Radioactive Airborne Species Formed in the Air in High Energy Accelerator Tunnels

High-power and high-energy proton accelerators have been proposed for basic scientific research in several countries, and some of them are under construction. In Japan, the high intensity proton accelerator (J-PARC), consisting of a 200-MeV Linac as well as 3-GeV and 50-GeV synchrotrons, of which the maximum beam power is about 1MW, is to be completed at the end of 2007. This accelerator was designed for a broad range of frontier sciences using pulsed-intense neutrons from the 3GeV synchrotron and neutrino and K beam from the 50-GeV synchrotron. At these accelerators, the air in the beam line tunnel is exposed to extremely high radiation fields during machine operation, where primary and secondary high-energy particles coexist. A number of radioactive nuclides are abundantly produced in the air, mainly through a variety of nuclear reactions between these high-energy particles and the elements of air. These nuclides generally behave as hot atoms with high kinetic energies and charges, and after completion of hot-atom reactions, part of them form radioactive aerosols and others are present in a gaseous form. Studying the physico-chemical properties of these radioactive airborne species is an interesting subject from radiochemical standpoint. Moreover, airborne radioactive nuclides become a source of external and internal exposures for workers when entering the accelerator tunnels after a shutdown of machine operation. To evaluate internal doses from radioactive nuclides induced in the air, information on their particle sizes and chemical forms is indispensable. However, thus far, there is little information available on these physical and chemical properties of airborne radioactive nuclides formed in the air in accelerator beam-line tunnels. An extensive study has been conducted on the physicochemical properties of airborne radioactive nuclides. In this paper, a review is given of recent results.