Patent Number: 055747582
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for measuring gamma rays of trace amounts of radionuclides (radioisotopes), such as iodine-131, cobalt-60, etc., particularly in primary water of a nuclear reactor, which further contains other radionuclides such as nitrogen-13, fluorine-18, cobalt-58, etc. each emitting a pair of annihilation gamma-rays. More particularly, the invention is concerned with an improved gamma-ray spectrometric method for measuring selectively gamma-rays of the aforementioned radionuclides by diminishing the annihilation gamma-rays emitted by radionuclides coexisting in the primary water, thus significantly elevating the detection limits of the gamma-rays. 2. Statement of Related Art For instance, at nuclear power plants, with a view toward safe operation thereof, leakage of nuclear fuel assemblies is always kept under surveillance, for example, by measuring gamma-rays of .sup.131 I .sup.60 Co ,etc. contained in trace amounts in water of a primary coolant of an individual nuclear reactor. However, other radionuclides such as .sup.13 N, .sup.18 F, .sup.58 Co, etc., which are unstable radionuclides emitting .beta..sup.+(e.sup.+) or positron, are also contained in the primary water, and soon .beta..sup.+ decay at a low energy level by absorption in a substance and bonding with electrons therein at the end of their ranges. At that time, one positron and one electron are annihilated, emitting annihilation gamma-rays of 0.511 MeV in diametrically opposite directions. The coexistence of the radionuclides emitting the annihilation gamma-rays is a major disturbing factor for the measurement of the intended gamma-rays, particularly gamma-ray of .sup.131 I, which has a close energy level (0.364 MeV) to the annihilation gamma-rays. Additionally, Compton scattering caused inevitably in a gamma-ray spectrometry also interferes with the intended measurement. As a consequence, it is essential for gamma-ray spectrometric measurement of gamma-rays of the intended radionuclides (.sup.131 I, .sup.60 Co, etc. ) in the primary water that the annihilation gamma-rays be minimized while Compton backgrounds or continua of the resulting gamma-spectra due to the gamma-rays and annihilation gamma-rays are suppressed. Hitherto, iodine-131 and other radionuclides emitting gamma-rays in water of a primary coolant has been measured by means of a germanium (Ge) detector or a scintillation detector of NaI(T1) (sodium iodide activated by thallium) or Bi.sub.4 Ge.sub.3 O.sub.12 (bismuth germanate known as BGO), or a gamma-ray sepctrometric measurement system wherein a scintillation detector is disposed around a germanium detector. The method using the Ge detector was poor in detection limit of .sup.131 I owing to the effect of Compton backgrounds produced from .sup.131 I gamma-ray, .sup.60 Co gamma-ray the annihilation gamma-rays, etc., so that trace amounts of .sup.131 I and other radionuclides emitting gamma-rays in the primary water couldn't be measured. Only in the event that .sup.131 I, .sup.60 Co, etc. were leaked from a fuel assembly into the primary water, increased concentrations of them enabled the measurement. Again, the method using the NaI(T1) detector was too inferior to the Ge detector method in resolution power. The method using both Ge detector and scintillation detector has been improved more or less over the preceding methods, but it has still not been possible to measure extremely slight concentrations of .sup.131 I, .sup.60 Co and others. Thus, any of the known gamma-ray spectrometric methods has not been satisfactory and feasible because the annihilation gamma-rays from coexsiting radionuclides in the primary water have interfered with the measurement of the intended radionuclides, e.g., .sup.131 I, etc. Another method for measuring .sup.131 I and other radionuclides by chemical analysis has been known, but has yielded disadvantageously awkward radioactive wastes, which should be handled or disposed of with great care. Hence, this is not suitable for frequent or continuous measurement. In view of the drawbacks or problems as encountered in the prior art measurement methods of gamma-rays in primary water of a nuclear reactor as stated above or gamma-rays in another radioactive substances, the present invention is designed to provide a gamma-ray spectrometric measurement method which enables to significantly enhance detection limits of gamma-ray-emetting radionuclides, particularly in the primary water. That is to say, it is a primary object of the invention to provide an improved method for measuring selectively gamma-rays of radionuclides (iodine-131, cobalt-60, etc.), particularly in the primary water contained in micro-quantities by excluding disturbing factors to the measurement, namely, the aforesaid annihilation gamma-rays emitted by other radionuclides coexisting in the primary water, and Compton effects due to the gamma-rays and annihilation gamma rays as far as possible. Another object of this invention is to provide a high-sensitive measurement method capable of detecting such extremely slight amounts of the radionuclides emitting gamma rays in the primary water that it has been not possible to detect hitherto. Further object is to provide a reliable measurement method which enables continuous surveillance of leakage of a nuclear fuel assembly, thereby assisting in early prevention of the risk. SUMMARY OF THE INVENTION The invention for achieving the foregoing objects resides generally in a method for measuring selectively gamma-rays of radionuclides of microquantities, particularly in primary water of a nuclear reactor, coexisting with radionuclides each emitting a pair of annihilation gamma-rays in diametrically opposite directions, using 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 10 shield detector for detecting photons of Compton-scattered 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 being disposed to surround the primary detector except for its portion facing to the pipe on which the gamma-rays and the annihilation gamma-rays are incident. The 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; and counting the pulses from the secondary detector in anticoincidence with the pulses from the primary detector thereby to reject the recording of the pulses of the annihilation gamma-rays, thus minimizing the annihilation gamma-rays; and subsequently measuring count numbers of the gamma-rays on the basis of the analysis of the pulses. More preferably, the method further comprises, simultaneously with the foregoing detecting step, detecting the photons of the Compton-scattered and escaped gamma-rays on the shield detector as pulses and counting the pulses from the shield detector in anticoincidence with pulses from the primary detector thereby to reject the recording of the pulses of the Compton-scattered gamma-rays, thus additionally diminishing the Compton gamma-rays. According to the method of this invention, when the photons of the annihilation gamma-rays emitted in diametrically opposite directions are coincidently detected on the primary and secondary detectors located in an opposed relation to each other, the resulting pulses are rejected by anticoincidence counting operation of the anticoincidence circuit, whereby the annihilation gamma-rays are vastly reduced from the primary detector. Consequently, it is possible to elevate significantly the detection limits of the intended gamma-rays of iodine-131, cobalt-60, etc. Further according to a preferred embodiment, when the photons of Compton-scatterered gamma-rays are coincidently detected on the primary and shield detectors, the resulting pulses are rejected by anticoincidence counting operation of the anticoincidence circuit, whereby the Compton gamma-rays are also significantly diminished from the primary detector, which enables to further enhance the detection limits of the intended gamma-rays of radionuclides.