Particle Size of Radioactive Aerosols Generated During Machine Operation in High-energy Proton Accelerators

INTRODUCTION Radioactive nuclides are formed through nuclear reactions in the air of accelerator tunnels during machine operation. Especially, in high-energy accelerator facilities, various radionuclides are produced from the air through many kinds of nuclear reactions, such as nuclear spallation reactions. In addition, nonradioactive aerosols are abundantly generated in the air due to high radiation doses in the operating accelerators. Under such a condition, the produced radioactive atoms form radioactive aerosols. These aerosols might be inhaled by workers who enter the tunnel just after beam stoppage. The particle size is very important information both for estimating the internal exposure doses and for designing the exhaust equipment of accelerator facilities. We installed a flow-type chamber in the slow-extracted proton beam line (EP2) of the 12-GeV proton synchrotron at High Energy Accelerator Research Organization (KEK) in order to intensively investigate the characteristics of the radioactive airborne species. The chamber makes it possible to precisely analyze radioactive aerosols formed by the irradiation of high-energy particles. Air can be supplied to the chamber under controlled conditions, and returning irradiated air is analyzed concerning the basic properties of aerosols observed in the air. In this work, focusing on typical radionuclides produced in the tunnel air, such as C, N, Be and Na, their aerosol size distributions are reported. In addition, the internal doses due to inhalation of radioactive aerosols were estimated using the obtained parameters for the aerosol size.