Studies on photoneutrons produced by 10 MV x rays

The purposes of the present study are to elucidate major photoneutron sources within the treatment head of a 10 MeV linac, photoneutron yields in their sources, and their primary photoneutron spectra and are to consider the additional risks to patients undergoing treatment, machine operators and other personnel due to the photoneutrons and a method of shielding calculations for the photoneutrons. For these purposes, various measurements and theoretical calculations were performed, and the following results were obtained. For the Toshiba LMR-15 linac, the calculated relative photoneutron yields in the target, main collimator, beam falttening filter, and movable diaphragms were 50.5%, 39.2%, 5.5%, and 4.8%, respectively, and, for the Toshiba LMR-13 linac, were 42.9%, 44.2%, 6.2%, and 6.6%, respectively. The calculated neutron dose equivalent rates from the major sources at the four locations around the treatment head of the LMR-15 linac were in agreement with the measured values. The calculated average neutron energies and neutron fluence to dose equivalent conversion factors of the primary photoneutron spectra were about 1.4 MeV and 0.11 (mrem.h-1.n-1.cm2.s), respectively. These values can be regarded as each upper limit for tungsten and lead. In the x-ray beams, the ratio of neutron dose equivalent (rem) to photon absorbed dose (rad) with the beam flattening filter was 0.026% for the LMR-13 linac. Substituting the tungsten target and the lead flattening filter for copper, the neutron dose equivalent rates at the various locations inside and outside the test cell for the LMR-15 linac decreased to 16 - 38% of the values measured under the ordinary condition to the linac. (J.P.N.)