JPH0684989B2 - Radioactivity concentration measuring device - Google Patents

Description

Description: TECHNICAL FIELD OF THE INVENTION The present invention relates to a radioactivity concentration measuring device for measuring the concentration of radioactive substances such as atypical solid radioactive waste filled in a fixed container by nucleus type.

[Technical background of the invention and its problems] Among atypical solid wastes such as pipes and concrete waste generated from nuclear facilities, those that cannot undergo incineration treatment or volume reduction treatment such as acid decomposition are It is stored as it is in a fixed container such as a drum, and is disposed of at a waste storage place. When moving such waste to another facility or evaluating the permitted storage capacity (radioactivity conversion) of the facility,
It is necessary to measure the amount of radioactivity contained in the waste. In such a case, NaI (Tl) that can discriminate the energy of gamma rays generally emitted from radioactive materials
A scintillation detector, a germanium semiconductor detector, or the like is used to perform measurement from outside the container without taking out solid waste inside.

However, there is a great error in quantifying the radioactivity of such waste. In general, the distribution of radioactivity of the radioactive waste packed in the container is local, and the distribution of substances that act as gamma-ray absorbers is not uniform in many cases, and the distance from the detector and gamma-ray absorption This is because the variation in body distribution directly affects the measured value. Such an error becomes remarkable as the measurement target becomes a low energy gamma ray emitting nuclide.

In order to eliminate these effects as much as possible, especially 100 KeV
FIGS. 5 to 8 show the method conventionally used for the analysis of low energy emission nuclides before and after. FIG. 5 shows a system in which the container 1 to be measured is placed on the turntable 2 and the detector 3 is moved in the longitudinal direction of the container 1 to be measured to perform the measurement. FIG. 6 is similar to FIG. In this method, the cylindrical container 1 to be measured is moved on a roll 5 rotated by a motor 4, and the measurement is performed. Each of these aims at averaging variations in distance due to non-uniform radioactivity distribution and variations in gamma ray absorber distribution by rotating the container 1 to be measured.

FIG. 7 shows an improved version of the above method, in which a parallel slit type collimator 6 is placed in front of the detector 3 to limit the visual field in the longitudinal direction of the container 1 to be measured, and the container 1 to be measured together with the rotating mechanism is lifted and lowered 7 It is moved by and measured. In this method, the components of the gamma rays emitted from the radioactive substance in the container 1 to be measured that cross the detector 3 obliquely in the longitudinal direction are cut, so that the inside of the container is different from the methods in FIGS. 5 and 6. It is possible to reduce the influence of the nonuniformity.

FIG. 8 shows a modification of the method shown in FIG. 7,
The container 1 to be measured is divided into several segments for measurement by the parallel slit type collimator 6 placed on the front surface of the detector 3, and the container 1 to be measured is elevated stepwise together with the rotation mechanism, and the radioactivity is increased for each segment. Evaluate
The total amount is used to measure the amount of radioactivity in the container 1 to be measured, and the external gamma ray source 8 other than the nuclide to be measured is placed at a position facing the detector 3 with the container 1 to be measured interposed therebetween to determine the gamma ray transmittance. The average of the densities inside the segments is calculated, and the gamma ray absorption is corrected for each segment.

In the conventional method, it is possible to correct the gamma-ray absorption to some extent with respect to the radioactivity distribution or density distribution in the longitudinal direction of the container, but if the radioactivity distribution or density distribution exists in the container radial direction, There may be a large difference in the evaluation value of. Fig. 9 shows an example of a case where radioactivity is locally unevenly distributed. In a steel drum of a 200-liter container, a filling material with a specific gravity of 1.25 is placed, and one point-like radiation source is embedded in it. Fig. 7 shows the measurement results when measured by the method of Fig. 7, and the curve A is the ratio of the evaluation value when the radiation source is located on the surface of the drum can to the evaluation value when the radiation can is evenly distributed in the drum can. Is a graph of the gamma ray energy of the nuclide to be measured,
Represents the ratio of the evaluation values when the radiation source is located at the center of the drum. As is clear from this figure, when the radioactive material is unevenly distributed in spots, the lower the gamma ray energy, the greater the tendency for the activity to be overestimated or underestimated. Cause an error. This is because the lower the energy of gamma rays, the more the rate of absorption of gamma rays by the filler increases.

When there is a density distribution or radioactivity distribution in the radial direction of the container as described above, it may be possible to adopt a transmission CT method using a gamma ray source and a single photon emission CT method that are mainly used in nuclear medicine. Transmission CT is for rotating or translating a standard container to obtain multiple transmittances in the same cross section, and reconstructing the density distribution from that information.Single photon emission CT uses an extremely thin collimator in front of the detector. The detector is limited to a narrow beam in the field of view, and a large number of scanning data are obtained by rotating or translating the standard container in the same way as the transmission CT, and based on this, the radioactivity distribution of the container cross section is reconstructed. It is what configures.

As an example of using such transmission CT and single photon emission CT methods, Fig. 10 shows a measuring device developed for nondestructive measurement of high-level vitrified solids. The measurement apparatus is provided with an external gamma ray source 8 and one or more gamma ray detectors 3 each having a slit collimator 6 in front of which a measurement sample 1 is sandwiched, and the measurement sample includes a lifting motor 9 and a rotation motor 10. And, by the horizontal traveling motor 11, not only the rotation but also the vertical movement and the horizontal movement are performed.

In order to obtain the amount of radioactivity in a container using an apparatus of this type, the fixed container 1 is moved up and down with respect to the gamma ray detector 3 as shown in FIG. Divide into 12 and further each segment
In 12, it is necessary to divide the mesh into thin meshes for evaluation, but by making this divided mesh finer, it is possible to reduce measurement errors due to variations in distance due to density distribution and radioactivity distribution and variations in absorption of gamma rays.
However, this method requires an extremely large amount of measurement data. For example, when the inside of the container is divided into 10 × 10 mesh, the required measurement data are 200 to 300 points for each of the density distribution and the radioactivity distribution. Furthermore, when the energy spectrum data necessary for nuclide analysis is used, the measurement data from each detector is about 256 with NaI (Tl) scintillation detector and 2000 to 4000 with germanium semiconductor detector. Yes, and requires huge memory. For this reason, when determining the radioactivity for each nuclide, the output signal of the detector is measured by the single-channel wave height discriminating circuit, as shown in Fig. 12, as the wave height corresponding to the photopeak due to the gamma rays emitted from the specific nuclide. The method is limited to counting only windows. In this case, an energy window b corresponding to gamma rays from an external gamma ray source used for transmittance measurement
Is set, and one or a plurality of energy windows a corresponding to the measured nuclides in the container on a one-to-one basis are set.

However, this method is effective when focusing on a small number of specific nuclides in the container, but as shown in FIG. 13, in the case of nuclides with energy close to each other that cannot be separated from each other or the nuclide to be measured is In many cases, as shown in Fig. 14, there is a risk of overestimation due to the effect of Compton tail c from the interfering nuclide. In addition, it is difficult to simultaneously measure a large number of nuclides because a large number of single-channel wave height discrimination circuits and counting circuits are required.

In addition, since this method requires a large number of measurement points, the measurement time per measurement is extremely short, and the field of view of the detector is limited by a collimator with a narrow opening, so the detection limit of radioactivity is relatively low. When it is high and the contained radioactivity is low, there is a drawback that the error becomes large.

[Object of the Invention] The present invention has been made in consideration of such a point, and there is little measurement error due to the radial density distribution and radioactivity distribution of a fixed container, and the radioactivity measurement does not increase so much. It is intended to provide a device.

[Summary of the Invention] That is, the present invention provides a plurality of γ-ray detectors for energy spectrum measurement radially arranged with an external radiation source and a test sample sandwiched therebetween, a rectangular collimator placed on the front surface thereof, and a measurement sample. A mechanism that measures the γ-rays emitted from the measurement sample by rotating or rotating vertically or intermittently by the driving mechanism in the spiral or intermittently from the surroundings in each of the horizontally and vertically divided regions. , Splitting the output signal from the energy spectrum measuring γ-ray detector into both a signal corresponding to γ-ray energy generated by an external source and a signal corresponding to γ-ray energy generated from a specific nuclide in the measurement sample. Then, the mechanism for logging each data for each angle in the circumferential direction and the output pulse of the γ-ray detector for energy spectrum measurement are summed up to obtain 1 A signal processing unit that stores as γ-ray spectrum data for the circumference and a density distribution focusing on one division in the horizontal direction from the count rate data for each circumferential direction corresponding to γ-rays generated from an external radiation source, and the measurement sample Obtain the radioactivity distribution of the specific nuclide focusing on the horizontal division from the count rate data for each circumferential direction corresponding to the γ-ray energy generated from the specific nuclide in
The radioactivity conversion count of other nuclides calculated from these distributions, and the photoelectric peak count rate of the γ-ray spectrum data, from the radioactivity concentration of other nuclides, characterized by comprising a computing device It is a radioactivity concentration measuring device.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail based on the embodiments shown in the drawings.

1 and 2 show an embodiment of the radioactivity concentration measuring apparatus of the present invention. The detection unit of the present invention is a standard container that is a sample.
A rotary table 22 on which 21 is mounted, an elevating table 23 that elevates and lowers the sample together with the rotary table 22, an external gamma ray source 24 that transmits gamma rays to the sample, and a gamma ray emitted from the sample and a sample from the external gamma ray source 24. It is mainly composed of a plurality of gamma ray detectors 25 for detecting gamma rays that have passed through and a rectangular collimator 26 placed in front of the gamma ray detectors 25. The preamplifier 27 and the linear amplifier 28 connected to the gamma ray detector 25 and the signals passed through these amplifiers are input,
One is a pulse having a window width corresponding to gamma rays from an external gamma ray source 24, and the other is a preset pulse which is considered to be universally contained in waste in a container such as cobalt-60 or cesium-137. A pulse with a window width corresponding to the nuclide is extracted. Two single-channel wave height analyzers 29, 29a, counters 30, 30a for counting the output pulses of the respective single-channel wave height analyzers 29, 29a, and a counter for inputting the output of each counter and outputting it to the arithmetic unit 31 The scanner 32, a sum amplifier 33 for inputting and adding detection signals from the detectors that have passed through the linear amplifier, an analog-digital converter 34 connected to the sum amplifier 33, and this analog-digital converter 34.
And a process memory 35 for storing the added data from the above as spectrum data. The arithmetic unit 31 inputs the data from the counter scanner 32 and the process memory 35, and also inputs the position detection signal from the angle and height position detection mechanism 36, and determines the density distribution by the transmitted gamma ray from the external gamma ray source 24 and the specific nuclide. The radioactivity distribution of the nuclide is calculated from the emission gamma ray of each, and the radioactivity conversion coefficient for other nuclides is calculated using these density distribution and the radioactivity distribution of the specific nuclide. Calculate the radioactivity of the nuclide.

Next, the operation of the radioactivity concentration measuring apparatus configured as described above will be described. The measurement container 21 placed on the rotary table 22 at the time of measurement is a gamma ray detector 25 in which the visual field in the longitudinal direction of the measurement container 21 is limited by the rectangular collimator 26 having a rectangular opening, and the elevating table 23. Container to be measured
By the translation of 21 itself, it is divided into thin cylindrical segments and measured. In each of these segments, the container 21 to be measured is rotated at a fixed angle, and the count data by each gamma ray detector 25 for each fixed angle is logged according to the position detection signal from the angular position detection mechanism. Thus, for each constant angle, the number of energy windows provided for the detection signal of each gamma ray detector 25, that is, the number corresponding to the product of 2 and the number of gamma ray detectors 25, that is, the same number as the number of single-channel wave height analyzers. Data is logged and one rotation worth of data is obtained, and the detection signals of each gamma ray detector 25 are summed by the sum amplifier 33, and are processed as spectrum data for rotation in the process memory.
Stored in 35. The data for each one segment is processed by the arithmetic unit 31 each time, or after being stored in an appropriate external storage unit, is collectively processed by the arithmetic unit 31. The arithmetic unit 31 calculates the radioactivity concentration of each nucleus based on the data processing method described below.

First, as shown in FIG. 3, the cross section of the container 21 to be measured is divided into square meshes for consideration. At this time, the external gamma ray source 4 by the Kth gamma ray detector 25 at the rotation angle θ
The ratio of the count data Ik (θ) corresponding to the transmitted gamma ray of the above and the count data I _{0 in the} state without the container 1 to be measured is
_ij is a rayrayk connecting the external gamma ray source 4 and the gamma ray detector 5
If (θ) is the distance across the mesh (i, j), it is expressed by the following equation.

Here, μ _ij is the linear absorption coefficient in the mesh (i, j). Since the linear absorption coefficient is proportional to the density, the density can be reconstructed by solving the equation (I) for the distribution of μ _ij .

Moreover, as shown in FIG. 4, the radioactivity distribution a _ij is expressed by the following equation.

Σ f _ij a _ij = Nk (θ) (II) (i, j) ∈ rayk (θ) where Nk (θ) is the count rate data by the Kth gamma ray detector 25 at the rotation angle (θ). Yes, rayk (θ)
Is the field of view of the gamma ray detector 25, and f _ij is a weighting coefficient considering the distribution of the linear absorption coefficient μ _ij between the mesh (i, j) and the gamma ray detector 25 and the expected angle ε of the gamma ray detector 25. is there. The radioactivity distribution a _ij can be obtained by solving this equation (II). However, this is just the radioactivity distribution of the specific nuclide selected as a representative,
Other contained nuclides are not considered.

Therefore, next, by using the density and the radioactivity distribution of the specific nuclide obtained in the above process, the radioactivity conversion coefficient for other nuclides is obtained. Calculation of such conversion factor is
For example, it is performed by two-dimensional and rotational direction integration considering the radioactivity distribution and the density distribution.

On the other hand, the counting rate for each nucleus type can be obtained by analyzing the spectrum data obtained by performing the measurement on the entire segment, but this data includes the influence of the distribution of radioactivity and density. Therefore, from the count rate data of these nuclides, the above-mentioned conversion factor is used to calculate the radioactivity of any nuclide excluding the effects of density and radioactivity distribution.

In order for the above relationship to be established, it is necessary that there is a correlation between the distribution of the specific nuclide and the other nuclide in the container, but this is often established. For example, in nuclear power plants, the dominant nuclide with a relatively long half-life is cobalt-60,
It is universally present in the waste generated by nuclear power plants.

Further, in the device having the above-mentioned configuration, the same gamma ray detector was used for both the spectrum measurement and the density and radioactivity distribution measurement, but the device of the present invention is not limited to this, for the spectrum measurement, the density It is also possible to use a gamma ray detector other than that for the radioactivity distribution. In this case, the gamma ray detector for spectrum measurement does not require a collimator on the front surface of the detector, and independent measurement with high data sensitivity is possible.

[Effects of the Invention] As is clear from the above description, in the present invention, since the radioactivity distributions of other nuclides are estimated from the radioactivity distributions of the representative nuclides, it is necessary to determine the density and the radioactivity distribution externally. It is sufficient to measure only the transmitted gamma rays of the gamma ray source and the representative gamma rays of a specific nuclide, and the measured data can be considerably reduced, and the radioactivity of the nuclide type with less error considering the influence of the density distribution and the radioactivity distribution. The concentration can be measured efficiently.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the radioactivity concentration measuring apparatus of the present invention, FIG. 2 is a side view of the detection unit shown in FIG. 1, and FIGS. 3 and 4 are diagrams for evaluating the density distribution. 5 to 8 are schematic views showing a conventional radioactivity measuring device, and FIG. 9 is a can surface in a drum can and a can using the device shown in FIG. Fig. 10 is a graph showing the change in the radioactivity evaluation value with respect to the gamma ray energy of the nuclide to be measured when a point-like radiation source is embedded in the center, Fig. 10 is a transmission CT developed for nondestructive measurement of high level vitrified solid Explanatory diagram showing a measuring device for single photon emission CT, FIG. 11 is an explanatory diagram showing an example of segment division of a sample in the device shown in FIG. 10, FIG. 12 and FIG.
Fig. 13 is a graph showing the energy window for gamma rays from the nuclide to be measured and the energy window for gamma rays from an external gamma ray source, and Fig. 14 shows the existence of the Compton tail of the interfering nuclide in the energy window corresponding to the gamma rays of the nuclide to be measured. It is a graph. 21 …… Container to be measured 22 …… Rotating stand 23 …… Lifting stand 24 …… External gamma ray source 25 …… Gamma ray detector 26 …… Collimator 27 …… Preamplifier 28 …… Linear amplifier 29, 29a …… Single channel Wave height analyzer 30, 30a …… Counter 31 …… Computing device 32 …… Counter scanner 33 …… Sum amplifier 34 …… Analog-digital converter 35 …… Process memory 36 …… Angle and height position detection mechanism

Claims (1)

[Claims] 1. A plurality of γ-ray detectors for measuring an energy spectrum, which are radially arranged with an external radiation source and a test sample sandwiched therebetween, and a rectangular collimator placed in front of the detector, and the measurement sample is rotated and vertically. A mechanism for measuring the γ-rays emitted from the measurement sample by rotating vertically or intermittently in a spiral or intermittent manner by a drive mechanism from the surroundings in each of a plurality of divided regions in the horizontal and vertical directions, and the energy spectrum measurement. The output signal from the γ-ray detector for use is divided into both a signal corresponding to the γ-ray energy generated by the external radiation source and a signal corresponding to the γ-ray energy generated from the specific nuclide in the measurement sample, in the circumferential direction. A mechanism for logging each data for each angle and the output pulse of the γ-ray detector for measuring the energy spectrum are added together, and the γ-ray spectrum for one rotation is obtained. The signal processing unit that stores the data as a data source, and the density distribution focusing on one division in the horizontal direction from the count rate data for each circumferential direction corresponding to the γ-rays generated from the external radiation source, and from the specific nuclide in the measurement sample Obtain the radioactivity distribution of a specific nuclide focusing on one horizontal division from the count rate data for each circumferential direction corresponding to the generated γ-ray energy, and calculate the radioactivity equivalent counts of other nuclides calculated from these distributions. A radioactivity concentration measuring device comprising an arithmetic unit for calculating radioactivity concentrations of other nuclides from the photoelectric peak count rate of the γ-ray spectrum data.