Patent Number: 055747582
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIGS. 1 and 2, a detecting device is shown, which comprises a primary detector 1 and a secondary detector 2 located in mutually opposed manner relative to the axis of a pipe P, e.g., a coolant pipe, through which for example, water of a non-reproductive cooler (not shown) flows, and a shield detector 3 surrounding substantially the whole perimeter of the primary detector 1 except for its portion facing the pipe. The primary detector 1 is constructed preferably of a semiconductor detector or a scintillation detector. The secondary detector 2 and the shield detector 3 are constructed each of one or more scintillation detectors. The semiconductor detector to be used includes, for example, HP Ge (high-purity germanium) detector, Si(Li) (lithium drift silicon) detector, CdTe (cadmium telluride) detector, GaAs (gallium arsenide) detector, HgI.sub.2 (mercuric iodide) detector, etc. The scintillation detector usable for this invention includes, for example, NaI(T1) (sodium iodide activated by tallium) detector, CsI(T1) (cesium iodide activated by thallium) detector, Bi.sub.4 Ge.sub.3 O.sub.12 (bismuth germanate known as BGO) detector, or the like. The secondary detector 2 is configured in a sector-form so as to surround a half to one-third of the periphery of the coolant pipe P. The primary, secondary, and shield detectors 1,2,3 are surrounded by lead shields 4 except for their directions in which gamma-rays and annihilation gamma-rays in the primary water are incoming and detected so that the incident gamma-rays and annihilation gamma-rays may be collimated and the incident dose of the gamma-rays and annihilation gamma-rays may be restricted. The detecting device including the primary detector 1, the secondary detector 2 and the shield detector 3 is connected with an anticoincidence circuit 11 for operating an anticoincidence counting between the primary and secondary detectors 1,2 and between the primary and shield detectors 1,3, and a multichannel pulse height analyzer 12, thus forming a gamma-ray spectrometric system as a whole, as shown in FIG. 3 in which essential components only are depicted with other components omitted since they are well-known per se in the art. In the gamma-ray spectrometry system thus constructed, the primary detector 1 serves to detect photons of gamma-rays from .sub.131 I, .sub.60 Co, and the like as intended as well as photons of the one annihilation gamma-rays as pulses; the secondary detector 2 serves to detect photons of the other annihilation gamma-rays as pulses; and the shield detector 3 serves to detect photons of the gamma-rays, which are Compton-scattered and escaped from the primary to the shield detectors, as pulses. When the photons of the gamma-rays or annihilation gamma-rays are detected on a germanium detector, an interaction of the photons with the germanium material yields gamma-ray spectra which usually include each a photoelectric peak due to full energy absorption event and a continuous sepctrum called Compton continuum or background due to once or twice scattering and subsequent escaping of the scattered photons outside the detector. Thus, the full energy absorption event is never attended with the escaping of scattered photons. The photopeak is utilized to determine the gamma-ray energy which is important for identification and quantitative determination of a radionuclide whereas the Compton backgound is a disturbing factor for the gamma-ray spectrometric measurement. Consequently, when the escaped photons are detected coincidently by means of the shield detector 3 and the primary detector 1 and an anticoincidence counting is operated, that tends to reject selectively the Compton scattering events only without affecting the full energy events. Further simultaneously when the annihilation gamma-rays are coincidently detected on the primary detector 1 and the secondary detector 2, and an anticoincidence counting is operated, that assists in rejecting selectively the events due to the annihilation gamma-rays without affecting the full energy events. More specifically, the rejection is conducted by passing the pulses from the primary detector 1 through electron gates of the anticoincidence circuit 11 which are adapted to be closed when pulses are detected on the secondary detector 2 and the shield detector 3, coincident with the detection on the primary detector 1. The rejection by anticoincidence counting operation yields the result that the annihilation gamma-rays and Compton background gamma-rays are significantly reduced, which enables it to increase the photopeak-to-background ratio in the spectrum and accordingly, to determine the count numbers of the intended gamma-rays with more precision. In the multichannel pulse height analyzer 12, the pulses detected by conversion of the radiation energy to voltage or current in proportion to the energy are divided into thousands of intervals (namely, multichannels) over the whole voltage or current pulse range, and number ratios of the pulses belonging to the respective channels are determined, thus yielding an energy distribution of the gamma-rays, i.e. gamma-spectra, from which count numbers of the intended gamma-rays are determined. One example of a method of the invention will be explained when applied to primary water of a nuclear reactor, i.e. non-reproductive cooler by fitting a coolant pipe P connecting to the cooler on its inflow side with the detecting device as described above including the primary, secondary, and shield detectors 1,2,3. As the primary detector 1, a germanium detector was used, which had a good energy resolution having a half band width of up to 2.0 KeV when 1.33 MeV gamma-ray of .sup.60 Co was taken as a standard and a counting efficiency of at least 75%. The shield detector used has such dimensions that make the Compton background (continuum) in the 131I area of the spectrum smaller than 1/10 of that without Compton suppression. When pulses from the primary detector 1 were counted in anticoincidence with pulses from the secondary detector 2 and the shield detector 3 by the operation of the anticoincidence circuit 11, the annihilation gamma-rays and Compton gamma-rays could be significantly diminished. Then, count numbers of the gamma-rays from .sup.131 I, .sup.60 Co, and others in the primary detector 1 were determined from the resulting gamma-spectra by analysis with the multi-channel pulse height analyzer 12. As a result, the detection limit of I gamma-ray area was enhanced to less than 1.5 Bq/cm.sup.3, more than 10 times as high as that (15 Bq/cm.sup.3) of a conventional method without reduction of the annihilation gamma-rays. This invention has been so far described, by way of example, with a primary water of a nuclear reactor, but the method can be naturally used for the analysis of: steam of a secondary system (from a steam generator), drain water of a primary coolant, chemical analysis of a primary coolant, etc. in the nuclear power field. However, this invention is also applicable to other fields, namely, researches in high energy physics, micro-analysis in accelerator engineering, etc. As described above, the conspicuous elevation of the detection limits of gamma-rays makes it possible to conduct continuous measurement of highly low-concentrations of radionuclides in primary water of a nuclear reactor, with the result that security of the nuclear reactor can be ensured by early detection of leakage of the fuel assembly. Further, it is possible to decrease the frequency of chemical analysis for detecting the leakage which has been hitherto performed in nuclear power plants.