Method for measuring gamma-rays of radionuclides, particularly in primary water of nuclear reactor

A method for measuring selectively gamma-rays of radionuclides, in primary water of a nuclear reactor, which further contains radionuclides each emitting a pair of annihilation gamma-rays in diametrically opposite directions, by the use of a gamma-ray spectrometric system which includes a primary detector for detecting photons of the gamma-rays and photons of the one annihilation gamma-rays in the one direction, a secondary detector for detecting at least photons of the other annihilation gamma-rays in the opposite direction, a shield detector for detecting Compton-scattered photons of the gamma-rays escaped from the primary detector to the shield detector, and an anticoincidence circuit connecting with the primary, secondary, and shield detectors, the primary detector and the secondary detector being located in opposed manner relative to the axis of a coolant pipe through which the primary water flows, the shield detector surrounding the primary detector except for its portion facing the pipe on which the gamma-rays and the annihilation gamma-rays are incident, which method comprises:

• detecting the photons of the gamma-rays and the photons of the one annihilation gamma-rays on the primary detector as pulses while detecting the photons of the other annihilation gamma-rays on the secondary detector as pulses;
• counting the pulses of the secondary detector in anticoinci- dence with the pulses of the primary detector thereby to reject the recording of the pulses of the annihilation gamma-rays from the primary detector, thus minimizing the annihilation gamma-rays on the primary detector;
• and subsequently determining count numbers of the gamma-rays.