Measurement of induced radioactivities for the evaluation of internal exposure at high energy accelerator facilities

At high-intense and high energy accelerator facilities, accelerator components are exposed to primary and/or secondary high energy particles during machine operation. As a result, these become radioactive and the radioactivities are accumulated with operation time. When workers engage in maintenance work such as cutting, welding, etc. in the areas with residual activities. These become a source of internal exposure through the inhalation of radioactive airbornes as well as a source of external exposure. The estimation of external doses to workers is relatively easy by directly measuring the radiation fields by pertinent radiation counters. While the internal dose depends very much on the kinds of radioactive nuclides and their concentrations in air. In a routine survey for internal dose evaluation, airborne activities are filtered and their activities on the filter are measured with a GM counter with an automatic sample changer at KEK (the High Energy Accelerator Research Organization). Ordinarily many filter samples have to be measured with a relatively short counting time, so this gross beta counting is a practical way in a routine procedure. In order to evaluate the internal dose from these countings, it is necessary to examine precisely the kinds of radioactivities and their concentrations collected on the filters by a Ge semiconductor detector, and the correlation between the gross beta counting and the actual dose has to be made clear in advance. However, kinds of radioactivities and their concentrations depend very much on production rates of individual nuclides and time variations after beam-off. First, in order to elucidate the production rates of individual nuclides and their concentrations after beam-off, metal samples of Al, Fe, Cu, Steel, etc., which are principal materials used in accelerator facilities, were irradiated at various places in the tunnel of KEK-500MeV and 12GeV proton synchrotrons. By using these irradiated samples, we examined extensively a GM gross counting method for evaluation of internal doses and compared it with a Ge counting method under various conditions. The application, of this gross beta counting method to actual situations at high energy accelerator facilities is discussed. Further, Fe-55 is abundantly produced in accelerator components, however both above methods can not be applicable to the estimation of internal doses due to this isotope because Fe-55 decays through EC process with low energy X-rays and no gamma-ray. A low energy photon spectroscopy (LEPS) using a Ge-semiconductor was applied to the measurement of Fe-55 in activated components. In routine work, it is very convenient if the radioactivity of Fe-55 can be deduced from the concentrations of other long-lived isotopes such as Mn-54, Cr-51, concomitantly present in those irradiated materials. We examined extensively the possibility of direct evaluation of Fe-55 internal doses. (author)