JP6282540B2 - Radioactivity concentration measurement system - Google Patents

Description

  The present invention relates to a radioactivity concentration measurement system for measuring, for example, the radioactivity concentration of sediment on the bottom of water.

  As is well known, an accident occurred at a nuclear power plant after the Great East Japan Earthquake, and as a result, radioactive materials were released into the environment over a wide area, and the released radioactive materials over time. It fell to the ground and the surface of the water, leading to the situation of depositing on the ground and the bottom of the water. Since then, the amount of radioactive material fall as described above has been investigated.

  The survey result of the amount of radioactive material fall is generally indicated by the radioactivity concentration (radioactivity per unit weight or area) of sediments such as soil deposited on the ground or at the bottom of the water. Therefore, the radiation measuring apparatus used in the survey is required to output the radioactivity concentration as the measurement result.

  On the other hand, generally, the detection result of the radiation detector is a radiation dose detected by the detector called a count number. Therefore, in order for the radiation measuring apparatus to output the radioactivity concentration as a measurement result, the following two means can be considered.

  The first means is to calculate the radioactivity concentration based on the radiation count obtained from the detection result of the detector. That is, predetermined conversion (conversion) is performed on the radiation count (see, for example, Patent Document 1).

  The second means is to collect a measurement object, measure the weight and the radiation dose (count number), and divide the obtained radiation dose by the weight (see, for example, Patent Document 2).

JP2013-205152A JP 2013-257159 A

  However, in the first means, the calculated radioactivity concentration may cause an error with respect to the actual radioactivity concentration. Further, the second means requires a means for collecting the measurement object and a means for measuring the weight, which may increase the manufacturing cost.

  In view of the above circumstances, an object of the present invention is to provide a radioactivity concentration measurement system capable of suppressing measurement errors while suppressing manufacturing costs.

  The inventors of the present application have made extensive studies in order to solve the above problems, and one of the causes of the error in the radioactive concentration calculated by the first means with respect to the actual radioactive concentration is the first means. In the calculation of, the knowledge that the distribution of radioactivity concentration in the actual sediment was not taken into consideration was obtained.

The present invention provides a detection means that is disposed above a deposit and detects radiation emitted from the deposit, and calculates a radioactivity concentration based on a count number of the radiation obtained from the detection result of the detection means. A radioactivity concentration measurement system that outputs the radioactivity concentration calculated by the calculation means as a measurement result, wherein the radiation emitted from a plurality of virtual point sources in the deposit is the radioactivity concentration measurement system. It is assumed that the incident light is incident on the detection means, and the deposit is connected to a virtual circular surface extending in a transverse direction perpendicular to the depth direction around the center position of the detection means, and an upper end is connected to the outer periphery of the virtual circular surface. It is assumed that a virtual cylindrical shape surrounded by a virtual cylindrical surface extending in the vertical direction is formed, and the center position of the detection means is set within the plane extending in the lateral direction of the deposit. Free in the center Jo the equally spaced, in the plane extending in the depth direction of the deposit, and assumed to be equally spaced on each disposed and depth direction and the transverse direction into a mesh, further, the Assuming a hypothetical radioactive concentration distribution in the depth direction in the deposit, from one point source of the plurality of point sources toward a hemisphere of a predetermined size centered on the one point source The emission intensity of each of the plurality of emitted radiations is obtained for each emission angle, and the emission intensity of the radiation incident on the detection means among the plurality of radiations is obtained according to the emission angle, and is obtained. The received intensity from one point source of the plurality of point sources is calculated based on the emitted intensity, and the theoretical calculation value is calculated as the sum of the received intensities from all the point sources based on the radioactivity concentration distribution. Calculate Calculation means are provided, said calculation means, by using the theoretical calculated value, and calculates the radioactivity concentration.

  With this configuration, the radioactivity concentration is calculated using a theoretically calculated value calculated as the sum of the received intensities from all virtual point sources based on the virtual radioactivity concentration distribution. Therefore, it is possible to calculate the radioactive concentration reflecting the radioactive concentration distribution of the deposit by using the theoretical calculation value. Thereby, it can suppress that the radioactive concentration output as a measurement result produces an error with respect to an actual radioactive concentration. Further, since no means for collecting the measurement object and no means for measuring the weight are required, the manufacturing cost can be suppressed. That is, according to the radioactivity concentration measurement system of the present invention, it is possible to suppress measurement errors while suppressing manufacturing costs.

  In the above configuration, the calculation means calculates the theoretical calculation value based on each of a plurality of virtual radioactivity concentration distributions, and the theory used for the calculation means is selected from the plurality of the theoretical calculation values. The calculation value may be selectable.

  With this configuration, the radioactivity concentration can be calculated by selecting a theoretically calculated value calculated based on a virtual radioactivity concentration distribution that is considered to be close to the radioactivity concentration distribution at the actual measurement site. Thereby, a more accurate measurement result of the radioactivity concentration can be obtained.

  In any one of the configurations described above, when the count number is I and the theoretical calculation value is S, the mathematical formula used by the calculation unit to calculate the radioactivity concentration includes I / S as a factor. You may go out.

  With this configuration, a more accurate measurement result of radioactivity concentration can be obtained. In this configuration, the coefficient which is a constant of the mathematical formula is obtained by calculating the standard count number obtained from the detection result of the detection means of the radiation actually radiated from the standard radiation source and the actual depth direction in the standard radiation source. If it is obtained on the basis of the standard theoretical calculation value calculated by the calculation means as the sum of the received intensities from all the point sources based on the radioactive concentration distribution, a more accurate measurement result of the radioactive concentration is obtained. be able to. Here, the standard radiation source is an existing radiation source whose radioactivity concentration, radioactivity concentration distribution, size, density, etc. are known (the same applies hereinafter).

  In any one of the configurations described above, the deposit may be deposited on a water bottom.

  In the case of sediment deposited on the bottom of the water, it is often difficult to collect the sample itself. Therefore, the present invention which does not need to collect a sample is preferable.

  ADVANTAGE OF THE INVENTION According to this invention, the radioactivity concentration measuring system which can suppress a measurement error can be provided, suppressing manufacturing cost.

1 is a schematic configuration diagram of a radioactivity concentration measurement system according to an embodiment of the present invention. It is a longitudinal cross-sectional view of the radioactivity measurement part containing a detection means. It is a flowchart which shows the method of calculating a theoretical calculation value. It is a perspective view for demonstrating the conditions at the time of calculating a theoretical calculation value. It is the top view and side view for demonstrating the conditions at the time of calculating a theoretical calculation value. It is a flowchart which shows the content of the process W1 of FIG. It is a figure for demonstrating process W14 of FIG. It is a figure for demonstrating the process W2 of FIG. It is a figure which shows the specific example of virtual radioactivity concentration distribution. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration. It is a figure for comparing the measurement density | concentration by the radioactive concentration measurement system which concerns on the Example of this invention, and an actual density | concentration.

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

  FIG. 1 is a schematic configuration diagram of a radioactivity concentration measurement system according to an embodiment of the present invention. The radioactivity concentration measurement system 1 of this embodiment includes a detection unit 2, a calculation unit 3, a calculation unit 4, a storage unit 5, and an output unit 6 as main components.

  The detection means 2 is disposed above the deposit P and detects radiation emitted from the deposit P. In the present embodiment, the deposit P is deposited on the bottom of the water. The calculation means 3 calculates the radioactivity concentration based on the radiation count obtained from the detection result of the detection means 2.

  In the present embodiment, the type of radiation detected is γ-rays attributed to cesium 134 and cesium 137, but is not particularly limited thereto, and may be γ-rays attributed to other radioactive substances. In that case, a detection means suitable for detecting the radiation may be used.

  As will be described in detail later, the calculation means 4 assumes a plurality of virtual point sources in the deposit P, and assumes a hypothetical radioactive concentration distribution in the depth direction in the deposit P. Based on the density distribution, the theoretical calculation value is calculated as the sum of the received intensities from all virtual point sources. The calculation means 3 calculates the radioactivity concentration using the theoretical calculation value calculated by the calculation means 4.

  The storage unit 5 stores the theoretical calculation value calculated by the calculation unit 4. In the present embodiment, the calculation means 4 calculates theoretical calculation values based on each of a plurality of types of virtual radioactivity concentration distributions, and stores the plurality of theoretical calculation values in the storage means 5. And it is comprised so that the theoretical calculation value used for a calculation means may be selected from the some theoretical calculation values memorize | stored in the memory | storage means 5. FIG. In this embodiment, the selection of the theoretical calculation value is performed according to the situation at the measurement site based on the judgment of the user. However, the present invention is not limited to this and may be automatically selected. In addition, all the theoretical calculation values may be used without selecting the theoretical calculation values.

  The calculation unit 3, the calculation unit 4, and the storage unit 5 can be configured by an electronic information processing apparatus such as a personal computer, for example.

  The output means 6 is, for example, a display means such as a monitor or a printer, and outputs the radioactivity concentration calculated by the calculation means 3. Thereby, the radioactivity concentration measurement system 1 outputs the radioactivity concentration calculated by the calculation means 3 as a measurement result.

  Next, mathematical formulas used by the calculation means 3 for calculating the radioactivity concentration will be described.

  Here, the radioactivity concentration is B, the radiation count obtained from the detection result of the detection means 2 is I, and the theoretical calculation value is S. Note that S has no unit (dimensionless number).

For example, the radioactivity concentration B expressed by the amount of becquerel per unit weight ([Bq / kg]) is B1, and the radiation count obtained from the detection result of the detection means 2 is I [cpm]. In this case, the following formula 1 including the density ρ [g / cm ³ ] of the deposit to be measured is used.
(Formula 1) B1 = α × I / (ρ × S) = (α / ρ) × (I / S)
α is a coefficient that becomes a constant when the deposit is actually measured. Here, ρ is a variable that varies depending on the measurement target, but may be a constant (for example, 1). In this case, the coefficient that becomes the constant of Equation 1 is α / ρ.

Further, for example, the radioactivity concentration B expressed by the amount of becquerel per unit area [Bq / m ² ] is B2, and the radiation count obtained from the detection result of the detection means 2 is I [cpm]. In such a case, the following formula 2 is used.
(Formula 2) B2 = β × I / S
β is a coefficient that becomes a constant when the deposit is actually measured. B2 is calculated from the amount of radioactive substance (Bq) sensed by the detection means 2 and the area projected on the boundary surface between the sediment P and water. For example, this is considered to be an effective index for grasping the radioactivity level in a certain wide area.

  Thus, Equations 1 and 2 include (I / S) as a factor.

  The coefficient α in Expression 1 and the coefficient β in Expression 2 are constants when the deposit is actually measured, and are obtained in advance using an existing radiation source (standard radiation source).

  The coefficient α and the coefficient β are the count number (standard count number) obtained from the detection result of the radiation detecting means 2 actually emitted from the standard radiation source and the actual radioactive concentration in the depth direction of the standard radiation source. It is obtained based on the theoretical calculation value (standard theoretical calculation value) calculated by the calculation means 4 as the sum of the received intensities from all virtual point sources based on the distribution. The calculation method of the standard theoretical calculation value by the calculation means 4 is a method of calculating the theoretical calculation value as the sum of the received intensities from all virtual point sources based on the virtual radioactivity concentration distribution in the deposit P by the calculation means 4. , Instead of “virtual activity concentration distribution in the sediment P”, “actual activity concentration distribution in the depth direction in the standard radiation source” is used.

Specifically, the coefficient α and the coefficient β are the radioactivity concentration of the standard radiation source B1o [Bq / kg], B2o [Bq / m ² ], the standard count number Io [cpm], and the standard theoretical calculation value So. When the density of the standard radiation source is ρo [g / cm ³ ], the following values are obtained according to the mathematical formulas 1 and 2.
α = B1o × ρo × So / Io
β = B2o × So / Io

  Next, a specific example of the detection unit 2 will be described.

  FIG. 2 is a longitudinal sectional view of a radioactivity measurement unit including the detection means 2 of the present invention. The radioactivity measurement unit includes a cylindrical detection instrument 7, a pressure-resistant container 8 that houses the detection instrument 7 in a sealed state (watertight state), and a frame assembly (not shown) that surrounds and holds the pressure-resistant container 8. Is the main component. And the cable 9 of the signal wire | line which sends the signal from the detection instrument 7 is pulled out outside in the state which maintained the inside of the pressure-resistant container 8 in the airtight state.

  The detection instrument 7 has a radiation detector 10 located in the lower part and a wave height analyzer 11 connected to the upper part, and the radiation detector 10 has a radiation detection means 2 (as a radiation detection sensor) at its lower end part. In this embodiment, the wave height analyzer 11 is a nuclide discriminating means (multichannel analyzer: MCA) for discriminating between cesium 137 and cesium 134. Built in. Therefore, the detection instrument 7 according to this embodiment is configured to separately measure the amounts of cesium 137 and cesium 134 as radioactive substances.

  The pressure vessel 8 includes a cylindrical tubular body 12 made of a pressure resistant resin such as transparent acrylic covering the outer peripheral side of the detection instrument 7 through a gap, and O-rings 13 at both upper and lower ends of the tubular body 12. , 14 and a disc-shaped upper lid body 15 and a lower lid body 16 which are closely fitted to each other. In this case, both the upper lid body 15 and the lower lid body 16 are made of a pressure-resistant resin (for example, MC nylon (registered trademark)) softer than the material of the cylindrical body 12, and are fastened by the fastening bands 17 and 18. The tubular body 12 is fastened and fixed to both upper and lower ends.

  Inside the pressure vessel 8, an instrument fixing member that is tightly fitted to the inner peripheral surface of the cylindrical body 12 with a constriction 19 between the wave height analyzer 11 and the radiation detector 10 in the detection instrument 7 interposed therebetween. 20, and this instrument fixing member 20 fixes and supports the detection instrument 7 in the pressure vessel 8. The instrument fixing member 20 has, for example, a structure in which a plate-like body having a circular shape in plan view is divided into two at the center, and the pair of plate-like bodies are fastened with bolts 21.

  Further, in the pressure vessel 8, an upper fixing pipe 22 that covers the outer peripheral side of the wave height analyzer 11 is disposed between a recess 15 a formed on the lower surface of the upper lid 15 and the instrument fixing member 20. A lower fixing pipe 23 that covers the outer peripheral side of the radiation detector 10 is disposed between the recess 16 a formed on the lower surface of the lower lid 16 and the instrument fixing member 20. A cylindrical shield 24 made of lead or the like, which is a material that shields radiation, is disposed in the gap between the cylindrical body 12 of the pressure vessel 8 and the lower fixed pipe 23.

  In other words, the lower end portion of the radiation detector 10 in the detection instrument 7 is inserted into the recess 16 a of the lower lid body 16 of the pressure-resistant container 8, and the outer peripheral side of the radiation detector 10 is covered with the shield 24. . Thereby, the radiation detection means 2 of the radiation detector 10 is as close as possible to the lower end surface of the pressure-resistant container 8 (lower end surface of the lower lid body 16), and the radiation from the outer peripheral side is shielded by the shielding body 24, You will receive mainly radiation from below.

  In addition, a signal line 9 a that extracts data obtained by the radiation detection means 2 and the wave height analyzer 11 as an electrical signal is connected to the upper end of the detection instrument 7.

  Specifically, the data extracted to the signal line 9a is a radiation dose (count number) for each energy by γ-rays, which is obtained from the detection result of the detection means 2 via the pulse height analyzer 11. is there.

  The signal line 9 a is drawn out as a cable 9 through a plug 25 fixed to the center portion of the upper lid 15. In this case, the central region of the upper lid 15 (the peripheral region of the plug 25) is sealed in a watertight state. Therefore, the internal space of the pressure vessel 8 is completely sealed and is not crushed even when the water depth is about 100 m to 150 m.

  Further, the cable 9 for sending a signal from the detection instrument 7 is configured to be connected to an electronic information processing apparatus (calculation means 3) such as a personal computer provided on the surface of the water (for example, on the ship or in the ship). . In addition, the detection instrument 7 has a temperature compensation function that compensates for a sensitivity change with respect to a temperature change caused by a difference in water depth.

  Next, a method in which the calculation means 4 calculates the theoretical calculation value S will be described.

  In the present embodiment, as shown in FIG. 3, the calculation method of the theoretical calculation value S by the calculation means 4 includes the step W1 of calculating the reception intensity from the point source and the sum of the reception intensity for each depth of the point source. A step W2 for calculating, and a step W3 for calculating the sum of the received intensities from all the point source based on the virtual radioactivity concentration distribution.

  In the present embodiment, the theoretical calculation value S is the sum of the received intensities from all point sources calculated in step W3 based on the virtual radioactivity concentration distribution. The total received intensity is displayed as a ratio (relative intensity) to the radiation emission intensity of the dotted line source. In the present embodiment, the sum of the emission intensities of all the radiations emitted from one point source is 1.

In the present embodiment, as shown in FIG. 4, the radiation A emitted from a plurality of virtual point sources F in the deposit P deposited in the vertical direction (vertical direction in the present embodiment) has been described with reference to FIG. It is assumed that the light enters the detection means 2 of the radiation measurement unit. In the following description, it is assumed that a virtual point source F is arranged as shown in FIG. 4 unless otherwise noted. That is, the deposit P is composed of a virtual circular surface extending in the lateral direction perpendicular to the depth direction around the center position of the detection means 2 and a virtual cylindrical surface extending in the depth direction with the upper end connected to the outer periphery of the virtual circular surface. Assume that it is surrounded by a virtual cylinder. As shown in the U1 part, the point source F is assumed to be radially arranged at equal intervals around the center position of the detection means 2 in the laterally extending surface (horizontal plane in the present embodiment). To do. Furthermore, as shown in the U2 part, the point source F is arranged in a mesh shape on a surface extending in the vertical direction (vertical surface in the present embodiment), and is arranged at equal intervals in each of the vertical direction and the horizontal direction. Assume that

  In the present embodiment, as shown in FIG. 5, the X-axis is arranged so as to pass through one point source F, and in order to specify the direction of the radiation A emitted from one point source F, the radiation on the horizontal plane Consider an angle of A with respect to the X axis (hereinafter referred to as horizontal angle θ) and an angle of X-ray of radiation A on the vertical plane (hereinafter referred to as elevation angle φ). The depth of the point source F is L, and the distance between the point source F and the center axis O of the detection means 2 is D. The received intensity of the radiation emitted from one point source F existing at the depth L and the separation distance D is R (L, D). As shown in FIG. 5, when the X axis is arranged so as to pass through the point source F, the separation distance D becomes the X coordinate of the point source F.

  For the convenience of explanation, it is assumed that there are m values of the depth L, L = l1, l2,..., Lm, and n values of the separation distance D are present, and D = d1, d2,. , Dn. The value of the depth L and the value of the separation distance D are equally spaced. Since the positions of the point source F are determined by all the depths L and all the separation distances D, there exist m × n.

  In the present embodiment, in order to calculate the reception intensity R (L, D) from the point source F in Step W1, Steps W11 to W18 shown in FIG. 6 are performed.

  In step W11, the depth L and the separation distance D of one point source are set. In step W12, the horizontal angle θ of the radiation is set, and in step W13, the elevation angle φ of the radiation is set.

  In step W14, the emission intensity E (φ) of one radiation A emitted from one point source F is calculated. For example, as shown in FIG. 7, a hemispherical surface Ca having a predetermined size centered on one point source F is considered. Then, consider the intersection V of the radiation A for each hemispherical surface Ca and elevation angle φ. The elevation angle φ is changed at equal intervals. Then, the hemispherical surface Ca is cut into a circle perpendicular to the radius passing through the center and the apex of the hemispherical surface Ca so as to include the intersection V of each elevation angle φ. As a result, a plurality (eight in the illustrated example) of round spherical surfaces C1, C2,..., C8 are formed. In addition, when considering the longitudinal section passing through the center and the apex of the hemispherical surface Ca, the lengths of the ring-shaped spherical surfaces C1, C2,..., C8 are the same on the outline of the hemispherical surface Ca. Further, when considering a longitudinal section passing through the center and apex of the hemispherical surface Ca and the intersection V, the intersection V exists at the center position of the ring-shaped spherical surfaces C1, C2,. To do. Then, it is assumed that the intervals G between the plurality of intersection points V in the respective ring-shaped spherical surfaces C1, C2,..., C8 are the same in any of the ring-shaped spherical surfaces C1, C2,.

  Therefore, the number of intersection points V on each ring-shaped spherical surface is proportional to the area of each of the ring-shaped spherical surfaces C1, C2,. Therefore, the emission intensity E (φ) of the radiation A emitted from one point source F in the direction of the elevation angle φ is proportional to the area of each of the ring-shaped spherical surfaces C1, C2,. . Therefore, for example, assuming that the ratio of the area of each of the circular spherical surfaces C1, C2,..., C8 is c1: c2:..., C8, the emission intensity of all the radiation A emitted from one point source F. , The emission intensity E (φ) of the radiation A at each elevation angle φ is c1, c2,..., C8.

  In step W15, the reception intensity Ro (θ, φ) of one radiation A emitted from one point source F is calculated. In step W15, first, as shown in FIG. 5, one radiation A emitted from one point source at a horizontal angle θ and an elevation angle φ is deposited P, water, pressure vessel 8, shield 24, and pipe 23. The distance passing through each of the storage container 10a and the detection means 2 is calculated. These distances are calculated from the dimensions and positions of the pressure vessel 8, the shield 24, the pipe 23, the storage container 10 a, and the detection means 2, the depth L of the point source F, the separation distance D, the elevation angle φ, and the horizontal angle θ. . These distances, the radiation transmittance (attenuation rate) of the deposit P, water, the pressure vessel 8, the shield 24, the pipe 23, and the detection means 2, and the emission intensity E (φ of the radiation A calculated in the step W14 ), The reception intensity Ro (θ, φ) of one radiation A is calculated.

  In this embodiment, when calculating the passage distance of the radiation A in the step W15, for example, as shown in FIG. 5, the Y axis is perpendicular to the X axis and is along the horizontal plane, and is along the central axis O of the detection means 2. Arrange the Z-axis. If it does so, the intersection of the radiation A and the detection means 2 will be calculated by the coordinate using these, the radiation A will calculate the passage distance in the detection means 2 from there, and the containment container 10a, the pipe 23, and the shielding body from this distance 24, the passage distance of water and sediment P can be calculated.

  In step W16, the reception intensity Rt (θ) of the radiation A emitted from one point source F is calculated for each horizontal angle θ. More specifically, in step W16, first, while maintaining the horizontal angle θ constant, the elevation angle φ is changed at equal intervals, and the steps W13 to W15 are repeated to obtain the reception intensity Ro (θ, φ) of one radiation. Obtained for each elevation angle φ. Next, the sum of these received intensities Ro (θ, φ) is calculated and set as the received intensity Rt (θ) at a constant horizontal angle θ.

  In step W17, the reception intensity R (L, D) from one point source F is calculated. More specifically, in step W17, first, the horizontal angle θ is changed at equal intervals while the depth L and the separation distance D are kept constant, and the steps W12 to W16 are repeated to set the reception intensity Rt (θ) to the horizontal. Obtained for each angle θ. Next, the sum of these received intensities Rt (θ) is calculated and set as a received intensity R (L, D) from one point source F having a constant depth L and separation distance D.

More specifically, the horizontal angle θ is k and the horizontal angles θ = θ1, θ2,..., Θk, and the elevation angle φ is l and the elevation angles φ = φ1, φ2,. Then
Rt (θ1) = Ro (θ1, φ1) + Ro (θ1, φ2) + .. + Ro (θ1, φl)
Rt (θ2) = Ro (θ2, φ1) + Ro (θ2, φ2) +... + Ro (θ2, φl)
・
・
Rt (θk) = Ro (θk, φ1) + Ro (θk, φ2) + .. + Ro (θk, φl)
R (L, D) = Rt (θ1) + Rt (θ2) +... + Rt (θk)

Thus, the reception intensity R (L, D) from one point source F having the depth L and the separation distance D is obtained. In step W18, the reception intensity R (L, D) from the point source F with the separation distance D changed is calculated. More specifically, in step W18, the separation distance D is changed, and the steps W11 to W17 are repeated, and the reception intensity R (L, D) is determined for the point source F whose position is determined at all the separation distances D. calculate. For example, when the depth L = 11, n reception intensities R (L, D) shown below are calculated in step W18.
R (l1, d1), R (l1, d2), ..., R (l1, dn)

  When the process W18 is completed, the process W1 is completed. Here, returning to FIG. 3, the process W2 subsequent to the process W1 will be described. In step W2, the sum of received intensities is calculated for each depth L of the point source F. Ra (L) is the sum of the received strengths. Since the reception intensity R (L, D) calculated in the process W1 is the reception intensity from one point source F determined by the depth L and the separation distance D, there may be a plurality of point sources F at the same separation distance D. Not considered. Therefore, the total received intensity Ra (L) is obtained as follows.

  First, for example, as shown in FIG. 8, it is assumed that there are n (the number of separation distances D) point source F on the X axis, and each of the detection means 2 including n point sources F on the X axis. Consider circular regions H1, H2,..., Hn centered on the central axis O. The annular regions H1, H2,..., Hn are defined so that the point source F is arranged at the center in the X-axis direction of each of the annular regions H1, H2,. Here, it is assumed that the point source F is evenly arranged in the horizontal plane. Therefore, the number of point source F existing in each of the annular regions H1, H2,..., Hn (including portions other than those on the X axis not shown) is equal to the annular regions H1, H2,. It is proportional to each area of Hn.

Therefore, the total received intensity Ra (L) is calculated by multiplying each received intensity of the point source F on the X-axis by the ratio of each area of the annular region H including the point source F, and calculating the sum of these. . For example, if the ratio of each area of the annular regions H1, H2,..., Hn is h1: h2:...: Hn, Ra (L) is expressed by the following equation when the depth L = l1. Is done.
Ra (l1) = R (l1, d1) × h1 + R (l1, d2) × h2 +,..., + R (l1, dn) × hn

  Then, the value of the depth L is changed, the process W1 is performed, the reception strength R (L, D) corresponding to the changed depth L is calculated, and this reception strength R (L, D) is used. Then, the sum Ra (L) of the received intensity is calculated. By repeating this, the total received intensity Ra (L) for all depths L is calculated.

  In the next step W3, the theoretical calculation value S is calculated as the sum of the received intensities from all the point sources F based on the virtual radioactivity concentration distribution in the depth direction in the deposit P.

  As the hypothetical radioactivity concentration distribution in the depth direction in the deposit P, for example, as shown in FIGS. 9A to 9D, a predetermined radioactivity concentration in a predetermined longitudinal range (depth range). The thing which has is mentioned. In the illustrated example, the radioactivity concentration is displayed as a relative value. 9 (A) to 9 (C) are examples in which the radioactivity concentration is evenly distributed to depths of 5 cm, 10 cm, and 15 cm, respectively, and FIG. 9 (D) shows a decrease in radioactivity concentration as the depth increases. This is an example.

The calculation method of the theoretical calculation value S is considered as follows. Until the process W2, since it is assumed that the point sources F are evenly arranged in the vertical direction (depth direction), the sum of the received intensity sum Ra (L) for each depth L is simply calculated. Then, the sum of the received intensities from all the point source F in the case where the radioactive concentration distribution in the depth direction is uniform is calculated. Therefore, in order to reflect the virtual radioactivity concentration distribution in the depth direction, the theoretical calculation value S is multiplied by the relative ratio of the virtual concentration distribution in the depth direction to each of the sum Ra (L) of the received intensity, Calculated as the sum of these. For example, if the relative ratio of the virtual density distribution in the depth direction is j1: j2:...: Jm, S is expressed by the following equation.
S = Ra (l1) × j1 + Ra (l2) × j2 +... + Ra (lm) × jm

For example, in the example of FIG. 9D, if the interval of L is 1 cm, l1 = 1, l2 = 2, l3 = 3, l4 = 4,..., L15 = 15, and j1: j2: If j3: j4: ...: j15 = 1: 0.8: 0.5: 0.5: ...: 0.1, S is expressed by the following equation.
S = Ra (1) + 0.8Ra (2) + 0.5Ra (3) + 0.5Ra (4) + ... + 0.1Ra (15)

  In the present embodiment, four types of theoretical calculation values corresponding to the respective concentration distributions are calculated and stored in the storage unit 5 by performing the process W3 for the respective concentration distributions of FIGS. 9 (A) to (D). Let 9A to 9D, the user selects a theoretical calculation value corresponding to what is considered to be close to the situation at the measurement site, and the radioactivity concentration is calculated by the calculation means.

  In the case of the radioactivity concentration measuring system 1 configured as described above, the theoretical calculation value S calculated as the sum of the received intensities from all the virtual point sources F based on the virtual radioactivity concentration distribution is used. Calculate the radioactivity concentration. Therefore, by using the theoretical calculation value S, it is possible to calculate the radioactivity concentration reflecting the radioactivity concentration distribution of the deposit P. Thereby, it can suppress that the radioactive concentration output as a measurement result produces an error with respect to an actual radioactive concentration. Further, since no means for collecting the measurement object and no means for measuring the weight are required, the manufacturing cost can be suppressed. That is, according to the radioactivity concentration measurement system 1 of the present embodiment, it is possible to suppress measurement errors while suppressing manufacturing costs.

  The present inventors evaluated the radioactivity concentration measurement system according to the example of the present invention.

  This evaluation was performed as follows. With the radioactivity concentration measurement system according to the example of the present invention, the radioactivity measurement unit described in FIG. 2 was placed on the actual bottom of the water, and the radioactivity concentration was measured (calculated). Then, the bottom sediment (soil) where the radioactivity measuring unit was arranged was collected and analyzed by a germanium semiconductor detector, and the actual radioactivity concentration was determined from the radiation dose and weight. Similar measurements were taken at four locations.

  The results for cesium 137 are shown in FIGS. The results for cesium 134 are shown in FIGS. The results in the case where cesium 134 and cesium 137 are combined (not distinguished) are shown in FIGS. 10, FIG. 12, and FIG. 14 show the measured concentration and the actual radioactive concentration by the radioactive concentration measuring system according to the embodiment of the present invention for each of the four measurement points. 11, 13, and 15, the horizontal axis represents the actual radioactivity concentration, and the vertical axis represents the concentration measured by the radioactivity concentration measurement system according to the embodiment of the present invention. Is shown. From these figures, it can be understood that the measured concentration by the radioactive concentration measuring system according to the embodiment of the present invention and the actual radioactive concentration substantially coincide.

  The present invention is not limited to the above description, and various modifications are possible within the scope of the technical idea. For example, in the above description, the deposit to be measured is deposited on the bottom of the water, but may be deposited on the ground.

DESCRIPTION OF SYMBOLS 1 Radioactivity concentration measurement system 2 Detection means 3 Calculation means 4 Calculation means A Radiation F Point source P Deposit

Claims (5)

A detecting unit that is disposed above the deposit and detects radiation emitted from the deposit; and a calculating unit that calculates a radioactivity concentration based on a count number of the radiation obtained from a detection result of the detecting unit. , A radioactivity concentration measurement system that outputs the radioactivity concentration calculated by the calculation means as a measurement result,
It is assumed that radiation emitted from a plurality of virtual point sources in the deposit is incident on the detection means, and the deposit is perpendicular to the depth direction around the center position of the detection means. It is assumed that the virtual circular surface extending in the lateral direction and the virtual circular cylindrical surface surrounded by the virtual cylindrical surface extending in the depth direction with the upper end connected to the outer periphery of the virtual circular surface, and the plurality of point sources are In the plane extending in the lateral direction of the deposit, the detectors are arranged radially at equal intervals around the center position of the detection means, and in the plane extending in the depth direction of the deposit, the deposit is arranged in a mesh shape and has a depth. Assuming that they are equally spaced in each direction and
Furthermore, assuming a hypothetical radioactive concentration distribution in the depth direction in the deposit,
The emission intensity of each of a plurality of radiations emitted from one point source among the plurality of point sources toward a hemisphere having a predetermined size centered on the one point source is set to each emission angle. The emission intensity of the radiation incident on the detection means among the plurality of radiations is determined according to the emission angle, and one of the plurality of point sources is determined based on the determined emission intensity. Calculate the received intensity from one point source,
Calculating means are provided for calculating a theoretical calculation value as the sum of the reception intensity from all of the point source on the basis of the radioactivity concentration distribution,
The radioactivity concentration measurement system characterized in that the calculation means calculates the radioactivity concentration using the theoretical calculation value.   The calculation means calculates the theoretical calculation value based on each of a plurality of virtual radioactivity concentration distributions, and the theoretical calculation value used for the calculation means can be selected from the plurality of the theoretical calculation values. The radioactivity concentration measurement system according to claim 1, wherein   2. The mathematical formula used by the calculation means for calculating the radioactivity concentration, where I is the count number and S is the theoretical calculation value, includes I / S as a factor. Or the radioactivity concentration measurement system according to 2;   The coefficient which is a constant of the mathematical formula is the standard count number obtained from the detection result of the detection means of the radiation actually radiated from the standard radiation source, and the actual radioactive concentration distribution in the depth direction in the standard radiation source. The radioactivity concentration measurement system according to claim 3, wherein the radioactivity concentration measurement system is obtained based on a standard theoretical calculation value calculated by the calculation means as a sum of reception intensities from all the point sources based on the above.   The radioactive concentration measuring system according to any one of claims 1 to 4, wherein the deposit is deposited on a water bottom.

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