JPH0821879A - Radioactivity measuring method and device - Google Patents

Abstract

PURPOSE:To separately measure the radioactivity for each radioactive nuclide in the radioactivity measurement of radioactive waste contaminated by multiple radioactive nuclides. CONSTITUTION:Phosphors are applied by a phosphor coating device 10 to form a phosphor layer on the surface of a measured object 100. The phosphor layer contains multiple fluorescent materials as constituents. The fluorescence from the phosphor layer passes through an optical band-pass filter 22, then is detected by a detection section 20, and is stored in an image memory 34 as a digital image. The pass band of the optical band-pass filter 22 is changed in steps, and the fluorescence quantity distribution for each step is obtained. A computer 36 has a table indicating the relation between the wavelength band and detection efficiency for each radioactive nuclide. Simultaneous equations are established with the fluorescence quantity distribution measured in each wavelength band and the detection efficiency of each radioactive nuclide, and the radioactivity distribution for each radioactive nuclide is obtained when the computer 36 solves the simultaneous equations. The obtained radioactivity distribution is displayed on a display device 38.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring radioactivity, and more particularly, to a method and apparatus for measuring radioactivity on the surface of an object to be measured using a phosphor.

[0002]

2. Description of the Related Art Conventionally, measurement of radioactive contamination of radioactive waste has been carried out by a survey meter or a surface pollution meter. As a method of measuring the two-dimensional distribution of radioactive contamination, a method of providing a phosphor layer on the front surface of the image intensifier for detection or a method of detecting using an X-ray film or an imaging plate is available. is there. Furthermore, the applicant of the present invention has filed in Japanese Patent Application No. 5-112953.
We proposed a method to detect the radioactivity distribution on the surface of an object to be measured by directly applying the phosphor to the radioactive waste as the object to be measured and detecting the distribution of the fluorescence emitted from the applied phosphor.

[0003]

The radioactive waste is not necessarily contaminated with one type of radioactive nuclide, but may be contaminated with a plurality of radioactive nuclides. However,
When a radioactive waste is contaminated with a plurality of radionuclides, the conventional radioactivity measuring device cannot separately measure the radioactivity of each radionuclide.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to measure the surface radioactivity of radioactive wastes contaminated with a plurality of radionuclides separately for each radionuclide. It is an object of the present invention to provide a performance measuring method and device.

[0005]

In order to solve the above problems, the method for measuring radioactivity according to the present invention comprises a step of forming a phosphor layer formed by mixing a plurality of kinds of fluorescent substances on the surface of an object to be measured. The step of separately measuring the amount of fluorescence contained in a plurality of different wavelength bands for each wavelength band, the amount of fluorescence for each wavelength band, and the radionuclide of each radionuclide when the phosphor layer is used. It is characterized by including the step of obtaining the radioactivity for each radionuclide by performing a simultaneous calculation using the detection efficiency for each wavelength band, and separately determining the radioactivity on the surface of the DUT. And

Further, the radioactivity measuring apparatus according to the present invention has a plurality of wavelength bands different from each other among the fluorescence emitted from the phosphor layer formed by mixing a plurality of types of fluorescent substances formed on the surface of the object to be measured. The amount of fluorescence contained in each, the fluorescence amount measuring means to measure separately by using an optical or electrical bandpass filter, the fluorescence amount for each wavelength band, in the case of using the phosphor layer It has a calculation means for obtaining the radioactivity for each radionuclide by performing a simultaneous calculation using the detection efficiency for each wavelength band of each radionuclide, and has the radioactivity on the surface of the DUT for each radionuclide. It is characterized in that it is obtained separately.

Further, the present invention is characterized in that, in the above-mentioned radioactivity measuring apparatus, the number of types of fluorescent substances forming the phosphor layer and the number of wavelength bands are respectively equal to or more than the number of radionuclides to be measured. To do.

Further, in the radioactivity measuring apparatus according to the present invention, the peak wavelengths of the emission spectra of the fluorescent substances are different from each other, and the respective wavelength bands are bands in the vicinity of the peak wavelengths of the emission spectra of the fluorescent substances. Is characterized by.

Further, according to the present invention, in the above-mentioned radioactivity measuring apparatus, the fluorescence amount measuring means measures the two-dimensional fluorescence amount distribution on the surface of the object to be measured, and the computing means calculates each radionuclide based on the fluorescence amount distribution. The feature is that a two-dimensional radioactivity distribution is obtained for each.

[0010]

In the present invention, a phosphor layer is first formed on the surface of the substance to be measured using a mixture of a plurality of fluorescent substances. This phosphor layer is excited by the radiation emitted from the object to be measured and emits fluorescence. The fluorescence emitted at this time includes fluorescence from each of the fluorescent substances that are components of the phosphor layer.

Next, the amount of fluorescence emitted from the phosphor layer in this manner is measured. At this time, in the present invention, the total amount of fluorescence from the phosphor layer is not measured, but the wavelength band of fluorescence to be measured is limited, and the amount of fluorescence in the limited wavelength band is measured. Moreover, in the present invention,
A plurality of different wavelength bands are set as the wavelength bands to be measured, and the fluorescence amount is measured for each of them. The wavelength band to be measured can be limited by limiting the fluorescence itself with an optical bandpass filter and measuring only the light that has passed through the optical bandpass filter. Alternatively, the output signal of the detector that detects fluorescence is band-limited by an electric bandpass filter,
It can also be performed by extracting only the signal corresponding to the desired fluorescence wavelength band.

Now, the amount of fluorescence measured in this way for each wavelength band will be considered. As described above, the fluorescence emitted by the phosphor layer is a mixture of the fluorescences of the respective fluorescent substances as its components. However, since the emission spectrum of a fluorescent substance is usually different depending on its type, when different wavelength bands are selected, a fluorescent substance having a large amount of fluorescence and a fluorescent substance having a small amount of fluorescence are different in the wavelength band.

The fluorescent substances have different sensitivities to each radionuclide depending on their type. The difference in the sensitivity of the fluorescent substance is the difference in the detection efficiency when detecting the radioactivity based on the fluorescence of the fluorescent substance. However, here
The amount of fluorescence detected per unit of radioactivity of the radionuclide is called the detection efficiency. For example, when radioactivity is measured using a fluorescent substance that is highly sensitive to α rays, the detection efficiency for radionuclides that frequently emit α rays becomes high. When the DUT contains multiple types of radionuclides, it is possible to obtain detection efficiencies above a certain level for all radionuclides by mixing fluorescent substances with high sensitivity into each of these radionuclides. it can.

In the present invention, the amount of fluorescence is measured in a plurality of wavelength bands, but when a phosphor layer made of a plurality of fluorescent substances is used, it is different from the case where a wavelength band in which a certain fluorescent substance has a large amount of fluorescence is selected. The detection efficiency of each radionuclide in each wavelength band differs from that in the case of selecting a wavelength band in which the fluorescence amount of the fluorescent substance is large. For example, the fluorescent substance a has high sensitivity to the nuclide X, and the fluorescent substance b has the nuclide Y.
In the wavelength band A, the detection efficiency of the nuclide X is higher than the detection efficiency of the nuclide Y in the wavelength band A. growing. If the amount of fluorescence from the fluorescent substance b is larger than that from the fluorescent substance a in the wavelength band B different from A, the detection efficiency of the nuclide Y becomes larger than the detection efficiency of the nuclide X in the wavelength band B. .

As described above, the present invention utilizes the wavelength band dependence of the detection efficiency of each radionuclide in the case of using a phosphor layer composed of a plurality of fluorescent substances, to thereby determine the radioactivity of each radionuclide. Ask separately.

That is, the amount of fluorescence measured in a certain wavelength band is the sum of the amount of fluorescence caused by each radionuclide, and the amount of fluorescence caused by each radionuclide depends on the radioactivity of each radionuclide. Since it is multiplied by the detection efficiency of each radionuclide in the wavelength band, the detection efficiency for each wavelength band of each radionuclide for the phosphor layer to be used is obtained by experiments, etc., and the detection efficiency and each wavelength band are determined. Using the amount of fluorescence measured for each, by solving a simultaneous equation by solving the radioactivity of each radionuclide as an unknown number,
Calculate the radioactivity of each radionuclide.

Wavelength bands having different detection efficiencies of the radionuclide contained in the object to be measured are set to the same number or more as the types of the radionuclide, and simultaneous equations are set up using the measurement results in such wavelength bands. Then, the equation can be solved to obtain the radioactivity of each radionuclide.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT A preferred embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the radioactivity measuring apparatus according to the present invention. In FIG.
The radioactivity measuring apparatus includes a phosphor coating device 10 that coats a phosphor to form a phosphor layer on the surface of the object to be measured 100 such as radioactive waste, and fluorescence emitted from the phosphor layer of the object to be measured 100. And a processing unit 3 that performs arithmetic processing based on the output signal of the detection unit 20 to obtain and display a radioactivity distribution for each radionuclide on the surface of the DUT 100.
0.

The phosphor coating device 10 sprays the phosphor onto the object to be measured 100, for example, by spraying. By controlling the moving path and the moving speed of the spray according to the shape of the DUT 100, it is possible to form a phosphor layer having a substantially uniform thickness on the surface of the DUT 100.
When the spray is used in this way, not only when the DUT 100 has a simple shape such as a plane shape or a spherical shape, but also when the measured object 100 has a complicated shape with irregularities, the phosphor layer is formed all over the surface. Can be formed. In this example,
The phosphor coating device 10 coats phosphors containing a plurality of types of fluorescent substances as components.

The detection unit 20 includes an optical bandpass filter 22 that passes only the fluorescence of a predetermined wavelength band among the fluorescence emitted from the phosphor layer on the surface of the DUT 100, and the optical bandpass filter 22. It is composed of a two-dimensional weak light detector 24 which detects the passing fluorescence as a two-dimensional distribution. As the two-dimensional weak light detector 24, for example, a CCD image pickup device can be used.

The processing unit 30 also includes an A / D circuit 32 for converting the output of the two-dimensional weak light detector 24 into a digital signal, and an A / D circuit 32.
An image memory 34 that stores the output from the D circuit 32 as a digital image, a computer 36 that reads out the digital image stored in the image memory 34 and performs arithmetic processing to obtain the radioactivity distribution for each radionuclide, and And a display device 38 for displaying the radioactivity distribution. In addition to the arithmetic processing, the computer 36 also controls the phosphor coating device 10 and the signal reading control of the two-dimensional weak light detector 24.

Next, a method of measuring the radioactivity distribution using the radioactivity measuring apparatus of this embodiment will be described. Here, ²⁴¹ A that mainly emits α-rays as radionuclides on the surface of the object to be measured.
Two types of ⁹⁰ Sr, which mainly emit m and β rays, are distributed, and ZnS (Cu) is highly sensitive to α rays and Y ₂ Si0 ₃ is highly sensitive to β rays.
A case will be described in which the radioactivity of these nuclides is separately measured using a phosphor mixed with (Ce) at a predetermined ratio.

First, the phosphor coating device 10 coats the mixed phosphors to a predetermined thickness. In this embodiment, the phosphor is automatically applied under the control of the computer, but may be manually operated by spraying. In this embodiment, the phosphor is applied by spraying, but the application is not limited to this, and may be applied, for example, by immersing the object to be measured in a liquid phosphor.

When the phosphor layer is formed on the surface of the object to be measured 100, the detecting section 20 detects the fluorescence emitted from the phosphor layer. Here, only the fluorescence that has passed through the optical bandpass filter 22 is detected. In this operation, the fluorescence is detected by the two-dimensional weak light detector 24 as a two-dimensional distribution of the amount of fluorescence.

The two-dimensional distribution of the amount of fluorescence thus detected is sent to the processing unit 30 and processed to obtain the radioactivity distribution. In the processing unit 30, first, the two-dimensional distribution is converted into a digital image signal by the A / D circuit 32. This digital image signal is stored in the image memory 34 as a digital image.

The above operation is performed twice, and the fluorescence amount distribution is obtained for the two wavelength bands. That is, in the present embodiment, the passband of the optical bandpass filter 22 is near 400 nm (hereinafter, this wavelength band is referred to as B1
Measurement), and the second time, the pass band (hereinafter, this wavelength band is referred to as B2) is set to near 580 nm and measurement is performed to obtain the fluorescence amount distribution in each. In addition, near 400 nm and 580 nm
As shown in FIG. 2, the measurement in the near wavelength band is performed.
Since Y ₂ SiO ₃ (Ce) often emits fluorescence with a wavelength near 400 nm, and ZnS (Cu) often emits fluorescence near 580 nm, more fluorescence can be detected by selecting such a band. This is because the accuracy of the measured value can be improved.

When the fluorescence amount distributions in the two wavelength bands are obtained in this way, the computer 36 then performs an arithmetic process using the fluorescence amount distributions.

In this arithmetic processing, the computer 3
Reference numeral 6 sequentially reads the digital images of the fluorescence amount distribution stored in the image memory 34, and performs calculation on the digital image, for example, for each pixel based on the following formula.

[0030]

[Equation 1] D _B1 = η _{Am, B1} × X _Am + η _{Sr, B1} × X _Sr・ ・ ・ (1) D _B2 = η _{Am, B2} × X _Am + η _{Sr, B2} × X _Sr・ ・ ・ (2) D _B1 : Fluorescence amount measured in wavelength band B1 D _B2 : Fluorescence amount measured in wavelength band B2 X _Am : Radioactivity of ²⁴¹ Am X _Sr : Radioactivity of ⁹⁰ Sr η _{Am, B1} : Wavelength band B1 Detection efficiency of ²⁴¹ Am when measured at η _{Am, B2} : Detection efficiency of ²⁴¹ Am when measured at wavelength band B2 η _{Sr, B1} : Detection efficiency of ⁹⁰ Sr when measured at wavelength band B1 η _{Sr, B2} : Detection efficiency of ⁹⁰ Sr when measured in wavelength band B2 Here, η _{Am, B1} , η _{Am, B2} , η _{Sr, B1} and η _{Sr, B2} are obtained in advance by experiments or the like. That is, in this example, the detection efficiencies of various radionuclides in the radioactivity measuring apparatus are obtained in advance. This is, for example, forming a phosphor layer with the same components and thickness as in the actual measurement of radioactivity on a standard sample of each nuclide whose radioactivity is known, and the amount of fluorescence at that time is determined for each wavelength band. It can be obtained by measuring. Figure 3 is thus obtained
The wavelength band dependence characteristics of the detection efficiency of ²⁴¹ Am and ⁹⁰ Sr are shown. For example, since ²⁴¹ Am emits mainly α rays, ZnS (Cu), which is highly sensitive to α rays, emits a large amount of light.
The detection efficiency is highest in the wavelength band near 80 nm, and the detection efficiency is low in the emission band of Y ₂ SiO ₃ (Ce). The detection efficiencies of each radionuclide for each wavelength band thus obtained are stored in the computer 36 as a table. It should be noted that the wavelength band dependence characteristic of the detection efficiency does not have to be obtained for all the emission wavelength bands of the phosphor layer as shown in FIG. 3, but may be obtained for at least the number of wavelength bands required for the arithmetic processing.

Thus, in the above equations (1) and (2), D _B1 and D _B2 are obtained by the detection work,
Further, η _{Am, B1} , η _{Am, B2} , η _{Sr, B1} and η _{Sr, B2} can be taken out from the table in the computer 36 and used. Therefore, the above equations (1) and (2) are expressed as X _Am and X _Sr.
Is a simultaneous equation with unknown. By solving this, the radioactivity of ²⁴¹ Am and ⁹⁰ Sr can be obtained, respectively.

²⁴¹ Am and ⁹⁰ thus obtained
The respective radioactivity distributions of Sr are displayed on the display device 38. In this display, the radioactivity distributions of ²⁴¹ Am and ⁹⁰ Sr are represented by different colors, and the intensity of radioactivity is represented by the brightness of each color, so that each distribution is displayed on one image. To do. With such a display, it is possible to grasp at a glance both the overall radioactivity distribution and the radioactivity distribution for each radionuclide. Needless to say, the radionuclide may be displayed as a separate image.

In this example, wavelength bands near 400 nm and 580 nm were selected as the wavelength bands for measuring the amount of fluorescence, but in principle, if other wavelength bands are selected, the radioactivity of each nuclide can be similarly obtained. However, in order to obtain the radioactivity of each nuclide with high accuracy, two wavelength bands in which the ratio of the detection efficiency of each nuclide greatly differs for each wavelength band and the amount of emission of the phosphor layer is large as in the present embodiment. It is better to choose.

Although the example in which the measured object contains two types of radionuclides is shown here, the method of the present invention can be used even when the number of radionuclides is larger than this. In this case, prepare the same number of fluorescent substances as the types of radionuclides, set the same number of wavelength bands, measure each wavelength band, set up the same equation as above, and solve the simultaneous equations. Good. In order to improve the measurement accuracy, prepare fluorescent substances with high sensitivity for each radionuclide to be measured, mix them to form a phosphor layer, and set the wavelength band near the peak of the emission spectrum of each fluorescent substance. (That is, the fluorescence amount may be measured in the wavelength band in which the amount of emitted light is large).

In this embodiment, the optical bandpass filter 22 is provided in front of the two-dimensional weak light detector 24, and the optical bandpass filter is used to limit the band of fluorescence. The output signal of the weak light detector 24 is band-limited by an electric bandpass filter,
The configuration may be such that only the signal corresponding to the desired fluorescence wavelength band is extracted.

[0036]

As described above, according to the present invention,
Even when the object to be measured contains a plurality of radionuclides, the amount of radioactivity for each radionuclide and its distribution can be determined separately.

Further, by mixing each of the radionuclides to be measured with a highly sensitive fluorescent substance, a sensitivity higher than a certain level can be obtained for all of the radionuclides, and the measurement accuracy is improved.

Further, as a wavelength band for measuring the amount of fluorescence,
By selecting the vicinity of the peak wavelength of the emission spectrum of each fluorescent substance, the amount of fluorescence detected increases, and the measurement accuracy improves.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a radioactivity measuring apparatus according to the present invention.

FIG. 2 Fluorescent substances ZnS (Cu) and Y ₂ Si0 ₃ (C
It is a graph of the light emission distribution characteristic of e).

FIG. 3 is a graph showing the wavelength band dependence characteristics of the detection efficiency of ²⁴¹ Am and ⁹⁰ Sr in the present example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Phosphor coating device 20 Detection part 22 Optical bandpass filter 24 Two-dimensional weak light detector 30 Processing part 32 A / D circuit 34 Image memory 36 Computer 38 Display device 100 Measured object

Claims (5)

[Claims] 1. A step of forming a phosphor layer formed by mixing a plurality of types of fluorescent substances on the surface of an object to be measured, and separately measuring the amount of fluorescence contained in a plurality of different wavelength bands for each wavelength band. And the amount of fluorescence for each wavelength band, by performing a simultaneous calculation using the detection efficiency for each wavelength band of each radionuclide when using the phosphor layer, for each radionuclide And a step of determining the radioactivity of the radioactivity, the radioactivity on the surface of the object to be measured is classified and determined for each radionuclide. 2. The amount of fluorescence contained in each of a plurality of wavelength bands different from each other among fluorescence emitted from a phosphor layer formed by mixing a plurality of types of fluorescent substances formed on the surface of an object to be measured, Fluorescence amount measuring means for separately measuring using an optical or electrical bandpass filter, the fluorescence amount for each wavelength band, and detection for each wavelength band of each radionuclide when the phosphor layer is used Radiation characterized by having a calculation means for calculating the radioactivity for each radionuclide by performing simultaneous calculations using the efficiency and, and calculating the radioactivity on the surface of the DUT separately for each radionuclide. Noh measuring device. 3. The radioactivity measuring apparatus according to claim 2, wherein the number of the wavelength bands is equal to or more than the number of radionuclides to be measured. 4. The radioactivity measuring apparatus according to claim 2 or 3, wherein each of the fluorescent substances has a different peak wavelength of an emission spectrum, and each of the wavelength bands has an emission spectrum of each of the fluorescent substances. A radioactivity measuring device characterized by being in a band near a peak wavelength. 5. The radioactivity measuring device according to claim 2, wherein the fluorescence amount measuring means measures a two-dimensional fluorescence amount distribution on the surface of the object to be measured, and the computing means comprises: A radioactivity measuring device characterized by obtaining a two-dimensional radioactivity distribution for each radionuclide based on this fluorescence amount distribution.