. Most of the nuclides mainly produced by thermal neutron-capture reactions and their specific activities are important from the viewpoint of accelerator clearance. In previous work, the specific activities of the 36Cl in the concretes at the various accelerator facilities have been measured and it was suggested that the 36Cl in the concrete is useful as an indicator for thermal neutron fluence because of a characteristic of very long half life (301 kyr). However, in the concretes of the accelerator facilities over several hundreds of MeV, the 36Cl are considerably produced by spallation from other concrete components, such as K and Ca, in addition to the thermal neutron capture of 35Cl. The contribution of the 36Cl productions from the spallation is unclear due to the lack of the cross sections for the neutron-induced reactions. In this work, therefore, we measured the 36Cl production rates in concrete from Cl, K, and Ca targets in irradiation with secondary neutrons, which were produced by a bombardment of primary 500-MeV protons with W targets, at high-energy neutron-irradiation course of KENS. Samples of NaCl, K2CO3, and CaCO3 were set into 7. irradiation spaces located on the depth raging from O to 320 cm from the concrete surface and irradiated for approximately one week. After the irradiation, separations of Cl from the samples were carried out radiochemically and the production rates of 36Cl were determined by the AMS. The production rates from Cl, K, and Ca exponentially decreased with an increase of the depth from the concrete surface, and the profiles were very similar each other. Although the production rates from Cl were two orders higher than those from Ca in the same depth, it is suggested that the contribution of the 36Cl production from spallation of Ca is comparable to those from the thermal neutron capture of 35Cl in the concrete at the high-energy accelerator facilities as KENS.