In order to estimate the total radioactivity induced in a concrete shield by thermal neutron-capture reactions at high-energy
accelerator facilities, the observed activity of 36Cl in the concrete is expected to serve as an indicator of the thermal neutron fluence. Since 36Cl can also be produced from K and Ca by spallation reactions, we measured these production rates in order to clarify the
contribution of each reaction. The Cl, K, and Ca targets were irradiated with neutrons having a maximum energy of 500 MeV.
As a result, the production rates of 36Cl from Cl were only two orders higher than those from K and Ca. It was found that the 36Cl production ratios from Cl, K, and Ca were 6.7%, 6.8%, and 86.5%, respectively, and Ca was the main source of 36Cl production.