JP7207646B2 - Radioactivity measurement method - Google Patents

Description

Application of Article 30, Paragraph 2 of the Patent Act Issued on August 31, 2018 Abstracts of the 22nd Annual Meeting of the Mushroom Society of Japan, p. 61 The Mushroom Society of Japan Proceedings Editorial Committee [Publications] September 12, 2018 The 22nd Annual Meeting of the Mushroom Society of Japan, Hakodate Arena

TECHNICAL FIELD The present invention relates to a technique for measuring radioactivity of an object.

The accident at the Fukushima Daiichi Nuclear Power Station caused by the Tohoku-Chihou-Taiheiyo-Oki Earthquake has seriously damaged shiitake mushroom cultivation and log production in Fukushima Prefecture and surrounding prefectures. In order to maintain the reliability of the quality of raw wood and shiitake mushrooms cultivated using this at the site, for example, the index value set by the Forestry Agency (for example, radioactive cesium is "50 Bq / kg"). It is necessary to exclude raw wood with high radioactivity. Therefore, it is conceivable to adopt a device that inspects radioactive contaminants in an object in a non-destructive manner by measuring the radiation dose of the object at 662 keV derived from ¹³⁷ Cs (cesium 137) as a nuclide (for example, , see Patent Document 1).

Japanese Patent Application Laid-Open No. 8-220029

However, a large amount of radiation at 662 keV derived from ¹³⁷ Cs is also emitted from objects around the target in areas including Fukushima Prefecture and its surrounding prefectures. Therefore, in order to eliminate the influence of the radioactivity of the object and measure the low-level radioactivity derived from the object, it is necessary to use a relatively thick and heavy lead shield. There is Therefore, in order to measure the radioactivity of objects such as standing trees that cannot be moved from the spot, a radioactivity measuring device with a heavy lead shield is transported to a location where the object is located, such as a log production forest. doing so involves a great deal of effort.

Therefore, the present invention is a method that can improve the measurement accuracy while reducing the load of measuring the radioactivity even when the concentration of radioactivity derived from an object that cannot be moved from its place, such as a standing tree, is low. etc.

The present inventor has found that 32 keV X-rays derived from ¹³⁷ Cs, rather than 662 keV gamma rays derived from ¹³⁷ Cs as in the prior art, can be shielded by a metal plate thin enough to be easily transported, and X-rays It was found that high-precision measurement of radioactivity is possible even if the dose is very small.

The radioactivity measuring apparatus of the present invention based on the above findings comprises an X-ray detector that outputs a signal corresponding to the incident dose of at least 32 keV X-rays derived from ¹³⁷ Cs as predetermined X-rays, and the X-ray detector. An X-ray detection unit having an openable and closable housing made of metal having a shielding property against the predetermined X-rays, and a metal member having a shielding property against the predetermined X-rays, and is flexible. a plate-shaped shielding member having properties, a storage device for storing the predetermined reference dose of X-rays according to the output signal of the X-ray detector, and the predetermined dose according to the output signal of the X-ray detector. an arithmetic processing unit configured to evaluate the radioactivity concentration of the object based on the difference between the dose of X-rays and the reference dose of the predetermined X-rays stored in the storage device; and a terminal device comprising:

The method for measuring radioactivity of the present invention based on the above knowledge detects the radioactivity of an object using an X-ray detector that outputs a signal corresponding to the incident dose of at least 32 keV X-rays derived from ¹³⁷ Cs as predetermined X-rays. A method for measuring the X-ray detector, wherein the X-ray detector is housed in an openable and closable housing made of metal having a predetermined X-ray shielding property, and the X-ray detector is the object. A flexible plate-shaped shielding member, which is arranged in an open state so as to be exposed to the X-rays and is made of a metal member having a shielding property against the predetermined X-rays, is provided at least in the housing. a first preparation step of realizing a first state in which the object is arranged to surround the object except for an exposed region of the X-ray detector; a first measuring step of measuring a predetermined dose of X-rays, wherein the housing is arranged in a closed state such that the X-ray detector is shielded from the object, and the shielding member is positioned close to the object; a second preparation step of realizing a second state different from the first state in that an object is not surrounded; Based on the difference between the predetermined dose of X-rays measured in the second measuring step of measuring, the predetermined dose of X-rays measured in the first measuring step, and the predetermined dose of X-rays measured in the second measuring step, and an evaluation step of evaluating the radioactivity concentration of the object.

According to each of the radioactivity measuring device and the radioactivity measuring method of the present invention, the "first state" is achieved. In the first state, the openable/closable housing that houses the X-ray detector is in an open state, and the X-ray detection unit is arranged so that the X-ray detector is exposed to the object. Further, a flexible plate-shaped shielding member is arranged so as to surround the object except for at least the exposed area of the X-ray detector in the housing. Both the housing and the shielding member are made of metal having a shielding property against predetermined X-rays. By adopting 32 keV X-rays derived from ¹³⁷ Cs as the predetermined X-rays, the thickness of the metal constituting the housing and the shielding member can be reduced, and the weight of the housing and the shielding member can be reduced. , the housing and the shielding member can each have a high shielding effect against the predetermined X-rays.

Therefore, in the first state, in addition to X-rays (=A) including predetermined X-rays (=A ₀ ) originating from the object, X-rays (=B) originating from objects around the object A signal corresponding to the dose (=I(A+B-B ₀ )) of the X-rays (=B−B ₀ ) from which the predetermined X-rays (=B ₀ ) are at least partially removed from the X-ray detector output from

Furthermore, a "second state" is realized. In the second state, the housing is closed and the X-ray detection unit is arranged such that the X-ray detector is shielded from the object. Furthermore, the shielding member does not surround the object. Other points are common to the first state.

Therefore, in the second state, in addition to the X-rays (=A−A ₀ ) excluding the predetermined X-rays (=A ₀ ) among the X-rays (=A) derived from the object, of the X-rays ( ₌ B) originating from the object, the dose ( ₌ I(A-A ₀ +BB ₀ )) is output from the X-ray detector. The dose of X-rays in the second state may be stored in advance in the storage device as a "reference dose".

Therefore, the X-ray dose (=I(A+B−B ₀ )) corresponding to the output signal of the X-ray detector in the first state and the X-ray dose corresponding to the output signal of the X-ray detector in the second state The difference between (=I(A−A ₀ +B−B ₀ )) corresponds to the dose (=I(A ₀ )) of the predetermined X-rays (=A ₀ ) originating from the object. . 32 keV X-rays derived from ¹³⁷ Cs having such energy (or wavelength) that the difference is likely to become apparent is adopted as the predetermined X-rays. As a result, based on the dose (=I(A ₀ )) of the predetermined X-rays (=A ₀ ) originating from the object, the measurement accuracy of the radioactivity concentration of the object can be improved.

Furthermore, as described above, the shielding member has a predetermined shielding property against X-rays, but is made of a metal plate material that is thin enough to have flexibility. It can be made lighter than a thick member. As a result, the switching between the first state and the second state is simplified with the deformation of the shielding member. In addition, the thickness of the metal member forming the housing can be reduced to the extent that a predetermined X-ray shielding property can be ensured. For this reason, efforts are being made to reduce the burden of transportation of the radioactivity measuring apparatus including the housing and the shielding member, and thus the burden of radioactivity measurement. Therefore, even for an object that cannot be moved from its place, such as a standing tree, the measurement load can be reduced while maintaining high radioactivity measurement accuracy.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory diagram of the configuration of a radioactivity measuring device as an embodiment of the present invention; 4 is a perspective view of the housing in an open state; FIG. The perspective view of the housing|casing in a closed state. 1 is a flow chart of a radioactivity measurement method as one embodiment of the present invention. Explanatory drawing about a 1st state. Explanatory drawing about a 2nd state. Explanatory drawing about the X-ray dose measurement result in a 1st state and a 2nd state. FIG. 5 is an explanatory diagram regarding the difference in X-ray dose measurement results between the first state and the second state; Explanatory drawing about the relationship between radioactivity concentration and the dose of predetermined X-rays. FIG. 4 is an explanatory diagram relating to the relationship between the thickness of various metals and the predetermined X-ray transmittance; BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a configuration explanatory diagram of a radioactivity information processing system as one embodiment of the present invention; 1 is a functional explanatory diagram of a radioactivity information processing system as an embodiment of the present invention; FIG. Explanatory drawing about an example of the information regarding the radioactivity concentration of a target object. Explanatory drawing about an example of the information regarding the economic value of a target object.

(Radioactivity measuring device (configuration))
A radioactivity measuring apparatus as one embodiment of the present invention shown in FIG.

The X-ray detection unit 10 has an X-ray detector (scintillation counter) composed of a crystal scintillator 11 and a photomultiplier tube 12, and a housing 14 that accommodates the X-ray detector. The housing 14 is formed of a substantially rectangular cylindrical metal plate with one end shielded. A closed state (see FIG. 2B) in which the lid member 18 formed of a plate is closed is switched.

The crystal scintillator 11 is made of cesium iodide with a thickness of 1 cm, for example, and emits light in response to incidence of gamma rays (X-rays). Photomultiplier tube 12 detects the light emitted by crystal scintillator 11 . Thereby, the X-ray dose is detected as the number of events.

The housing 14 and the lid member 18 are made of, for example, one or more metals selected from the group consisting of Pb, Fe, Al, Cu, Ti, Ni, and Zn, or plate materials of alloys thereof. FIG. 8 shows the relationship between the thickness of various metals and the shielding rate of 32 keV characteristic X-rays (corresponding to "predetermined X-rays") derived from ¹³⁷ Cs. From FIG. 8, even if the thickness of Pb, Fe, Cu, Ni, and Zn is 0.10 cm or less, the predetermined X-ray transmittance is less than 10%. For this reason, the housing 14 and the cover member 18 made of a metal plate of Pb, Fe, Cu, Ni, Zn, or the like are highly resistant to a predetermined X-ray even if the thickness of the metal plate is about 0.10 mm. It has shielding properties.

In order to make the shielding effect of the X-ray detector (scintillation counter) by the housing 14 uniform regardless of the orientation, it is preferable that the thickness of the metal plate material constituting the housing 14 is uniform. For example, instead of having the side walls of the housing 14 with a thickness of 2 mm and the bottom having a thickness of 1 mm, the housing 14 can be entirely made of a 1 mm steel plate. preferable.

The shielding member 20 is formed of a substantially rectangular metal plate which is notched in a substantially rectangular shape on one short side in a developed view. The shielding member 20 may generally have a generally cylindrical shape with both short sides spaced apart from each other and having slits extending in the axial direction on the side surfaces. The shielding member 20 is made of, for example, one or more metals selected from the group consisting of Pb, Fe, Al, Cu, Ti, Ni, and Zn, or plate materials of alloys thereof. The shielding member 20, like the housing 14 and the lid member 18, has high shielding properties against predetermined X-rays even if the metal plate has a thickness of about 0.10 mm. In addition, since the shielding member 20 is made of a thin metal plate having a thickness of about 0.10 mm, the shielding member 20 is flexible enough to be deformed into a substantially cylindrical shape so that both short sides are in contact with each other (see FIG. 1). ).

The shape of the shielding member 20 may be changed variously according to the outer shape of the target object X, and its deformation mode may also be changed variously. The shielding member 20 is formed of a single metal plate that is partially detachably connected after being deformed into a shape that conforms to the outer shape of the object X, or by a mechanical connection mechanism such as a bolt-nut. It may be composed of a plurality of independent flat or curved metal plates that are detachably connected and assembled into a shape that follows the contour of the object. For example, the substantially cylindrical shielding member 20 may be assembled by detachably connecting a pair of substantially semi-cylindrical metal plates having the same diameter and length by a mechanical connecting mechanism at the edges. good.

The terminal device 100 is composed of at least one of a personal computer, a smart phone, a tablet terminal, and the like. The terminal device 100 may be fixed with respect to the housing 14 . The terminal device 100 includes a storage device 102 , an input device 111 , an output device 112 and an arithmetic processing device 120 .

The storage device 102 is configured by a memory and stores various information such as the X-ray dose corresponding to the output signal of the X-ray detector in the second state (described later) as a reference dose. The input device 111 is composed of at least one of touch buttons, a keyboard, and a mouse, and further composed of an input interface circuit to which an output signal from the X-ray detector is input by wire or wirelessly. The output device 112 is composed of a display device (image output device), or a display device and an audio output device. The input device 111 and the output device 112 may be composed of touch panels.

The arithmetic processing unit 120 is configured by a CPU (multi-core processor or single-core processor, etc.), and calculates the difference between the X-ray dose corresponding to the output signal of the X-ray detector and the reference dose stored in the storage device 102. is configured to evaluate the radioactivity concentration of the object X based on. Processing unit 120 is “configured” to perform assigned processing means that processing unit 120 reads necessary data and software from various memories, such as the memory that makes up storage device 102, It means that it is programmed or designed to execute arithmetic processing on the data in accordance with the software.
(Radioactivity Measuring Method) A radioactivity measuring method as one embodiment of the present invention using the radioactivity measuring apparatus having the above configuration will be described.

A "first state" is realized in the first preparation step (FIG. 3/STEP02). In the first state, the housing 14 is open, and the X-ray detection unit 10 is arranged such that the crystal scintillator 11 constituting the X-ray detector is exposed to the object X through the housing opening 16. (See Figure 2A). In the first state, a flexible plate-shaped shielding member 20 is deformed and arranged so as to surround the object X except for the exposed area (opening 16) of the X-ray detector in the housing 14. ing. When a standing tree such as a log for cultivating shiitake mushrooms or a log for building materials is selected as the object X, the X-ray detection unit 10 and the shielding member 20 are arranged with respect to the object X as shown in FIG. 4A. ing. In order to maintain the positions and attitudes of the X-ray detection unit 10 and the shielding member 20 with respect to the object X, mechanical fixing mechanisms or connecting mechanisms such as bolts and nuts, as well as support mechanisms such as a mounting table, are used.

A plurality of X-ray detection units 10 may be used from the viewpoint of improving X-ray detection sensitivity. In this case, the shape, size and number of the openings 26 of the shielding member 20 may be changed variously corresponding to the plurality of X-ray detection units.

In the first measurement step, the X-ray dose corresponding to the output signal of the X-ray detector (scintillation counter) is measured in the first state (FIG. 3/STEP04). As a result, the number of events (corresponding to the specific X-ray dose) at each energy (wavelength) is measured, as indicated by the solid line in FIG.

As described above, both the housing 14 and the shielding member 20 have shielding properties against predetermined X-rays. Therefore, in the first state, in addition to X-rays (=A) including 32 keV X-rays (=A ₀ ) derived from ¹³⁷ Cs as predetermined X-rays derived from the object X, X-rays (=BB ₀ ) obtained by at least partially removing predetermined X-rays (=B ₀ ) among X-rays (=B) derived from surrounding objects (soil and fallen leaves near trees) A signal corresponding to the dose (=I(A+B−B ₀ )) is output from the X-ray detector.

Further, the "second state" is realized in the second preparation step (FIG. 3/STEP06). In the second state, the housing 14 is closed and the X-ray detection unit 10 is arranged so that the X-ray detector is shielded from the object X. FIG. Furthermore, the shielding member 20 does not surround the object X. Other points are common to the first state. When a standing tree is selected as the object X, the X-ray detection unit 10 is arranged with respect to the object X as shown in FIG. 4B.

In the second measurement step, the X-ray dose corresponding to the output signal of the X-ray detector (scintillation counter) is measured in the second state (FIG. 3/STEP08). As a result, the number of events (corresponding to the specific X-ray dose) at each energy (wavelength) is measured, as indicated by the dotted line in FIG.

In the second state, in addition to the X-rays (=A−A ₀ ) excluding the predetermined X-rays (=A ₀ ) among the X-rays (=A) derived from the object X, The dose ( ₌ I(A−A _{0 +} B A signal corresponding to -B ₀ )) is output from the X-ray detector. The X-ray dose in the second state may be stored in the storage device 102 in advance as a "reference dose".

In the evaluation step, the level of the object X is evaluated based on the difference between the X-ray dose measured in the first measurement step and the X-ray dose measured in the second measurement step (FIG. 3/ STEP 10). For example, FIG. 6 shows the difference in the number of events at each energy (wavelength) as the difference.

FIG. 7 shows the relationship between the radioactivity concentration and the number of events of a given X-ray (32 keV). In obtaining the relationship, in a laboratory with little background (X-rays (= B) derived from objects around object X), a log of about 280 Bq / kg was used as object X, and the radioactivity concentration was measured with a metal plate. Measurement tests were conducted at radioactivity concentrations in the range of 20 to 280 Bq/kg while adjusting . As a result, it was confirmed that there was a proportional relationship between the radioactivity concentration and the number of gamma-ray events.

X-ray dose (=I(A+B−B ₀ )) corresponding to the output signal of the X-ray detector in the first state and X-ray dose (= The difference between I(A−A ₀ +BB ₀ )) and , corresponds to the dose (=I(A ₀ )) of the predetermined X-rays (=A ₀ ) originating from the object X. X-rays having such energy (or wavelength) that the difference is likely to become apparent are employed as the predetermined X-rays. Therefore, based on the dose (=I(A ₀ )) of a given X-ray (=A ₀ ) originating from the object X, ie the number of events at 32 keV, according to the relationship shown in FIG. The measurement accuracy of the radioactivity concentration of X is improved.

Furthermore, as described above, the shielding member 20 is made of a metal plate material thin enough to have flexibility while shielding against X-rays of 32 keV originating from ¹³⁷ Cs as predetermined X-rays. , the weight can be reduced more than a thick member that has high shape retention and is difficult to deform. This facilitates switching between the first state and the second state with deformation of the shielding member 20 . In addition, the metal member forming the housing 14 can be made thin enough to ensure the desired shielding performance against X-rays. For this reason, the burden of transportation of the radioactivity measuring apparatus including the housing 14 and the shielding member 20 is reduced, and the burden of radioactivity measurement is reduced. Therefore, even for an object that cannot be moved from its place, such as a standing tree, the measurement load can be reduced while maintaining high radioactivity measurement accuracy. (Radiation information processing system (structure))

A radioactivity information processing system as one embodiment of the present invention shown in FIG. Each of the first terminal device Q1 and the second terminal device Q2 and the radiation information processing server 20 are configured to be able to communicate with each other via a network. The first terminal device Q1 may be composed of a terminal device 100 that constitutes a radioactivity measuring device. The second terminal device Q2 may be configured by a terminal device separate from the terminal device 100 that constitutes the radioactivity measuring device, but has the same configuration as the terminal device 100 concerned.
The radioactivity information processing server 200 includes a storage device 202 , an input interface 211 , an output interface 212 and an arithmetic processing device 220 .

The storage device 202 is composed of memory and stores various information. The storage device 202 may be composed of a database server (external storage device) having mutual communication function with the radioactivity information processing server 200 . The input interface 211 is composed of a receiving device having a data receiving function via a network. The output interface 212 is composed of a transmission device having a data transmission function via a network. The arithmetic processing unit 220 is configured by a CPU (multi-core processor, single-core processor, etc.) and configured to execute assigned arithmetic processing (described later).
(Radiation information processing system (function))

According to the radioactivity information processing system configured as described above, the information on the radioactivity concentration of the object X is generated by the first terminal device Q1 (FIG. 10/STEP 112). For generating the information, for example, a radioactivity measuring apparatus and method according to an embodiment of the present invention are employed (see FIGS. 1 to 7).

Information about the radioactivity concentration of the object X is transmitted from the first terminal device Q1 to the radioactivity information processing server 200 together with point information about the point where the object X exists (Fig. 10/arrow D1). The point information is the latitude and longitude of the point where the object X exists (or latitude, longitude and altitude). On the map displayed on the output device 112 of the first terminal device Q1, point information may be specified by a point designated by a user's operation (for example, tapping). Point information may be represented by a plurality of combinations of latitude and longitude representing the extent of the area to which the point belongs and, if necessary, the name of the area.

The radioactivity information processing server 200 receives the information on the radioactivity concentration of the object X and the location information, associates them, and causes the storage device 202 to store and hold them. In addition, based on the radioactivity concentration of the object X, the arithmetic processing unit 220 updates the radioactivity concentration and the economic value of the object X stored in the storage device 202 or an external storage device (database). According to the relationship, the economic value of the object X is evaluated and information on the economic value is generated (FIG. 10/STEP 202). The information on the economic value of the object X is stored and held in the storage device 202 in association with point information associated with the information on the radioactivity concentration that is the basis of the information.

When a predetermined operation (for example, an operation according to predetermined application software) is performed through the input device 111 of the second terminal device Q2, a request for information regarding the radioactivity concentration of the object X existing in the specific area is generated. (FIG. 10/STEP 122). The specific area is, for example, an area designated by a user's operation (for example, double-tapping) on the map displayed on the output device 112 of the second terminal device Q2. The request is transmitted from the second terminal device Q2 to the radioactivity information processing server 200 together with the specific area information (Fig. 10/arrow D2).

The radioactivity information processing server 200 receives the request and the specific area information, and the processing unit 220 calculates the radioactivity concentration of the object X associated with the point information corresponding to the points included in the specific area. information is retrieved from the storage device 202 (FIG. 10/STEP 204). Then, the search information is transmitted from the radioactivity information processing server 200 to the second terminal device Q2 that is the source of the request (Fig. 10/arrow D3). The second terminal device Q2 receives the information on the radioactivity concentration, and the processing unit 220 causes the output device 212 to output the information (FIG. 10/STEP 124).

As a result, on the display device constituting the output device 212, for example, as shown in FIG. A map containing the specified area is displayed. Further, by tapping, for example, the first region S1 among the regions shown in FIG. The text "the number of standing trees Δ" indicating the number of existing objects X and the text "radioactivity concentration ▴" indicating the radioactivity concentration derived from the object X are displayed.

When a predetermined operation (for example, an operation according to predetermined application software) is performed through the input device 111 of the second terminal device Q2, a request for information on the economic value of the object X existing in the specific area is generated. (FIG. 10/STEP 126). The request is transmitted from the second terminal device Q2 to the radiation information processing server 200 together with the specific area information (Fig. 10/arrow D4).

The radioactivity information processing server 200 receives the request and the specific area information, and the processing unit 220 calculates the economic value of the object X associated with the point information corresponding to the points included in the specific area. information is retrieved from the storage device 202 (FIG. 10/STEP 206). Then, the search information is transmitted from the radioactivity information processing server 200 to the requesting second terminal device Q2 (Fig. 10/arrow D5). The second terminal device Q2 receives the information on the economic value, and the processing unit 220 causes the output device 212 to output the information (FIG. 10/STEP 128).

As a result, on the display device constituting the output device 212, as shown in FIG. 11B, for example, the level of the economic value of the object X is represented by the shading of the areas S1 to S5 where the object X exists. A map containing the specified area is displayed. Further, by tapping, for example, the second area S2 among the areas shown in FIG. The text "number of standing trees 0" indicating the number of existing objects X and the text "economic value ●" indicating the economic value derived from the object X are displayed.
(Another embodiment of the present invention)

In the above embodiment, X-rays at 32 keV derived from ¹³⁷ Cs were used as predetermined X-rays to measure the radioactivity of the object X. However, as another embodiment, for example, X-rays of about 100 keV or less For example, X-rays of other nuclide and energy may be used as predetermined X-rays, and the radioactivity of the object X may be measured.

In the above embodiment, information on the radioactivity concentration of the object X is generated by the first terminal device Q1 (see FIG. 10/STEP 112). An output signal (see FIG. 5/solid line) may be transmitted to the radioactivity information processing server 200, and information on the radioactivity concentration of the object X may be generated by the processing unit 220 thereof. In this case, the storage device 202 of the radioactivity information processing server 200 stores the output signal from the X-ray detector in the second state (see dotted line in FIG. 5) or reference information on the radioactivity concentration of the object X corresponding thereto. It may be stored in memory.

In the above embodiment, the radioactivity information processing server 200 generated the information on the economic value of the object X (see FIG. 10/STEP 202). may generate information about the economic value of object X.

In the above-described embodiment, the information on the radioactivity concentration and economic value of the object X is transmitted from the radioactivity information processing server 200 to the second terminal device Q2 (see FIG. 10/arrows D3 and D5). As an embodiment of , at least one of the information on the radioactivity concentration and the economic value of the object X may be directly transmitted from the first terminal device Q1 to the second terminal device Q2.

10: X-ray detection unit, 11: Crystal scintillator (X-ray detector), 12: Photomultiplier tube (X-ray detector), 14: Housing, 16: Housing opening, 18: Lid member, 20: Shield Member 26 Shielding opening 100 Terminal device 102 Storage device 111 Input device 112 Output device 120 Arithmetic processing device 200 Radiation information processing server Q1 First terminal device Q2 Second terminal device.

Claims (1)

A method for measuring the radioactivity of an object using an X-ray detector that outputs a signal corresponding to the incident dose of at least X-rays at 32 keV derived from ¹³⁷ Cs as predetermined X-rays,
A housing that houses the X-ray detector and is made of a metal that shields the predetermined X-rays and that can be opened and closed is opened so that the X-ray detector is exposed to the object. and a shielding member made of a metal member having a shielding property against the predetermined X-rays, wherein X-rays other than the predetermined X-rays do not enter the space surrounded by the shielding member. A plate-shaped shielding member that is permissibly thin and flexible is arranged to surround the object and form the space except for at least the exposed area of the X-ray detector in the housing. A first preparation step for realizing a first state in which
a first measuring step of measuring the predetermined dose of X-rays according to the output signal of the X-ray detector in the first state;
A second state different from the first state in that the housing is arranged in a closed state so that the X-ray detector is shielded from the object, and the shielding member does not surround the object. a second preparatory step for realizing two states;
a second measuring step of measuring the predetermined dose of X-rays according to the output signal of the X-ray detector in the second state;
Evaluate the radioactivity concentration of the object based on the difference between the predetermined X-ray dose measured in the first measurement step and the predetermined X-ray dose measured in the second measurement step. A method of measuring radioactivity, comprising:

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