Abstract
Measurements of neutron dose equivalent rate with a LiJ scintillation counter and polyethylene moderator spheres (7.5, 30 and 40 cm diameter) have been performed in the treatment room of the linear accelerator Saturne-25 for 23 and 8 MV photon radiation. All values were normalized upon an absorbed dose rate of 0.8 Gy/min at 3 cm depth in a water phantom. At 50 cm central beam distance, at the level of the accelerator rotation axis, the neutron dose equivalent rate for 23 MV corresponds to a ratio of 0.0011 Sv/Gy; it is 4 times lower than on the beam axis, 4 times higher than in the room corners, and 80 times higher than for 8 MV. Only a small dependence upon photon beam direction and field size was found, probably due to a compensation between changes of leaking and scattered radiation. The neutron spectrum, determined by measurements without and with three moderator spheres, primarily consists of fractions of fast and thermal neutrons. The neutron fluence distribution in a 30 cm moderator sphere, measured with LiF-TLD, is in agreement with the neutron spectrum.
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