Experience in radiation control for 300 MeV electron linear accelerator of Tohoku University

Since the 300 MeV electron linear accelerator in Tohoku University has been operated for nine years, the radiation control so far is reviewed centering around the relation between residual radioactivity and personal exposure. The annual gross exposure dose increased in proportion to the operating time until 1970, and it seemed to be one figure higher than that of other linear accelerator facilities. Then the policy change was carried out to improve the exposure condition. It is likely that the failure of the accelerator may increase due to the superannuation, thus exposure increases owing to the repair works. Induced activity is found mainly at the slit of each beam channel and its periphery, radioisotope irradiation disk, and ND target. From radioactive dust, 54Mn and small amounts of 58Co and 60Co have been observed due to the activation of stainless steel. Most of radiation exposure is caused at the time of accelerator maintenance, and the exposure amounts to 1 to 2 rem when vacuum leak occurs in beam handling because sufficient cooling time is hardly taken, and rather long time is required to seek the leaking place. Radioactive gases are mainly 15O and 13N which were demonstrated to be fully below the permissible level in the measurement immediately after the completion of installing the accelerator. The disposal of large wastes accompanying the reconstruction of the accelerator is now becoming a problem. (Wakatsuki, Y.)