Thermal and epi-thermal neutron fluence rates around the isocenter of the electron linear accelerator at Hiroshima Prefectural University were measured using indium foils when the 10 MV X-rays were generated at the copper target. Gold foils are frequently used for measurements of thermal and epi-thermal neutron fluence rates. The indium foils are more sensitive than gold ones, and the half-life (54 min) of 116mIn is convenient for measuring the gamma rays. The thermal and epi-thermal neutron fluence rates in a tissue-equivalent block phantom were measured not only by indium foils but also by gold ones. The both results were in approximately agreement with each other. Thermal neutron fluence in phantom human was, for example, 2.6×1010 m−2 per 1 Gy of treatment dose at 5 cm depth from the surface along the base line. The epi-thermal neutron fluence rate was less than 1/10 of the thermal one. The results suggest that the neutrons induce few activities in patient body. The results also suggest that the 28Al radioactivities induced in the couch may cause about 0.4 mGy of annual radiation doses to the medical staffs.
