Method of correcting photoneutron in boric acid concentration measuring instrument

For the on-line measurement of B-14 concentration in heavy water, neutrons released from a neutron source are moderated by heavy water and absorption of the moderated neutrons into B-14 is utilized. However, radioactive impurities are present in the heavy water and, among them, a portion of γ-ray radiated from Na-24 and Mn-56 reacts with heavy water to form photoneutrons, which constitute an error in B-14 concentration measurement. Accordingly, in order to amend the photoneutrons, the flow of heavy water containing boric acid is stopped to measure a detector output by a plurality of times while varying the measuring time. Then, the effective component is obtained by calculation for the neutron source and nuclides at a time just after closing the valve. In this way, it is not necessary to previously obtain a detector output in the absence of photoneutrons, and only the change with lapse of time of the coefficient rate for the sum of the original neutron source and the photoneutron source may be measured. In a case if both of them change abruptly, remarkable, continuous measurement is possible by using a counter tube with a Cd filter in combination. (N.H.)