JP5810066B2 - Method for estimating the amount of radioactive material - Google Patents

Description

  The present invention relates to a method for estimating the amount of radioactive material.

The radioactive substance can be generated by, for example, fission. Since radioactive materials emit radiation that is harmful to the human body, it is necessary to store them strictly, for example, in a container in order to suppress harm to the human body.
Therefore, measuring the radiation dose of a subject placed in a container or the like containing a radioactive substance is important in terms of managing harmful substances.

  Conventionally, the measurement of the amount of radiation emitted from a subject containing a radioactive substance has been performed by, for example, a radiation measuring instrument.

  Specific examples of the measurement include, for example, a string-shaped radiation detection unit that spirally wraps around an object including a radioactive substance and detects incident radiation and converts it into a signal, and is connected to both ends of the detection unit. Using a radiation measuring instrument having a signal processing unit for calculating and processing a signal generated in the detection unit, a signal generated by radiation incident on a part of the radiation detection unit is sent to both ends of the string-like detection unit, There is known a technique in which a signal processing unit calculates a radiation emission location and intensity on a subject surface based on a time difference and intensity of signals received at both ends (Patent Document 1).

  According to the measurement as described above, the radiation dose on the surface of the subject can be grasped. However, it is difficult to grasp the amount of radioactive substance contained in the subject only by measuring the radiation dose on the surface of the subject as described above.

  In contrast, the amount of radioactive material contained in the specimen is obtained by taking out a part of the specimen, eluting the radioactive substance from the specimen with water, etc., and measuring the amount of the eluted radioactive substance with a radiation measuring instrument. The method of grasping is known.

  However, the method for determining the amount of radioactive substance in the subject in this way has a problem that the amount of radioactive substance in the subject cannot be determined unless at least a part of the subject is taken out.

Japanese Patent Laid-Open No. 09-080156

  Therefore, there is a demand for a method for estimating the amount of radioactive material that can grasp the amount of radioactive material contained in the subject without taking out a part of the subject.

  An object of the present invention is to provide a method for estimating the amount of radioactive substance that can grasp the amount of radioactive substance contained in a subject without taking out a part of the subject in view of the above-described problems and demands.

上記式において、ｄは、前記γ線放射単位体の底面の直径［ｃｍ］を示し、η _ｊ は、前記γ線エネルギーの放出率を示し、ρ _ｍ は、被検体の密度［ｇ／ｃｍ ^３ ］を示し、ρ _ａ は、空気の密度［ｇ／ｃｍ ^３ ］を示し、ｒは、被検体とγ線検出部との距離［ｃｍ］を示し、ｈ _ｊ は、放射性物質中における放射性元素の存在割合であり、ｉは、γ線検出部に最も近いγ線放射単位体から遠ざかる方向へ向けて並ぶγ線放射単位体の順序を示し、Ｂ _ｉ はｉ番目のγ線放射単位体に対するビルドアップ係数を示し、Ｐmは、被検体の減衰係数［ｃｍ ^２ ／ｇ］を示し、Ｐaは、空気の減衰係数［ｃｍ ^２ ／ｇ］を示し、ｅｘｐは、底がｅの指数関数を示し、Ｑは、前記算出用係数［Ｂｑ／ｋｇ］を示し、
前記γ線放射単位体の並び方向の延長線上に前記γ線検出部の検出位置を配することにより、γ線を測定する、ことを特徴とする。
In order to solve the above-mentioned problems, the method for estimating the amount of radioactive substance according to the present invention measures a γ dose at a part of the surface of a subject containing a radioactive substance with a radiation measuring instrument equipped with a γ ray detector, and the γ dose. A method for estimating the amount of radioactive substance that estimates the amount of radioactive substance contained in the subject from the measured value of:
An assumption step that assumes that a plurality of γ-ray emitting units that emit γ-rays are present in the subject in a straight line so as to penetrate the subject from a part of the surface of the subject;
In order to determine the amount of γ-rays emitted from each of the γ-ray emission unit bodies and reaching the detection position of the γ-ray detection unit, the concentration of the radioactive substance contained in the subject is set as a calculation coefficient, and the subject The γ-rays emitted from each γ-ray radiation unit are calculated based on the assumption that the concentration of the radioactive material contained in the γ-ray radiation unit is the same as the concentration of the radioactive substance contained in each γ-ray radiation unit. Based on the fact that the γ-rays radiated from the respective γ-ray radiation units are attenuated according to Lambert-Beer's law and reach the detection positions, Each γ-ray dose determination step that determines the amount of γ-rays that reaches the light source using the calculation coefficient;
Performing a calculation step of calculating the calculation coefficient based on the assumption that the total amount of γ rays determined in each γ dose determination step and the measured value of the γ dose are the same ;
In the assumption step, D equal cylindrical γ-ray radiation units are continuously arranged along the cylindrical axis direction of the γ-ray radiation unit from one end to the other end of the subject, and adjacent γ-ray radiation units. Assuming that the γ-ray radiation unit exists in the subject so that the bottom surfaces of the bodies overlap each other, and the length of the γ-ray radiation unit in the cylinder axis direction is 1 cm,
In each γ dose determination step, the radioactive substance is set to a specific radioactive element, and the emission rate of γ-ray energy of each γ ray emitted from the specific radioactive element is set.
In the calculation step, the total amount of γ rays determined in each γ dose determination step is represented by the following equation:

In the above formula, d represents the diameter [cm] of the bottom surface of the γ-ray radiation unit body, η _j represents the emission rate of the γ-ray energy, and ρ _m represents the density of the subject [g / cm ³ Ρ _a represents the density of air [g / cm ³ ], r represents the distance [cm] between the subject and the γ-ray detector, and h _j represents the radioactive element in the radioactive substance. an existence ratio, i denotes the order of γ-radiation unit body arranged toward a direction away from the nearest γ-radiation unit body γ-ray detection unit, B _i build for the i-th γ-radiation unit body Pm represents the attenuation coefficient [cm ² / g] of the subject , Pa represents the attenuation coefficient [cm ² / g] of air , exp represents the exponential function with the bottom being e, Q represents the calculation coefficient [Bq / kg],
The γ-ray is measured by arranging the detection position of the γ-ray detection unit on an extension line in the arrangement direction of the γ-ray radiation units .

  In the method for estimating the amount of radioactive material having the above-described configuration, by measuring the γ dose at a part of the surface of the subject, and performing the assumption step, the γ dose determination step, and the calculation step. The amount of radioactive substance contained in the subject can be grasped without taking out a part of the subject.

In the estimation method of radioactive materials of the present invention, the subject said γ-ray detector of γ-rays as γ-rays emitted from other than the partially suppress the reaching the detection position of the γ-ray detector of the surface of the It is preferable to cover with an attenuating material and measure γ rays.

In the method for estimating the amount of radioactive substance of the present invention, the analyte is adsorbed to contain a radioactive substance by passing water in which the radioactive substance is dissolved into an adsorption tower packed with an adsorbent. It is preferably a mixture of adsorbent and water in the tower .
Moreover, in the estimation method of the radioactive substance amount of this invention, it is preferable that the said radioactive substance is radioactive cesium or a radioactive iodine.

  As described above, the method for estimating the amount of radioactive substance of the present invention has an effect that the amount of radioactive substance contained in the subject can be grasped without taking out a part of the subject.

The figure which represented typically the mode of the measurement of a gamma dose, and the cross section of a subject and a gamma ray detection part. The graph showing the estimation result and measurement result of a radioactive substance amount.

  Hereinafter, an embodiment of a method for estimating a radioactive substance amount according to the present invention will be described with reference to the drawings. FIG. 1 is a schematic diagram schematically showing a state of measurement of γ dose and a γ-ray radiation unit body in the method for estimating the amount of radioactive material of the present embodiment.

In the method for estimating the amount of radioactive substance according to the present embodiment, the γ dose at a part of the surface of the subject 1 containing the radioactive substance is measured by a radiation measuring instrument equipped with a γ-ray detection unit 3, and the measured value of the γ dose is used to determine the above-mentioned value. A method for estimating the amount of radioactive material for estimating the amount of radioactive material contained in a subject 1,
Assuming step that a plurality of γ-ray emitting units 2 that emit γ-rays are present in the subject 1 in a straight line so as to penetrate the subject from a part of the surface of the subject 1;
In order to determine the amount of γ-rays emitted from each of the γ-ray emission unit bodies 2 and reaching the detection position 3a of the γ-ray detection unit 3, the concentration of the radioactive substance contained in the subject 1 is set as a calculation coefficient. And the concentration of the radioactive substance contained in the subject 1 and the concentration of the radioactive substance contained in each γ-ray radiation unit body 2 are radiated from each γ-ray radiation unit body 2 based on the assumption that the same. Γ-rays are determined using the coefficient for calculation, and the γ-rays radiated from the respective γ-ray emission unit bodies 2 are attenuated according to Lambert-Beer's law and reach the detection position 3a. Each γ-ray dose determination step for determining the amount of γ-rays reaching the detection position 3a from the γ-ray emission unit body 2 using the calculation coefficient;
The calculation step of calculating the calculation coefficient based on the assumption that the total amount of γ rays determined in each of the γ dose determination steps and the measured value of the γ dose are the same.

  In the method for estimating the amount of radioactive substance, as shown in FIG. 1, the γ dose at a part of the surface of the subject 1 containing the radioactive substance is measured by a radiation measuring instrument equipped with a γ-ray detector 3. Specifically, as shown in FIG. 1, for example, the γ-ray amount is measured by detecting γ-rays at the detection position 3 a located closest to the subject 3 in the γ-ray detection unit 3.

The radioactive substance is not particularly limited as long as it emits γ-rays. Specific examples of the radioactive substance include radioactive cesium such as cesium 134 ( ¹³⁴ Cs) and cesium 137 ( ¹³⁷ Cs), Alternatively, radioactive iodine such as iodine 131 ( ¹³¹ I) and the like can be given. These radioactive materials can be generated, for example, by fission in a nuclear power plant.
The type of radioactive substance can be known in advance by a radiation measuring instrument. Specifically, for example, it can be known in advance by analyzing the energy of the detected γ-ray.

The subject 1 is not particularly limited as long as it includes at least the radioactive substance as described above.
Specifically, the analyte 1 is, for example, an adsorption tower containing a radioactive substance by passing water in which a radioactive substance such as cesium 137 is dissolved in an adsorption tower filled with an adsorbent. Examples thereof include a mixture of an adsorbent and water.

As the adsorbent, for example, zeolite, an ion exchange resin that adsorbs cations, or a ferrocyanide metal compound can be employed.
Note that the subject 1 is generally known in advance in terms of size and material constituting the subject 1.

  A general thing can be used as said radiation measuring instrument. Specifically, as the radiation measuring instrument, for example, a detector equipped with a Geiger-Muller counter (GM tube) as the γ-ray detector 3 and a NaI (TI) scintillation detector equipped with a scintillator as the γ-ray detector 3 Or a germanium semiconductor detector provided with a semiconductor as the γ-ray detector 3 can be used.

  The radiation measuring device is configured to identify γ-ray energy specific to each radioactive substance and measure the amount of γ-ray energy.

The radiation measuring instrument usually further includes a measurement value display unit for displaying the amount of γ rays detected by the γ ray detection unit 3 as a numerical value.
The measurement value display unit is connected to the γ-ray detection unit 3 and is configured to convert the amount of γ-rays detected by the γ-ray detection unit 3 into a numerical value and display it. Specific examples of the unit of the converted numerical value include μSv / h (microsievert / hour).

In the above-described measurement of γ-rays, the amount of radioactive substance can be estimated more accurately, and as shown in FIG. It is preferable to measure γ rays by arranging the detection position 3a.
In order to estimate the amount of radioactive substance more accurately, the gamma-ray radiation units 2 arranged in a straight line using a radiation measuring device having the γ-ray detection unit 3 in which the detection direction of γ-rays is fixed can be obtained. It is preferable to arrange the γ-ray detection unit 3 so as to exist ahead of the γ-ray detection direction of the γ-ray detection unit 3 and measure γ-rays.

Further, in the above-described measurement of γ-rays, the γ-ray detection unit 3 is configured to suppress γ-rays radiated from a part other than the surface of the subject 1 from reaching the detection position 3 a of the γ-ray detection unit 3. It is preferable to measure γ rays in a state where is covered with the γ ray attenuating material 4.
By measuring γ-rays with the γ-ray detector 3 covered with the γ-ray attenuating material 4 as described above, the influence of γ-rays from directions other than the incident direction of the γ-rays to be measured can be suppressed. The γ-rays emitted from a part of the surface of the subject 1 can be detected more reliably at the detection position 3a of the γ-ray detection unit 3, so that the amount of radioactive substance can be estimated more accurately. There is.

The γ-ray attenuating material 4 is a material that can attenuate γ-rays. The γ-ray attenuating material 4 usually has a thickness that attenuates γ-rays. As the γ-ray attenuating material 4, lead, iron, concrete or the like is usually used.
The γ-ray attenuating material 4 is not particularly limited as long as it attenuates γ-rays. The γ-ray attenuating material 4 may be, for example, a liquid or powdery material, or may be one in which these are put in a container or the like so as to have a thickness that attenuates γ-rays. .

  In the measurement of γ-rays, the amount and amount of radioactive substance can be estimated more accurately, so that the shape and size of a part of the surface of the subject 1 and the detection position 3a of the γ-ray detector 3 facing the part of the surface are measured. It is preferable that the shape and size in are the same.

In the measurement of γ-rays, a part of the surface of the subject 1 and the γ-ray detection unit 3 of the radiation measuring device may or may not be separated. In terms of preventing attenuation of γ rays in the air, it is preferable that a part of the surface of the subject 1 is not separated from the γ ray detector 3 of the radiation measuring instrument.
In the measurement of γ rays, the distance between a part of the surface of the subject 1 and the γ ray detector 3 of the radiation measuring instrument is 10 cm or less in that attenuation of γ rays in the air can be further suppressed. Is preferred.

  In the assumption step, as shown in FIG. 1, a plurality of γ-ray emission units 2 that emit γ-rays are linearly arranged so as to penetrate the subject from a part of the surface of the subject 1. Assume that the subject 1 exists.

  In the assumption step, it is assumed that the amount of radioactive material can be estimated more accurately, and that the γ-ray emission units 2 are in the same shape and the same size as shown in FIG. It is preferable.

The number of the γ-ray radiation unit bodies 2 is not particularly limited, and is appropriately set depending on the size and shape of the subject 1.
The shape of the γ-ray radiation unit body 2 is not particularly limited, and specific examples of the shape include a rectangular parallelepiped shape and a cylindrical shape.
The size of the γ-ray radiation unit body 2 is not particularly limited, and is appropriately set depending on the size and shape of the subject 1.
It is preferable that the γ-ray emission unit bodies 2 are all set to have the same volume.

The γ-ray radiation unit 2 is a cylindrical body having a bottom surface with a diameter of 1 to 5 cm and a cylinder axis with a length of 1 to 5 cm in that the estimation accuracy of the amount of radioactive material can be improved. Is preferably set.
It is preferable that the cylindrical γ-ray radiation unit bodies 2 are continuously arranged along the cylinder axis direction and are assumed to exist in the subject 1 such that the bottom surfaces of adjacent γ-ray radiation unit bodies 2 overlap each other. .

  In each γ-ray dose determination step, first, the γ-ray radiation unit body 2 is included in the subject 1 in order to determine the amount of γ-rays emitted from each of the γ-ray emission unit bodies 2 and reaching the detection position 3a of the γ-ray detector 3. Based on the assumption that the concentration of the radioactive substance is set as a coefficient for calculation, and the concentration of the radioactive substance contained in the subject 1 is the same as the concentration of the radioactive substance contained in each γ-ray radiation unit 2 The γ-rays radiated from the respective γ-ray emission unit bodies 2 are determined using the calculation coefficient.

  In each γ dose determination step, for example, the concentration of the radioactive substance contained in the subject 1 is set as a calculation coefficient (Q), and the calculation coefficient (Q) and the γ-ray radiation unit 2 By multiplying the volume or weight (M) by the γ-ray emission rate (η), the γ dose emitted from each γ-ray radiation unit body 2 can be determined using the calculation coefficient. The γ dose determined using the calculation coefficient can be expressed by a mathematical formula.

Specifically, in each γ dose determination step, the γ dose radiated from each γ-ray radiation unit body 2 can be expressed as in the following formula (1).
Coefficient for calculation Q × M × η (1)

  In each of the γ-ray dose determination steps, the γ-rays radiated from the γ-ray radiation units 2 are then attenuated according to Lambert-Beer's law and reach the detection position 3a. The amount of γ rays reaching the detection position 3a from the line radiation unit 2 is determined using the calculation coefficient.

In general, when I ₀ radiations are incident on a material with a thickness x from a certain direction, the number I of radiations that pass through the material with a thickness x is expressed by the following formula (Lambert-Beer's law): 2).

I = B I ₀ e ^-Px (2)
(In the formula, e is the number of Napiers, P is an attenuation coefficient, and B is a correction coefficient related to a scattering effect of γ rays called a build-up coefficient.)

In each γ dose determination step, I ₀ in the above equation (2) is the direction of the detection position 3a in the γ dose emitted from each γ-ray radiation unit body 2 expressed by the above equation (1). Corresponds to what is emitted.

  In the measurement of γ-rays, when the subject and the γ-ray detection unit 3 are separated from each other, the γ-rays are also attenuated by air while reaching the detector. The amount of γ rays reaching the γ ray detector 3 is determined using the calculation coefficient in consideration of not only the attenuation of the rays but also the attenuation of γ rays by air.

  The respective γ dose determination steps will be described in more detail with specific examples.

In each γ-ray dose determination step, when the main component of the radioactive substance contained in the subject is known, the radioactive substance is identified as a specific element in that the amount of radioactive substance contained in the subject 1 can be estimated more reliably. It is preferable to set to.
Further, in each of the γ dose determination steps, if the main component of the radioactive substance is known, the γ-ray energy of the γ-ray emitted from the specific element as the radioactive substance and its It is preferable to set the release rate.

Specifically, in each of the γ dose determination steps, for example, radioactive materials can be set to cesium 134 and cesium 137.
In each γ-ray dose determination step, for example, the γ-ray energy of cesium 134 and cesium 137 and the emission rate of the corresponding γ-ray energy can be set as follows.

Cesium 134 (set 3 types)
Gamma ray energy release rate A. 0.569 MeV (Mega Electron Volt) 0.154
B. 0.605 MeV 0.976
C. 0.796MeV 0.855
Cesium 137
γ-ray energy release rate D. 0.662 MeV 0.851

More specifically, in each of the γ dose determination steps, for example, when the radioactive substance is set to cesium 134 and cesium 137, the above-described three kinds of cesium 134 and the release rate of γ-ray energy of each of cesium 137, The calculation coefficient Q, the volume or weight (M) of the γ-ray emission unit 2, and the abundance ratio of each cesium (h ₁ : abundance ratio of cesium 134, h ₂ : abundance ratio of cesium 137) By multiplying, the amount of each γ-ray energy radiated from each γ-ray radiation unit 2 is determined using a calculation coefficient and expressed by a mathematical expression, and each γ-ray energy expressed using a calculation coefficient By summing the amounts, the γ dose emitted from each γ-ray radiation unit body 2 can be determined using a calculation coefficient and expressed by a mathematical expression.

For example, in each γ-ray dose determination step, the amount of γ-ray energy radiated from each γ-ray radiation unit body 2 is calculated according to the following formulas (3) to (6) that are more specific to the formula (1). The γ-ray dose emitted from each γ-ray radiation unit body 2 is calculated using the calculation coefficient, by using the calculation coefficient, and by summing the respective γ-ray energy amounts expressed using the calculation coefficient. Can be represented.
Cesium 134 (γ-ray energy 0.569 MeV, emission rate η _A 0.154)
A. Coefficient for calculation Q × M × h ₁ × η _A (3)
Cesium 134 (γ-ray energy 0.605 MeV, emission rate η _B 0.976)
B. Coefficient for calculation Q × M × h ₁ × η _B (4)
Cesium 134 (γ-ray energy 0.796 MeV, emission rate η _C 0.855)
C. Coefficient for calculation Q × M × h ₁ × η _C (5)
Cesium 137 (γ-ray energy 0.662 MeV, emission rate η _D 0.851)
D. Coefficient for calculation Q × M × h ₂ × η _D (6)

In the above formulas (3) to (6), the abundance ratios (h ₁ , h ₂ ) of cesium can be set by analyzing the energy of γ rays measured in advance by a radiation measuring instrument.

  Furthermore, in each said gamma dose determination process, it attenuate | damps according to Lambert-Beer's law, for example based on each gamma ray energy amount in each gamma ray radiation unit body 2 expressed like Formula (3)-(6). Then, each γ-ray energy amount reaching the γ-ray detection unit 3 is expressed using a coefficient for calculation, and these γ-ray energy amounts are added to each γ-ray emission unit 2 to the γ-ray detection unit 3. The γ-ray dose after attenuation that arrives can be expressed by a mathematical formula including a coefficient for calculation.

  In the above equation (2) according to the Lambert-Beer law, as can be seen from the equation (2), at least an attenuation coefficient, a distance through which γ rays pass, and a buildup coefficient are used.

The attenuation coefficient and the build-up coefficient are generally known for each material or for each γ-ray energy.
Specifically, for example, the attenuation coefficient of water, the attenuation coefficient of concrete, and the attenuation coefficient of air in the γ-ray energy of cesium 134 and cesium 137 are as shown in Table 1 below.
In Table 1, the value of each attenuation coefficient is expressed in units of cm ² / g. In order to express this value in units of cm ⁻¹ , the value of each attenuation coefficient expressed in units of cm ² / g may be multiplied by the density of each substance in units of g / cm ³ .
The value of the build-up coefficient (B) can be approximated as B = 1 when Px <1 in Equation (2), and can be approximated as B = Px when Px> 1.

The attenuation coefficient can be appropriately set to an appropriate numerical value based on a known attenuation coefficient according to the material constituting the subject 1.
Specifically, for example, since the main components of zeolite are silica and alumina, the same numerical value as the attenuation coefficient of concrete can be adopted as the attenuation coefficient of zeolite.
Further, for example, when the specimen 1 is a mixture of water and zeolite as an adsorbent, the attenuation coefficient of the mixture is the attenuation coefficient of water and the attenuation coefficient of concrete. An average value can be adopted.

In each γ-ray dose determination step, the distance through which γ-rays pass in the Lambert-Beer law is preferably the distance between the center point of each γ-ray radiation unit 2 and the detection position of the detection unit 3. .
The center point of each γ-ray radiation unit body 2 is the center of gravity when the γ-ray radiation unit bodies 2 are homogeneous.

Specifically, as shown in FIG. 1, cylinder-shaped D equal-γ-radiation unit body 2, from one end of the subject 1 to the other end, the cylinder axis Direction of γ-radiation unit body 2 The γ-ray radiation unit bodies 2 are present in the subject 1 so that the bottom surfaces of the adjacent γ-ray radiation unit bodies 2 overlap each other, and the length of the γ-ray radiation unit bodies 2 in the cylinder axis direction is 1 cm. Assuming that the subject 1 and the γ-ray detection unit 3 are separated by rcm, the distance through which the γ-rays emitted from the first γ-ray radiation unit 2 closest to the detection unit pass is It can be expressed as r + 0.5 cm.

In each γ dose determination step, the density value of the subject 1 or the air density value can be used as necessary. The density of the subject 1 can be appropriately set to an appropriate numerical value based on the density of the known material according to the material constituting the subject 1.
Specifically, for example, when the subject 1 is in a state where water and zeolite are mixed, and the volume ratio of water and zeolite in the subject 1 is substantially equal, the density of the subject 1 is Can set the average value of the density of water and the density of zeolite.

  In the calculation step, the calculation coefficient is calculated based on the assumption that the total amount of γ rays determined in each γ dose determination step and the measured value of the γ dose are the same.

Specifically, in the calculation step, the total amount of γ rays obtained in each γ dose determination step can be expressed by, for example, the following equation (7).
In the following formula (7), the subject 1 is a mixture of zeolite and water, and as shown in FIG. 1, D equal cylindrical γ-ray radiation units 2 are formed from one end of the subject 1 to the other end. It is assumed that the γ-ray radiation unit bodies 2 are present in the subject 1 so that the bottom surfaces of the adjacent γ-ray radiation unit bodies 2 are overlapped with each other until the bottom surfaces of the adjacent γ-ray radiation unit bodies 2 overlap each other. It can be used when the cylinder axis direction length in the γ-ray emission unit body 2 is 1 cm.

Specifically, d in the above equation (7) indicates the diameter of the bottom surface of the γ-ray radiation unit body 2 as shown in FIG. η _j indicates the release rate of each γ-ray energy as described above. ρ _m represents the density (g / cm ³ ) of the subject 1. ρ _a represents the density of air (g / cm ³ ). r indicates the distance (cm) between the subject 1 and the γ-ray detector 3. h _j is an abundance ratio of radioactive elements ( ¹³⁴ Cs or ¹³⁷ Cs) emitting each γ-ray in the radioactive substance (specifically, if the radioactive element is cesium, for example, j = 1, 2, 3 ¹³⁴ Cs, and j = 4 is ¹³⁷ Cs). i indicates the order of the γ-ray radiation unit bodies 2 arranged in a direction away from the γ-ray radiation unit body 2 closest to the γ-ray detection unit 3. B _i represents a build-up coefficient for the i-th γ-ray radiation unit body 2. Pm represents the attenuation coefficient (cm ² / g) of the subject 1. Pa represents the attenuation coefficient (cm ² / g) of air. exp indicates an exponential function with a base e. The term 4 * π (r + i−0.5) ² represents the total area of the spherical space that spreads while the γ-rays emitted from the i-th γ-ray emitting unit 2 travel the distance to the detector. Accordingly, the term πd ² /4/(4*π(r+i−0.5) ² ) indicates that the γ-rays emitted from the i-th γ-ray emitting unit 2 travel the distance to the detection position 3a. It is the ratio of the total area of the sphere space extending between and the area of the detection position. This is the ratio of the γ dose incident on the detection position 3a among the γ-rays emitted from the i-th γ-ray radiation unit body 2. It is an approximate value of the ratio. Q indicates the calculation coefficient (Bq / kg) described above.

More specifically, in the calculation step, as shown in the following formula (8), by assuming that the value represented by formula (7) and the measured γ dose φ are the same value, A calculation coefficient Q can be calculated.
For example, the calculation coefficient Q can be calculated by substituting a temporary value such that Equation (8) holds into Q and determining an appropriate temporary value by repeated calculation.
Further, for example, the calculation coefficient Q can be calculated by calculation by a computer after substituting a known numerical value in the following equation (8) into the equation (8).

  Note that when the equation (7) is calculated using the calculation coefficient Q obtained by the calculation, the calculated γ dose (total effective dose rate) φ ′ is obtained. The comparison between the calculated γ dose (total effective dose rate) φ ′ and the measured γ dose (γ dose rate) φ will be described later in Examples.

In the method for estimating the amount of radioactive substance according to the present embodiment, the gamma dose at a part of the surface of the subject 1 is measured, and the assumption step, each gamma dose determination step, and the calculation step are performed. Thus, the amount of radioactive substance contained in the subject 1 can be grasped without taking out a part of the subject 1.
Specifically, in the method for estimating the amount of radioactive material, it is possible to grasp the amount of radioactive material in a portion where at least the γ-ray radiation unit body 2 is assumed to exist.

  In the method for estimating the amount of radioactive substance, a plurality of surface portions of the subject 1 for measuring the γ dose are set, and the assumption step is performed so that the γ-ray radiation unit bodies 2 do not overlap each other. The amount of radioactive substance contained in a plurality of locations of the specimen 1 can be estimated.

  In the method for estimating the amount of radioactive substance, when an adsorbent such as zeolite that adsorbs a radioactive substance is used as a material constituting the subject 1, for example, water containing a radioactive substance such as radioactive cesium is used as the adsorbent for the subject 1. The amount of radioactive material can be estimated for each predetermined amount of water. Therefore, the amount of radioactive substance can be estimated over time, and changes in the amount of radioactive substance can be known over time.

The method for estimating the amount of radioactive material of the present embodiment is as illustrated above, but the present invention is not limited to the method for estimating the amount of radioactive material illustrated above.
In addition, various modes used in a general method for estimating the amount of radioactive substance can be employed within a range that does not impair the effects of the present invention.

  EXAMPLES Next, although an Example is given and this invention is demonstrated in more detail, this invention is not limited to these.

  As shown below, raw water containing cesium 134 and cesium 137 as radioactive materials is passed through an adsorption tower packed with zeolite as an adsorbent, cesium 134 and cesium 137 are adsorbed on the zeolite, and the adsorption tower A mixture of zeolite, water and radioactive substance in the inside was used as the specimen. Then, the amount of radioactive substance in the subject in the adsorption tower was estimated.

(Test Example 1)
[Preparation of specimen]
An adsorbent was filled in a hollow columnar adsorption tower (made of vinyl chloride resin) installed so that the cylinder axis was in the vertical direction.
Diameter of the bottom surface portion of the adsorption tower: 7 cm, Adsorption tower cylinder axis length: 100 cm,
Adsorbent: Zeolite (manufactured by New Tohoku Chemical Industries)
Flow rate of raw water: 150 L, flow direction: from bottom to top Note that in the radioactive substance estimation method, the thickness of the peripheral wall of the adsorption tower is thin, and the presence of the adsorption tower is not considered.

[Measurement of gamma rays]
A radiation measuring instrument (product name “scintillation survey meter (TCS-171B)” manufactured by Hitachi Aloka Medical Co., Ltd.) equipped with a NaI (TI) scintillation detector (cylindrical) as a γ-ray detector was used.
NaI (TI) scintillation detector: cylindrical shape with a diameter of 3 cm and a length of 20 cm As shown in FIG. 1, the tip part (site where the γ-ray enters 3 cm in diameter) as a detection position in the NaI (TI) scintillation detector A NaI (TI) scintillation detector was placed so as to face a circular portion having a diameter of 3 cm (d) on the surface of the specimen. The distance r between the tip portion of the NaI (TI) scintillation detector and the subject surface was 4.5 cm. Further, the portion of the NaI (TI) scintillation detector other than the tip portion was covered with a γ-ray attenuating material (iron) having a thickness of 15 cm.
The measured γ dose (γ dose rate) φ was determined by such γ-ray measurement. The value of γ dose φ was 0.28 μSv / h.

[Assumption process]
It was assumed that a cylindrical γ-ray radiation unit was present in the subject. Specifically, seven cylindrical γ-ray radiation units having a bottom surface diameter of 3 cm and a cylindrical axial length of 1 cm are arranged so as to penetrate the cylindrical subject in the horizontal direction, and γ-ray radiation is obtained. The unit bodies were assumed to exist with their bottom surfaces overlapping each other.
The NaI (TI) scintillation detector was arranged so that the tip portion of the NaI (TI) scintillation detector was placed ahead of the direction in which the γ-ray radiation units were arranged.
In addition, it was assumed that the center of the bottom surface of the cylindrical γ-ray radiation unit is located at a height of 5 cm from the lowest part of the subject.

[Each gamma dose determination process]
According to the following formula, the γ-ray dose reaching each γ-ray radiation unit to the γ-ray detector was determined.
^{πd 2/4 * Q * h} j * η j * ρ m /1000/(4*π(r+i-0.5) 2) * B i * exp (-Pm * ρ m * (i-0.5)) * exp (-Pa * ρ _a * r)
... Formula (9)

In equation (9), d = 3 cm, π = 3.14 (circumferential ratio), h _j (j = 1, 2, 3) (the proportion of ¹³⁴ Cs present) = 0.42, h _j ( j = 4) (the presence ratio of ¹³⁷ Cs) = 0.58, ρ _m = 2.30 g / cm ³ (adopting an average density of water and zeolite), r = 4.5 cm, ρ _a = 1.20 * 10 ^-3 g / cm ³ . About Pm, it is as having mentioned above about Formula (7). About Pa, it is as Table 1. Moreover, i is 1-7 cm. η _j is as described above for the equation (1). As for the value of B _i , Pm = 0.0834 cm ² / P even if the value of Pm * ρ _m * (i−0.5) corresponding to Px in the equation (2) is calculated to be the maximum. g (adopting the values of concrete in Table 1), ρ _m = 2.30 g / cm ³ , i = 7 cm, 1.25, while in most other cases Pm * ρ _m * (i- Since the value of 0.5) is 1 or less, B _i is set to 1. Also, the build-up coefficient in the air was set to 1 without considering the build-up coefficient because the value of Pa * ρ _a * r was much smaller than 1.

[Calculation process]
The amount of γ-rays determined in each γ-ray dose determination process is summed, and the calculated value (equation (8)) is assumed to be the same as the total value and the measured value of the γ-ray (measured γ-ray dose φ). The amount of radioactive material was calculated from this formula. That is, Q (Bq / kg) in the formula (8) was calculated.

(Test Example 2)
The amount of radioactive substance was estimated in the same manner as in Test Example 1 except that the water flow rate was 380 L. The measured value of γ dose φ was 0.28 μSv / h.

(Test Example 3)
The amount of radioactive material was estimated in the same manner as in Test Example 1 except that the water flow rate was 451 L. The measured value of γ dose φ was 0.32 μSv / h.

Table 2 shows calculation coefficients Q calculated in Test Examples 1 to 3.
On the other hand, the part of the adsorbent where γ rays were measured in the subject was taken out, and the radiation dose of the taken out adsorbent was measured. Specifically, the radiation dose was measured using a germanium semiconductor detector (product name “SEG-EMS” manufactured by Seiko EG & G Co.) over a measurement time of 2500 seconds. The measurement results are also shown in Table 2.
In addition, the adsorbent taken out for measuring the radiation dose is the portion from the lowermost side of the subject to the height of 10 cm (the water inlet side) immediately after reaching each water flow rate.

  Note that the γ dose (total effective dose rate) φ ′ calculated by the equation (7) using the estimated calculation coefficient Q and the measured γ dose φ were very close to each other. From this result, in the above test example, most of the radioactive substance in the measured γ dose φ is radioactive cesium, and it can be recognized that the calculation using the equation (8) is effective. Thus, the amount of radioactive cesium can be estimated by using the measured γ dose φ in equation (8).

(Test Example 4)
Except for the points shown below, the amount of radioactive material was estimated in the same manner as in Test Example 1.
That is, the total water flow amount was 500 L, and the amount of radioactive substance contained in the subject was estimated for every 20 L water flow amount. Further, at the 5 cm height, 50 cm height, and 95 cm height (3 locations) from the lower side of the subject in the adsorption tower, the center of the bottom surface of the cylindrical γ-ray radiation unit is arranged, respectively. A line radiation unit was assumed. Moreover, the average value (average adsorption amount) of the radioactive substance amount estimated in these three places was calculated | required.

(Test Example 5)
The same operation as in Test Example 4 was performed again to estimate the amount of radioactive substance.

The amount of radioactive substance estimated as described above is shown in the graph of FIG.
On the other hand, a part of the adsorbent is taken out when the water flow amount is 150L, 380L, and 451L, and the taken out adsorbent is used with a germanium semiconductor detector (product name “SEG-EMS” manufactured by Seiko EG & G). The radiation dose was measured over a measurement time of 2500 seconds. The measurement site (part taken out from the adsorption tower) of the subject is a portion from the bottom of the subject to a height of 10 cm, a portion from the bottom to a height of 45 to 55 cm, and a portion from the top to the bottom of 10 cm. There were three locations. The removed adsorbent is immediately after reaching each water flow rate. The average value (average adsorption amount) was also determined for the measured values at three locations.
The results of measuring the amount of radioactive material in this way are also shown in FIG.

  As can be understood from Table 2 and FIG. 2, according to the method for estimating the amount of radioactive material of the present invention, the amount of radioactive material contained in the subject can be estimated relatively accurately without taking out a part of the subject. Can do.

  The method for estimating the amount of radioactive material of the present invention is suitably used for managing and controlling the amount of radioactive material in a subject containing a radioactive material that may adversely affect the human body, for example. Specifically, for example, it is suitable for passing water containing a radioactive substance through the adsorbent, grasping the amount of radioactive substance in the adsorbent that increases with the amount of water flow, and predicting when the adsorbent breaks through. Used for.

1: subject,
2: γ-ray emission unit,
3: γ-ray detection unit, 3a: detection position,
4: Material for attenuating gamma rays.

Claims (4)

A radioactive substance that measures a γ dose on a part of the surface of a subject containing a radioactive substance with a radiation measuring instrument equipped with a γ-ray detector, and estimates the amount of the radioactive substance contained in the subject from the measured value of the γ dose A method for estimating quantity,
An assumption step that assumes that a plurality of γ-ray emitting units that emit γ-rays are present in the subject in a straight line so as to penetrate the subject from a part of the surface of the subject;
In order to determine the amount of γ-rays emitted from each of the γ-ray emission unit bodies and reaching the detection position of the γ-ray detection unit, the concentration of the radioactive substance contained in the subject is set as a calculation coefficient, and the subject The γ-rays emitted from each γ-ray radiation unit are calculated based on the assumption that the concentration of the radioactive material contained in the γ-ray radiation unit is the same as the concentration of the radioactive substance contained in each γ-ray radiation unit. Based on the fact that the γ-rays radiated from the respective γ-ray radiation units are attenuated according to Lambert-Beer's law and reach the detection positions, Each γ-ray dose determination step that determines the amount of γ-rays that reaches the light source using the calculation coefficient;
Performing a calculation step of calculating the calculation coefficient based on the assumption that the total amount of γ rays determined in each γ dose determination step and the measured value of the γ dose are the same ;
In the assumption step, D equal cylindrical γ-ray radiation units are continuously arranged along the cylindrical axis direction of the γ-ray radiation unit from one end to the other end of the subject, and adjacent γ-ray radiation units. Assuming that the γ-ray radiation unit exists in the subject so that the bottom surfaces of the bodies overlap each other, and the length of the γ-ray radiation unit in the cylinder axis direction is 1 cm,
In each γ dose determination step, the radioactive substance is set to a specific radioactive element, and the emission rate of γ-ray energy of each γ ray emitted from the specific radioactive element is set.
In the calculation step, the total amount of γ rays determined in each γ dose determination step is represented by the following equation:

In the above formula, d represents the diameter [cm] of the bottom surface of the γ-ray radiation unit body, η _j represents the emission rate of the γ-ray energy, and ρ _m represents the density of the subject [g / cm ³ Ρ _a represents the density of air [g / cm ³ ], r represents the distance [cm] between the subject and the γ-ray detector, and h _j represents the radioactive element in the radioactive substance. an existence ratio, i denotes the order of γ-radiation unit body arranged toward a direction away from the nearest γ-radiation unit body γ-ray detection unit, B _i build for the i-th γ-radiation unit body Pm represents the attenuation coefficient [cm ² / g] of the subject , Pa represents the attenuation coefficient [cm ² / g] of air , exp represents the exponential function with the bottom being e, Q represents the calculation coefficient [Bq / kg],
A method for estimating a radioactive substance amount , wherein γ-rays are measured by arranging a detection position of the γ-ray detection unit on an extension line in an arrangement direction of the γ-ray radiation units . A gamma ray is measured by covering the gamma ray detection unit with a gamma ray attenuating material so as to prevent gamma rays emitted from a part other than a part of the surface of the subject from reaching the detection position of the gamma ray detection unit. Item 2. The method for estimating the amount of radioactive material according to Item 1 . The analyte is a mixture of the adsorbent and water in the adsorption tower containing the radioactive substance by passing water in which the radioactive substance is dissolved in the adsorption tower filled with the adsorbent. The estimation method of the radioactive substance amount of Claim 1 or 2. The method for estimating a radioactive substance amount according to any one of claims 1 to 3, wherein the radioactive substance is radioactive cesium or radioactive iodine.

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