Evaluation of equivalent dose from neutrons and activation products from a 15-MV X-ray LINAC

摘要

A high-energy photon beam that is more than 10 MV can produce neutron contamination. Neutrons are generated by the [gamma,n] reactions with a high-Z target material. The equivalent neutron dose and gamma dose from activation products have been estimated in a LINAC equipped with a 15-MV photon beam. A Monte Carlo simulation code was employed for neutron and photon dosimetry due to mixed beam. The neutron dose was also experimentally measured using the Optically Stimulated Luminescence (OSL) under various conditions to compare with the simulation. The activation products were measured by gamma spectrometer system. The average neutron energy was calculated to be 0.25 MeV. The equivalent neutron dose at the isocenter obtained from OSL measurement and MC calculation was 5.39 and 3.44 mSv/Gy, respectively. A gamma dose rate of 4.14 mu Sv/h was observed as a result of activations by neutron inside the treatment machine. The gamma spectrum analysis showed Al-28, Na-24, Mn-54 and Co-60. The results confirm that neutrons and gamma rays are generated, and gamma rays remain inside the treatment room after the termination of X-ray irradiation. The source of neutrons is the product of the [gamma,n] reactions in the machine head, whereas gamma rays are produced from the [n,gamma] reactions (i.e. neutron activation) with materials inside the treatment room. The most activated nuclide is Al-28, which has a half life of 2.245 min. In practice, it is recommended that staff should wait for a few minutes (several Al-28 half-lives) before entering the treatment room after the treatment finishes to minimize the dose received.