JPH07218640A - Method and apparatus for measuring radioactivity - Google Patents

Abstract

PURPOSE:To measure radioactivity of each radioactive nuclide separately at the time of measuring radioactivity of waste contaminated with a plurality of radioactive nuclides. CONSTITUTION:An object 100 to be measured is coated with a phosphor by means of a coater 10 thus forming a phosphor layer on the surface of the object. Fluorescence emitted from the phosphor layer is detected, in the form of fluorescence distribution, by a detecting section 20 and stored in an image memory 34 in the form of digital image. The thickness of phosphor layer is altered stepwise and the fluorescence distribution is detected at each step and stored in the image memory 34. A computor 36 has a characteristic table representative of the relationship between the thickness of phosphor layer and the detection efficiency and the detection efficiency of each radioactive nuclide at each step of thickness is determined with reference to the table. The computor 36 then determines the radioactive distribution for each nuclide using the fluorescence distribution at each step of thickness and the detection efficiency of each nuclide. The radioactive distribution thus determined is presented on a display 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 containing a radioactive substance 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. In addition, as a method for 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 and a method of detecting using an X-ray film or an imaging plate are 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. Furthermore, among radioactive wastes, many radioactive wastes, such as radioactive wastes produced by dismantling of nuclear reactors, can be expected to be contaminated.

However, in the conventional radioactivity measuring apparatus, the radioactive waste is contaminated with a plurality of radioactive nuclides in this way,
Even if these radionuclides were known in advance, it was not possible to separately measure the radioactivity for each radionuclide.

The present invention has been made to solve the above-mentioned problems, and in the measurement of the surface radioactivity of radioactive wastes contaminated with a plurality of radionuclides, when those radionuclides can be predicted, An object of the present invention is to provide a method and an apparatus for measuring radioactivity capable of measuring radioactivity separately for each radionuclide.

[0006]

In order to achieve the above object, the method for measuring radioactivity according to the present invention forms a phosphor layer on the surface of an object to be measured and measures the fluorescence amount of this phosphor layer. Thus, a method of separately measuring the radioactivity of the measured object for each radionuclide contained in the measured object,
While changing the thickness of the phosphor layer stepwise, the step of measuring the fluorescence amount of the phosphor layer in each step of the thickness of the phosphor layer, and the fluorescence amount measured in each step and in each step And a step of obtaining radioactivity for each radionuclide by performing calculation using the detection efficiency of each radionuclide.

Further, in order to achieve the above object, the radioactivity measuring apparatus according to the present invention is arranged on the surface of the object to be measured in order to form a phosphor layer while gradually increasing the thickness on the surface of the object to be measured. Phosphor layer forming means for forming a phosphor, a fluorescence amount measuring means for measuring the fluorescence amount of the phosphor layer in each step of the thickness of the phosphor layer, and the thickness of the phosphor layer for each radionuclide And a detection efficiency characteristic table that stores the detection efficiency characteristic that represents the relationship between the detection efficiency and the detection efficiency characteristic, using the fluorescence amount measured in each step and the detection efficiency for each radionuclide in each step obtained from the detection efficiency characteristic table. It is characterized by including a calculation means for obtaining radioactivity for each radionuclide by simultaneous calculation.

[0008]

In principle, the amount of fluorescence detected by the detector is determined by the balance between the amount of fluorescence generated in the phosphor layer and the amount of the generated fluorescence that is scattered or absorbed in the phosphor.

Radiation with weak penetrating power, such as α rays and low energy β rays, has a short range. Therefore, even if a thick phosphor layer is formed on the surface of the object to be measured, the phosphor layer near the object to be measured The fluorescence is generated only in the portion (i.e., the portion within the range of the radiation), and the fluorescence is not generated in the portion further away. Therefore, in the case of a radionuclide that emits radiation having such a weak penetrating power, as the thickness of the phosphor layer is increased, the amount of fluorescence generated increases up to a certain thickness, so the detected fluorescence amount also increases. However, when the thickness is larger than that, the influence of scattering and absorption in the phosphor layer increases, so that the amount of detected fluorescence decreases. Here, when the amount of fluorescence detected per unit of radioactivity is defined as the detection efficiency, the detection efficiency of the radionuclide that emits radiation having a weak penetrating power increases as the thickness of the phosphor layer increases from 0,
It exhibits the characteristic that it reaches a peak at a certain thickness and gradually decreases as the thickness increases.

On the other hand, radiation having a strong penetrating power such as γ-rays and high-energy β-rays has a long range, so that it is farther from the phosphor layer than the above-mentioned radiation having a weak penetrating power. Can be reached. Therefore, in the case of such a radionuclide that emits radiation having a strong penetrating power, the change characteristic of the detection efficiency when the thickness of the phosphor layer is increased is
Although the general tendency is the same as that of the radiation having low penetrating power, the peak of the detection efficiency appears in a thicker region than the radiation having weak penetrating power.

On the other hand, the radionuclide differs in the type and energy of radiation generated depending on its type. Therefore, the characteristics of detection efficiency differ for each radionuclide.
The present invention utilizes such a difference in the characteristics of detection efficiency to measure the radioactivity for each radionuclide.

The working phosphor layer is excited by the radiation emitted from the object to be measured and emits fluorescence. The thickness of the phosphor layer is changed stepwise, and the fluorescence amount is measured at each step. By performing a simultaneous equation calculation using the amount of fluorescence measured at each stage and the detection efficiency of each radionuclide at each stage, the radioactivity can be obtained separately for each radionuclide.

[0013]

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 construction of a radioactivity measuring apparatus according to the present invention. In FIG. 1, a radioactivity measuring apparatus includes a phosphor coating device 10 for coating a phosphor on a surface of an object 100 to be measured such as radioactive waste, and a phosphor layer of the object 100 to be measured. Detection unit 20 for detecting fluorescence emitted from the detection unit, and a processing unit 30 for performing 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. It consists of and.

The phosphor coating apparatus 10 sprays a phosphor on the object to be measured 100 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. If the spray is used in this way, the object to be measured 100
However, not only in the case of a simple shape such as a plane shape or a spherical shape, but also in the case of a complicated shape having unevenness, the phosphor layer can be formed on the entire surface thereof.

The detection unit 20 includes an optical lens 22 that collects the fluorescence emitted from the phosphor layer on the surface of the object 100 to be measured,
It is composed of a two-dimensional weak light detector 24 which detects the fluorescence collected by the optical lens 22 as a two-dimensional distribution. As the two-dimensional weak light detector 24, for example, C
A CD image sensor can be used.

Further, the processing section 30 has 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 this arithmetic processing, the computer 36 also controls the coating amount of 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, a case where ZnS (Ag) is used as the phosphor and the object to be measured contains two kinds of radionuclides, ²⁴¹ Am and ⁹⁰ Sr, as an example will be described.

First, the phosphor coating device 10 is used to coat the phosphor ZnS.
(Ag) is applied and the surface density of the measured object 100 is 5 m.
A phosphor layer is formed with a thickness of g / cm ² . In this operation, when the operator inputs a desired thickness (here, 5 mg / cm ² ) into the computer 36, the computer 36
The phosphor coating device 10 controlled by the above controls the phosphor coating while adjusting the spray amount and the moving speed of the spray so as to form the phosphor layer having the desired thickness.

In this embodiment, the phosphor is applied automatically under the control of the computer, but it may be applied manually by operating the spray. In this case, it is advisable to use a known thickness measuring means so that the thickness of the formed phosphor layer can be measured. Also, in this example,
Although the phosphor is applied by spraying, the application is not limited to this, and may be applied, for example, by immersing the object to be measured in a liquid phosphor. In this case as well, the thickness of the phosphor layer may be measured using a known thickness measuring means.

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. 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 section 30 and processed to obtain the radioactivity distribution. In the processing section 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.

In this way, the phosphor layer thickness is 5 mg / c.
When the fluorescence amount distribution at m ² is detected and stored in the image memory 34 as a digital image, next 5 mg / cm ²
Further, a phosphor is applied on the phosphor layer with a surface density of 10 mg / cm ² to form a phosphor layer with a total of 15 mg / cm ² . Then, the fluorescence amount distribution detection operation similar to the above is repeated.

In this way, the phosphor layer thickness is 5 mg /
When the fluorescence amount at the time when the 15 mg / cm ² cm ² of distribution is obtained, performs arithmetic processing by using the fluorescence intensity distribution by computer 36 in turn.

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 each pixel or each pixel group based on the following equation.

D ₅ = X _Am E _Am5 + X _Sr E _Sr5 (1) D ₁₅ = X _Am E _Am15 + X _Sr E _Sr5 (2) D ₅ : Phosphor layer thickness 5 mg / cm ² Fluorescence amount D ₁₅ : Fluorescence amount when the phosphor layer thickness is 15 mg / cm ² X _Am : ²⁴¹ Am radioactivity X _Sr : ⁹⁰ Sr radioactivity E _Am5 : Phosphor layer thickness 5 mg / ²⁴¹ A at cm ²
m detection efficiency E _Am15 : ²⁴¹ when the phosphor layer thickness is 15 mg / cm ²
Detection efficiency of Am E _Sr5 : ⁹⁰ Sr when the phosphor layer thickness is 5 mg / cm ^2.
Detection efficiency E _Sr15 : ⁹⁰ S when the phosphor layer thickness is 15 mg / cm ^2.
Detection efficiency of r Here, D ₅ and D ₁₅ have already been obtained in the above-described detection work. Also, E _Am5 , E _Am15 , E _Sr5 and E
_Sr15 is obtained in advance by experiments or the like. That is, in this example, ZnS (A
g) The detection efficiency when the thickness of the phosphor layer is variously changed is previously obtained by experiments and the characteristics are stored in the computer 36 as a detection efficiency characteristic table. FIG. 2 shows an example of such a detection efficiency characteristic, and shows the characteristic of the change in the detection efficiency of ²⁴¹ Am and ⁹⁰ Sr when the thickness of the ZnS (Ag) phosphor layer is changed. In FIG. 2, ²⁴¹
Since Am mainly emits α-rays having a weak penetrating power, the peak of the detection efficiency appears in the graph where the phosphor layer is relatively thin. On the other hand, in the graph of ⁹⁰ Sr,
The detection efficiency only gradually increases with an increase in the thickness of the phosphor layer, and the detection efficiency peak does not appear in the graph.
This is because the β ray radiated by ⁹⁰ Sr has a strong penetrating power, so that the peak of the detection efficiency is not reached in the thickness range shown in this graph.

As described above, in the above formulas (1) and (2), D ₅ and D ₁₅ are obtained by the detection work,
Further, E _Am5 , E _Am15 , E _Sr5, and E _Sr15 can be taken out from this detection efficiency characteristic table and used. Therefore, the above equations (1) and (2) are simultaneous equations in which X _Am and X _Sr are unknowns. If you solve this, ²⁴¹ Am and ⁹⁰ Sr
The radioactivity of each can be calculated.

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, the thickness of the phosphor layer is 5 mg / cm.
^{Although 2} and 15 mg / cm ² are used, these values have no particular meaning, and in principle, any value may be used. Here, since the tendency of the detection efficiency characteristics of ²⁴¹ Am and ⁹⁰ Sr changes greatly, 5 mg / cm ² and 15 mg / cm ²
I chose a thickness of cm ² . From the viewpoint of measurement accuracy, it is desirable to select a thickness in which the fluorescence of the detection efficiency characteristic of each radionuclide greatly changes.

Here, an example is shown in which the object to be measured contains two types of radionuclides, but the method of the present invention can be used even when the number of radionuclides is larger than this. In this case, for example, assuming that the number of radionuclides is n, the step of the thickness of the phosphor layer is changed to n steps to detect the fluorescence amount distribution, and based on this, an n-element simultaneous equation is set up and solved. , It is possible to obtain the radioactivity distribution for each radionuclide.

Further, in the present embodiment, the phosphor layer was formed by applying the phosphor by the phosphor coating device. However, for example, when the object to be measured has a flat shape, the phosphor film is used. it can. In this case, the thickness of the phosphor layer can be adjusted by increasing or decreasing the number of phosphor films.

[0032]

As described above, according to the present invention,
When the object to be measured contains a plurality of radionuclides and the radionuclides can be predicted, the intensity and distribution of radioactivity for each radionuclide can be determined separately.

[Brief description of drawings]

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

FIG. 2 is a graph showing the characteristics of changes in the detection efficiency of ²⁴¹ Am and ⁹⁰ Sr when the thickness of the ZnS (Ag) phosphor layer is changed.

[Explanation of symbols]

 10 Phosphor coating device 20 Detection unit 22 Optical lens 24 Two-dimensional weak light detector 30 Processing unit 32 A / D circuit 34 Image memory 36 Computer 38 Display device 100 Object to be measured

Claims (4)

[Claims] 1. A method of forming a phosphor layer on the surface of an object to be measured and measuring the amount of fluorescence of this phosphor layer to separate radionuclides contained in the object to be measured to measure radioactivity. There is a stepwise fluorescence amount measurement step of measuring the fluorescence amount of the phosphor layer in each step of the thickness of the phosphor layer while changing the thickness of the phosphor layer stepwise, and each of the steps And a calculation step of obtaining the radioactivity for each radionuclide by performing simultaneous calculation using the fluorescence amount measured in step 1 and the detection efficiency of each radionuclide in each step described above. Measuring method. 2. The method for measuring radioactivity according to claim 1, wherein a radioactivity distribution for each radionuclide is obtained. 3. The radioactivity measuring method according to claim 1, wherein the phosphor layer is formed by applying a phosphor to the object to be measured. Measuring method. 4. A phosphor layer forming means for forming a phosphor on the surface of the object to be measured so as to form the phosphor layer while gradually increasing the thickness on the surface of the object to be measured, and the thickness of the phosphor layer. Detection efficiency characteristic table that stores the detection efficiency characteristics that represent the relationship between the detection efficiency and the thickness of the phosphor layer for each radionuclide, and the fluorescence amount measurement means that measures the fluorescence amount of the phosphor layer at each stage And, using the detection amount for each radionuclide in each stage obtained from the fluorescence amount and the detection efficiency characteristic table measured in each stage, calculating means for calculating the radioactivity for each radionuclide by simultaneous calculation, and A radioactivity measuring device comprising: