For boron neutron capture therapy, the thermal neutron beam is worth using (1) as thera-peutic neutron irradiation without useless and unfavorable exposure of normal tissues around tumor and (2) for microanalysis system to measure ppm-order 10B concentrations in tissue and to search for the location of the metastasis of tumor. In the present study, the thermal neutron flux distribution in a phantom, when beam-type thermal neutrons were incident on it, was measured at the KUR Neutron Guide Tube. The measurements were carried out by two different methods using indium foil. The one is an ordinary foil activation technique by using the 115In (n, γ) 116m1In reactions, while the other is to detect γ-rays from the 115In(n, γ)116m2In reactions during neutron irradiations with a handy-type Ge detector. The calculations with DOT 3.5 were performed to examine thermal neutron flux in the phantom for various beam size and phantom size. The experimental and calculated results are in good agreement and it is shown that the second type measurement has a potential for practical application as a new monitoring system of the thermal neutron flux in a living body for boron neutron capture therapy.
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