JP6978091B2 - Density measuring device and density measuring method - Google Patents

Description

The present invention relates to a density measuring device and a density measuring method for detecting the density of a board such as a construction site.

At a construction site, etc., the density of the substance of the board (measurement target) of the construction site, etc. is measured using a density measuring device.
The number of times the density is measured is specified for each area to be measured, and is called the "specified number of points". A measurer such as a construction contractor moves within the measurement target to position the measurement point corresponding to the specified number of points, transports and installs the density measuring device on the ground surface or underground of the measurement point, and determines the density of the measurement point. The measurement is repeated until the specified number of points is reached.

The density measuring device includes a radiation source that generates radiation and a radiation detector that detects radiation (more accurately, electrons emitted by reacting with atoms of a substance).
There are three types of radiation: alpha rays, beta rays, and gamma rays, but gamma rays are often used in density measuring devices because of their penetrating power. Radioactive sources such as cobalt-60 (Co-60), cesium-137 (Cs-137), barium 133 (Ba-133), californium 252 (Cf-252), etc. as radioactive sources (gamma ray sources) that generate gamma rays. Is used.

Examples of the radiation detector used in the density measuring device include a Geiger-Muller (GM) counter tube, a scintillation type gamma ray detector, a semiconductor detector, etc., but the description thereof is omitted because it is not a main part of the present invention. ..

The measured gamma rays are mainly represented by a "counting rate" indicating the number (counts) of gamma rays incident on the radiation detector per unit time.
It is known that there is a certain relationship between the gamma ray dose (counting rate) detected by the radiation detector and the density of the substance to be measured, and these relationships are expressed by a calibration formula. That is, the density of the measurement target is calculated from the gamma ray dose detected by the radiation detector based on the calibration formula prepared in advance.

By the way, radiation containing gamma rays is also emitted from space, air, and soil, and naturally exists in nature. Radiation emitted from the natural world is called background (natural radiation). The background contains, for example, gamma rays due to potassium-40 (K-40).

In the measurement target, if the background is incident from the outside of the density measuring device, an error occurs in the measured value of the density measuring device (specifically, the radiation detector), and the radiation (gamma ray) dose of the measurement target cannot be measured accurately.
Therefore, it is known that the correction process excluding the influence of the background is performed in the density measuring apparatus (for example, JP-A-2018-151225, JP-A-06-323980).

In JP-A-2018-151225, the radiation source is first removed and then the background is measured, and then the radiation source is reattached to measure the object to be measured. As described above, a method of calculating (subtracting) the difference between the measured value of the background and the measured value of the measured object to obtain the radiation dose of only the measured object is usually performed.
Further, in Japanese Patent Application Laid-Open No. 06-323980, since the value of gamma ray is different every day, the standard value is obtained by first measuring the gamma ray dose every day of measurement. Specifically, first measure the standard background count rate NS _BG excluding the gamma ray source (radioactive source), and then attach the gamma ray source (radioactive source) and measure the standard gamma ray count rate NS. There is. Then, the correction processing is performed with _{the count rate NS B} of the gamma ray as a standard value _{as NS B} = NS-NS _BG.
Next, the measurement points corresponding to the specified number of points are positioned, and at the measurement points, the count rate NF _BG of the background gamma rays excluding the radiation source and the gamma ray dose NF with the gamma ray source (radioactive source) attached are measured. The correction processing is performed with _{the count rate NF B} of the gamma ray as the measured value _{as NF B} = NF-NF _BG.
Then, the gamma count ratio RG = NF _B / NS _B is calculated, and this is applied to the calibration formula to calculate the density of the measurement target.

Japanese Unexamined Patent Publication No. 2018-151225 Japanese Unexamined Patent Publication No. 06-323980

According to Japanese Patent Application Laid-Open No. 2018-151225, the actual radiation (gamma ray) dose can be obtained by a simple correction of subtraction.
According to Japanese Patent Application Laid-Open No. 06-323980, the background count rate is corrected by subtracting the background count rate from the measured radiation (gamma ray) count rate in both the standard value and the measured value. Also in JP-A-06-323980, the actual radiation (gamma-ray) dose can be obtained by a simple correction of subtraction, as in JP-A-2018-151225.

However, for radiation measurement in a measurement target such as a construction site, as in JP-A-2018-151225, usually, the background is measured only once, and then the underground measurement of the construction site is performed up to the specified score. A repeating measurement flow is adopted. That is, the correction process of subtracting the background count rate measured only once from the count rate measured multiple times at each measurement point up to the specified number of points is premised on a constant background.
In addition, in a site where the background is not constant, such as a vast site or a site where the soil quality is not constant, the correction process of subtracting based on the one-time background measurement accurately obtains the actual counting rate. Cannot be calculated.

It is conceivable that the background measurement is performed not only once but multiple times as in JP-A-06-323980. If the background is measured multiple times, the background count rate can be accurately grasped even in the field where the background is not constant.
However, if the background is measured a plurality of times, the measurement time is long and the burden on the measurer increases. In addition, the density measuring device is equipped with a radiation source and a shield that blocks the influence of radiation from the radiation source, and the possibility of increasing the size and weight cannot be denied.

An object of the present invention is to provide a density measuring device capable of reducing the number of times of measuring the density of a board to be measured, reducing the burden on the measurer, and realizing miniaturization and weight reduction. There is.
Further, the present invention provides a density measuring method capable of reducing the number of times of measuring the density of the board to be measured to reduce the burden on the measurer and realizing miniaturization and weight reduction. It has another purpose.

In order to achieve the above object, the density measuring device detects the gamma rays from the radiation source and the background together, and automatically estimates the counting rate (energy region) of only the gamma rays from the background from the detected values.
That is, according to the present invention according to claim 1, at least a radiation source that generates radiation to the measurement target and a radiation detector that detects the incident radiation are provided, and the density of the measurement target is measured by radiation. In the density measuring device, the radiation is gamma rays, the radiation detector can detect gamma rays from the radiation source and the background, and has a control device, and the control device is detected by the radiation detector. In the gamma ray spectrum, the energy value of the gamma ray corresponding to twice the half price width of the peak is calculated and set as a threshold value, only the energy region of the gamma ray equal to or higher than the threshold value is extracted, and the energy region of the extracted gamma ray is less than the threshold value. The energy region of the gamma ray is estimated, and the energy region of the gamma ray less than the estimated threshold value and the energy region of the extracted threshold value or more are added up to obtain the total energy region of the gamma ray by the background.
Further, according to the third aspect of the present invention, the generation step of generating and irradiating the measurement target for measuring the density with gamma rays, and the detection of detecting the gamma rays generated in the generation step and the gamma rays due to the background. In the step and the gamma ray spectrum detected in the detection step, the calculation step of calculating and setting the energy value of the gamma ray corresponding to twice the half price width of the peak as a threshold, and extracting only the energy region of the gamma ray above the threshold. The energy region of gamma rays below the threshold is estimated from the energy region above the threshold extracted in the extraction step, and the energy region of gamma rays below the estimated threshold and the energy region of gamma rays above the threshold extracted in the extraction step. It includes at least an estimation process that adds up the energy regions to make the total energy region of gamma rays by the background.

In the present invention according to claims 1 and 3, even if gamma rays from a radiation source and gamma rays from a background are detected together, the entire energy region of the background can be automatically estimated. Therefore, the influence (value) of gamma rays due to the background can be separated from the detected value (counting rate), and the influence (value) of only gamma rays from the radiation source can be grasped.
As a result, it is not necessary to remove the radiation source from the density measuring device (which embodies the density measuring method) and measure the background gamma rays in advance, and the number of measurements itself can be reduced. By reducing the number of measurements, the burden on the measurer can be reduced. In addition, a container for burying the radiation source in the ground is not required for background measurement, and the density measuring device can be made smaller and lighter.

The schematic side view of the density measuring apparatus which concerns on one Example of this invention (the apparatus which embodies the density measuring method which concerns on embodiment of this invention) is shown. The graph showing the energy spectrum (spectrum) of the measured cesium-137 and potassium-40 is shown. The flow chart of the density measurement (estimation) by the density measuring apparatus is shown. In the background, a graph showing a relational expression and a correlation for deriving the count rate in the entire energy region from the count rate in the energy region of gamma rays above the threshold value is shown.

In a density measuring device that includes at least a radiation source that emits radiation to a measurement target and a radiation detector that detects incident radiation, and measures the density of the measurement target by radiation, the radiation is gamma rays, and the radiation is said to be gamma rays. The detector has a control device as well as capable of detecting gamma rays from the radiation source and background, and the control device doubles the peak half-value width in the gamma ray spectrum detected by the radiation detector. The energy value of the corresponding gamma ray is calculated and set as a threshold, only the energy region of the gamma ray above the threshold is extracted, the energy region of the gamma ray below the threshold is estimated from the extracted energy region, and the energy region of the gamma ray below the threshold is estimated. The energy region of the gamma ray and the energy region of the above-extracted threshold value or more are added together to form the total energy region of the gamma ray by the background.

Hereinafter, examples of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a schematic side view of a density measuring device according to an embodiment of the present invention (a device embodying a density measuring method according to an embodiment of the present invention).

The density measuring device 10 (which embodies the density measuring method) is, for example, a scattering type RI (radioisotope) density measuring device. The density measuring device 10 may be provided with a water content measuring device (not shown).
The density measuring device (RI (radioisotope) density measuring device) 10 can move on the ground surface 22 of the construction site in order to measure the density of the substance of the board (measurement target, underground) 20 of the construction site or the like. is set up. It should be noted that the filler (not shown) may be filled without forming a gap between the bottom surface of the density measuring device 10 and the ground surface 22 at the construction site, but this is not the purpose of the present invention. In FIG. 1, the ground surface 22 is represented by a alternate long and short dash line.

As can be seen from FIG. 1, the density measuring device 10 includes at least a radiation source 12 that generates radiation to the measurement target (underground) 20 and a radiation detector 14 that detects incident radiation. Has been done. Further, in FIG. 1, a shield 16 that shields radiation is provided between the radiation source 12 and the radiation detector 14, but the configuration is not limited to this.

The radiation source 12 is tightly enclosed and stored in the storage portion 30, and is provided, for example, on the side portion of the density measuring device 10. In addition to the radiation source 12, for example, a neutron source (not shown) may be enclosed in the storage unit 30, and a substance to be enclosed can be added as needed. The radiation source 12 generates and scatters radiation (gamma ray γ, which will be described later) toward the ground 20 to be measured from a position slightly separated from the ground surface 22.

As the radiation source 12, a gamma ray source that generates gamma ray γ is used in the examples. The reference numeral γ includes gamma rays generated from the radiation source 12 and reflected by the substance 20-1 in the ground (more accurately, the electrons emitted by reacting with the atoms of the substance 20-1), and is due to the radiation source. Represents gamma rays.
Further, in the embodiment, cesium-137 (Cs-137) is used as the radiation source (gamma ray source) 12 that generates gamma ray γ. Cesium-137 is commonly used as a radiation source for scattered RI (radioisotope) density measuring devices.

The radiation detector 14 is capable of detecting gamma rays from the radiation source 12 and the background, and has a control device 14-1. Further, the radiation detector 14 is fixed to the bottom of the density measuring device 10, that is, near the ground surface 22 so as to be able to detect radiation, particularly gamma rays.
Not only the radiation (gamma ray) generated by the radiation source (gamma ray source) 12 but also the radiation (gamma ray; background) in the natural world from the outside of the radiation detector is slightly incident into the density measuring device 10. That is, in the radiation detector 14, both the gamma ray γ by the radiation source 12 using cesium 137 and the gamma ray γ'by the background are simultaneously incident and detected. The code γ'represents a background gamma ray.

The control device 14-1 is connected to the radiation detector 14. The control device 14-1 is composed of, for example, a CPU 14-1'(processor) having an information processing function, controls the density measuring device 10, and also executes various programs, such as a control unit 14-1a and a flash memory. Storage unit 14-1b that is composed of the storage medium of the above and stores information, input unit 14-1c that accepts input from input means 14-1c'such as a touch panel, keyboard, and buttons, and output (display) means 14 such as a display. It has an output unit 14-1d and the like that output to -1d'. In addition, the control device 14-1 may have a communication unit (not shown) that communicates with the outside via a network, and is not limited to this configuration.

In the storage unit 14-1b of the control device, for example, a density measurement program (not shown) and a calibration formula for calculating the density are stored. The control unit 14-1a calculates the count rate for the energy of gamma rays γ and γ'detected by the radiation detector 14, for example, by executing a density measurement program (not shown) stored in the storage unit 14-1b. A calculation means 14-1a1 (step S2-1; calculation step, which will be described later. FIG. (See), Extraction means 14-1a2 (step S2-2; extraction step) that extracts only the energy region of gamma rays above the threshold, estimates the energy region of gamma rays below the threshold from the extracted energy region, and is less than the estimated threshold. It functions as an estimation means 14-1a3 (step S2-3; estimation step) in which the energy region of gamma rays and the energy region equal to or higher than the extracted threshold are added up to form the total energy region of gamma rays by the background.

FIG. 2 shows a graph showing the measured energy spectra of cesium-137 and potassium-40. The vertical axis is the count rate (count number; cpm), and the horizontal axis is the energy of gamma rays (keV).
The dashed line in FIG. 2 shows the spectrum of cesium-137 detected by the radiation detector 14 using cesium-137 as the radiation source. The solid line in FIG. 2 shows the spectrum of potassium-40.
The spectrum of cesium-137 shown by the broken line in FIG. 2 represents the sum of the energies of the two gamma rays detected by the radiation detector 14, that is, the gamma ray γ from the radiation source 12 and the gamma ray γ'from the background. ing. Looking only at the gamma ray value (counting rate) detected by the radiation detector 14, the value derived from the gamma ray γ from the radiation source 12 (counting rate) and the value derived from the gamma ray γ'by the background (counting rate). Rate) and cannot be separated.
The symbols X1 and X2 in FIG. 2 represent the peaks of the energy spectrum, respectively, and are designated as the first peak and the second peak from the lowest energy of the gamma ray. The energy (horizontal axis) of gamma rays at the second peak X2 is approximately 662 keV.

In the present invention, it is considered that most of the naturally occurring radiation (background) is due to potassium-40 (K-40), and the spectrum of potassium-40 as compared with cesium-137 is also shown in FIG. As can be seen from FIG. 2, the spectra of cesium-137 and potassium-40 are approximately the same in the region where the energy of gamma rays is high (region α in FIG. 2). That is, it can be seen that in the region α where the energy of gamma rays is high, the behavior of the gamma ray count rate of cesium-137 and the background gamma ray count rate are almost the same.

Here, as a threshold for branching the behavior of the spectra of cesium-137 and potassium-40, in the embodiment, the threshold is calculated and set by the half-value width of the spectrum at the peak of the higher energy of the gamma ray in the spectrum of cesium-137. There is. Specifically, first, in the spectrum forming the second peak of cesium-137, a width that becomes half of the second peak value, that is, a half width is calculated. Then, a value that doubles the full width at half maximum is added to the energy of the gamma ray at the second peak (about 662 keV), and this value is used as the threshold value. When using the cesium-137 of the example, the threshold is set to 800 to 900 keV (approximately 800 keV in FIG. 2 of the example).

That is, if the energy of gamma rays is measured using cesium-137 as the radiation source 12 and only the energy region α of gamma rays equal to or greater than the threshold is extracted in the measured gamma ray energy spectrum (broken line in FIG. 2), the energy is less than the threshold. The energy region β of potassium 40 can be estimated. The relational expression used for estimation will be described later. The energy region of potassium-40 is almost the same as the background energy region as described above. Therefore, in the spectrum when the energy of gamma rays is measured using cesium-137, all the energy regions α and β of the gamma rays γ'due to the background (potassium-40) should be estimated from the energy region α of the gamma rays above the threshold value. Can be done.

FIG. 3 shows a flow chart of density measurement (estimation) by a density measuring device.
As can be seen from FIG. 3, the density measuring method by the density measuring device 10 for estimating the entire energy region of the gamma ray γ'by the background is outline, a measuring step (step S1), and an extraction / estimation step (step S2). ) Is included. As will be described later, the measurement step (step S1) and the extraction / estimation step (step S2) are performed by the control device 14-1 of the radiation detector.

The measurement step (step S1) is a step of installing the density measuring device 10 on the ground surface 22 of the measurement target, generating gamma rays γ from the radiation source 12, and measuring the density of the measurement target 20. The measurement step (step S1) is a detection step (step S1) in which the generation step (step S1-1) of generating gamma rays γ from the radiation source 12 and the gamma rays γ and γ'by the radiation source and the background are detected and measured together (step S1). -2) and are included.
The extraction / estimation step (step S2) is a calculation step (step S2-1) in which spectra of detected gamma rays γ and γ'are created to calculate and set a threshold, and extraction is performed to extract only an energy region equal to or higher than the threshold. The energy region of gamma rays below the threshold is estimated from the energy region above the threshold extracted in the step (step S2-2) and the extraction step, and the energy region of gamma rays below the estimated threshold and the energy region of gamma rays above the threshold extracted in the extraction step It includes an estimation step (step S2-3; steps S2-3a, b) in which the energy regions are added up to obtain the total energy region of gamma rays by the background.

A density measurement (estimation) method using a density measuring device will be described with reference to FIGS. 1 to 3.
First, the density measuring device 10 is installed on the ground surface 22 of the construction site. Then, radiation (gamma ray γ) is generated from the radiation source 12 using cesium-137, and is scattered and radiated to the ground 20 to be measured (step S1-1, generation step). Then, the gamma ray γ incident on the radiation detector 14 (more accurately, the electron emitted by reacting with the atom of the substance 20-1) and the gamma ray γ'from the background incident on the outside of the density measuring apparatus 10 Is detected by the radiation detector 14 (step S1-2, detection step). As a result, on-site gamma rays, that is, gamma rays γ from the radiation source 12 and gamma rays γ'from the background are simultaneously detected and measured by the density measuring device 10.

The control device 14-1 of the radiation detector calculates the count rate (count number, unit cpm) with respect to the energy of the detected gamma rays γ and γ'and creates a spectrum. The spectrum is preferably smoothed in order to clarify the peak.
Then, as described above, the control unit 14-1a of the control device calculates the width at half of the second peak value, that is, the half width in the detected gamma ray spectrum. Then, a value that doubles the full width at half maximum is added to the energy of the gamma ray at the second peak (about 662 keV in FIG. 2), and this value is calculated and set as a threshold value (step S2-1, calculation step). When using the cesium-137 of the example, the threshold value is 800 to 900 keV (FIG. 2). The spectrum created in the calculation step (step S2-1) may be displayed on the output means (display) 14-1d'via the control unit 14-1a of the control device.

The control unit 14-1a of the control device extracts only the energy region equal to or larger than the threshold value calculated in the calculation step (step S2-1) (step S2-2, extraction step). That is, only the energy region α of gamma rays equal to or greater than the threshold value is extracted from the spectrum created in the calculation step (step S2-1).

Next, the control unit 14-1a of the control device estimates the energy region β of the gamma ray less than the threshold value from the energy region α extracted in the extraction step (step S2-2) (step S2-3a). The relational expression used for estimation will be described later.
Then, the control unit 14-1a of the control device extracts the energy region α of the gamma ray above the threshold value extracted in the extraction step (step S2-2) and the gamma ray β below the threshold value in the estimation step (step S2-3). Add up and use this as the total energy region of the background (steps S2-3b). The output means (display) 14-1d'may display the entire energy region of a series of backgrounds, which is the sum of the energy region α of gamma rays above the threshold value and the energy region β of gamma rays below the threshold value, as a spectrum. Alternatively, the energy region from the radiation source 12 is calculated on the output means (display) 14-1d'excluding the total energy region of the background estimated from the energy region of the measured gamma ray, and this is displayed as a spectrum. You may.
When the specified score is specified, the steps of steps S1-1 to S2-3 are repeated until the specified score is reached.

FIG. 4 shows a graph showing a relational expression and a correlation for deriving the count rate in the entire energy region from the count rate in the energy region of gamma rays above the threshold value in the background.
When the cesium-137 of the example was used as the radiation source 12 and the threshold value was set to 900 keV by the density measurement (estimation) method by the density measuring device 10, the relational expression of FIG. 4 was obtained. As can be seen in FIG. 4, the relational expression can be expressed as a linear function. ^{R 2} in the figure is the coefficient of determination of the approximate curve. The coefficient of determination R ² is 0.9522, and it is understood that there is a high correlation between the counting rate in the energy region α of gamma rays above the threshold and the counting rate in the total energy regions α and β in the background.
That is, if the relational expression shown in FIG. 4 is used, in the estimation step (step S2-3), the energy region β of the gamma ray less than the threshold value is estimated from the energy region α extracted in the extraction step (step S2-2). Further, it is possible to add up the background energy regions α and β below the estimated threshold value and above the extracted threshold value to obtain the total energy region of the background.

In the density measuring device 10 (embodying the density measuring method), the control unit 14-1a of the control device creates a spectrum from the measured gamma rays γ and γ'and calculates a threshold value (step S2-1), and measures the measurement. From the energy regions α above the thresholds of the gamma rays γ and γ', the energy regions of all the backgrounds including the energy region β of the gamma rays γ'due to the background are estimated (steps S2-2, 3). That is, the entire energy region of the background is automatically estimated, the influence of gamma rays due to the background (counting rate) is separated from the measured energy region of gamma rays, and the influence of only gamma rays from the radiation source (counting rate). ) Can be grasped.
Therefore, it is not necessary to measure the background before the measurement on the measurement target, the entire measurement time can be significantly shortened, and the burden on the measurer can be reduced.

In measuring the measurement target 20, it is sufficient to install the density measuring device 10 on the ground surface 22 of the measurement target and irradiate the gamma ray γ from the radiation source 12. That is, it is not necessary to remove the storage portion 30 of the radiation source 12 and bury it in the ground which is the measurement target 20, and the burden on the measurer can be reduced from this point as well. Further, since it is sufficient to install the density measuring device 10 on the ground surface 22 to be measured, it can be installed on a self-propelled device traveling on the ground surface to be measured, which also reduces the burden on the measurer. be able to.
Further, a container for burying the radiation source 12 in the ground becomes unnecessary, and the density measuring device can be made smaller and lighter. Further, since it is not necessary to provide the container of the radiation source 12 so as to be removable for burying, there is no risk of losing the radiation source.

The above-mentioned examples are for explaining the present invention, and do not limit the present invention in any way, and all the modifications, modifications, etc. within the technical scope of the present invention are included in the present invention. Needless to say.

Although the present invention is suitable for a density measuring device (which embodies a density measuring method), the present invention can also be applied to an RI density water content measuring device including both a density measuring device and a water content measuring device. It can also be applied to a self-propelled device equipped with a density measuring device (which embodies the density measuring method).

10 Density measuring device 12 Radiation source 14 Radiation detector 14-1 Control device γ, γ'Gamma ray by radiation source, gamma ray by background α Energy region of gamma ray above threshold β Energy region of gamma ray below threshold S1-1 Generation step S1-2 Detection process S2-1 Calculation process S2-2 Extraction process S2-3 Estimation process

Claims (3)

A radiation source that emits radiation to the object to be measured,
A radiation detector that detects incident radiation, and
In a density measuring device that measures the density of the object to be measured by radiation
Radiation is considered to be gamma rays,
The radiation detector has a control device as well as capable of detecting gamma rays from the radiation source and the background.
The control device calculates and sets the energy value of the gamma ray corresponding to twice the half price width of the peak as a threshold in the gamma ray spectrum detected by the radiation detector, and extracts only the energy region of the gamma ray above the threshold. Then, the energy region of the gamma ray below the threshold is estimated from the extracted energy region, and the energy region of the gamma ray below the estimated threshold and the energy region of the gamma ray above the extracted threshold are added up to obtain the background gamma ray. A density measuring device for the entire energy range. Cesium-137 is used as the radiation source.
The density measuring device according to claim 1, wherein the control device calculates and sets a threshold value based on the half width of the spectrum at the peak of the spectrum of cesium-137, which has the higher energy of gamma rays. The generation process of generating and irradiating gamma rays to the measurement target for measuring density,
A detection step for detecting gamma rays generated in the generation step and gamma rays due to the background, and
In the gamma ray spectrum detected in the detection step, a calculation step of calculating and setting the energy value of the gamma ray corresponding to twice the half width of the peak as a threshold value, and a calculation step.
An extraction step of extracting only the energy region of gamma rays above the threshold value and
The energy region of gamma rays below the threshold value is estimated from the energy region above the threshold value extracted in the extraction step, and the energy region of gamma rays below the estimated threshold value and the energy region above the threshold value extracted in the extraction step are added up. And the estimation process to make the total energy region of gamma rays by the background,
At least a density measurement method.

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