JPH0711573B2 - Radioactivity measuring method and apparatus - Google Patents

Description

TECHNICAL FIELD The present invention relates to a radioactivity measuring method for nondestructively measuring radioactivity in a radioactive waste storage container (hereinafter simply referred to as a storage container) such as a drum can and a method for measuring the radioactivity. Γ in a storage container, which is related to the device, and whose density and radioactivity distribution state inside the storage container are unknown.
The present invention relates to a radioactivity measuring method and apparatus suitable for easily quantifying radioactivity of a radionuclide.

[Prior art]

As a conventional method for measuring radioactivity in the case where the density in the storage container is unknown, there are Japanese Patent Laid-Open Nos. 56-115974 and 61-61.
The one disclosed in Japanese Patent Laid-Open No. 107183 is known.

[Problems to be Solved by the Invention]

The radioactivity measuring method disclosed in JP-A-56-115974 is
The weight in the storage container is measured, and the measured value is divided by the volume to be measured to obtain the average density in the storage container, and the radiation in the storage container is measured using the average density. However, this method has a problem that it cannot be applied when the content density changes greatly in the long axis direction of the storage container.

The radioactivity measuring method disclosed in JP-A-61-107183,
The density is obtained by using an external radiation source having a known radioactivity provided at a position facing the radiation detector with the storage container interposed therebetween, and the radioactivity in the storage container is measured using the density. Radioactivity in the storage container is measured for each cross section having a height in the long axis direction of the storage container determined by a collimator installed in front of the radiation detector, and the radioactivity in the entire storage container is quantified. Therefore, in this method, in order to discriminate between the γ-rays emitted from the external radiation source and the γ-rays emitted from the inside of the storage container, two γ-ray measurements with and without the external radiation source per section are performed. Therefore, there is a problem that the measurement time becomes long.

An object of the present invention is to provide a radioactivity measuring method and apparatus for measuring radioactivity in a storage container with a short measuring time, by which the radioactivity in the storage container can be quantified by measuring the radioactivity once per section even if the content density in the storage container is unknown. To provide.

Another object of the present invention is to provide a simple method for measuring radioactivity and a device therefor capable of quantifying radioactivity in a storage container and determining an unknown content density in the storage container. is there.

Further, a third object of the present invention is to provide a radioactivity measuring method and apparatus capable of quantifying radioactivity in a storage container without obtaining the content density in the storage container.

[Means for Solving the Problems]

The feature of the present invention is that the subject (contents in the storage container) is incident through a collimator installed in front of the radiation detector.
Obtaining the energy spectrum of the radiation from, the spectrum index based on the scattered ray intensity and the non-scattered ray intensity of the radiation obtained from the wave height distribution of the energy spectrum, the density related value related to the density of the subject from the spectrum index To determine the radioactivity of the subject from the density-related value and the non-scattered ray intensity.

〔Example〕

Preferred embodiments of the present invention will be described below with reference to the drawings.

1 and 2 show a radiation measuring apparatus according to the first embodiment of the present invention. In this embodiment, a drum can is used as a storage container for the subject, and for the sake of simplicity, a case where only a nuclide that emits γ-rays having a single energy E ₀ is present in the drum can.

In FIGS. 1 and 2, reference numeral 1 denotes a drum can as a subject, which can be rotated in the R direction by a rotating device (not shown) and can be moved in the longitudinal direction L of the drum can by an elevating device (not shown). It is possible. The γ-ray emitted from the inside of the drum 1 is measured by the radiation detector 2. As the radiation detector 2, for example, a NaI (Tl) scintillator can be used. A vertical collimator 4 is provided on the front surface of the radiation detector 2 so that the radiation detector 2 can measure only the γ-rays emitted from the radioactivity in the one cross section 3 of the drum longitudinal direction L. A horizontal collimator 5 is provided on the front surface of the radiation detector 2 to correct the detection efficiency for the position of the γ-ray emitting nuclide in the cross section 3.

The output of the radiation detector 2 is input to the wave height analysis device 6, and in the wave height analysis device 6, the non-scattered line 7 emitted inside the drum can 1 and transmitted through the drum can 1 and the scattered line 8 once scattered and then transmitted inside the drum can 1. Find the energy spectrum due to. The output of the wave height analysis device 6 is input to the photoelectric peak calculation device 9, and the photoelectric peak calculation device 9 obtains the count rates due to the non-scattered line 7 and the scattered line 8. These count rates represent the intensities of the unscattered ray 7 and the scattered ray 8, respectively. The output of the photoelectric peak calculation device 9 is input to the spectrum index calculation device 10, and this spectrum index calculation device 10 obtains the intensity ratio of the scattered rays 8 and the non-scattered rays 7 as an example of the spectrum index. The output of the spectrum index calculation device 10 is input to the density related value calculation device 11, and this density related value calculation device 11 obtains the average density of the contents in the drum can cross section 3 from the spectrum index. The outputs of the photoelectric peak calculator 9 and the density-related value calculator 11 are input to the radioactivity calculator 12, which calculates the radioactivity in the cross section 3 from the count rate and average density of the non-scattered rays 7. In addition to the calculation, the total radioactivity in the drum can is calculated by adding the radioactivity of all sections. The result is output from the output device 13 including a printer or a CRT.

The photoelectric peak calculation device 9, the spectrum index calculation device 10, the density-related value calculation device 11 and the radioactivity calculation device 12 can be configured by a microcomputer.

Next, the operation of the radioactivity measurement value device thus configured will be described.

From the γ-ray intensity measured around the drum can, the γ inside the drum can
In order to determine the radioactivity of the radionuclide, it is necessary to correct the measured γ-ray intensity based on the detection efficiency of the measurement system that depends on the radioactivity distribution and the density distribution of the drum contents. Therefore, before quantifying the radioactivity in a series of drums, the measurement system consisting of the drums 1, the collimators 4 and 5, and the radiation detector 2 is set at least once.

The radiation distribution in the longitudinal direction of the drum can and the density distribution of the contents of the drum can are corrected by setting the opening width of the vertical collimator 4 so that the radiation detector 2 allows only one cross section in the longitudinal direction of the drum can. This is done by measuring the entire cross section of the drum while moving up and down in the axial direction.

On the other hand, the correction in the cross section of the drum can is performed as follows. Regarding the radioactivity distribution, the radiation detector 2
In detection efficiency for non-scattered radiation to be measured has an opening width of 1 ₀ of the horizontal collimator 5 such that substantially uniform, the opening width
_Address this by measuring with 10. Optimum aperture width 1 ₀ is carried out by providing a drum having a density put a known reference material.
As the reference substance, for example, water, cement, air or the like is used. For the contents of density distribution, using the optimum aperture width 1 ₀ as the opening width of the horizontal collimator 5, the value related to the contents of density or content density (hereinafter, referred to these values and the density associated values) and spectral index The radioactivity is corrected so as to be able to be accurately quantified by previously obtaining the relationship with and as a calibration curve.

After setting up the measurement as described above, the radioactivity is quantified as follows for each cross section of the drum.

First, in the wave height analyzer 6, the drum can 1 is rotated once by the rotating device, and the energy spectrum of the γ-ray measured by the radiation detector 2 is obtained. The wave height distribution of this energy spectrum changes depending on the content density in the drum can 1, and the wave height distribution when there are only nuclides that emit only γ-rays of single energy E ₀ in the drum can 1 is shown in FIG. As shown in. Generally, when the density of the contents is high, γ-rays are less likely to pass therethrough and the intensity P of non-scattered rays becomes weaker, but the relative value of the intensity C of scattered rays to the intensity P of non-scattered rays becomes strong. If standardized by the photoelectric peak value of the non-scattered line of energy E ₀ that has reached the radiation detector 2 without being scattered by the contents even once, the wave height distribution becomes a broken line when the contents have a high density ρ _H , and the contents are In the case of low density ρ _L , it becomes a solid line. In FIG. 3, the spectrum due to the non-scattered radiation 7 is a photopeak area of energy E ₀ near the energizing range Delta] E ₁ including an energy E ₀ of the photoelectric peak area,
The sum P of count rates within this energy range ΔE ₁ corresponds to the intensity of the non-scattered ray 7. The energy range ΔE ₁ can be set to the energy resolution of the radiation detector 2, for example.
Therefore, in the photoelectric peak calculation device 9, the energy range ΔE ₁ including the energy E ₀ in the photoelectric peak region is given,
The sum P of count rates within that range is obtained. That is, the intensity of the non-scattered ray 7 is obtained.

Further, in FIG. 3, the region below the photoelectric peak region is caused by the component scattered by the non-scattered ray 7 in the radiation detector 2 and the scattered ray scattered in the drum can cross section 3. Of these, the former component is unique to the radiation detector 2, and the ratio of the component to the non-scattered ray intensity does not change even if the density of the drum can contents changes. Therefore, if the energy range ΔE ₂ is given to a certain energy range below the photoelectric peak region, for example, to the region directly below the photoelectric peak region and directly in contact with this, the sum C of count rates within this energy range ΔE ₂ becomes Corresponding to the strength of. The energy range ΔE ₂ can also be taken as the energy resolution of the radiation detector ₂ , for example. In this way, the photoelectric peak calculation device 9 obtains the intensity C of the scattered radiation 8.

The spectrum index calculation device 10 obtains a ratio C / P between the scattered and non-scattered lines C and P of the photoelectric peak calculation device 9 as a spectrum index.

As shown in FIG. 3, the spectral index C / P changes as shown in FIG. 4 depending on the density in the drum can cross section 3. FIG. 4 is determined by setting a γ-ray source having a constant radioactivity in a drum can, changing the density of contents in the drum can, and determining the relationship with the spectral index at that time. The density-related value calculation device 11 stores the calibration formula shown in FIG. 4 which shows the relationship ρ = f (C / P) (1) between the content density ρ and the spectrum index C / P. FIG. 5 is a diagram showing a processing flow when the density-related value calculation device 11 is configured by a microcomputer. The content density ρ is obtained according to this processing flow. That is, the sum P of the count rates in the energy ranges ΔE ₁ and ΔE ₂
And C are used to obtain the spectrum index C / P, and the density ρ of the content is obtained from the spectrum index C / P and the calibration formula (1). Then, the value ρ is output to the radioactivity computing device 12.

In the radioactivity calculator 12, the output of the photoelectric peak calculator 9 and the density-related value calculator 11 is used to determine the drum can cross section 3
The radioactivity inside is calculated as follows.

Assuming that the radioactivity in the drum can cross section 3 is A, the radioactivity A and the intensity P of the non-scattered radiation generally satisfy the following equation.

P = η A exp (−μ _m ρt) (2) Here, μ _m is the mass absorption coefficient of γ-rays that depends on energy, and γ-rays with energies of about 300 Kev or more hardly depend on the substance. t is an average distance through which the γ-rays penetrate through the drum can cross section 3, and can be set to, for example, the radius r of the drum can 1. Further, η is the detection sensitivity which is one element of the detection efficiency. The detection sensitivity η can be represented by the product of the geometric efficiency and the intrinsic efficiency of the radiation detector mainly depending on the non-scattered ray energy, and is almost constant even if the content density changes. If the opening width of each of the vertical collimator and the horizontal collimator is determined, the geometric efficiency becomes constant. Therefore, in the measurement system having the set collimator aperture width, the nuclide whose radioactivity is known A ₀ is added to the reference substance having the density ρ ₀ , and the intensity P ₀ of the non-scattered ray is obtained. If P ₀ is obtained, the detection sensitivity η can be determined by the equation (3) obtained by modifying the equation (2).

Therefore, if the detection efficiency η is obtained for various nuclides, in other words, the energy E of the non-scattered rays corresponding to each nuclide in a one-to-one manner, the effect formula η = g (E) (4) You can ask for it.

The radioactivity calculation device 12 stores in advance a calibration formula for the detection sensitivity η, a mass absorption coefficient μ _m, and an average transmission distance r. First,
The radioactivity computing device 12 obtains the detection efficiency η from the energy E of the non-scattered radiation input from the photoelectric peak computing device 9 and the calibration formula (4). Then, using the obtained detection efficiency η, the intensity P of the non-scattered radiation from the photoelectric peak computing device 9 and the content density ρ from the density related value computing device 11, A = P exp (μ _m ργ) / η ... The radioactivity A of the cross section 3 is calculated from the equation (5).

In this way, the radioactivity calculator 12 calculates the radioactivity in the drum can cross section 3. Such calculation of the radioactivity is performed for all the cross sections of the drum can 1, and the radioactivity calculating device 12 adds the radioactivity of all the cross sections to calculate the radioactivity in the drum can.

As described above, according to the present embodiment, the measurement conditions such as the opening width of each of the vertical collimator and the horizontal collimator, the detection sensitivity, and the average transmission distance can be set once before quantifying the radioactivity in a series of drums. For example, by measuring the γ-rays emitted from the inside of the drum can once for each cross section, the radioactivity of different drum cans can be quantified one after another, and the measurement time can be shortened. Also, since the density inside the drum can can be obtained without using an external radiation source, the device can be simplified.

The above is an example of the case where a single nuclide is present in the drum can which is the subject, but the present invention is also applicable to the case where a plurality of nuclides are present in the drum can. Hereinafter, the second embodiment of the present invention will be described with reference to FIG. FIG. 6 shows the wave height distribution of γ rays measured by the radiation detector 2 when a plurality of nuclides are present in the drum.
Here, E ₀₁ , E ₀₂ , and E ₀₃ are energies of the non-scattered rays 7 of different nuclides. In the photoelectric peak computing device 9 of this embodiment,
The non-scattered ray intensity P _0i (i = 1,2,
3) and obtain the scattered ray intensity for the highest energy. For the nuclide of E ₀₁ , which has the highest non-scattered ray energy, the non-scattered ray intensity P
₀₁ and scattered radiation intensity C ₀₁ are calculated. The intensities P ₀₂ and P ₀₃ for non-scattered ray energy lower than E ₀₁ are above and above a smooth curve or straight line (shown by a broken line in FIG. 6) that touches the photoelectric peak region on the measured spectrum. It is calculated as the sum of the count values of the area or the area. this is,
This is because the spectrum of scattered rays emitted from the inside of the drum, which is the object of measurement, has a gentle shape, whereas the spectrum of non-scattered rays has a peak shape (for example, normal distribution). . That is, it can be considered that the values of P ₀₂ and P ₀₃ thus obtained almost depend on the non-scattered ray intensity.

Next, the spectrum index calculation device 10 pays attention only to E ₀₁ shown in FIG. 6, which has the largest non-scattered line energy, and finds the intensities of the scattered and non-scattered lines obtained by the photoelectric peak calculation device 9.
The spectral index C / P is _calculated from C ₀₁ and P ₀₁ .

The density-related value calculation device 11 obtains the content density ρ from the spectrum index value according to the processing flow of FIG.

In the radioactivity computing device 12, first, from the intensity P _0i of the non-scattered ray of each nuclide obtained by the photopeak computing device 9 and the equation (4) stored therein, with respect to the energy E _0i of each non-scattered ray. The detection efficiency η _0i is obtained. Next, the obtained detection efficiency η
_0i , the intensity of non-scattered rays P _0i, and the content density ρ obtained by the density-related value arithmetic unit 11, the radioactivity A _0i (i = 1,2,3) of each nuclide is calculated using the equation (5). Ask. At this time, the mass absorption coefficient μ _m was made constant as in the case of a single nuclide, but in order to improve the measurement accuracy, a calibration formula was prepared in advance for the energy E of the non-scattered ray, and It is also possible to set it as the value corresponding to the energy E of.

As described above, according to the present embodiment, even when a plurality of nuclides are present, the radioactivity can be calculated for each nuclide type.

In the above embodiment, the density related value computing device 11 calculates the content density ρ. However, the content density ρ
Sometimes it is not necessary to ask explicitly. Therefore, a third embodiment will be described below in which the radioactivity inside the drum can is quantified using a value related to the content density without explicitly obtaining the content density ρ.

In the equation (2), when the energy of γ-ray is about 300 Kev or more, the mass absorption coefficient μ _m hardly depends on the density of the internal matter, and the average distance t through the cross section of the drum can is constant with the radius r of the drum can. since there, exp (-μ _m ρr)
Has a one-to-one correspondence with the content density ρ. Therefore, considered as content density related _{value, F = exp (μ m ργ} ) / η ... (8). When the content density related value F is expressed in a different form from the formula (2), F = A / P (9). Therefore, radioactivity is known in contents whose density is unknown A ₀
The non-scattered ray intensity P ₀ and the scattered ray intensity C ₀ are obtained by inserting the nuclide. Both intensities from radioactivity A ₀ , spectral indication C ₀ / P ₀
And the content density related value E ₀ are obtained, and the relationship between the spectrum instruction C ₀ / D ₀ and the content density related value F ₀ is obtained. By obtaining this relationship for contents whose values are unknown but with various densities, a calibration formula F = h ( C / P) (10) can be obtained. Further, by modifying the equation (9), A = PF (11) is obtained. Therefore, even for a drum having an unknown radioactivity, the amount of radioactivity can be quantified by the non-scattered ray intensity P and the content density related value F obtained from the spectral index C / P. The present embodiment has excellent advantages that it is not necessary to previously obtain the detection sensitivity η for each nuclide, and that it is not necessary to measure the content density when making the calibration formula (10).

The first and second embodiments are examples in which the non-scattered ray intensity P is within the photoelectric peak region and the scattered ray intensity C is below the photoelectric peak region. The method of taking is not limited to this. Below, scattered ray intensity C
And another embodiment showing another method of taking the non-scattered ray intensity P (fourth example).
Example) will be described with reference to FIG. In this embodiment, the drum can as the subject has a single energy E ₀ of γ
Assuming that there is a nuclide that emits a ray,
FIG. 8 shows the wave height distribution of γ rays obtained by the wave height analyzer 6 from the output of the radiation detector 2. Where line m is
It is a straight line connecting the start point and the end point of the photoelectric peak region on the wave height distribution curve. In this case, if the energy range ΔE including the energy E ₀ is given, the area of the peak region of the straight line m or more within the range of ΔE is the non-scattered ray intensity P, and the other area is the scattered ray intensity C. Become. The method of taking ΔE is arbitrary.

In the above description, a single nuclide has been described, but in the above embodiment as well, by introducing the concept shown in the second embodiment, it can be applied to the case where a plurality of nuclides are present in the drum can. .

In the fourth embodiment, in the photoelectric peak calculation device 9 and the density related value calculation device 11, the intensity P of the non-scattered line and the spectrum index C / P are obtained from the regions thus divided by the straight line m. It is a thing.

Further, in the first to fourth embodiments, the ratio C / P of the scattered ray intensity C and the non-scattered ray intensity P is used as the spectral index, but the method of taking the spectral index is not limited to this. As described in the explanation of FIG. 3, generally, when the density of the contents becomes high, γ rays are easily scattered, the intensity C of scattered rays becomes strong, and the intensity P of non-scattered rays becomes weak.
Energy reaching the radiation detector 2 without being scattered by the contents
If standardized by the non-scattered ray intensity E _{0, that is,} the non-scattered ray intensity P, the scattered ray intensity C becomes relatively weak when the content has a high density, and conversely when the content has a low density. Become stronger. From the above, it is necessary that the ratio C / P or P / C of both the scattered ray intensity C and the non-scattered ray intensity P is a constituent item as a spectral index for obtaining the density-related value. Shows. Therefore, as a spectral index, C / P or P
You can use any / C function.

〔The invention's effect〕

As apparent from the above, according to the present invention, even if the content density in the storage container is unknown, the radioactivity with a short measurement time that can quantify the radioactivity in the storage container with one radioactivity measurement per cross section A measurement method and an apparatus therefor can be provided.

Further, it is possible to provide a simple radioactivity measuring method and device having a device configuration capable of quantifying the radioactivity in the storage container and determining the unknown content density in the storage container.

Furthermore, it is possible to provide a radioactivity measuring method and apparatus capable of quantifying radioactivity in a storage container without obtaining the content density in the storage container.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing a radioactivity measuring apparatus according to a preferred embodiment of the present invention, FIG. 2 is an elevational view of a part thereof, and FIG. 3 is a radiation detector in the radioactivity measuring apparatus. Figure showing how to take wave height distribution and non-scattered ray intensity and scattered ray intensity,
4 and FIG. 4 are diagrams showing the density-related values for the spectral index when the density-related values are the average density of the subject, and FIG. 5 is the density-related value calculation device for calculating the average density from the calibration curve shown in FIG. FIG. 6 is a view showing a flow chart showing a procedure for carrying out the procedure, FIG. 6 is a view showing how to obtain non-scattered ray intensity and scattered ray intensity in a radioactivity measuring apparatus according to another embodiment of the present invention,
FIG. 7 is a diagram showing the density-related value for the spectral index when the density-related value is the ratio of radioactivity and non-scattering intensity.
The figure is a diagram showing how to obtain non-scattered ray intensity and scattered ray intensity in a radioactivity measuring apparatus according to still another embodiment of the present invention. 1 ... drum can, 2 ... radiation detector, 3 ... drum can longitudinal direction cross section, 4,5 ... collimator, 6 ... wave height analyzer, 7 ... non-scattered radiation, 8 ... scattered radiation, 9 ... Photoelectric peak calculator, 10 ... Spectral index calculator, 11 ...
Density related value calculator, 12 …… Radioactivity calculator, 13 …… Output device.

Claims (9)

[Claims] 1. A scattered radiation intensity and a non-scattered radiation intensity of the radiation obtained from a wave height distribution of the energy spectrum obtained from a subject incident from a collimator installed in front of the radiation detector. Radiation characterized by obtaining a spectrum index based on the spectrum index to determine the density of the object or a density-related value related to the density from the spectral index, and quantifying the radioactivity of the object from the density-related value and the non-scattered ray intensity Noh measurement method. 2. The intensity ratio of scattered rays and non-scattered rays of the radiation obtained from the energy spectrum of the radiation from the subject incident from a collimator installed in front of the radiation detector and obtained from the wave height distribution of the energy spectrum. To obtain a spectral index defined by a function of, to obtain a density-related value related to the density or density of the object from the relationship between the spectral index and the density of the object or density-related value related to the density obtained in advance Therefore, the radioactivity measuring method characterized in that the radioactivity of one cross section of the subject is quantified by one measurement. 3. An intensity ratio of scattered radiation and non-scattered radiation of the radiation obtained from an energy spectrum of radiation from a subject incident from a collimator installed in front of the radiation detector and obtained from a wave height distribution of the energy spectrum. Obtaining a spectral index defined by the function of, without determining the density of the analyte from the relationship between the spectral index obtained in advance and the density-related value related to the density of the analyte, the radioactivity of the analyte is quantified. A method for measuring radioactivity, which comprises: 4. The sum of count rates within a predetermined energy range below the light intensity peak region is set as scattered ray intensity, and the count rate within a predetermined energy range within the photoelectric peak region is set as non-scattered ray intensity. The method for measuring radioactivity according to claim 1. 5. A photoelectric peak area is divided by a straight line connecting the start point and the end point of the photoelectric peak area on the wave height distribution curve, and the count rate excluding the photoelectric peak area above the straight line within a predetermined energy range is calculated. The radioactivity measuring method according to claim 1, wherein the sum is the scattered ray intensity, and the sum of the count rates of the photoelectric peak regions above the straight line is the non-scattered ray intensity. 6. The radioactivity measuring method according to claim 1, wherein a subject is a drum having a drive mechanism for rotating the drum and moving it vertically. 7. A radiation detector for detecting radiation emitted from a subject and a collimator installed in front of the radiation detector, means for obtaining an energy spectrum of the radiation, and a pulse height distribution of the energy spectrum. Means for obtaining the scattered ray intensity and non-scattered ray intensity of the radiation from, means for obtaining a spectral index determined from the scattered ray intensity and the non-scattered ray intensity, and a density related value related to the density of the subject from the spectral index A radioactivity measuring apparatus comprising: a means for obtaining the radioactivity of the subject based on the density-related value and the non-scattered ray intensity. 8. The radioactivity measuring apparatus according to claim 7, wherein the subject is a drum having a drive mechanism for rotating the drum and moving it vertically. 9. A radioactivity measuring device for a drum can, comprising a drum can driving mechanism having only a mechanism for rotating the drum can and moving it up and down, and the radioactivity measuring device according to claim 7.