JP2015180872A - Radioactivity measuring apparatus and radioactivity measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a measuring method capable of quickly, accurately measuring measurement target radioactivity while suppressing the effect of environmental radiation, and a measuring apparatus enabling such measurements, ensuring light weight and excellent handleability.SOLUTION: A measuring apparatus 10 is a directional apparatus comprising a detection element 20 and a shield body 30, and measuring the radioactivity of a radioactive material emitting characteristic X-rays and gamma rays. The detection element 20 detects the characteristic X-rays incident on a detection surface 22. The shield body 30 is provided to surround at least the periphery of the detection element 20, transmits the gamma rays and shields scattered gamma rays that are scattered rays of the gamma rays and the characteristic X-rays. The measuring apparatus 10 counts the characteristic X-rays incident on the detection surface 22 directed to a measuring target 100 containing a radioactive material.

Description

  The present invention relates to a radioactivity measurement apparatus and a radioactivity measurement method for measuring the degree of contamination of a measurement target by a radioactive substance.

  In order to decontaminate buildings and soil contaminated with radioactive substances such as radioactive cesium, an operation of cleaning or removing these surface layers is performed. In order to determine the degree of contamination before decontamination and the effect of decontamination, it is necessary to measure the contamination density of the surface layer of the soil. There are two methods commonly used at present: a method of obtaining a sample of a part to be measured and measuring it at a facility equipped with measuring equipment, and a method of performing at a site where decontamination work is performed. While the method of obtaining a sample and measuring it at a facility is a highly reliable method, it cannot be measured at a contaminated site, so it takes a lot of time and money, and the application range is limited. In this respect, it is preferable to directly determine the effect of the decontamination work at the decontamination site.

  As a method for determining the decontamination effect at the decontamination site, a method for obtaining a contamination density from the air dose rate at the site using a survey meter and a method for counting beta rays are known. Non-Patent Document 1 describes measuring an air dose rate of gamma rays using a scintillation-type survey meter as a method for obtaining an index of the state of contamination by radioactive substances. However, since gamma rays fly not only from the measurement object such as soil but also from the surrounding four sides, the radioactivity of the measurement object cannot be accurately quantified even if the air dose rate is measured.

  In order to accurately quantify the degree of contamination of the surface of the measurement target, it is preferable to shield the environmental radiation coming from the surroundings and only the radiation emitted from the measurement target enters the detection element. However, in the case of cesium 137, which is radioactive cesium, for example, the energy of main gamma rays is as extremely high as 662 keV. Therefore, in order to shield such gamma rays, a shield having a thickness of 50 to 100 mm in terms of lead is required. For this reason, the total weight of the radioactivity measuring apparatus is as high as several hundred kilograms, and it is difficult to measure radioactivity at the decontamination site because of poor transportability and handling.

  On the other hand, in the method of counting beta rays, there is a problem that the radioactivity of a portion slightly hidden from the surface layer to be measured or a portion hidden by an intermediate such as a fallen leaf cannot be measured. This is because the beta ray is greatly absorbed even by a shield having a thickness of about 1 mm of paper, and the count rate is significantly reduced.

  On the other hand, Patent Document 1 describes a radioactive substance detection apparatus (radioactivity measurement apparatus) that counts characteristic X-rays originating from radioactive substances. This apparatus counts the characteristic X-rays incident from the measurement object by shielding the characteristic X-rays flying from the surroundings with respect to the detection element directed to the measurement object without shielding the gamma rays. For example, the characteristic X-ray energy of barium originating from radioactive cesium is as low as 32.1 keV and 36.5 keV, so that the thickness of the shield can be reduced. Patent Document 1 describes that by using stainless steel (SUS) having a thickness of 1 mm as a shield, it is possible to shield 98% of 32.1 keV characteristic X-rays and transmit 94% of 662 keV gamma rays. Has been.

International Publication No. 13/105519 Pamphlet

“Guidelines on Survey and Measurement Methods for Environmental Pollution Status in the Pollution Status Priority Survey Area” Ministry of the Environment, December 2011, 1st Edition

  In addition to the main gamma rays (hereinafter sometimes referred to as “main gamma rays” or simply “gamma rays”) and characteristic X-rays that have large energy released from radioactive materials, the environmental radiation at the contaminated site includes scattered gamma rays generated by Compton scattering. include. The energy of scattered gamma rays is smaller than that of main gamma rays and is continuously distributed. When multiple Compton scattering occurs, the energy of the scattered gamma rays gradually decreases and may reach an energy region close to the characteristic X-ray. Compton scattering is caused by several factors. In the case of gamma rays radiated from radioactive materials present on the surface of the soil, there is a phenomenon in which scattered gamma rays generated by elastic scattering of electrons from the soil material jump out of the surface of the soil into the air. Scattered gamma rays in which Gamma rays traveling in the depth direction of the soil are Compton scattered at a scattering angle of 180 degrees take an energy peak called a backscattering peak. In the case of radioactive cesium, the energy of the backscattering peak is about 200 keV. The backscattering peak is generally not as sensitive as the photoelectric peak, and the backscattered rays are further scattered, and the photons of the scattered gamma rays are widely detected below the backscattered peaks. For this reason, the background value (baseline) of the spectrum of the characteristic X-ray is increased by increasing the count value of the photons of 32.1 keV or 36.5 keV which are the energy of the characteristic X-ray.

In the apparatus of Patent Document 1, a characteristic X-ray originating from radioactive cesium is shielded by 98% by using stainless steel having a thickness of 1 mm as a shield. The surface density of this shield is 0.78 g / cm ² . If it is going to implement | achieve the same shielding capability with lead, the thickness of a shielding body will be about 0.004 mm. Moreover, the upper limit of the thickness of the shield in Patent Document 1 is 0.22 times the mean free path of the main gamma ray, and when measuring radioactive cesium, it is converted to 3.8 mm and converted to lead. Then, it is 1.7 mm.

  However, Patent Document 1 does not consider the presence of scattered gamma rays, and the surface density of the shielding body cannot shield multiple scattered gamma rays having a peak near 100 keV. For this reason, in addition to the scattered gamma rays incident from the measurement object containing radioactive cesium, the scattered gamma rays included in the environmental radiation around the detection element also enter the detection element. On the other hand, since characteristic X-rays from the surroundings are shielded by the shield, the characteristic X-rays are incident only from the measurement object. In other words, in the case of measuring the radioactivity of radioactive cesium by counting 30 keV to 200 keV photons with the apparatus of Patent Document 1, in addition to the characteristic X-rays and scattered gamma rays incident from the direction of the sensing element, The characteristic X-rays do not enter and only scattered gamma rays enter. For this reason, in the count value of the characteristic X-ray by the apparatus of Patent Document 1, scattered gamma rays are counted at a rate higher than the ratio of the characteristic X-ray and scattered gamma rays in environmental radiation.

  Here, in order to measure the radioactivity of the measurement target with the desired accuracy, it is necessary to count a sufficient amount of photons so that the scattered gamma rays can be distinguished from the characteristic X-rays. There is a trade-off relationship. For this reason, it is difficult to achieve this trade-off at a high level in the apparatus of Patent Document 1, and when measuring radioactivity with a desired accuracy, the exposure amount of the measurer is increased due to a longer counting time. Increases and becomes a problem. The above problem occurs when measuring the radioactivity of various radioactive substances, not limited to radioactive cesium.

  The present invention has been made in view of the above-described problems, and a measurement method capable of accurately measuring the radioactivity of a measurement target in a short time while suppressing the influence of environmental radiation, and such measurement are possible. In addition, the present invention provides a measuring apparatus that is lightweight and excellent in handleability.

  According to the present invention, there is provided a directivity measuring device that measures the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays, the detection element detecting a characteristic X-ray incident on a detection surface, and at least a side of the detection element. A shield provided around the circumference and transmitting gamma rays and shielding scattered gamma rays and characteristic X-rays, which are scattered rays of the gamma rays, and directed to the measurement object containing the radioactive substance There is provided a radioactivity measuring apparatus characterized by counting the characteristic X-rays incident on a surface.

  Further, according to the present invention, there is provided a method for measuring the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays using a collimated detection element that detects characteristic X-rays incident on a detection surface, wherein the radioactive substance is Directing the detection surface with respect to the measurement object to be contained, transmitting gamma rays flying to the detection surface from a direction different from the collimating direction, and shielding the scattered gamma rays and characteristic X-rays that are the scattered rays of the gamma rays, There is provided a radioactivity measurement method characterized by discriminating and counting the characteristic X-rays from the characteristic X-rays flying from the collimating direction to the detection surface, the scattered gamma rays and the gamma rays.

  According to the above invention, since characteristic X-rays are detected instead of gamma rays (main gamma rays), it is not necessary to shield the gamma rays with the shielding body, the weight of the shielding body is reduced, and the handleability is excellent. Furthermore, since the scattered gamma rays and the characteristic X-rays are shielded by the shield, unlike the apparatus of Patent Document 1, the characteristic X-rays can be counted without being substantially affected by the scattered gamma rays included in the environmental radiation. For this reason, even if the scattered gamma rays enter the detection element together with the characteristic X-rays from the measurement target (directing direction), the baseline due to the scattered gamma rays is lowered as compared with Patent Document 1. Thereby, the trade-off between the reduction of the characteristic X-ray counting time and the measurement accuracy of radioactivity can be realized with a high balance.

  According to the present invention, there is provided a measuring method capable of measuring the radioactivity of a measurement target with high accuracy in a short time while suppressing the influence of environmental radiation, and a measuring device that is capable of such measurement, is lightweight, and is easy to handle. Provided.

It is a cross-sectional schematic diagram which shows the radioactivity measuring apparatus of 1st embodiment of this invention. It is explanatory drawing which shows the radiation which is shielded with a shield, and the radiation which injects into a detection element. It is a cross-sectional schematic diagram which shows the radioactivity measuring apparatus of 2nd embodiment of this invention. It is a graph which shows the radiation spectrum of the 1st Example acquired using the radioactivity measuring apparatus of 2nd embodiment. It is a graph which shows the state which fitted the radiation spectrum of 1st Example with the approximate function. It is a graph which shows the radiation spectrum of the comparative example 1 acquired using the radioactivity measuring apparatus which is not equipped with the shield. It is a graph which shows the state which fitted the radiation spectrum of the comparative example 1 with the approximate function. It is a cross-sectional schematic diagram which shows the radioactivity measuring apparatus of 3rd embodiment of this invention. It is explanatory drawing which shows the radiation which is shielded with a shield or a measurement base, and the radiation which injects into a detection element. It is a graph which shows the radiation spectrum of the 2nd Example acquired using the radioactivity measuring apparatus of 3rd embodiment. It is a graph which shows the state which fitted the radiation spectrum of the 3rd Example which measured the radioactivity in the contaminated area using the radioactivity measuring apparatus of 3rd embodiment, and this radiation spectrum was fitted with the approximation function. It is a cross-sectional schematic diagram which shows the radioactivity measuring apparatus of 4th embodiment of this invention. It is a graph which shows the radiation spectrum of the 4th Example acquired using the radioactivity measuring apparatus of 4th embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In all the drawings, the same constituent elements are denoted by the same reference numerals, and redundant description is omitted as appropriate.

<First embodiment>
FIG. 1 is a schematic cross-sectional view showing a radioactivity measuring apparatus (hereinafter sometimes abbreviated as “measuring apparatus 10”) according to a first embodiment of the present invention. FIG. 1 shows a longitudinal section of the measuring device 10 cut along the measuring direction of the sensing element 20. The measuring apparatus 10 is installed with the detection surface 22 of the detection element 20 facing the surface layer 110 of the measurement target 100.

  First, the outline | summary of the measuring apparatus 10 of this embodiment is demonstrated.

The measuring device 10 is a directional device that measures the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays.
The measuring device 10 includes a detection element 20 and a shield 30. The detection element 20 detects a characteristic X-ray incident on the detection surface 22. The shield 30 is provided so as to surround at least the side periphery of the detection element 20, transmits gamma rays, and shields scattered gamma rays and characteristic X-rays that are scattered rays of gamma rays.
The measuring apparatus 10 of the present embodiment is characterized in that it counts characteristic X-rays incident on the detection surface 22 directed to the measurement object 100 containing a radioactive substance.

  Next, the measuring apparatus 10 of this embodiment will be described in detail. In addition, as shown in FIG. 1, although this embodiment demonstrates the state by which the measuring object 100 is arrange | positioned below in the figure, the up-down direction of the figure does not necessarily mean the direction of gravity. However, for convenience, the side of the measuring device 10 on which the detection surface 22 is disposed is referred to as the front end side, the lower side or the lower end side, and the side on which the photoelectric conversion device 24 and the amplifier 26 are disposed is the base end side, above or It may be called the upper end side.

  Further, in the present embodiment, the case where the measurement target 100 is a flat plane will be described as an example, but the present invention is not limited to this. The surface of the measuring object 100 may be uneven, or an intermediate (not shown) exemplified by fallen leaves and vegetation may exist.

  The measuring device 10 is a measuring device that measures the degree of contamination of the surface layer 110 of the measuring object 100 due to radioactivity. Here, the degree of contamination by radioactivity is a quantitative index indicating the degree of radioactivity contamination. In addition to the characteristic X-ray count rate, the radioactivity or radioactivity surface density of radiation such as characteristic X-rays and gamma rays, Or it can be converted from these numbers. Hereinafter, for the sake of convenience, this degree of contamination is expressed as “radioactivity”.

  Examples of the measurement object 100 include the ground (paved road surface, soil surface, gravel surface, etc.), the wall surface of the building, the surface of the building, the bark surface of the tree, and the like. When the measuring object 100 is an upright surface like a wall surface of a building, the measurement direction of the measuring device 10 is the normal direction of the upright surface, for example, the horizontal direction. Further, the “surface layer” of the measurement target 100 is not limited to the physical surface of the measurement target 100 but includes the inside of the measurement target 100 in the depth direction. When the measurement object 100 is soil, the surface layer 110 is defined from the surface of the measurement object 100 to the ground of about several mm to several tens of mm. When the measurement object 100 is a tree, the back side of the peeled bark or the inside of the gap between the bark also corresponds to the surface layer 110.

  The sensing element 20 of the present embodiment is an inorganic crystal scintillator. The inorganic crystal scintillator used for the sensing element 20 is not particularly limited. Thallium activated sodium iodide, cesium iodide, gadolinium silicate, lutetium silicate, bismuth germanate, cerium fluoride, barium fluoride, lead fluoride, lead tungstate, europium activated lithium iodide, europium activated iodine And strontium chloride. As cesium iodide, pure cesium iodide, thallium activated cesium iodide or sodium activated sodium iodide can be used.

  As the detection element 20, in addition to a scintillation detector, a proportional counter, a semiconductor detector, or the like can be used. The detection element 20 detects at least a characteristic X-ray of 30 keV or more and less than 40 keV emitted from barium originating from radioactive cesium. The detection element 20 may detect radiation of 40 keV or more and 1000 keV or less. Hereinafter, characteristic X-rays, gamma rays, and scattered gamma rays are collectively referred to as radiation.

  The sensing element 20 has a detection efficiency of 40% or more, preferably 50% or more for a characteristic X-ray of 30 keV or more and less than 40 keV. The detection element 20 has a detection efficiency of 50% or less, preferably 40% or less, for photons of 200 keV or higher. As described above, by giving the energy characteristic suitable for the characteristic X-ray, it is possible to reduce the influence of the main gamma ray and the scattered ray gamma ray incident on the detection element 20. Specifically, when the sensing element 20 is NaI (Tl), the influence of the main gamma rays and the scattered gamma rays can be significantly suppressed by setting the thickness to 5 mm or less. In particular, the scattered gamma rays are counted as characteristic X-rays. The effect on the rate can be significantly suppressed. In terms of the detection efficiency of the detection element 20, this is achieved by setting the detection element 20 to a thickness that makes the detection efficiency 40% or less for a photon of 200 keV or higher.

  The half-life of radioactive cesium cesium 137 and cesium 134 is about 30 years and about 2 years, respectively. Cesium 137 decays to barium 137m with a short half-life due to beta decay, and then becomes non-radioactive barium. Cesium 134 disintegrates directly into non-radioactive barium 134. At the time of beta decay of radioactive cesium, orbital electrons are emitted instead of gamma rays, and barium emits characteristic X-rays when electrons fall into the holes. The characteristic X-ray energy of barium originating from radioactive cesium is 32.1 keV and 36.5 keV. The detection element 20 detects one or both of 32.1 keV and 36.5 keV, which are barium characteristic X-rays.

  The measuring device 10 includes a photoelectric conversion device 24 such as a photomultiplier tube or a photodiode, and an amplifier 26. The photoelectric conversion device 24 is a device that converts light emitted when radiation (photons) enters the detection surface 22 of the detection element 20 into electric pulses that are electric energy. The amplifier 26 amplifies the electric pulse generated by the photoelectric conversion device 24. The amplifier 26 is provided with a signal terminal 27 for outputting the amplified electric pulse signal to the data processing device 90 and a power supply terminal 28 for power supply. The power supply terminal 28 is connected to a power supply device (not shown). In addition, when the detection element 20 is a semiconductor detector (solid state detector), the photoelectric conversion device 24 is unnecessary. The data processing device 90 is an external device used by being connected to the measuring device 10 of the present embodiment. By using the measurement device 10, the data processing device 90, and the power supply device (not shown) of the present embodiment, a radioactivity measurement method described later is realized.

  The data processing device 90 is connected to the signal terminal 27, acquires the pulse signal (input pulse) amplified by the amplifier 26 from the signal terminal 27, and performs various operations and data output. A general-purpose computer device can be used as the data processing device 90. The data processing device 90 includes a storage device (not shown), a control device (CPU), and input / output interface equipment.

The data processing device 90 includes a wave height analysis unit 92, a calculation unit 94, and an output unit 96. The pulse height analysis unit 92 detects and counts the maximum energy of the input pulse to generate a radiation spectrum. The pulse height analysis unit 92 can be a multi-channel analyzer that divides and counts the pulse height of an input pulse into a plurality of channels. The calculation unit 94 calculates radioactivity such as radioactivity surface density (unit: Bq / cm ² ) and radioactivity (unit: Bq) based on the generated radiation spectrum. The arithmetic unit 94 is realized by a control device (CPU) of a computer device that constitutes the data processing device 90. The output unit 96 is means for outputting the calculated result to a display or the like.

  A collimator 40 is attached in front of the detection surface 22 in the directivity direction. The collimator 40 of the present embodiment has a cylindrical shape concentric with the detection surface 22 and has a plurality of concentric openings 41. The partition wall 42 that partitions adjacent openings 41 has a surface density capable of shielding the characteristic X-rays. Specifically, the partition wall 42 can be made of stainless steel having a thickness of 0.25 mm or more. The partition walls 42 are arranged in concentric circles and have a short cylindrical shape with different opening diameters.

  The height dimension of the partition wall 42 is equal to the interval between the adjacent partition walls 42, and can be, for example, 5 mm or more and 20 mm or less. The height dimension of the partition wall 42 is the length of the partition wall 42 in the direction of the directional axis of the detection element 20 (the normal direction of the detection surface 22). Thereby, the collimator 40 (partition wall 42) passes the characteristic X-rays traveling parallel to the partition wall 42 or along the partition wall 42 at a shallow crossing angle less than a predetermined value. Then, the characteristic X-rays traveling at a deep intersection angle greater than or equal to a predetermined distance with respect to the partition wall 42 are shielded. Thereby, a characteristic X-ray having an incident angle of 45 degrees or more traveling in a two-dimensional plane passing through the axis of the collimator 40, and a characteristic having an incident angle of approximately 60 degrees or more traveling in an arbitrary direction in the three-dimensional space inside the collimator 40. All X-rays are shielded. The incident angle of radiation such as a characteristic X-ray is an angle formed by the directional axis direction of the detection element 20 and the traveling direction of the radiation, and the directional axis direction of the detection element 20 is zero degrees. The measuring apparatus 10 according to the present embodiment includes the collimator 40, so that the traveling direction of the characteristic X-ray incident on the detection surface 22 is collimated so as to be 0 degree or more and 60 degrees or less. When the average incident angle of the characteristic X-ray passing through the opening 41 of the collimator 40 was calculated by the EGS (Electron Gamma Shower) method which is a Monte Carlo simulation method, it was about 30 degrees.

  The detection element 20, the photoelectric conversion device 24, and the amplifier 26 are accommodated in the housing 12. The housing 12 has a hollow cylindrical shape and is made of a thin metal plate. As the housing 12, stainless steel or the like having a thickness of about 1 mm can be used as in the shielding body of Patent Document 1.

  The measurement apparatus 10 includes a gantry 70 that arranges the detection element 20 so as to face the surface layer 110 of the measurement target 100. The specific mechanism and shape of the gantry 70 are not particularly limited.

FIG. 2 is an explanatory diagram showing radiation shielded by the shield 30 and radiation incident on the detection element 20. For the sake of explanation, the lower end of the measuring apparatus 10 is arranged to be spaced above the surface layer 110 of the measuring object 100, but is not limited thereto. The lower end of the measuring device 10 may be brought into contact with the surface layer 110.
However, the visual field of the measuring device 10 can be expanded by separating the lower end of the measuring device 10 above the surface layer 110 of the measuring object 100. For this reason, since the radioactivity of the wide range of measuring object 100 can be measured by one measurement, it is possible to reduce the exposure dose of the measurer. You may measure by making the lower end of the measuring apparatus 10 spaced apart from the surface layer 110 of the measuring object 100, for example, 2 times or more the opening diameter of the collimator 40.

  The shield 30 of the present embodiment is provided so as to surround at least the side periphery of the detection element 20. The surface density of the shield 30 in the region surrounding the side periphery of the detection element 20 (FIG. 1: the side periphery 32) allows gamma rays originating from the radioactive substance contained in the measurement object 100 to pass therethrough and the scattered rays of the gamma rays. Scatter scattered gamma rays and characteristic x-rays.

  The shield 30 includes the collimator 40 and the detection element 20, and is provided in a range that covers at least a part of the side periphery of the photoelectric conversion device 24.

  The shield 30 shields the scattered gamma ray peak of the radioactive substance so as to have a transmittance of less than 50%. Further, the transmittance of gamma rays of 60 keV to 300 keV with respect to the shield 30 is less than 50%.

  For the shield 30, a metal or a material containing a metal may be used. As the metal material, heavy metals such as iron, stainless steel, lead, cadmium, or tungsten can be used.

When measuring radioactive cesium as a radioactive substance, the thickness of the shield 30 is preferably 0.7λ or more in terms of the mean free path (λ) of 300 keV gamma rays. The shielding ability of the shield 30 against gamma rays of 60 keV or more and 300 keV or less is preferably an area density of 2.2 g / cm ² or more in terms of lead. In other words, the shielding ability of the shielding body 30 against gamma rays in the above-described wavelength region is preferably equal to or more than the shielding ability of a lead plate (2 mm in terms of thickness) having a surface density of 2.2 g / cm ^2. . That is, when the shield 30 is made of lead, the surface density is preferably 2.2 g / cm ² or more (thickness 2 mm or more).

  As a result, scattered gamma rays originating from radioactive cesium are shielded with a transmittance of less than 50%. Since the characteristic X-rays originating from radioactive cesium have lower energy than the scattered gamma rays as described above, the characteristic X-rays are also shielded at a lower transmittance than the scattered gamma rays by using the shield 30 of this embodiment. The

The upper limit of the shielding ability of the shield 30 with respect to gamma rays of 60 keV or more and 300 keV or less is not particularly limited. Due to the demand for weight reduction of the measuring device 10, the surface density is 12 g / cm ² or less in terms of lead, 10 mm or less in terms of lead thickness, and 5 λ or less in terms of the mean free path (λ) of 300 keV gamma rays. preferable. More preferably, the upper limit of the shielding capability of the shield 30 is 6 g / cm ² or less in surface density in terms of lead, and 5 mm or less in terms of lead thickness. That is, when the shield 30 is made of lead, the surface density is preferably 6 g / cm ² or less (thickness 5 mm or less).

  As shown in FIG. 2, the shield 30 surrounding the side periphery of the sensing element 20 transmits the main gamma ray R1 and shields the scattered gamma ray R2 and the characteristic X-ray R3. Therefore, the main gamma ray R1 passes through the detection element 20 regardless of the traveling direction. In general, the main gamma ray R1 has an extremely high energy as compared with the characteristic X-ray R3. For this reason, even if the main gamma ray R1 enters the sensing element 20 and the photoelectric conversion device 24 generates an electric pulse, this is the detection peak of the characteristic X-ray R3 in the radiation energy spectrum (hereinafter referred to as the radiation spectrum). The effect is negligible.

  The shield 30 comes from a direction different from the directivity direction of the detection element 20, that is, the scattered gamma ray R <b> 2 and the characteristic X-ray R <b> 3 that come from the periphery of the measuring device 10 without passing through the opening 41 of the collimator 40 enter the detection element 20. To reduce.

  The characteristic X-ray R3 radiated from the surface layer 110 of the measurement target 100 enters the detection element 20 through the opening 41 of the collimator 40. Further, back scattered gamma rays R4 generated by scattering gamma rays radiated from the surface layer 110 in the ground direction with soil materials such as alumina and silicon dioxide enter the detection element 20 through the openings 41 of the collimator 40. In addition, when the main gamma ray R <b> 1 collides with the metallic shield 30, a characteristic X-ray R <b> 5 of the shield 30 is generated and enters the detection element 20. When the shield 30 is made of lead, the energy (wavelength) of the characteristic X-ray R5 is about 74 keV, which is close to the energy (wavelength) of the backscattered gamma ray R4 originating from radioactive cesium. However, since the dose of the characteristic X-ray R5 originating from the shield 30 is smaller than the dose of the characteristic X-ray R3 originating from the radioactive substance and the backscattered gamma ray R4, the influence on the detection peak of the characteristic X-ray R3 is backward. It is smaller than the influence of the scattered gamma ray R4.

  According to the measuring apparatus 10 of the present embodiment, the characteristic X-ray R3 can be counted to quantitatively measure the degree of contamination of the measurement object 100 such as radioactivity and radioactivity surface density. At this time, since it is not necessary to shield the main gamma ray R1 by the shield 30, the weight of the measuring apparatus 10 can be reduced. Here, the scattered gamma ray R2 may be close in energy to the characteristic X-ray R3. As a representative example, the characteristic X-ray R3 energy of barium originating from radioactive cesium is 32.1 keV and 36.5 keV, while the scattered gamma ray R2 of radioactive cesium is distributed from 90 keV to 200 keV. Close to X-ray R3. Therefore, the characteristic X-ray R3 and the scattered gamma ray R2 from other than the desired collimating direction can be shielded by surrounding the side periphery of the detection element 20 with the shield 30 as in the present embodiment. Thereby, when the characteristic X-ray R3 incident on the detection surface 22 from the collimating direction is counted by the detection element 20, it is possible to suppress the influence of the backscattered gamma ray R4, and the counting time by the measuring apparatus 10 can be shortened. .

The shield 30 is provided in a region surrounding at least the side periphery of the detection element 20 in the measurement apparatus 10. The shield 30 may be provided on the entire measurement apparatus 10 except for the opening 41 of the collimator 40. However, regarding the region excluding the side periphery of the detection element 20, the shielding ability of the shield 30 does not need to be a surface density of 2.2 g / cm ² or more in terms of lead.

  Returning to FIG. 1, the shield 30 of the present embodiment includes a side peripheral portion 32 that surrounds the side periphery of the detection element 20, and a base end portion that is connected to the side peripheral portion 32 on the opposite side of the detection surface 22. 34. The surface density at the base end portion 34 is smaller than the surface density at the side peripheral portion 32.

  A top surface portion 36 that covers a part of the base end side of the photoelectric conversion device 24 and the amplifier 26 is provided on the upper end side of the base end portion 34. The surface density of the top surface portion 36 is even smaller than the surface density of the base end portion 34. For example, in terms of lead, the thickness of the side peripheral portion 32 can be 5 mm, the thickness of the base end portion 34 can be 2 mm, and the thickness of the top surface portion 36 can be 1 mm.

The side circumferential portion 32 shields scattered gamma rays Ra flying in the thickness direction of the side circumferential portion 32 from the periphery of the measuring device 10 toward the sensing element 20, and therefore requires a relatively high shielding ability, converted into lead. The surface density is preferably 2.2 g / cm ² or more. On the other hand, the scattered gamma ray Rb that passes through the base end portion 34 that is connected to the base end side of the side peripheral portion 32 and travels toward the detection element 20 passes through the base end portion 34 at an inclination angle θb, and further, The light passes through the photoelectric conversion device 24 toward the detection element 20. In other words, the base end part 34 should just shield the scattered gamma ray contained in environmental radiation which passes through the base end part 34 diagonally. For this reason, even if the surface density of the base end portion 34 is smaller than that of the side peripheral portion 32, sufficient shielding ability is provided. Similarly, the scattered gamma ray Rc that passes through the top surface portion 36 toward the detection element 20 passes through the top surface portion 36 obliquely at an inclination angle θc (> θb), and further passes through substantially the entire length of the photoelectric conversion device 24 to be detected. It will face the element 20. For this reason, the top surface portion 36 has a sufficient shielding ability equivalent to the side peripheral portion 32 and the base end portion 34 even if the surface density is smaller than that of the base end portion 34. For this reason, the surface density of the base end portion 34 can be made less than half that of the side peripheral portion 32, and the surface density of the top surface portion 36 can be made about half that of the base end portion 34.

  In the measuring apparatus 10 of the present embodiment, the detection element 20 is disposed in the vicinity of the lower end of the housing 12, and the side peripheral portion 32 of the shield 30 is provided around the detection element 20. A base end portion 34 of the shield 30 is provided in the middle portion of the housing 12, and a top surface portion 36 of the shield 30 is provided at the upper end portion of the housing 12. For this reason, weight reduction of the measuring apparatus 10 is achieved, and the center of gravity of the measuring apparatus 10 is located on the lower end side from the center. Thereby, when the detection surface 22 of the detection element 20 is directed downward as shown in FIG. 1, the measurement apparatus 10 can be stably installed.

<Second embodiment>
FIG. 3 is a schematic cross-sectional view showing the measuring apparatus 10 according to the second embodiment. The measuring apparatus 10 of the present embodiment is different from the first embodiment in the collimating direction by the collimator 40. In the collimator 40 of the second embodiment, the partition wall 42 is inclined in a funnel shape (conical frustum shape) so as to increase in diameter toward the distal end side. In the center of the collimator 40, a truncated cone-shaped shielding region 44 is provided.

  The inclination angle between the extending direction of the partition wall 42 and the direction of the directing axis of the detection element 20 is 60 degrees, that is, the partition wall 42 is inclined by 30 degrees with respect to the surface layer 110. When the average incident angle of the characteristic X-ray passing through the opening 41 of the collimator 40 was calculated by the EGS method, it was about 60 degrees.

  A lid member 46 for shielding beta rays, internal conversion electrons, and Auger electrons is provided on the front end surface of the opening 41 of the collimator 40. Internally converted electrons are electrons that are emitted instead of gamma rays when a nucleus in an excited state transitions to the ground state, and its energy is smaller than that of main gamma rays. Auger electrons are electrons emitted instead of characteristic X-rays when electrons transition from the outer shell to the inner shell of an atom in an excited state. Since the transmission power of these electrons is smaller than that of the characteristic X-ray, by providing the lid member 46, it is possible to transmit the characteristic X-ray and shield these electrons. As the lid member 46, a resin material of about 1 mm or more and 10 mm or less can be used. The resin material is not particularly limited, and a polyester resin such as polyethylene terephthalate or an acrylic resin can be used.

The shield 30 of the present embodiment is common to the first embodiment in that the shielding ability of the side peripheral portion 32 surrounding the side periphery of the detection element 20 is an area density of 2.2 g / cm ² or more in terms of lead. To do. And since the shield 30 of this embodiment protects the circumference | surroundings of the collimator 40 which is larger diameter than the lower end of the housing 12, the lower end side of the side peripheral part 32 is from the upper end side provided in a row with the base end part 34. Is different from the first embodiment in that it has a large diameter.

  In the measuring apparatus 10 of the first embodiment and the second embodiment, a shield 30 having a surface density larger than that of the housing 12 is attached to the outside of the housing 12 made of stainless steel having a thickness of 1 mm. In common. Thereby, the radioactivity of the measuring object 100 can be accurately measured in a state where the characteristic X-rays and the scattered gamma rays included in the environmental radiation are shielded.

  Here, since the occurrence rate of the characteristic X-rays radiated by the radioactive substance is constant for each nuclide, the radioactivity of the radioactive substance can be obtained by obtaining the peak area of the characteristic X-ray in the radiation spectrum.

  Hereinafter, a method for counting characteristic X-rays using the measuring apparatus 10 of the first embodiment or the second embodiment, and a method for calculating the radioactivity of a radioactive substance based on the result of counting characteristic X-rays (also referred to as the present method) Explain).

This method is a method of measuring the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays using the collimated sensing element 20. The detection element 20 is an element that detects a characteristic X-ray incident on the detection surface 22 as described above.
In this method, the detection surface 22 is directed toward the measurement object 100 containing a radioactive substance, and the characteristic X-rays, scattered gamma rays and gamma rays flying from the collimating direction to the detection surface 22 are discriminated and counted. At this time, the main gamma rays flying from the direction different from the collimating direction to the detection surface 22 are transmitted, and the scattered gamma rays and the characteristic X-rays, which are the main gamma ray scattering rays, are counted.

  The collimating direction is a direction connecting the opening 41 of the collimator 40 and the detection surface 22. In the first embodiment shown in FIG. 1, the direction passing upward through the collimator 40 corresponds to the directional axis direction of the detection element 20. In the second embodiment shown in FIG. 3, a direction inclined by a predetermined angle (for example, 60 degrees) with respect to the direction of the directional axis of the detection element 20 corresponds to the collimating direction.

As described above, in the present method, the scattered gamma rays and characteristic X-rays flying from the collimating direction are counted in a state where the scattered gamma rays and characteristic X-rays flying from a direction different from the collimating direction are shielded. Such a process is called a counting process. Hereinafter, for convenience, a direction different from the collimating direction may be referred to as “side”.
The method includes the above-described counting step and a calculation step of calculating the radioactivity of the radioactive substance based on the counting result of the characteristic X-ray.

  In the counting step, counting is performed in a state where scattered gamma rays flying from the side are sufficiently shielded by the shield 30. The shielding ability (surface density) of the shield 30 is determined by acquiring a radiation spectrum in advance and applying a function fitting method to confirm that the characteristic X-ray can be distinguished from the scattered gamma rays with high accuracy. .

  Specifically, among the radiation spectrum generated based on the result of counting of characteristic X-rays and scattered gamma rays, a predetermined spectral region including a characteristic X-ray peak (characteristic X-ray peak) is obtained from a linear term, an exponential term, and a Gaussian function. Fitting with the approximate function The spectral region may include a characteristic X-ray peak, with a minimum point adjacent to the lower side as a lower limit and a minimum point adjacent to the upper side as an upper limit. The upper limit may be set at a symmetrical position with respect to the lower limit wavelength with the characteristic X-ray peak as the center. Then, when such fitting is performed, it is preferable to determine the extent to which scattered gamma rays flying from the side are shielded so that the exponential term in the value of this approximate function is smaller than the linear term at the characteristic X-ray peak.

  As shown in FIG. 2, in addition to the characteristic X-ray R3, backscattered gamma rays R4 are incident on the sensing element 20 of the measurement object 100. In the case of radioactive cesium, the characteristic X-ray peaks are 32.1 keV and 36.5 keV, and the peak of the backscattered gamma ray R4 (backscattered peak) exists from 100 keV to 200 keV. In addition, a characteristic X-ray R5 originating from the shield 30 is generated by the main gamma ray R1. When the shield 30 is made of lead, the peak of the characteristic X-ray R5 is 74 keV, which is close to the backscattering peak. Therefore, a radiation peak (adjacent peak) is generated above the characteristic X-ray peak in the peak of the characteristic X-ray R5 or in the energy (wavelength) region where the count values of the characteristic X-ray R5 and the backscattered gamma ray R4 overlap. When the level of such an adjacent peak is high, the baseline at the characteristic X-ray peak becomes high and the counting error becomes large, so that it becomes difficult to distinguish the characteristic X-ray peak.

  In this method, an approximate function is obtained by the least square method for the region between the local minimum points on both sides of the characteristic X-ray peak, that is, the region where the Gaussian function takes a significant value, and the function at the characteristic X-ray peak in such an approximate function is obtained. Determine the magnitude of the linear and exponential terms in the value. In general, in the approximate function, the value of the linear term increases at a constant rate, whereas the value of the exponent term increases dramatically as the variable (energy) increases.

  On the other hand, the present method is characterized in that the coefficient of the exponent term is made extremely small so that the exponent term becomes smaller than the linear term at the characteristic X-ray peak. This is because the background (baseline) of the radiation spectrum is lowered by sufficiently shielding the scattered gamma rays R2 (see FIG. 2) from the side, and thereby the coefficient of the exponent term in the approximate function is extremely small. This means that the exponential term remains at a smaller value than the linear term at the characteristic X-ray peak. Thereby, according to this method, since the characteristic X-ray R3 can be accurately discriminated by sufficiently shielding the scattered gamma ray R2 from the side, the radioactivity of the measuring object 100 can be measured with high accuracy. In addition, since the characteristic X-ray R3 can be easily discriminated, the radioactivity can be measured with a desired accuracy even if the count value of the characteristic X-ray R3 is small, and the measurement with a small amount of exposure of the operator is possible in a short time. . The first embodiment of the method will be described below with reference to FIGS.

(First Example)
FIG. 4 is a graph showing the radiation spectrum SP1 of the first example obtained using the measuring apparatus 10 (see FIG. 3) of the second embodiment. A NaI (Tl) scintillator having a thickness of 0.5 mm was used as the sensing element 20.

  A pavement surface mainly containing radioactive cesium as a radioactive substance is used as a measurement object 100, and radiation is counted at a counting time of 770 seconds to generate a radiation spectrum SP1. The horizontal axis represents photon energy, and the vertical axis represents the count value. FIG. 5 is a graph showing a state in which the radiation spectrum of the first embodiment is fitted with the approximate function F1, and the graph of the approximate function F1 is overlaid on the enlarged view of the low energy portion of the graph of FIG. is there.

The total surface density of radioactivity obtained by counting the main gamma rays with a NaI (Tl) scintillator having a thickness of 2 inches (about 50 mm) was 33 [Bq / cm ² ].

  As shown in FIG. 5, a characteristic X-ray peak P1 originating from radioactive cesium was observed at a position of about 33 keV. In addition, a lead characteristic X-ray R5 peak (lead characteristic X-ray peak P2) was observed at a position of about 74 keV. Then, an adjacent peak (scattered gamma ray peak) P3, which is a radiation peak generated by overlapping the count value of the lead characteristic X-ray R5 and the count value of the scattered gamma ray R4 of radioactive cesium, was observed at a position of about 80 keV.

  The approximate function F1 shown in FIG. 5 is obtained by approximating the characteristic X-ray peak P1 (about 33 keV) with the lower limit L being about 20 keV and the upper limit H being about 50 keV by function fitting using the least square method. Is what we asked for. Therefore, in the spectral region exceeding the upper limit H, the approximate function F1 does not necessarily approximate the radiation spectrum SP1.

The approximation function F1 is composed of a constant term CT, a linear term LT, an exponential term ET, and Gaussian functions G1 and G2, as shown by the following expression (1), and approximates the radiation spectrum SP1 by a nonlinear least square method. To do.
[Equation 1]
F1 = a + b · X + c · EXP (ηX) + G1 + G2 (1)
In Equation (1), X is the radiation energy with the lower limit L as the origin, a is the constant term CT, b · X is the linear term LT, and c · EXP (ηX) is the exponent term ET. The Gaussian functions G1 and G2 have the center, peak, and half width as variables.

For convenience, FIG. 5 shows the Gaussian functions G1 and G2 separated from the approximate function F1. That is, the graph of the approximate function F1 illustrated in FIG. 5 corresponds to the sum of the constant term CT, the linear term LT, and the exponent term ET. A straight line LN shown in FIG. 5 is a straight line having a lower limit L as an origin and an inclination of b. The difference between the straight line LN and the constant term CT corresponds to the linear term LT, and the difference between the approximate function F1 and the straight line LN corresponds to the exponent term ET. The contribution rate of the exponential term ET and the linear term LT in the value of the approximate function F1 at the characteristic X-ray peak P1 has the relationship of the following formula (2) as compared with the arrow in FIG.
[Equation 2]
Linear term LT> exponential term ET (2)

As described above, in the radiation spectrum SP1 of the first example acquired using the measurement apparatus 10 of the second embodiment, the value of the approximate function F1 at the characteristic X-ray peak P1 as a result of sufficiently suppressing the adjacent peak P3. The exponent term ET is smaller than the linear term LT. Thereby, it is confirmed that the shielding ability of the shield 30 in the measuring apparatus 10 (surface density 2.2 g / cm ² in terms of lead) is sufficiently large and the characteristic X-ray peak P1 can be clearly distinguished. It was done.

(Comparative Example 1)
FIG. 6 is a graph showing a radiation spectrum SP2 of Comparative Example 1 obtained by an apparatus (hereinafter referred to as a comparative example apparatus) in a state where the shield 30 is removed from the measuring apparatus 10 of the second embodiment. FIG. 7 is a graph showing a state in which the radiation spectrum SP2 of Comparative Example 1 is fitted with the approximate function F2. The shielding ability of scattered gamma rays and characteristic X-rays in the comparative apparatus is equivalent to the radioactive substance detection apparatus described in Patent Document 1 in that the shield 30 is not attached and the housing 12 is made of stainless steel having a thickness of 1 mm. Is.

  As shown in FIG. 7, also in Comparative Example 1, a characteristic X-ray peak P1 (about 33 keV) originating from radioactive cesium, a characteristic X-ray peak P2 (about 74 keV) of lead, and an adjacent peak P3 (about 80 keV) were observed. .

The total radioactivity surface density obtained by counting the main gamma rays with a NaI (Tl) scintillator having a thickness of 2 inches (about 50 mm) is 28 [Bq / cm ² ], which is substantially the same value as in the first embodiment. there were.

The approximate function F2 is expressed by an expression common to the approximate function F1 of the first embodiment, as shown by the following expression (3).
[Equation 3]
F2 = a + b · X + c · EXP (ηX) + G1 + G2 (3)
However, the coefficient of each term and the position of the lower limit L are different between the radiation spectra SP1 and SP2.

For convenience, FIG. 7 shows the Gaussian functions G1 and G2 separated from the approximate function F2. As compared with the arrow in FIG. 7, in Comparative Example 1, the contribution ratio of the exponent term ET and the linear term LT in the value of the approximate function F2 at the characteristic X-ray peak P1 is the relationship of the following equation (4): The result was the reverse of the first example.
[Equation 4]
Linear term LT <exponential term ET (4)

  This is due to the fact that the amount of scattered gamma rays that cannot be ignored from the side is incident on the sensing element 20 without using the shield 30 and the baseline is raised. As a result, the radiation spectrum SP2 has a sharply rising shape on the upper side of the characteristic X-ray peak P1, and it is difficult to distinguish the characteristic X-ray peak P1. As an index indicating the difficulty in discriminating the characteristic X-ray peak P1, the index term ET at the characteristic X-ray peak P1 is larger than the linear term LT as described above.

  In order to perform function fitting with approximately the same reliability as in the first embodiment, when generating the radiation spectrum SP2 of Comparative Example 1, radiation was counted in a counting time of 1600 seconds, which is twice or more that in the first embodiment. As described above, when the comparative apparatus corresponding to the radioactive substance detection apparatus described in Patent Document 1 is used, it is difficult to discriminate the characteristic X-ray peak P1, and it takes a long time to count the radiation. It was.

  As described above, in this method, the counting step is performed in a state where the scattered gamma rays and the characteristic X-rays flying from the side are sufficiently shielded by the shield 30. Next, in this method, a calculation process is performed for calculating the radioactivity of the radioactive substance based on the result of counting the characteristic X-rays.

  In the calculation step, first, a radiation spectrum is generated based on the result of counting of characteristic X-rays and scattered gamma rays originating from the radioactive substance contained in the measurement object 100.

  Next, in the calculation step, Gaussian functions G1 and G2 are obtained by function fitting with the above-described approximation function F1 for each generated radiation spectrum SP1, and the area of the characteristic X-ray peak P1 is calculated from these areas. However, as described in the first embodiment, by generating the radiation spectrum SP1 using the measurement apparatus 10, the influence of the adjacent peak P3 is reduced and the baseline at the characteristic X-ray peak P1 is lowered. For this reason, since the influence of the exponent term ET on the approximate function F1 is reduced, it is possible to omit performing function fitting using the approximate function F1 in the calculation step. That is, instead of function fitting by the approximate function F1 including the exponent term ET and the Gaussian functions G1 and G2, the area of the characteristic X-ray peak P1 may be simply calculated by area calculation from the radiation spectrum SP1. In calculating the area, the area of the characteristic X-ray peak P1 can be calculated using the Kobel method with respect to a spectral region centered on the characteristic X-ray peak P1 and having the minimum values adjacent to both sides as the lower limit L and the upper limit H. The Kobel method is a method of obtaining the peak area by integrating the count values included in the characteristic X-ray peak P1 and subtracting the area of the baseline portion, in other words, the area of the region surrounded by the radiation spectrum SP1 and its tangent line. (Total of count values) is obtained. Moreover, when performing function fitting in a calculation process, you may use the approximate function different from said approximate function F1 used in order to evaluate the shielding capability of the shielding body 30 in a counting process. Further, in the calculation step, function fitting may be performed for a spectral region different from the spectral region (lower limit L and upper limit H) that has been function-fitted with the approximate function F1.

That is, in the calculation step of the present method, using the function fitting, the spectral region of the generated radiation spectrum SP1 is fitted with an approximate function F1 including at least the linear term LT and the Gaussian functions G1 and G2, and this fitting is performed. The radioactivity of the radioactive substance may be calculated based on the Gaussian functions G1 and G2 obtained by the above.
Instead, in the calculation process of the present method, a radioactive material is used based on the area surrounded by the generated radiation spectrum SP1 and the lower tangent in the spectral region including the characteristic X-ray peak P1 using the Kobel method. May be calculated.

In this method, the radioactivity or radioactivity surface density of the measurement object 100 may be calculated based on the area of the characteristic X-ray peak P1. Specifically, the radioactivity (Bq) or the radioactivity surface density (the radioactivity surface density (Bq) is obtained by applying a conversion coefficient predetermined for the geometrical arrangement of the measuring apparatus 10 and the measurement object 100 to the area of the characteristic X-ray peak P1. Bq / cm ² ).

  According to this method, it is possible to make a non-destructive determination at a measurement site (in situ) whether the concentration is a radioactive contamination that needs to be decontaminated before decontamination work. Further, according to this method, it is possible to determine whether or not the target contamination concentration has been achieved after the decontamination work. In addition, it is possible to quantitatively determine how much radioactive contamination has been improved by the decontamination work. On the other hand, with the conventional method using a survey meter, even if the radioactive contamination is improved in the decontamination target area, the effect of decontamination cannot be quantified due to the influence of environmental radiation flying from the surrounding area. It was. In other words, according to the present method performed using the measuring apparatus 10, the effect of local decontamination work such as decontamination of a house or partial decontamination of land in an open space is quantified as decontamination radioactivity. It is possible.

<Third embodiment>
FIG. 8 is a schematic cross-sectional view showing the radioactivity measurement apparatus (measurement apparatus 10) of the third embodiment of the present invention. FIG. 9 is an explanatory diagram showing radiation shielded by the shield 30 or the measurement base 50 and radiation incident on the detection element 20 in the measurement apparatus 10 of the present embodiment.

  The measurement apparatus 10 of the present embodiment is different from the first embodiment in that the measurement apparatus 10 includes a measurement base 50 that photoelectrically absorbs backscattered gamma rays R4 of a radioactive substance contained in a measurement target (sample) 120. The measurement base 50 is installed on the opposite side of the detection surface 22 with the measurement target (sample) 120 interposed therebetween.

  According to the present embodiment, the backscattered gamma ray R4 radiated from the measurement target 120 is photoelectrically absorbed by the measurement base 50, so that the base near the characteristic X-ray peak P1 is further increased from the radiation spectrum SP1 (see FIGS. 4 and 5). The line can be pulled down.

  The measurement base 50 is installed to face the detection element 20 and is an instrument for placing the measurement target (sample) 120 thereon. That is, the measuring apparatus 10 of the present embodiment is a destructive inspection apparatus that cuts a sample from an object such as soil containing a radioactive substance and measures radioactivity. However, as with the measurement apparatus 10 of the first and second embodiments, the radiation can be obtained by counting the radiation in a short time while reducing the weight of the shield 30. Radioactivity measurement is possible.

  8 and 9 show a state in which the measurement base 50 is placed on the surface layer 110 of the soil 102 and the measurement device 10 is installed thereon. The measurement object 120 is obtained by cutting the vicinity of the surface layer 110 of the soil 102 and optionally covering with a protective film (not shown). The surface density of the protective film may be sufficiently small so as not to substantially shield the characteristic X-ray R3 emitted from the measurement target 120. Therefore, the characteristic X-ray R3 and the main gamma ray R1 emitted from the measurement object 120 are also emitted from the soil 102.

  The measurement base 50 has a surface density that shields the scattered gamma rays R2 and characteristic X-rays R3 of the radioactive substance existing on the opposite side of the measurement object 100 (sample) with the measurement base 50 interposed therebetween. As a result, the scattered gamma rays R2 radiated from the soil 102 toward the surface layer 110 are shielded as shown in FIG. 9, so that the counting noise when counting the characteristic X-rays R3 by the sensing element 20 is further reduced. .

  The measurement base 50 includes a base plate 52 and a shielding plate 54 that shields the characteristic X-ray R5 emitted from the base plate 52. The shielding plate 54 is provided between the base plate 52 and the measurement target 100. The characteristic X-ray peak of the shielding plate 54 is smaller than the characteristic X-ray peak of the base plate 52.

  The base plate 52 is preferably made of a metal material having a large specific gravity such as lead so as to shield the backscattered gamma rays R4 radiated from the soil to the ground surface. On the other hand, when the main gamma ray R1 collides with the base plate 52 (lead), a characteristic X-ray R5 is generated, which can be a noise for counting the characteristic X-ray R3 originating from the radioactive material. On the other hand, the measuring apparatus 10 of the present embodiment provides the characteristic X-ray R5 derived from the base plate 52 (lead) by providing the shielding plate 54 between the base plate 52 and the measurement target 100. Can be shielded. As the shielding plate 54, a metal material such as stainless steel (iron) or copper can be used. The characteristic X-ray of lead is about 74 keV, while iron is about 6 keV and copper is about 8 keV. In other words, the energy of the characteristic X-ray of the shielding plate 54 is preferably significantly smaller than the energy of the characteristic X-ray R3 of the radioactive substance of the measurement target 100. Thereby, the characteristic X-ray derived from the shielding plate 54 does not generate noise with respect to the counting of the characteristic X-ray R3 of the radioactive substance.

(Second embodiment)
FIG. 10 is a graph showing the radiation spectrum SP3 of the second example acquired from the measurement object 120 using the measurement apparatus 10 (see FIGS. 8 and 9) of the third embodiment. A NaI (Tl) scintillator having a thickness of 0.5 mm was used as the sensing element 20. As the measurement base 50, a lead base plate 52 having a thickness of 10 mm and a copper shielding plate 54 having a thickness of 2 mm attached to the upper surface thereof were used.

  The second example and the following Comparative Examples 2 and 3 were performed not in a contaminated site but in a room where the background of radiation was substantially negligible.

As Comparative Example 2, the radiation of the measurement object 120 was counted using a 6 cm thick brick for the measurement base 50. The radiation spectrum SP4 of the counting result is displayed together with FIG. The brick imitates the surface layer 110 of a contaminated site such as concrete or soil.
As Comparative Example 3, the radiation of the measurement object 120 was counted using a light material such as foamed plastic for the measurement base 50. The radiation spectrum SP5 of the counting result is also displayed in FIG. Foamed plastic is composed only of light elements such as carbon and oxygen, and has the same degree as that of air as a generation factor of backscattered gamma rays R4.

  As shown in FIG. 10, when the measurement base 50 is a brick (Comparative Example 2: SP4), many scattered rays near 100 keV are observed, and it has been found that the influence of backscattering from the measurement target 120 is significant. . On the other hand, in the case of the second example (SP3) and comparative example 3 (SP5), there were few scattered rays around 100 keV. In the second embodiment, the backscattered gamma ray R4 from the measurement object 120 is absorbed by the measurement base 50 (base plate 52: lead), and the base plate 52 (lead) further absorbs the backscattered gamma ray R4. Since the characteristic X-rays that are sometimes emitted are shielded by the shielding plate 54, in Comparative Example 3, the main gamma rays from the measurement object 120 toward the measurement base 50 (foamed plastic) are transmitted through the measurement base 50 without being backscattered. It means that.

  From the above results, when measuring the radioactivity of the measurement target 120 on the surface layer 110 of the soil 102 at the contaminated site, the measurement base 50 is provided with the measurement base 50 as in the measurement apparatus 10 of the present embodiment. It has been found that noise caused by the emitted backscattered gamma ray R4 can be suppressed. Further, as shown in FIG. 10, the count value of the characteristic X-ray peak P1 (about 35 keV) caused by the radioactive substance is the same in the second example (SP3), comparative example 2 (SP4), and comparative example 3 (SP5). It was about. Therefore, according to the measurement apparatus 10 of the present embodiment, it is possible to reduce the influence of backscattering without reducing the count value of the characteristic X-ray peak P1, and thus it becomes clear that the characteristic X-ray R3 can be further discriminated. It was.

(Third embodiment)
FIG. 11 is a graph showing the radiation spectrum SP6 of the third example in which the radioactivity is measured in the contaminated area using the measuring apparatus 10 of the third embodiment, and a state in which the radiation spectrum SP6 is fitted with the approximate function F3. is there. However, only the base plate 52 was used as the measurement base 50, and the shielding plate 54 was not used. The approximation function F3 is an equation for approximating the radiation spectrum SP3 by the nonlinear least square method, and is the same as the above equation (1).

  As shown in FIG. 11, in the radiation spectrum SP6, the characteristic X-ray peak P2 of lead became an adjacent peak. The contribution ratio of the exponent term ET and the linear term LT in the value of the approximate function F3 at the characteristic X-ray peak P1 is linear term LT> exponential term ET, as in the equation (2) of the first embodiment. Thereby, it was confirmed that the characteristic X-ray peak P1 can be clearly distinguished also in the second embodiment.

<Fourth embodiment>
FIG. 12 is a schematic cross-sectional view showing a radioactivity measurement apparatus (measurement apparatus 10) according to a fourth embodiment of the present invention. The upper part of the measuring device 10 not shown is the same as that of the first embodiment (FIG. 1).

  The measurement apparatus 10 of the present embodiment is different from the first embodiment in that the side periphery 32 surrounding at least the side periphery of the detection element 20 of the shield 30 has a multilayer structure, and each layer is made of a dissimilar metal material. Is different.

  The number of multilayer layers constituting the side peripheral portion 32 is not particularly limited, and may be two layers or three or more layers. That is, the side peripheral portion 32 may be composed of two layers, that is, a first layer 32a corresponding to the outer layer and a second layer 32b corresponding to the inner layer, or as shown in FIG. 32a, the third layer 32c corresponding to the intermediate layer, and the second layer 32b corresponding to the inner layer may be arranged in this order from the outer peripheral side. It should be noted that the number of multilayer layers constituting the side periphery 32 is not limited to this.

  The shield 30 of the present embodiment is made of an outer layer (first layer 32a) made of a first metal material and an inner layer of the outer layer (first layer 32a) and made of a second metal material. And an inner layer (third layer 32c). The second metal material has a smaller atomic weight than the first metal material.

Here, the first layer 32a corresponding to the outermost periphery of the shield 30 transmits the main gamma ray R1 and shields the barium scattering gamma ray R2 and the characteristic X-ray R3 (see FIG. 2). The first metal material constituting the first layer 32a generates the characteristic X-ray R5 when the main gamma ray R1 collides (see FIG. 2).
On the other hand, by arranging the second layer 32b on the inner peripheral side from the first layer 32a, the second layer 32b corresponding to the inner layer is a characteristic X-ray R5 of the first metal material emitted from the first layer 32a corresponding to the outer layer. Shield. Thereby, the characteristic X-ray R5 is suppressed from entering the detection element 20.

  In the case of the shield 30 having a two-layer structure in which the third layer 32c, which is an intermediate layer between the first layer 32a and the second layer 32b, is not provided, the second layer 32b has a sufficient characteristic X-ray R5 of the first metal material. It is preferable to have a surface density that can be shielded. Thereby, it is not necessary to shield the characteristic X-ray R5 by the housing 12 holding the detection element 20 inside the shield 30, and the housing 12 can be designed arbitrarily and can be thinned.

  On the other hand, the shield 30 of the present embodiment shown in FIG. 12 has an intermediate layer (first layer) that shields the characteristic X-rays of the first metal material between the outer layer (first layer 32a) and the inner layer (second layer 32b). Further provided is a three layer 32c). The third layer 32c is made of a third metal material having an atomic weight between the first metal material and the second metal material.

  As a result, the characteristic X-ray R5 generated when the main gamma ray R1 collides with the outer layer (first layer 32a) is shielded by the third layer 32c (third metal material) adjacent to the inner peripheral side of the first layer 32a. The

  The combination of the first to third metal materials is not particularly limited, but the first metal material constituting the outer layer is lead, the second metal material constituting the inner layer is copper, iron or stainless steel, and the intermediate layer is constituted. The third metallic material to be used can be tin or cadmium.

  The thickness of the intermediate layer (third layer 32c) is equal to or greater than the thickness of the second layer 32b corresponding to the inner layer and smaller than the thickness of the first layer 32a corresponding to the outer layer. Thereby, the surface density of the first layer 32a made of the first metal material having the largest atomic weight and having the largest thickness is raised to the power of the surface density of the third layer 32c corresponding to the intermediate layer. Similarly, the surface density of the third layer 32c increases exponentially with respect to the surface density of the second layer 32b corresponding to the inner layer.

  Here, if a lead characteristic X-ray R5 (particularly Kα-ray) is incident on the sensing element 20, the count value of 32.1 keV or 36.5 keV photons, which is the energy of the barium characteristic X-ray R3, is increased for discrimination. Raising the background will be a problem. On the other hand, by shielding the characteristic X-ray R5 with the intermediate layer (third layer 32c) made of the third metal material as in this embodiment, this time, the third metal material (third layer 32c) For example, a characteristic X-ray of a predetermined energy is emitted from tin. The energy of tin Kα ray is about 25 keV, and Kβ ray is about 28 keV, which is lower than the characteristic X-ray R5 of lead. Since the characteristic X-ray of the third metal material is weak, it can be easily shielded by the inner layer (second layer 32b) adjacent to the inner peripheral side of the third layer 32c.

  That is, the surface density of the first layer 32a (outer layer), the third layer 32c (intermediate layer), and the second layer 32b may be reduced in a power manner for each adjacent layer from the outer layer side toward the inner layer side. . Thereby, while suppressing the entire mass of the shield 30, the characteristic X-rays emitted from each layer can be well shielded by the layer adjacent to the inner peripheral side.

(Fourth embodiment)
FIG. 13 is a graph showing radiation spectra SP7 to SP11 of the fourth example acquired using the radioactivity measuring apparatus (measurement apparatus 10) of the fourth embodiment. The horizontal axis represents photon energy, and the vertical axis represents the count value.

In the measurement apparatus 10 of the fourth embodiment, as the top surface portion 36 (see FIG. 1) that covers the upper portion of the housing 12, specifically, a part of the proximal end side of the photoelectric conversion device 24 and the amplifier 26 in the shield 30. A lead cylinder having a thickness of 2 mm was provided. Moreover, the following 1st layer is used as the base end part 34 provided between the side peripheral part 32 and the top | upper surface part 36 in the side peripheral part 32 which surrounds the side periphery of the detection element 20 among the shielding bodies 30, and the upper part. A third layer 32c was provided from 32a (see FIG. 12).
First layer 32a (outer layer): 4 mm or 6 mm thick lead cylinder Third layer 32 c (intermediate layer): 0.5 mm thick tin cylinder Second layer 32 b (inner layer): 0.5 mm thickness Or 1.5mm copper cylinder

  The radiation spectra SP7 to SP11 are spectra created by counting the radiation under the following conditions with the measurement device 10, the counting time, and the distance from the measuring object 100 to the sensing element 20 being common. The radiation spectra SP7 to SP10 are obtained by setting a radiation source having a predetermined total radioactivity surface density as the measurement object 100, and the radiation spectrum SP11 is a background spectrum in which no radiation source is provided.

The radiation spectrum SP7 is measured without adding any of the first layer 32a to the third layer 32c to the side peripheral portion 32 and the base end portion 34 (comparative example).
The radiation spectrum SP8 is measured by providing only the first layer 32a (lead having a thickness of 4 mm) at the side peripheral portion 32 and the base end portion 34.
In the radiation spectrum SP9, a first layer 32a (4 mm thick lead) is provided on the side peripheral portion 32 and the base end portion 34, and a second layer 32b (inner layer: copper having a thickness of 1.5 mm) is provided on the inner peripheral side thereof. Measured.
The radiation spectrum SP10 includes a first layer 32a (lead having a thickness of 4 mm) on the side peripheral portion 32 and a base end portion 34, and a third layer 32c (intermediate layer: tin having a thickness of 0.5 mm) on the inner peripheral side thereof. Furthermore, the second layer 32b (inner layer: copper having a thickness of 0.5 mm) is provided on the inner peripheral side and measured.
The radiation spectrum SP11 includes a first layer 32a (lead having a thickness of 6 mm) on the side peripheral portion 32 and a base end portion 34, and a third layer 32c (intermediate layer: tin having a thickness of 0.5 mm) on the inner peripheral side thereof. Furthermore, the second layer 32b (inner layer: copper having a thickness of 0.5 mm) is provided on the inner peripheral side and measured.

From the graph of FIG. 13, the following was confirmed.
First, the radiation spectrum SP11 has a count value sufficiently smaller than that of the other radiation spectra SP7 to SP10 over the entire energy band of photons, and the background when there is no radiation source is very low, and the minute radioactivity. It was confirmed on the premise that it is suitable for the measurement.
Next, with respect to the radiation spectrum SP7 in which the side peripheral portion 32 and the base end portion 34 are not mounted as the shield 30, lead having a thickness of 4 mm is mounted on the side peripheral portion 32 and the base end portion 34 as the first layer 32a. In the radiation spectrum SP8, it was confirmed that the count value significantly decreased except for an energy band of about 74 keV. However, a lead characteristic X-ray R5 (see FIG. 6) was generated in the vicinity of 74 keV.

  In the radiation spectrum SP9 in which 1.5 mm thick copper is additionally mounted as the second layer 32b (inner layer) on the inner peripheral side of the first layer 32a, the count value of the characteristic X-ray R5 of lead in the vicinity of 74 keV is greatly reduced. Then, it was found that the characteristic X-ray R5 is shielded by the second layer 32b.

  In the radiation spectrum SP10 in which tin having a thickness of 0.5 mm is attached as the third layer 32c (intermediate layer) between the first layer 32a and the second layer 32b, the second layer 32b is compared with the radiation spectrum SP9. Although the thickness of the (inner layer) was reduced from 1.5 mm to 0.5 mm, the overall count value was further reduced. That is, in the radiation spectrum SP10, even if the thickness of the copper in the second layer 32b is reduced by 1 mm as compared with the radiation spectrum SP9, the thickness of the copper in the second layer 32b can be reduced by 1 mm. It was found that the count value can be suppressed. The thickness of tin used as the third layer 32c is preferably 0.5 mm and more preferably 1.0 mm or more from the viewpoint of shielding the lead characteristic X-ray R5. That is, the thickness of the third layer 32c (intermediate layer) may be larger than the thickness of the second layer 32b (inner layer).

  From the above results, compared to the case where the shield 30 is a single layer, at least the side peripheral portion 32 has a multilayer structure, and the surface density is decreased from the outer layer toward the inner layer, so that a desired shielding ability can be obtained. It was found that it can be realized with a small mass. In particular, when lead is used for the outer layer (first layer 32a), a characteristic X-ray having energy lower than that of barium characteristic X-ray R3 originating from radioactive cesium is radiated to the layer (intermediate layer) disposed inside the outer layer (first layer 32a). It has been found that it is preferable to use a material, and it is preferable to use tin from the viewpoint of availability. And when using tin for the 3rd layer 32c (intermediate layer), it turned out that it is good to provide the 2nd layer 32b which shields the characteristic X ray of tin as an inner layer further on the inner peripheral side. The second layer 32b may be a thin plate of a general-purpose metal material such as copper, iron or stainless steel, and the thickness of the second layer 32b (inner layer) should be less than or equal to the thickness of the third layer 32c (intermediate layer). Can do. Therefore, according to the measurement apparatus 10 (see FIG. 12) used in this example, it was confirmed that the background of the radiation spectrum was satisfactorily suppressed with a low-mass and low-cost configuration.

  The present invention is not limited to the above-described embodiments and examples, and includes various modifications and improvements as long as the object of the present invention is achieved.

The above embodiments and examples include the following technical ideas.
(1) A directivity measuring device that measures the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays, and includes a detection element that detects a characteristic X-ray incident on a detection surface, and at least a side periphery of the detection element. A shield that transmits gamma rays and shields scattered gamma rays and characteristic X-rays that are scattered rays of the gamma rays, and is incident on the detection surface directed to the measurement object containing the radioactive substance The radioactivity measuring apparatus characterized by counting the characteristic X-rays.
(2) The radioactivity measuring apparatus according to (1), wherein the shield shields the peak of the scattered gamma rays of the radioactive substance so as to have a transmittance of less than 50%.
(3) The radioactivity measuring apparatus according to (1) or (2), wherein a transmittance of gamma rays of 60 keV or more and 300 keV or less to the shield is less than 50%.
(4) The radioactivity measuring apparatus according to (3), wherein the shielding ability of the shield against the gamma rays is an area density of 2.2 g / cm ² or more in terms of lead.
(5) The shield includes a side peripheral portion that surrounds a side periphery of the detection element, and a base end portion that is connected to the opposite side of the detection surface with respect to the side peripheral portion. The radioactivity measuring apparatus according to any one of (1) to (4), wherein a surface density at the portion is smaller than a surface density at the side peripheral portion.
(6) From the above (1) to (5), comprising a measurement base that is installed on the opposite side of the detection surface across the measurement target and photoelectrically absorbs backscattered gamma rays of the radioactive substance contained in the measurement target The radioactivity measuring apparatus as described in any one of Claims.
(7) The measurement base according to (6), wherein the measurement base has a surface density that shields the scattered gamma rays and the characteristic X-rays of the radioactive substance existing on the opposite side of the measurement target across the measurement base. Radioactivity measuring device.
(8) The measurement base includes a base plate, and a shielding plate that is provided between the base plate and the measurement target and shields characteristic X-rays radiated from the base plate, The radioactivity measuring apparatus according to (6) or (7) above, wherein the characteristic X-ray peak of the shielding plate is smaller than the characteristic X-ray peak of the base plate.
(9) A method for measuring the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays using a collimated detection element that detects characteristic X-rays incident on a detection surface, the measurement object containing the radioactive substance The gamma ray flying to the detection surface from a direction different from the collimating direction is transmitted to the detection surface, and the scattered gamma rays and characteristic X-rays, which are scattered gamma rays, are shielded from the collimating direction. A radioactivity measurement method comprising: discriminating and counting the characteristic X-rays from the characteristic X-rays flying to the detection surface, the scattered gamma rays and the gamma rays.
(10) A counting step of counting the scattered gamma rays and the characteristic X-rays flying from the collimating direction in a state where the scattered gamma rays and the characteristic X-rays flying from a direction different from the collimating direction are shielded, and counting the characteristic X-rays Calculating the radioactivity of the radioactive substance based on the result, and in the counting step, the peak of the characteristic X-ray among the radiation spectrum generated based on the counting result of the characteristic X-ray and the scattered gamma ray When fitting an approximate function consisting of a linear term, an exponential term, and a Gaussian function with respect to a spectral region having lower and upper limits as local minimum points adjacent to the lower side and the upper side, respectively, the approximation function of the approximate function at the peak of the characteristic X-ray The exponential term in value is the linear Radioactivity measurement method according to (9), characterized in that for shielding the scattered gamma rays to be less than.
(11) In the calculation step, the radiation spectrum is generated based on the counting result of the characteristic X-ray and the scattered gamma ray, and an approximate function including at least a linear term and a Gaussian function for the spectral region of the generated radiation spectrum The radioactivity measurement method according to (10), wherein the radioactivity of the radioactive substance is calculated based on the Gaussian function obtained by the fitting.
(12) In the calculation step, the radiation spectrum is generated based on the counting result of the characteristic X-ray and the scattered gamma ray, and the generated radiation spectrum and a tangent line in a spectral region including a peak of the characteristic X-ray The radioactivity measurement method according to (10), wherein the radioactivity of the radioactive substance is calculated based on an area surrounded by.

Moreover, said embodiment and Example further include the following technical thoughts.
(I) The shield is formed of an outer layer made of a first metal material and a second metal material arranged on the inner peripheral side of the outer layer and having an atomic weight smaller than that of the first metal material. The radioactivity measurement apparatus according to any one of (1) to (5), further including an inner layer.
(Ii) A characteristic X-ray of the first metal material made of a third metal material having an atomic weight between the first metal material and the second metal material between the outer layer and the inner layer. The radioactivity measurement apparatus according to (i), further including an intermediate layer that shields the light.
(Iii) The radioactivity measuring apparatus according to (ii) above, wherein the thickness of the intermediate layer is equal to or greater than the thickness of the inner layer and smaller than the thickness of the outer layer.
(Iv) The first metal material is lead, the second metal material is copper or iron, and the third metal material is tin or cadmium as described in (ii) or (iii) above Radioactivity measuring device.

DESCRIPTION OF SYMBOLS 10 Measuring apparatus 12 Housing 20 Detection element 22 Detection surface 24 Photoelectric conversion device 26 Amplifier 27 Signal terminal 28 Power supply terminal 30 Shielding body 32 Side periphery 32a First layer 32b Second layer 32c Third layer 34 Base end part 36 Top surface part 40 Collimator 41 Opening 42 Bulkhead 44 Shielding region 46 Lid member 50 Measurement base 52 Base plate 54 Shielding plate 70 Mounting base 90 Data processing device 92 Wave height analysis unit 94 Calculation unit 96 Output unit 100, 120 Measurement object 102 Soil 110 Surface layers θb, θc Inclination angle CT Constant term ET Exponential term LT Linear term LN Straight line H Upper limit L Lower limit P1 Characteristic X-ray peak of radioactive material P2 Lead characteristic X-ray peak P3 Scattering gamma ray peak (adjacent peak)
R1 Main gamma ray R2, Ra ~ Rc Scattering gamma ray R3 Characteristic of radioactive material X-ray R4 Backscattering gamma ray R5 Characteristic of shield (lead) X-ray SP1-SP5 Radiation spectrum

Claims (16)

A directivity measuring device that measures the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays,
A sensing element for detecting a characteristic X-ray incident on the sensing surface;
A shielding body that is provided so as to surround at least a side periphery of the sensing element, and that transmits gamma rays and shields scattered gamma rays and characteristic X-rays that are scattered rays of the gamma rays,
The radioactivity measurement apparatus characterized by counting the characteristic X-rays incident on the detection surface directed to a measurement object containing the radioactive substance.   The radioactivity measuring apparatus according to claim 1, wherein the shield shields the peak of the scattered gamma rays of the radioactive substance so as to have a transmittance of less than 50%.   The radioactivity measuring apparatus according to claim 1 or 2, wherein a transmittance of gamma rays of 60 keV or more and 300 keV or less to the shield is less than 50%. The radioactivity measuring apparatus according to claim 3, wherein the shielding ability of the shield against the gamma rays is an area density of 2.2 g / cm ² or more in terms of lead. The shield includes a side peripheral portion that surrounds a side periphery of the detection element, and a base end portion that is connected to the side peripheral portion on the opposite side of the detection surface,
The radioactivity measuring apparatus according to claim 1, wherein a surface density at the base end portion is smaller than a surface density at the side peripheral portion. The shield is
An outer layer made of a first metal material;
An inner layer that is arranged on the inner peripheral side of the outer layer and is made of a second metal material having an atomic weight smaller than that of the first metal material and can shield the characteristic X-rays of the first metal material emitted by the outer layer; The radioactivity measurement apparatus according to claim 1, further comprising:   Made of a third metal material having an atomic weight between the first metal material and the second metal material between the outer layer and the inner layer to shield the characteristic X-ray of the first metal material The radioactivity measurement apparatus according to claim 6, further comprising an intermediate layer.   The radioactivity measuring apparatus according to claim 7, wherein the thickness of the intermediate layer is equal to or greater than the thickness of the inner layer and smaller than the thickness of the outer layer.   The radioactivity measurement according to claim 7 or 8, wherein the first metal material is lead, the second metal material is copper, iron, or stainless steel, and the third metal material is tin or cadmium. apparatus.   10. The measurement base according to claim 1, further comprising a measurement base that is installed on the opposite side of the detection surface across the measurement target and photoelectrically absorbs backscattered gamma rays of the radioactive substance included in the measurement target. Radioactivity measuring device.   The radioactivity measurement apparatus according to claim 10, wherein the measurement base has a surface density that shields the scattered gamma rays and the characteristic X-rays of the radioactive substance existing on the opposite side of the measurement object across the measurement base. .   The measurement base includes a base plate, and a shield plate that is provided between the base plate and the measurement target and shields characteristic X-rays radiated from the base plate, The radioactivity measurement apparatus according to claim 10 or 11, wherein a characteristic X-ray peak is smaller than a characteristic X-ray peak of the base plate. A method of measuring the radioactivity of a radioactive substance that emits characteristic X-rays and gamma rays using a collimated sensing element that detects characteristic X-rays incident on a detection surface,
Directing the detection surface relative to a measurement object containing the radioactive substance;
In a state in which the gamma rays flying to the detection surface from a direction different from the collimating direction are transmitted and the scattered gamma rays and the characteristic X-rays that are the scattered rays of the gamma rays are shielded,
A radioactivity measurement method comprising: discriminating and counting the characteristic X-rays from the characteristic X-rays, the scattered gamma rays and the gamma rays flying from the collimating direction to the detection surface. A counting step of counting the scattered gamma rays and the characteristic X-rays flying from the collimating direction in a state where the scattered gamma rays and the characteristic X-rays flying from a direction different from the collimating direction are shielded;
A calculation step of calculating the radioactivity of the radioactive substance based on the counting result of the characteristic X-ray,
In the counting step, a spectral region having a lower limit and an upper limit as the minimum points adjacent to the lower side and the upper side of the peak of the characteristic X-ray among the radiation spectra generated based on the counting result of the characteristic X-ray and the scattered gamma ray, respectively. When fitting with an approximate function consisting of a linear term, an exponential term, and a Gaussian function, the scattered gamma ray is shielded so that the exponential term in the value of the approximate function at the peak of the characteristic X-ray is smaller than the linear term The radioactivity measurement method according to claim 13, wherein: In the calculation step,
Generating the radiation spectrum based on the counting result of the characteristic X-ray and the scattered gamma ray;
Fitting with an approximate function including at least a linear term and a Gaussian function for the spectral region of the generated radiation spectrum;
The radioactivity measurement method according to claim 14, wherein the radioactivity of the radioactive substance is calculated based on the Gaussian function obtained by the fitting. In the calculation step,
Generating the radiation spectrum based on the counting result of the characteristic X-ray and the scattered gamma ray;
The radioactivity measurement method according to claim 14, wherein the radioactivity of the radioactive substance is calculated based on an area surrounded by the generated radiation spectrum and a lower tangent in a spectral region including a peak of the characteristic X-ray.