Effect of (γ,n) reactions of heavy water moderator on beam quality in biomedical irradiation facility

The effect of (γ, n) reactions of heavy water on beam quality of a biomedical irradiation facility was studied. In an irradiation facility using thermal neutrons from a nuclear reactor, heavy water is widely employed as neutron moderator because of its good nuclear characteristics. It is well known that the threshold energy of (γ, n) reactions o f heavy water is 2.2 MeV and fast neutrons are generated from the reactions. In 1982, we proposed a technique for the evaluation of (γ, n) reactions using a computer code ANISN-JR. In the present work, with further improved ANISN-JR for the proper estimation of the leakage, the comparison calculations both with and without (γ, n)reactions were carried out and their results were compared with those of the foil activation experiments at the KUR Heaby Water Facility. In the ignorance of (γ, n) reactions, fast and epithermal neutron fluxes are extremely underestimated for deeper penetration than 80 cm, while the calculated results including (γ, n) reactions give good a greements with experiments. (author)