Measurement of leaking neutron ray and countermeasures in medical linear accelerators

In Japan, there are over 100 medical linear accelerators of 10 MV or more in operation. In such facilities, the radiation control against leaking photoneutrons is not adequate. The monitoring of the leaking dose equivalent out of therapeutic rooms by the instruments with ensured traceability is necessary. The measurement of neutron dose within therapeutic rooms is important in connection with the shielding problem. The effective means of reducing leaking neutron ray is to suppress the generation of photoneutrons. The turns in labyrinth largely attenuate leaking neutron ray. Fast neutrons are to be elastically scattered with hydrogen-containing materials such as polyethylene to lower the energy. To absorb the resultant slow neutrons, boron-containing materials are efficient. Combination of several countermeasures is the best to lower the leakage. (Mori, K.)