Development of low-gamma thermal neutron irradiation field and absorbed dose evaluation in its biological irradiation effect

Neutron capture medical treatment has been developed, and various technical problems had to be solved. This paper is described on the principle of the method, the characteristics of neutron field with reactors, the evaluation of dose absorbed by cell nuclei, and the examples of experiments performed at Kyoto University Reactors. The principle of the method of neutron capture medical treatment is to use heavy particles produced by the (n, α) reactions of B-10. The associated gamma-ray causes damage on normal organs, and must be suppressed. A bismuth scatterer was designed, and the mixing ratio of gamma-ray is reduced to 1/10. Tiles of LiF have been made to collimate the neutron beam and protect normal organs. The boron accumulation effect was investigated. The evaluation of absorbed dose was made by considering the absorption by cell nuclei. The calculated survival rate of cells was compared with the experimental data, and good agreement was seen. The measurement of small amount of B-10 was made by using a neutron guide tube and LiF tiles. Further improvement of the experimental apparatus is in progress. (Kato, T.)