Absolute measurement of D–T neutron flux with a monitor using activation of flowing water

Neutron activation with water flow based on the 16O(n, p)16N reaction (T1/2=7.13 s) has been proposed for the accurate fusion power monitor with reasonable time resolution in International Thermonuclear Experimental Reactor (ITER)-FEAT. The experiment for validation of the method for determining the neutron flux was carried out at the Fusion Neutronics Source (FNS) facility in Japan Atomic Energy Research Institute (JAERI). The stainless steel (SS316)/Water assembly was utilized to simulate a neutron field in the blanket region of ITER. The closed water loop was laid between the assembly and the shielded measurement area. The γ-rays from the irradiated water were measured by Bi4Ge3O12 (BGO) scintillator which was ∼12 m away from the irradiation point. The range of the flow velocity was 2–11 m/s. The neutron spectrum at the probe section in the SS316/Water assembly was calculated by the MCNP-4B code with the FENDL/E-2.0 library. The cross section data of the 16O(n, p)16N reaction was taken from the FENDL/A-2.0 file. The counting response of the BGO detector including the effect of the shielding lead blocks surrounding it was calculated by MCNP-4B. The neutron yield from the water activation has a good agreement with that from associated α particle detector.