JP6136777B2 - Nondestructive radioactivity measurement apparatus and radioactivity measurement method thereof - Google Patents

Description

  The present invention relates to a nondestructive radioactivity measuring apparatus and a radioactivity measuring method thereof.

A radiation measurement device that detects radiation is known (see, for example, Patent Document 1 and Patent Document 2).
Radioactive cesium and other radioactive materials generated in the nuclear power plant due to the Fukushima Daiichi nuclear power plant accident caused by the 2011 off the Pacific coast of Tohoku Earthquake that occurred on March 11, 2011, etc. in the atmosphere and water May be released. Agricultural and livestock products, marine products and other foods, and soil may be contaminated by radioactive materials.

  In general, in gamma ray measurement, self-absorption of gamma rays in the object to be measured cannot be ignored, and self-absorption needs to be corrected by some method. Strictly speaking, self-absorption is corrected by performing a simulation.

  Self-absorption depends on the material and shape of the object to be measured. Specifically, since the gamma ray count depends on the product of attenuation due to gamma ray absorption by the substance existing between the gamma ray generation point and the detection unit and the solid angle at which the generation point is desired by the detection unit, the gamma ray count depends on the product. It is necessary to correct the gamma ray count according to the shape.

In general radioactivity measurement equipment, the object to be measured is crushed, cut, chopped, minced, etc., packed into a marinade container of the specified shape, and always measured with the same object shape. (For example, refer nonpatent literature 1). In the measurement using this radioactivity measurement apparatus, correction was performed using an absorption correction coefficient prepared in advance by the measurement apparatus manufacturer. In this case, since the shape of the marinade container is the same, the absorption correction coefficient is the same.
Moreover, for example, when food is assumed as the material of the object to be measured, it can be approximated by “water having a specific gravity of 1”.
In the above-described radioactivity measuring apparatus (Becquerel monitor), it has been required to pack the object to be measured without gaps in the marinelli container so that it is not necessary to obtain the absorption correction coefficient for each measurement of the object to be measured.

JP 2008-281441 A JP2013-104726A

"European food radioactivity measurement manual", [online], March 2002, Ministry of Health, Labor and Welfare, Pharmaceutical Bureau, Food Health Department, Monitoring and Safety Division, Internet <URL: http://www.mhlw.go.jp/stf /houdou/2r9852000001558e-img/2r98520000015cfn.pdf>

  However, as described above, a general radioactivity measuring apparatus (Becquerel monitor) requires that the object to be measured be stuffed into the marinelli container without any gap, and stuffed the object to be measured such as food into the marinelli container. It took a lot of time and effort.

  In addition, in the inspection using the above-described radioactivity measuring apparatus, the food after inspection is in a fine state, and if all products are inspected, the food cannot be distributed. .

  For this reason, there is a demand for a radioactivity measuring apparatus that can measure the specific radioactivity of an object to be measured in the shape of the object to be measured such as food.

  However, even if you try to measure the specific activity of an object to be measured simply as it is, such as food, the shape of the food differs for each measurement, so it is very difficult to derive the absorption correction coefficient. Therefore, it was considered difficult to easily measure the specific activity of an object to be measured in a short time with high accuracy.

  The present invention has been made in view of the above-described problems, and can easily measure the specific activity of the object to be measured without finely destroying the object to be measured such as food. An object is to provide a device and a method for measuring the radioactivity thereof.

但し、Ａは比放射能［Ｂｑ／ｋｇ］、
Ｎは単位時間当りの計数値［counts／sec］、Ｍは質量［ｋｇ］、
Ｓ₀はガンマ線検出部の検出面の面積［ｍ²］、Ｓ₁は被測定物の底面積［ｍ²］、
Ｎ／Ｍは単位時間且つ単位質量当りの計数値［counts・sec^-1・kg^-1］、
ａは係数（正数）、ｂは係数（正数）。
尚、係数ａ、係数ｂは、標準試料を用いた測定により求めてもよい。
また、非破壊放射能測定装置は、標準試料を用いた測定により求められた係数ａ、係数ｂを記憶する記憶部、または、予め規定された係数ａ、係数ｂを記憶する記憶部を有していてもよい。算出部は、記憶部に記憶された係数ａ、係数ｂを用いて、被測定物の比放射能を算出してもよい。 The nondestructive radioactivity measuring apparatus of the present invention is a nondestructive radioactivity measuring apparatus that measures the specific radioactivity of an object to be measured without destroying the object to be measured such as food, and is placed on a mounting surface. A gamma ray detector that outputs a signal corresponding to the energy of gamma rays from the object to be measured, and a count value for a predetermined time by counting a signal indicating a predetermined range of energy based on the signal from the gamma ray detector. , A mass measuring unit for measuring the mass of the object to be measured, the count value by the counting unit, the mass of the object to be measured measured by the mass measuring unit, and formula (1) And a calculation unit for calculating the specific activity [Bq / kg] of the object to be measured.

Where A is the specific activity [Bq / kg],
N is a count value per unit time [counts / sec], M is a mass [kg],
S ₀ is the area [m ² ] of the detection surface of the gamma ray detector, S ₁ is the bottom area [m ² ] of the object to be measured,
N / M is a count value per unit time and unit mass [counts · sec ⁻¹ · kg ⁻¹ ],
a is a coefficient (positive number), b is a coefficient (positive number).
Note that the coefficient a and the coefficient b may be obtained by measurement using a standard sample.
The nondestructive radioactivity measuring apparatus has a storage unit for storing the coefficient a and the coefficient b obtained by measurement using a standard sample, or a storage unit for storing the predetermined coefficient a and the coefficient b. It may be. The calculation unit may calculate the specific activity of the measurement object using the coefficient a and the coefficient b stored in the storage unit.

  Further, the radioactivity measurement method of the nondestructive radioactivity measurement apparatus according to the present invention includes a first step in which the mass measurement unit measures the mass of the object to be measured, and the gamma ray detection unit and the counting unit are the object to be measured. A second step of outputting a count value of gamma rays from the measurement object for a predetermined time; and the calculation unit, the count value by the counting unit, the mass of the measurement object measured by the mass measurement unit, and And a third step of calculating a specific activity [Bq / kg] of the object to be measured based on the mathematical formula (1).

  As a result of trial and error, the inventor of the present application has found a very simple correction formula for absorption of gamma rays (a linear expression related to mass in Formula (1)). As a result of actual measurement, it has been confirmed that sufficiently practical measurement is possible by the quantitative formula of the above formula (1). It has been confirmed by actual measurement that the mathematical expression (1) is also a linear expression related to the reciprocal of the bottom area of the object to be measured.

ADVANTAGE OF THE INVENTION According to this invention, the nondestructive radioactivity measuring apparatus which can measure the specific radioactivity of the to-be-measured object can be provided in the state as it is, without destroying to-be-measured objects, such as foodstuffs finely.
Moreover, according to this invention, the radioactivity measuring method of a nondestructive radioactivity measuring apparatus can be provided.

1 is an overall configuration diagram showing an example of a nondestructive radioactivity measuring apparatus according to an embodiment of the present invention. The figure which shows an example of the nondestructive radioactivity measuring apparatus which concerns on embodiment of this invention, (a) is an electrical block diagram of a nondestructive radioactivity measuring apparatus, (b) is a functional block diagram of a control part. The figure for demonstrating an example of the absorption correction of the gamma ray in a to-be-measured object, (a) is the gamma-ray absorption effect with respect to the unevenness | corrugation of the upper end part of a to-be-measured object, (b) is the absorption of the gamma ray in a to-be-measured object, respectively. The figure for demonstrating. The figure which shows an example of the energy spectrum of a gamma ray. The flowchart which shows an example of operation | movement of the nondestructive radioactivity measuring apparatus which concerns on embodiment of this invention. The figure which shows an example of the specific activity which measured the to-be-measured object in the shape as it is (measured the whole), and the specific activity which measured to-be-measured object in fine states, such as a mince form. The perspective view of the gamma ray detection part of the nondestructive radioactivity measuring apparatus which concerns on embodiment of this invention. It shows an example of dependence of the area S ₀ of the detection surface of the bottom surface area S ₁ and the gamma ray detector of the object about the counts per unit of gamma ray time and unit mass (reciprocal). Diagram for explaining an example of a gamma ray measurement, (a) shows the case of _{S 0 / S 1 = 1.00,} (b) in the case of _{S 0 / S 1 = 1.66,} (c) the S ₀ / diagram for explaining the respective measurement for S ₁ = 11.7. For S ₀ / S ₁ = 1.00, shows the mass dependence of the count per unit time (count) [counts · sec ^-1]. For S ₀ / S ₁ = 1.00, shows an example of a graph obtained by converting the measured data in terms of expression. For S ₀ / S ₁ = 1.66, shows the mass dependence of the count per unit time (count). For S ₀ / S ₁ = 1.66, shows an example of a graph obtained by converting the measured data in terms of expression. For S ₀ / S ₁ = 11.7, shows the mass dependence of the count per unit time (count). For S ₀ / S ₁ = 11.7, illustrates an example of a graph obtained by converting the measured data in terms of expression.

  A nondestructive radioactivity measuring apparatus according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is an overall configuration diagram showing an example of a nondestructive radioactivity measuring apparatus 100 according to an embodiment of the present invention. FIG. 2 is a block diagram showing an example of the nondestructive radioactivity measuring apparatus 100. Specifically, FIG. 2A is an electrical block diagram of the nondestructive radioactivity measuring apparatus 100, and FIG. 2B is a functional block diagram of the control unit 51. FIG. 3 is a diagram for explaining an example of gamma ray absorption correction in the DUT 9.

  The non-destructive radioactivity measuring apparatus 100 according to the embodiment of the present invention can measure the specific activity [Bq] of the object 9 to be measured in its entirety without breaking the object 9 to be measured such as food. / Kg] is measured with relatively high accuracy.

<Outline of the present invention>
(A) The measurement object 9 such as food is kept in its shape without being made fine (entirely), and is placed in contact with the detection surface 1a in which the gamma ray detectors 1 are arranged in a plane. Place on the mounting surface 4a. For example, when a plurality of foods of the same type are placed on the placement surface 4a, it is preferably placed so as to satisfy the entire area of the placement surface 4a so as to minimize the gap between them. In the present embodiment, the detection surface 1a and the placement surface 4a are substantially coincident.

For example, it is assumed that the solid angle Ω at which the detection surface 1a of the gamma ray detection unit 1 is desired from the center of the upper end of the DUT 9 is approximately 2π [sr].
If it is assumed that there is no gamma ray absorption in the device under test 9, the gamma ray detector 1 detects approximately half of all gamma rays emitted from the device under test 9. In this case, the count of gamma rays detected by the gamma ray detection unit 1 does not depend on the relative position between the DUT 9 and the gamma ray detection unit 1. In this case, the gamma ray count detected by the gamma ray detector 1 does not depend on the shape of the object 9 such as food. In this case, the specific activity [Bq / kg] of the DUT 9 is proportional to the gamma ray count.

Specifically, when the density of the measurement object 9 is uniform (uniform) and the distribution of the radioactive substance in the measurement object 9 is uniform (uniform), As shown in FIG. 3A, the object to be measured 9 having an uneven shape may be handled by approximating it as a simplified shape having no unevenness and having the same volume (for example, a rectangular parallelepiped or a cylindrical shape).
Specifically, when the linear absorption coefficient μ of the object to be measured 9 is used, the absorption of gamma rays emitted in the vertical direction at the height h in the object to be measured 9 is exp (−μh), and the unevenness is Absorption at the position is exp (−μ (h + δh)) ≈exp (−μh) (1−μδh). Since Σδh = 0 when the height h is the average height of the object to be measured, the sum of the absorption effects is exp (−μh). For this reason, as shown in FIG. 3A, only the absorption effect of what approximates a rectangular parallelepiped needs to be considered.

(B) For example, gamma rays radiated from a radioactive substance in the object 9 to be measured that is approximate to a rectangular parallelepiped or the like receive absorption depending on the distance passing through the object 9 to be measured. As shown in FIG. 3B, the sum N ₀ [counts / sec] of the number of gamma rays emitted from each point toward the gamma ray detector 1 is expressed as N ₀ = n ₁ + n ₂ + n ₃ + · ... Then, the sum N [counts / sec] of the number of gamma rays in consideration of absorption is N = n ₁ exp (−μ · X ₁ ) + n ₂ exp (−μ · X ₂ ) + n ₃ exp ( -μ · X ₃ ) + ... N is the sum of all directions in which gamma rays are emitted. The inventor of the present application obtained an experimental result represented by the mathematical formula (A1) as a relational expression between N and N ₀ .

Here, S ₀ indicates the area of the detection surface 1 a of the gamma ray detection unit 1, and S ₁ indicates the bottom area of the object 9 to be measured. M [kg] is the mass of the object 9 to be measured, and is proportional to the volume of the object 9 to be measured (height h × bottom area of the object to be measured). Here, the coefficient a and the coefficient b are positive numbers. The coefficient a is related to the solid angle, and the coefficient b is related to the absorption coefficient.
The bottom area S ₁ of the measurement object 9 is an area of a region obtained by projecting the measurement object 9 onto the detection surface 1 a of the gamma ray detection unit 1. When the measurement object 9 is stored in a container such as a colander and measurement is performed, the bottom area of the container may be the bottom area S ₁ of the measurement object 9.

The specific activity A [Bq / kg] of the object to be measured 9 is A = const × N ₀ / M, and when the correction coefficient Y is expressed by equation (A2), the specific activity A is expressed by equation (1). Can do. Further, when N _et = N / M, Expression (1) can be expressed as Expression (A3).

The coefficient a and the coefficient b depend on the density of the object 9 to be measured. For example, when measuring a plurality of objects to be measured 9 of the same type together, the density may be set to a constant value for those having substantially the same degree of adhesion between the objects to be measured 9. When the main component of the object to be measured 9 is food, the specific gravity in a state containing moisture is 0.9 to 1.2, and the coefficients a and b may be constant values for measuring the specific activity of the food.
In addition, foodstuffs, such as agricultural and livestock products, marine products, etc. can be mentioned as the to-be-measured object 9 by the nondestructive radioactivity measuring apparatus 100 which concerns on embodiment of this invention.
When the specific gravity of the DUT 9 is large, the coefficients a and b are measured separately. The specific activity A [Bq / kg] also depends on the ratio of the area S ₀ of the detection surface 1a of the gamma ray detection unit 1 to the bottom area S ₁ of the object 9 to be measured, as shown in Equation (1). To do.

By the above-described (A) and (B), the object 9 such as food is not made into a fine, minced shape or the like, and the radiation 9 (gamma rays) from the object 9 is measured without changing the object 9 as it is. Detected and the count value per unit time and unit mass (N / M) [count · sec ⁻¹ · kg ⁻¹ ] of the radiation (gamma ray) from the object 9 to be measured, and the primary relating to the mass of the object 9 to be measured The specific activity A [Bq / kg] can be easily calculated by the product of the absorption correction coefficient Y in the equation.

Hereinafter, an example of the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention will be described in detail.
<Configuration of Nondestructive Radioactivity Measuring Device 100>
As shown in FIGS. 1 and 2, the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention includes a gamma ray detection unit 1, an amplifier 2, an analog-digital conversion unit 3 (ADC), and a mass measurement unit. 4, a control device 5, and the like.

  The mass measuring unit 4 includes a placement surface 4a and places the object 9 to be measured. The mounting surface 4a is a part of the mass measuring unit 4, and automatically measures the mass of the object 9 to be measured and outputs it to the computer (control device 5). The gamma ray detection unit 1 outputs a signal corresponding to the energy of gamma rays from the measurement object 9 placed on the placement surface 4a. The placement surface 4a is supported by the support portion 4b.

In the present embodiment, the gamma ray detection unit 1 is accommodated in the hollow shield 15. The shield 15 is made of lead, concrete, or the like. The shield 15 reduces the background in the gamma ray detection unit 1 by blocking radiation from outside the object to be measured 9.
In the present embodiment, the shield 15 is a mass measuring unit 4 having a planar mounting surface 4 a, a gamma ray detection unit 1, and an object to be measured 9 mounted on the mounting surface 4 a of the mass measuring unit 4. It is comprised so that it may cover.

  In the present embodiment, the shield 15 includes a bottomed quadrangular prism or cylinder-shaped main body portion 15a with an opening formed in the upper portion, and a lid portion 15b provided to be openable and closable with respect to the opening portion of the main body portion 15a. Have.

The gamma ray detection unit 1 includes a scintillator and a photomultiplier tube. The scintillator fluoresces upon receiving radiation (gamma rays in this embodiment) from a radioactive substance contained in the measurement object 9 such as food. Examples of the scintillator include NaI (Tl) in which a small amount of thallium (Tl) is added to sodium iodine (NaI), CsI (Tl) in which a small amount of thallium (Tl) is added to cesium iodide CsI, and the like. it can.
The photomultiplier tube converts the light energy of the fluorescence from the scintillator into electric energy and outputs a detection signal by the photoelectric effect.
Note that the gamma ray detection unit 1 is not limited to this configuration, and may be a semiconductor detector such as a germanium semiconductor detector or a silicon semiconductor detector as long as it can measure gamma ray energy.

  The amplifier 2 amplifies and outputs the level of the signal output from the gamma ray detection unit 1.

  The analog / digital conversion unit 3 (ADC) performs analog / digital conversion processing on the signal (analog signal) output from the amplifier 2 and outputs a detection signal to the control device 5 as a digital signal.

  The mass measuring unit 4 measures the mass of the measurement object 9 and outputs a signal indicating the mass of the measurement object 9 as a measurement result to the control device 5. As the mass measuring unit 4, a load cell type, an electronic type, or the like can be adopted.

  The control device 5 (computer) comprehensively controls each component of the nondestructive radioactivity measurement device 100. Further, the control device 5 is based on the signal detected by the gamma ray detection unit 1 according to the energy of the gamma rays from the radioactive substance in the measurement object 9 and the mass of the measurement object 9 by the mass measurement unit 4. The specific activity [Bq / kg] of the DUT 9 is calculated.

  Specifically, the control device 5 includes a control unit 51 (CPU), a RAM 52, a ROM 53, an operation input unit 54, an interface 55 (I / F), a communication unit 56, a time measuring unit 57, and a display unit. 58 and the like. Each component such as the control unit 51 (CPU) is electrically connected by a communication line 59 such as a bus.

  The control unit 51 (CPU) comprehensively controls each component of the control device 5. The control unit 51 (CPU) executes the program (PRG) stored in the storage unit such as the RAM 52 or the ROM 53, thereby allowing the nondestructive radioactivity measurement apparatus 100 (control apparatus 5) as a computer to be in accordance with the present invention. Realize the function.

The RAM 52 is used as a work area for executing a program.
The ROM 53 stores a measurement (control) program and the like, and the program is read and executed by the control unit 51.
The operation input unit 54 includes operation input devices such as operation buttons, switches, and a mouse, and outputs a signal corresponding to the operation of the operator to the control unit 51.

  The interface 55 (I / F) is connected to the gamma ray detection unit 1 through the analog-digital conversion unit 3 and the amplifier 2. Further, the mass measuring unit 4 is connected to the interface 55.

  The communication unit 56 is configured to be communicable with another computer via a wired or wireless communication path under the control of the control unit 51.

The timer unit 57 includes a timer circuit, measures a time such as a time, and outputs time information to the control unit 51.
The display unit 58 displays a predetermined display process according to the present invention, in this embodiment, the specific activity [Bq / kg] or the like of the object 9 to be measured, under the control of the control unit 51.

  The control unit 51 (CPU) implements the function according to the present invention in the control device 5 as a computer by executing a program (PRG) stored in a storage unit such as the RAM 52 or the ROM 53. Specifically, the control unit 51 (CPU) includes a gamma ray detection control unit 511, a counting unit 512, a mass measurement control unit 513, a calculation unit 514, and the like.

  The gamma ray detection control unit 511 controls the specific radioactivity measurement for the gamma ray detection unit 1, the amplifier 2, the analog-digital conversion unit 3, and the like.

  The counting unit 512 counts a signal indicating a level equal to or higher than a threshold value with a predetermined energy based on a signal from the gamma ray detection unit 1, and outputs a count value for a predetermined time (for example, unit time (1 second)).

<Energy spectrum of gamma rays>
FIG. 4 is a diagram showing an example of an energy spectrum of gamma rays. In FIG. 4, the horizontal axis represents gamma ray energy [keV], and the vertical axis represents the count value N.
For example, as shown in FIG. 4, the gamma ray detection unit 1 obtains an energy spectrum of radiation (gamma rays) emitted by radioactive decay of the radionuclide in the DUT 9. Specifically, an energy peak corresponding to the radionuclide, for example, an energy peak corresponding to ¹³⁷ Cs (662 keV), an energy peak corresponding to ¹³⁴ Cs (a plurality of energy peaks), and the like are obtained.
The counting unit 512 can perform counting by discriminating each radionuclide. The counting unit 512 can perform counting for each radionuclide except for background counting.

Specifically, for each radionuclide, the counting unit 512 discriminates and counts energy peaks corresponding to gamma rays emitted when the nuclide decays. At this time, a count value [counts / sec] per unit time obtained by subtracting the background is calculated.
The counting unit 512 may calculate and output a count value [counts / sec] per unit time (1 second) after counting the gamma rays by the gamma ray detection unit 1 for a time of about several seconds to several minutes. .
In the present embodiment, the counting unit 512 is configured to output a count value for an energy peak corresponding to ¹³⁷ Cs, but may be configured to output a count value for other nuclides. .

  In the present embodiment, the function of the counting unit 512 is realized in the computer by the control unit 51 executing the program. However, the present invention is not limited to this mode, and for example, has the same function as the counting unit 512. A dedicated device may be provided separately from the control device 5.

  As shown in FIG. 2, the mass measurement control unit 513 performs control related to mass measurement by the mass measurement unit 4.

The calculation unit 514 calculates the specific radioactivity of the measurement object 9 based on the count value obtained by the counting unit 512, the mass of the measurement object 9 measured by the mass measurement unit 4, and the mathematical formula (1). Specifically, the calculation unit 514 includes a count value N [counts / sec] per unit time by the counting unit 512, a mass M [kg] of the measurement object 9 measured by the mass measurement unit 4, and the measurement object 9. Based on the bottom area S ₁ and the mathematical formula (1) described above, the specific activity A [Bq / kg] of the DUT 9 is calculated.
This mathematical formula (1) is a gamma ray absorption correction coefficient represented by a linear expression related to the reciprocal of the mass M and the bottom area S ₁ of the DUT 9 obtained in advance by actual measurement.

In addition, gamma ray measurement was performed in advance using a standard sample (specific gravity 1.2) having a known cesium concentration, and the specific activity [Bq / kg] per unit mass and unit time and unit of radiation (gamma ray). The value of the ratio of the count value per mass [counts · sec ⁻¹ · kg ⁻¹ ] obtains a relationship represented by a linear equation for the reciprocal number of the mass M and the bottom area S ₁ of the object 9 to be measured. Based on the measurement result, the coefficient a and the coefficient b of the formula (1) are calculated, and the formula (1), the coefficient a, and the coefficient b are stored in a storage unit such as the RAM 52 and the ROM 53.

<Operation of Nondestructive Radioactivity Measuring Device 100>
Next, an example of the operation of the nondestructive radioactivity measuring apparatus 100 will be described.
FIG. 5 is a flowchart showing an example of the operation of the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention.

<Determining Coefficient a and Coefficient b by Radioactivity Measurement Using Standard Sample (Standard Radiation Source)>
First, gamma rays are measured by the nondestructive radioactivity measuring apparatus 100 using a standard sample (standard radiation source), and the coefficient a and coefficient b used in the nondestructive radioactivity measuring apparatus 100 are determined. Specifically, prepare a standard sample with the specified specific activity. At this time, a plurality of standard samples having different masses and bottom areas are prepared.
The nondestructive radioactivity measurement apparatus 100 according to the embodiment of the present invention measures the mass of each of the plurality of standard radiation sources by the mass measurement unit 4 and counts the gamma rays from the standard radiation source by the gamma ray detection unit 1 or the like. Then, the coefficient a and the coefficient b are determined based on the obtained count value and mass. In addition, when the mass of a standard sample is prescribed | regulated previously, you may use the mass.

<Measurement>
As shown in FIGS. 1 and 2, first, the object 9 having the bottom area S ₁ is placed on the detection surface 1 a or the placement surface 4 a of the gamma ray detector 1. At this time, the object to be measured 9 may be accommodated in a container having a prescribed bottom area (for example, a light monkey), and the bottom area of the container may be the bottom area S ₁ of the object 9 to be measured. In this case, a plurality of containers having different bottom areas are prepared, and the measurement is performed on a container suitable for the object to be measured 9, specifically, a container having an inner diameter substantially the same as or slightly smaller than the width of the object to be measured 9. by accommodating the object 9 may be substituted by the bottom area of the container as the bottom surface area S ₁ of the object to be measured 9. By doing so, the bottom area S ₁ of the DUT 9 can be easily defined. Further, when measuring a plurality of objects to be measured 9 of the same type (for example, a plurality of peaches), the bottom of the object to be measured 9 is accommodated in a container such as a monkey (a container having a known bottom area). The area S ₁ can be easily defined.
In addition, when measuring the several to-be-measured object 9 of the same kind collectively, it is preferable to mount on the mounting surface 4a (detection surface 1a) with the clearance gap between the to-be-measured objects 9 being the minimum.

  As shown in FIG. 5, in step ST <b> 1, the control unit 51 performs a process of measuring the mass [kg] of the DUT 9 by the mass measurement unit 4.

  In step ST3, the control unit 51 performs processing for detecting gamma rays by the gamma ray detection unit 1. The signal from the gamma ray detection unit 1 is amplified by the amplifier 2, subjected to analog / digital conversion by the analog / digital conversion unit 3, and then output to the control device 5 (computer).

  In step ST <b> 5, the counting unit 512 of the control unit 51 measures the energy spectrum based on the signal from the gamma ray detection unit 1 and takes a ROI (region of interest), thereby measuring the radioactive substance of the object 9 to be measured. The gamma rays from are counted and a count value for a predetermined time is output.

In step ST7, calculator 514 of the controller 51, the count value by the counting unit 512, the mass of the object to be measured 9 measured by the weight measuring unit 4, the bottom surface area S ₁ of the object 9, and Equation (1) Based on the above, the specific radioactivity of the DUT 9 is calculated. Specifically, the calculation unit 514 includes a count value N [counts / sec] per unit time by the counting unit 512, a mass M [kg] of the measurement object 9 measured by the mass measurement unit 4, and the measurement object 9. Based on the bottom area S ₁ and the formula (1) or the formula (A3), the specific activity A [Bq / kg] of the DUT 9 is calculated.

In step ST <b> 9, the control unit 51 performs processing for displaying the specific activity A [Bq / kg] of the DUT 9 calculated by the calculation unit 514 on the display unit 58.
Further, the control unit 51 may perform a process of transmitting the specific activity A [Bq / kg] of the DUT 9 calculated by the calculation unit 514 to another computer via the communication unit 56.

<Experiment>
In order to confirm the effect of the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention, the inventor of the present application made the measurement object 9 as it is (the present invention) and made the measurement object 9 finely minced. Specific radioactivity [Bq / kg] was measured in each state (comparative example).

  FIG. 6 shows the specific activity [Bq / kg] (invention) in which the object 9 is measured as it is (measured as a whole), and the specific activity in which the object 9 is measured in a fine state such as mince [ It is a figure which shows an example of the relationship of [Bq / kg] (comparative example). In FIG. 6, the abscissa indicates the specific activity [Bq / kg] (comparative example) obtained by measuring the object to be measured 9 in a fine state such as a minced shape, and the ordinate measures the object to be measured 9 as it is ( Specific activity [Bq / kg] (invention) measured as a whole).

Specific activity [Bq / kg] (invention) in which the object to be measured 9 is measured as it is (whole measurement), and specific activity [Bq / kg] in which the object 9 is measured in a fine state such as mince. It was found that (Comparative Example) substantially matches as shown in FIG.
That is, the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention measures the measured object 9 as it is (entirely), in a short time, easily and accurately compared to the comparative example. The specific activity [Bq / kg] of the product 9 can be obtained.

<Relationship Between Bottom Area S _{1 of} Object 9 and Area S ₀ of Detection Surface 1a of Gamma Ray Detecting Unit 1>
Next, the inventor of the present application changed the bottom area S ₁ of the DUT 9 and the area S ₀ of the detection surface 1a of the gamma ray detector 1 in the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention. In this case, the range in which gamma rays can be measured with high accuracy was examined.
FIG. 7 is a perspective view of the gamma ray detector 1 of the nondestructive radiation measuring apparatus 100 according to the embodiment of the present invention.

  As described above, the mathematical formula (A4) can be derived from the mathematical formula (1) used in the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention.

N is a count value per unit time [counts / sec] of gamma rays, M is a mass [kg] of the object 9 to be measured, A is a specific activity [Bq / kg], S ₁ is a bottom area of the object 9 to be measured, S ₀ is the area of the detection surface 1a of the gamma ray detector 1, and a and b are coefficients.

  As shown in the mathematical formula (A4), regarding the reciprocal of the actually measured gamma ray unit time and the count value per unit mass (N / M) and each parameter (coefficient), a conversion formula for each parameter (the mathematical formula (A4)) Examine the scope of application.

Specifically, the formula (A4) is a linear function of S ₀ / S ₁ . The specific radioactivity A and mass M of the standard sample were set to specified values, respectively, and the application range of the formula (A4) related to S ₀ / S ₁ was examined under the conditions of a plurality of different bottom areas S ₁ .

FIG. 8 is a diagram showing an example of the dependence of the bottom area S _{1 of the} object 9 to be measured and the area S ₀ of the detection surface 1a of the gamma ray detector ₁ with respect to the unit time of gamma rays and the count value (reciprocal) per unit mass. is there.
In FIG. 8, the vertical axis indicates the reciprocal of the actually measured gamma ray unit time and the count value (N / M) per unit mass, and the horizontal axis indicates S ₀ / S ₁ . S ₁ indicates the bottom area of the object 9 to be measured, and S ₀ indicates the area of the detection surface 1 a of the gamma ray detection unit 1.

As shown in FIG. 8, with respect to the mathematical formula (A4), it was found that the linearity was established when S ₀ / S ₁ was about 0.8 or more, and the linearity was not established when the value was less than 0.8.
That is, in the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention, when the condition “S ₀ / S ₁ is about 0.8 or more” is satisfied, the conversion formula (formula (A4)) is accurately calculated. Can be applied.

<Applicable range of geometric conditions of gamma ray detector and measurement object>
Next, the inventor of the present application examined the application range of the geometric conditions of the gamma ray detector 1 and the object 9 to be measured in the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention.

Specifically, the mathematical formula (A4) is a linear function related to the mass M of the DUT 9. A specific sample having a density of 1 [g / cm ³ ] = 10 ³ [kg / m ³ ] with specific activity A and bottom area S ₁ as specified values (fixed values), respectively, can be expressed in a number of different mass conditions. The application range of A4) was examined. The mass of the standard sample is defined by the product of the density (fixed value) of the standard sample, the bottom area (fixed value) of the standard sample, and the height (parameter).

FIG. 9 is a diagram for explaining an example of gamma ray measurement. Specifically, FIG. 9A shows a case where S ₀ / S ₁ = 1.00, FIG. 9B shows a case where S ₀ / S ₁ = 1.66, and FIG. 9C shows S ₀ / S. It is a figure for demonstrating each measurement in the case of ₁ = 11.7.

A gamma ray detector 1 having a plurality of detection surfaces 1a having different diameters R was prepared. As the cylindrical standard sample as the object 9 to be measured, one having a constant area (20 cm ² ) of the bottom area S ₁ is adopted, and the cylindrical standard sample is used as the gamma ray detector 1 as shown in FIG. Measurement was performed by stacking on the detection surface 1a.
Further, h is a distance (height) between the center of gravity of the added standard sample block and the detection surface 1a of the gamma ray detection unit 1. In this embodiment, cylindrical standard samples were stacked, and the measurement was performed up to the maximum value h _max of the height h = about 43.5 cm.
In this case, a gamma ray can be easily measured by accommodating the standard sample as the DUT 9 in a bottomed cylindrical or polygonal cylindrical monkey.

<S ₀ / S ₁ = 1.00>
FIG. 10 is a diagram showing the mass dependence of the count value (count number) [counts · sec ⁻¹ ] per unit time when S ₀ / S ₁ = 1.00. In FIG. 10, the vertical axis indicates the count value (count number) of gamma rays, and the horizontal axis indicates h / R.
As shown in FIG. 10, when h / R was about 3 or more (M = about 0.3 kg or more), the count value (count number) showed a substantially constant value (saturated state). That is, it was found that the count value (count number) was saturated when the height h was 16.25 cm or more.

As the height h of the standard sample as the object to be measured 9 increases, the distance of the optical path of the radiation emitted from the upper part of the standard sample to the gamma ray detection unit 1 increases, and the solid angle at which the gamma ray detection unit is desired from the upper part of the standard sample. Becomes very small, and the self-absorption of the DUT 9 increases, and the contribution to the count number becomes very small.
For this reason, when the mass M is increased to become a predetermined mass or more (a predetermined height h or more), the count number becomes a constant value (saturates).

  This saturation phenomenon can also be predicted from Equation (1). Specifically, in the formula (1), when the mass M is very large, the count value N [counts / sec] per unit time becomes the formula (A5) and approaches a constant value.

The saturation effect of the count number (count value) is precisely the solid angle at which the detection surface 1a of the gamma ray detector 1 is desired from each position in the object 9 and the gamma ray detector from each position in the object 9 It can be evaluated by complex three-dimensional numerical calculations incorporating self-absorption effects between 1 and 1.
In the nondestructive radioactivity measuring apparatus 100 according to the present invention, the specific radioactivity is easily calculated by the mathematical expressions (A4) and (1) which are linear functions of the mass M without performing such complicated numerical calculation. can do. If this mathematical formula (A4) is established with high accuracy even in the saturation region described above, the usefulness of the mathematical formula (1) can be shown.

FIG. 11 is a diagram showing an example of a graph obtained by converting the actually measured data shown in FIG. 10 by the conversion formula (formula (A4)) when S ₀ / S ₁ = 1.00. In FIG. 11, the vertical axis represents the gamma ray unit time and the count value (reciprocal) per unit mass, and the horizontal axis represents the mass M [kg] of the DUT 9 (standard sample).

In FIG. 11, the mathematical formula (A4) indicates that the mass M is larger than 0 kg, and that the mass in the saturation region described above holds.
This is because, when the density of the object to be measured 9 is substantially uniform (uniform) and the distribution of the radioactive substance is substantially uniform (uniform), the mass M is measured by measuring the mass M of the object to be measured 9. It shows that the specific activity can be measured with high accuracy by the formula (1) and the formula (A4) in a range larger than 0 kg. That is, there is no limitation on the mass of the specific radioactivity measurement method based on the mathematical formula (A3).
That is, in the nondestructive radioactivity measuring apparatus 100 according to the present embodiment, the mass M and the count of gamma rays of the object 9 are measured regardless of whether the object 9 is small or large. By doing so, specific radioactivity can be measured with high accuracy by the formulas (1) and (A4).

<S ₀ / S ₁ = 1.66>
FIG. 12 is a diagram showing the mass dependence of the count value (count number) [counts · sec ⁻¹ ] per unit time when S ₀ / S ₁ = 1.66. FIG. 13 is a diagram showing an example of a graph obtained by converting the actual measurement data shown in FIG. 12 by a conversion formula (formula (A4)) when S ₀ / S ₁ = 1.66.

As shown in FIG. 11, when h / R was about 3 or more (M = about 0.3 kg or more), the count value (count number) showed a substantially constant value (saturated state). That is, it was found that the count value (count number) was saturated when the height h was 19.5 cm or more. In the saturation region, h / R is about 3 or more as in the case of S ₀ / S ₁ = 1.00 described above.
This is because there exists a path of gamma rays that reach the vicinity of the end of the gamma ray detection unit 1 from the upper part of the measurement object 9 without being self-absorbed.
Thus, it was found that the formula (A3) also holds when the area S ₀ of the detection surface 1a of the gamma ray detection unit 1 is slightly larger than the standard sample of the object 9 to be measured.

<S ₀ / S ₁ = 11.7>
FIG. 14 is a diagram showing the mass dependence of the count value (count number) [counts · sec ⁻¹ ] per unit time when S ₀ / S ₁ = 11.7. FIG. 15 is a diagram showing an example of a graph obtained by converting the actual measurement data shown in FIG. 14 by a conversion formula (formula (A4)) when S ₀ / S ₁ = 11.7.

FIG. 14 shows that no saturation occurs even when the height h is 43.5 cm. This is due to the reason described later.
In the measurement under this condition, the diameter R of the detection surface 1 a of the gamma ray detection unit 1 is sufficiently larger than the diameter of the bottom surface of the object 9 to be measured.
As shown in FIG. 9C, among the radiation (gamma rays) radiated from the upper part of the object 9 to be measured, the radiation radiated along the substantially downward direction through the object 9 is measured. Due to the self-absorption of the object 9, the amount reaching the gamma ray detection unit 1 becomes very small.
As shown in FIG. 9 (c), among the radiation (gamma rays) emitted from the upper part of the object 9 to be measured, the radiation emitted in an oblique direction of a predetermined angle or more slightly passes through the object 9 to be measured. It passes through or does not pass through the object 9 to be measured, and reaches the gamma ray detector 1 without being self-absorbed (oblique effect).

Due to this oblique effect, the count value (count number) is not saturated even when the height h is 43.5 cm. Moreover, it is shown that the mathematical formula (A4) holds even when the detection amount due to the oblique effect increases.
That is, the usefulness of the conversion formula (formula (A4)) was shown.
From the above experimental results, it was shown that the conversion formula (A3) is useful without depending on S ₀ / S ₁ and the mass of the DUT 9.

As described above, the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention maintains the shape of the object 9 to be measured without destroying the object 9 to be measured such as food. Measure specific activity.
Specifically, in the present embodiment, the nondestructive radioactivity measuring apparatus 100 includes the object 9 to be measured and includes a flat mounting surface 4 a having the same width as the detection surface 1 a of the gamma ray detection unit 1. Based on the signal from the gamma ray detection unit 1 that outputs a signal corresponding to the energy of the gamma ray from the object to be measured 9 placed on the placement surface 4a so as to fill the entire area with a minimum gap, A counting unit 512 that counts a signal corresponding to a predetermined energy and outputs a count value for a predetermined time, a mass measuring unit 4 that measures the mass of the object 9 to be measured, and a count value per unit time by the counting unit 512 [ counts / sec], the mass [kg] of the object 9 measured by the mass measuring unit 4, the bottom area S _{1 of} the object 9 to be measured, and the specific radiation of the object 9 based on Equation (1) And a calculation unit 514 that calculates the performance [Bq / kg].
In this way, it is possible to provide the nondestructive radioactivity measuring apparatus 100 that can easily measure the specific activity of the object 9 without breaking the object 9 such as food.
Further, in this nondestructive radioactivity measuring apparatus 100, the specific activity can be measured as it is without making the object 9 fine, and the measured object 9 is distributed as it is. Can be made.

  In addition, the nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention includes a display unit 58 that displays the specific activity [Bq / kg] of the measurement target 9 calculated by the calculation unit 514. For this reason, the measurement object 9 is placed on the placement surface 4a in the main body 15a of the shield 15 by the operator, and the opening of the main body 15a is covered with the lid 15b. When the shield 15 is shielded and the measurement start button or the like is operated, the measurement is started, and after the measurement is completed, the specific radioactivity [Bq / kg] of the object to be measured 9 is displayed on the display unit 58. The specific radioactivity [Bq / kg] of the DUT 9 can be easily notified to the person.

The measured object 9 is preferably a food such as agricultural and livestock products and marine products. In this case, since the object to be measured 9 has a specific gravity of approximately 1 (0.9 to 1.2), the object to be measured 9 is placed on the placement surface 4a so as to fill the entire area with a minimum gap. The same primary equation can be used for the mass M [kg] of the DUT 9. When the bottom area of the object 9 to be measured is larger or smaller than the detection surface 1a, a primary correction equation corresponding to the bottom area is obtained and quantified.
Moreover, it is preferable that the to-be-measured object 9 has a substantially uniform distribution of the radioactive substance, and the absorption correction coefficient can be represented by a linear expression as represented by Expression (1).

Moreover, the nondestructive radioactivity measuring apparatus 100 which concerns on embodiment of this invention is the shield made from lead which covers the to-be-measured object 9 mounted in the mounting surface 4a of the gamma ray detection part 1 and the mass measurement part 4. FIG. 15
For this reason, the unnecessary background from the outside to the gamma ray detection part 1 can be interrupted | blocked, and the nondestructive radioactivity measuring apparatus 100 which can measure a specific radioactivity with high precision can be provided.

  Agricultural and livestock products and marine products that are radioactively contaminated by the Fukushima Daiichi Nuclear Power Plant accident may contain radioactive substances such as radioactive cesium. . Such food contamination problems are not only related to consumer safety, but also to producers' willingness to work and livelihoods. The nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention can inspect the entire object to be measured 9 such as food, gives the consumer peace of mind and motivates the producer. It can be expected that it will contribute to the recovery of the agriculture and fisheries industry.

As described above, the embodiments of the present invention have been described in detail with reference to the drawings. However, the specific configuration is not limited to these embodiments, and there are design changes and the like without departing from the gist of the present invention. Is included in the present invention.
Further, the embodiments described in the above drawings can be combined with each other as long as there is no particular contradiction or problem in the purpose and configuration.
Moreover, the description content of each figure can become independent embodiment, respectively, and embodiment of this invention is not limited to one embodiment which combined each figure.

  In the above embodiment, the gamma ray detection unit has the scintillator and the photomultiplier tube. However, the present invention is not limited to this form. As the gamma ray detection unit, a semiconductor detection device such as a germanium semiconductor detection device or a silicon semiconductor detection device may be employed.

In addition, the mass measurement part 4 may be comprised so that the mass of the to-be-measured object 9 mounted in the mounting surface 4a may be measured. By doing so, when the object to be measured 9 is placed on the placement surface 4a, the mass measuring unit 4 can measure the mass and the gamma ray intensity by the gamma ray detecting unit 1 and the counting unit 512 can be measured. .
The mass measuring unit 4 may be provided outside the shield 15.

  The nondestructive radioactivity measuring apparatus 100 according to the embodiment of the present invention includes the gamma ray detection unit as the radiation detection unit. However, the present invention is not limited to this configuration and is configured to detect radiation other than gamma rays. May be.

  Moreover, in the said embodiment, although the specific activity [Bq / kg] was calculated, a unit is not restricted to this form, You may convert into a desired unit.

As mentioned above, although embodiment of this invention was described, some or all of embodiment of this invention is described as the following additional remarks.
[Appendix 1]
A non-destructive radioactivity measuring apparatus that measures the specific radioactivity of a measurement object without destroying the measurement object such as food,
A gamma ray detector that outputs a signal corresponding to the energy of gamma rays from the object to be measured placed on the placement surface;
A counting unit that counts a signal indicating energy in a predetermined range based on the signal from the gamma ray detection unit and outputs a count value for a predetermined time;
A mass measuring unit for measuring the mass of the object to be measured;
Based on the count value obtained by the counting unit, the mass of the measured object measured by the mass measuring unit, the bottom area of the measured object, and the specific activity [Bq / Kg], a non-destructive radioactivity measuring device.
[Appendix 2]
The nondestructive radioactivity measuring apparatus according to appendix 1, wherein S ₀ / S ₁ is 0.8 or more.
[Appendix 3]
A radioactivity measurement method of the nondestructive radioactivity measurement apparatus according to appendix 1,
A first step in which the mass measuring unit measures the mass of the object to be measured;
A second step in which the gamma ray detection unit and the counting unit output a count value of gamma rays from the object to be measured for a predetermined time;
Based on the count value by the counting unit, the mass of the measurement object measured by the mass measurement unit, the bottom area of the measurement object, and the mathematical expression (1), the calculation unit And a third step of calculating a specific activity [Bq / kg]. A radioactivity measurement method for a nondestructive radioactivity measurement device.

DESCRIPTION OF SYMBOLS 1 Gamma ray detection part 1a Detection surface 2 Amplifier 3 Analog digital conversion part (ADC)
4 Mass measuring unit 4a Placement surface 5 Control device 9 Object to be measured (food such as agricultural and livestock products and marine products, soil, etc.)
15 Shield (lead shield etc.)
51 Control unit (CPU)
52 RAM
53 ROM
54 Operation input section 55 Interface (I / F)
56 Communication Unit 57 Timekeeping Unit 58 Display Unit 59 Communication Line (Bus)
511 Gamma ray detection control unit 512 Counting unit 513 Mass measurement control unit 514 Calculation unit

Claims (2)

A non-destructive radioactivity measuring apparatus that measures the specific radioactivity of a measurement object without destroying the measurement object such as food,
A gamma ray detector that outputs a signal corresponding to the energy of gamma rays from the object to be measured placed on the placement surface;
A counting unit that counts a signal indicating energy in a predetermined range based on the signal from the gamma ray detection unit and outputs a count value for a predetermined time;
A mass measuring unit for measuring the mass of the object to be measured;
Calculation for calculating the specific activity [Bq / kg] of the object to be measured based on the count value obtained by the counting unit, the mass of the object measured by the mass measuring unit, and the mathematical expression (1). And a nondestructive radioactivity measuring device.

Where A is the specific activity [Bq / kg],
N is a count value per unit time [counts / sec], M is a mass [kg],
N / M is a count value per unit time and unit mass [counts · sec ⁻¹ · kg ⁻¹ ],
S ₀ is the area [m ² ] of the detection surface of the gamma ray detector, S ₁ is the bottom area [m ² ] of the object to be measured,
a is a coefficient (positive number), b is a coefficient (positive number). A radioactivity measurement method of the nondestructive radioactivity measurement apparatus according to claim 1,
A first step in which the mass measuring unit measures the mass of the object to be measured;
A second step in which the gamma ray detection unit and the counting unit output a count value of gamma rays from the object to be measured for a predetermined time;
Based on the count value obtained by the counting unit, the mass of the measurement object measured by the mass measurement unit, and the mathematical expression (1), the calculation unit calculates the specific activity [Bq / kg of the measurement object. And a third step of calculating the radioactivity of the nondestructive radioactivity measurement apparatus.

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