Leakage radiation from a 10 MV medical linear accelerator, (2)

Neutron measurements for Mitsubishi ML-15 M II B Medical linac operated in the 10 MV photon mode were carried out at different locations in a treatment room by an iodine activation method with two Bonner spheres (11.5 cm diameter, 20 cm diameter). The method was established by Yamaguchi etal. and can determine neutron fluence and fluence to dose equivalent conversion factor. The measurement of 128I activity through 127I(n,r) 128I reactions in iodoform detectors were taken after exposing each iodoform detector with or without the use of a cadmium filter (1.00mm thick) in bare or in each of the Bonner spheres, and the results were used for determining the neutron fluence and the fluence to dose equivalent conversion factor. It also contains a method for estimating average energy of neutrons. 1. Major neutron sources considered within a treatment head are: Pt-target; main collimator and the parts around it; flattening filter and movable jaws in the case of 0 x 0 cm2 field. Estimated relative yields for each of the above sources were 55, 26, 5 and 14 % to total yield respectively. 2. Total leakage neutron source strength was found to be 6.4 x 1011 and 2.9 x 1011 neutron/Gy of X-ray in the case of using Pt-target and Cu-target respectively. 3. Estimated averege neutron energy was 0.43 MeV at a location near the front surface of the treatment head, and 0.36 MeV at the isocenter for 0 x 0 cm2 field. The average energies at other locations in the room were also estimated. 4. The distribution of neutron dose equivalent in the room was approximately inversely proportional to the square of the distance from the target. At the isocenter, percent neutron dose equivalent per unit X-ray output was 0.2 and 0.06 %Sv/Gy of X-ray for 30 x 30 cm2 (fully opened) and 0 x 0 cm2 field respectively. (author)