Nondestructive inspection method and apparatus comprising a neutron source and a gamma-ray detection device for determining a depth of a target component in an inspection target

A nondestructive inspection apparatus makes a neutron beam incident on an inspection target, detects a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the neutron beam, and determines a depth at which the target component exists, based on a result of the detecting. The nondestructive inspection apparatus includes a neutron source that emits a neutron beam to a surface of the inspection target, a gamma ray detection device that detects, as detection intensities, intensities of a plurality of types of specific gamma rays whose energy differs from each other, and a ratio calculation unit that determines a ratio between the detection intensities of a plurality of types of the specific gamma rays.

This application is a continuation of International Patent Application No. PCT/JP2018/038074 filed on Oct. 12, 2018, which is incorporated by reference herein as fully set forth.

TECHNICAL FIELD

The present invention relates to a nondestructive inspection method and apparatus for nondestructively determining a depth of a position where a target component exists in an inspection target, and a concentration of the target component at the depth.

BACKGROUND ART

Damage due to chlorine (chloride ions) is one of factors causing deterioration of infrastructures such as a road and a bridge. For example, chlorine contained in a sea breeze from a coast, or chlorine contained in an antifreezing agent applied in a cold area or a mountain area infiltrates into a concrete structure as an infrastructure. Then, when a concentration (hereinafter, referred to as a chloride ion concentration) of chloride ions around a reinforcing steel bar in the concrete structure exceeds a limit value (a value in a range of 1.2 kg/m3to 2.5 kg/m3), corrosion of the reinforcing steel bar occurs and progresses, causing the concrete structure to be deteriorated.

In order to maintain safety of a concrete structure, inspection is performed to grasp a deterioration state of the concrete structure. According to inspection of the prior art, at one location in the concrete structure, a concrete (referred to as a core) that exists in a range from the surface to the vicinity of the reinforcing steel bar is cut out, and the cut-out core is subjected to fluorescent X-ray analysis, electron probe microanalysis, potentiometric titration, or the like to measure a chloride ion concentration. In this manner, a chloride ion concentration is measured at each position in a range from the surface to a depth near the reinforcing steel bar in the concrete structure, and a deterioration condition of the concrete structure can be grasped.

CITATION LIST

Patent Literatures

SUMMARY OF INVENTION

Technical Problem

However, in extracting a core and measuring a chloride ion concentration thereof, there are the following problems (1) to (3). (1) A part of a concrete structure is damaged to extract the core, and thus, a location of the core extraction is limited. (2) It takes time to extract the core and perform pre-processing of measurement. (3) After a chloride ion concentration is measured for the core extracted from one location in the concrete structure, a chloride ion concentration cannot be measured for the same location. Thus, a change over the years in a deterioration condition of the same location cannot be grasped.

PTL 1 describes a technique of calculating a chloride ion concentration in a concrete, using an electromagnetic wave, but does not disclose that a depth of a position where chlorine exists is determined.

Thus, there is desired a technique capable of nondestructively detecting a depth of a position of a target component (e.g., chlorine) existing in an inspection target.

In view of it, an object of the present invention is to provide a technique capable of detecting a depth of a position of a target component existing in an inspection target without destruction of the inspection target, and a technique capable of evaluating a concentration of the target component at the depth.

Solution to Problem

A nondestructive inspection method according to one aspect of the present invention includes:

(A) making a neutron beam incident on an inspection target;

(B) detecting and identifying a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the neutron beam; and

(C) based on a result of the detecting, generating an index value indicating a depth at which the target component exists,

wherein a step of (B) includes detecting, as detection intensities, intensities of a plurality of types of specific gamma rays whose energy differs from each other, and

a step of (C) includes determining, as the index value, a ratio between the detection intensities of the plurality of types of specific gamma rays.

A nondestructive inspection apparatus according to one aspect of the present invention is an apparatus for making a neutron beam incident on an inspection target, detecting and identifying a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the neutron beam, and determining a depth at which the target component exists, based on a result of the detecting, the nondestructive inspection apparatus including:

a neutron source that emits a neutron beam to a surface of the inspection target;

a gamma ray detection device that detects, as detection intensities, intensities of a plurality of types of specific gamma rays whose energy differs from each other; and

a ratio calculation unit that determines a ratio between the detection intensities of the plurality of types of specific gamma rays.

A nondestructive inspection method according to another aspect of the present invention includes:

(A) making a pulse neutron beam incident on an inspection target;

(B) detecting a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the pulse neutron beam; and

(C) based on a result of the detecting, specifying, in relation to a reference time point, a time point at which the specific gamma ray is detected at (B).

A nondestructive inspection apparatus according to another aspect of the present invention is an apparatus for making a pulse neutron beam incident on an inspection target, detecting and identifying a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the pulse neutron beam, and determining a depth at which the target component exists, based on a result of the detecting, the nondestructive inspection apparatus including:

a neutron source that emits a pulse neutron beam to a surface of the inspection target;

a gamma ray detection device that detects the specific gamma ray generated by the pulse neutron beam incident on the inspection target; and

a time-point specifying unit that specifies, in relation to a reference time point, a time point at which the specific gamma ray is detected.

A nondestructive inspection method according to another aspect of the present invention is a method for making a neutron beam from a neutron source enter an inspection target, detecting and identifying a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the neutron beam, and determining a depth at which the target component exists, based on a result of the detecting, the nondestructive inspection including:

(A) preparing a gamma ray detection device, wherein the gamma ray detection device includes a gamma ray detector that detects the specific gamma ray and a gamma ray shielding portion, a gamma ray passage hole is formed in the gamma ray shielding portion, the gamma ray passage hole includes an opening through which gamma rays are allowed to enter, the gamma ray detector is arranged in the gamma ray passage hole so as to be at a position shifted to a deep side from the opening, and the opening and the gamma ray detector are positioned on a reference straight line;

(B) arranging the neutron source, the gamma ray detector, and the gamma ray shielding portion such that a path of a neutron beam emitted from the neutron source and an extension line of the reference straight line intersects with each other inside the inspection target;

(C) in a state of (B), making a neutron beam from the neutron source enter the inspection target, and detecting thereby-generated gamma rays by the gamma ray detector; and

(D) determining the number of times of detection of the specific gamma ray, based on detection data acquired by the gamma ray detector acquired at (C).

A nondestructive inspection apparatus according to another aspect of the present invention is an apparatus for making a neutron beam incident on an inspection target, detecting and identifying a specific gamma ray deriving from a target component in the inspection target, among gamma rays generated by the neutron beam, and determining a depth at which the target component exists, based on a result of the detecting, the nondestructive inspection apparatus comprising:

a neutron source that emits a neutron beam to a surface of the inspection target; and

a gamma ray detection device that detects a specific gamma ray generated by the neutron beam incident on inspection target,

wherein the gamma ray detection device includes a gamma ray detector for detecting the specific gamma ray, and a gamma ray shielding portion, and

a gamma ray passage hole is formed in the gamma ray shielding portion, the gamma ray passage hole includes an opening through which gamma rays are allowed to enter, the gamma ray detector is arranged in the gamma ray passage hole so as to be at a position shifted to a deep side from the opening, and the opening and the gamma ray detector are positioned on a reference straight line.

Advantageous Effects of Invention

According to the present invention, without destruction of an inspection target, it is possible to detect a depth of a target component existing in the inspection target, and evaluate a concentration of the target component at the depth.

DESCRIPTION OF EMBODIMENTS

Embodiments of the present invention are described with reference to the drawings. The same reference symbols are attached to the parts that are common in the respective drawings, and overlapping description is omitted. The following description does not limit the invention described in claims. For example, the present invention is not limited to one including all of the constituent elements described below.

First Embodiment

FIG.1illustrates a configuration of a nondestructive inspection apparatus10according to a first embodiment of the present invention. The nondestructive inspection apparatus10is an apparatus for emitting a neutron beam from an outside of an inspection target1to a surface1athereof, detecting and identifying, among gamma rays generated in the inspection target1by the neutron beam, a gamma ray (hereinafter, simply referred to also as a specific gamma ray) that derives from a target component in the inspection target1, and determining a depth where the target component exists, on the basis of the detection result. Note that a depth of the target component is a depth from the surface1aof the inspection target1.

In an embodied example, the inspection target1is a concrete structure that includes reinforcing steel bars in the inside thereof, and the target component is chlorine (or chloride ions). When the target component is chlorine, the chlorine may be the stably existing isotope35Cl of chlorine Cl, for example. Note that the inspection target1and the target component are not limited to the combination of the concrete structure and chlorine. In other words, according to the first embodiment, the inspection target1is not limited to a concrete structure, and the target component may be any component that emits a plurality of types of specific gamma rays by a neutron beam made incident on the inspection target1. For example, the target component may be calcium (40Ca as a majority), silicon (28Si as a majority), or the like. Note that hydrogen (1H) emits only one type of gamma rays, and is thus inappropriate for the target component in the first embodiment, but in the second and third embodiments described later, hydrogen may be a target component.

As illustrated inFIG.1, the nondestructive inspection apparatus10includes a neutron source3, a gamma ray detection device5, a ratio calculation unit7, a depth data storage unit9a, a depth detection unit11, and a concentration data storage unit9b.

The neutron source3emits a neutron beam to the surface1aof the inspection target1, making the neutron beam incident on the inspection target1. The neutron source3may emit a pulse neutron beam, or may continuously emit a neutron beam. In the example ofFIG.1, the neutron source3includes an ion source3a, an acceleration device3b, a beam adjuster3c, a target3d, a container3e, and a tubular shielding member3f.

The ion source3agenerates hydrogen ions (protons), for example. The acceleration device3baccelerates the protons generated by the ion source3a. In one example, the protons accelerated by the acceleration device3beach have energy of 7 MeV, for example. The beam adjuster3cincludes a plurality of magnetic field coils that adjust, to the target3d, a direction and an area of the proton beam accelerated by the acceleration device3b. The proton beam that has passed through the beam adjuster3cis incident on the target3d. As a result, reaction between the protons and the target3d(e.g., beryllium) generates neutrons. The target3dis arranged in the container3eformed of a material hard to transmit neutrons and gamma rays. In the container3e, there is formed a hole penetrating from the outer surface to the inside of the container3e. To this hole, the tubular shielding member3ffor neutron emission is attached. The tubular shielding member3fis formed of a material hard to transmit neutrons. Neutrons generated at the target3dpass through the inside of the tubular shielding member3f, and thereby form into a neutron beam to be incident on the inspection target1.

Such a neutron source3can be configured in a small size enough to be loaded on a vehicle such as a truck. Accordingly, the above-described nondestructive inspection apparatus10can be loaded on a vehicle such as a truck, and be transported to a place where the inspection target1(e.g., an infrastructure such as a road or a bridge) exists.

In the first embodiment, a neutron beam emitted by the neutron source3may include thermal neutrons and fast neutrons. Generally, thermal neutrons indicate neutrons having energy nearly at 25 meV and neutrons having energy lower than 25 meV in the case of a room temperature, and fast neutrons indicate neutrons having energy (equal to or higher than several hundred keV) sufficiently higher than that of a thermal neutron. Here, there is no strict definition on thresholds for the names of neutrons based on energy, and for this reason, in the definition of the present application, thermal neutrons may be neutrons having energy equal to or lower than several ten meV (e.g., 50 meV), and fast neutrons may be neutrons having energy equal to or higher than several hundred keV (e.g., 200 keV). Note that a neutron having energy between that of a thermal neutron and that of a fast neutron may be referred to as an epithermal neutron, and a neutron having energy equal to or lower than 0.01 eV may be referred to as a cold neutron.

Energy of respective neutrons emitted from the neutron source3has distribution of 1×10−3eV to 1×107eV, for example, but may be set to be appropriate values depending on a type of inspection target1.FIG.2illustrates one example of an energy spectrum of a neutron beam emitted from the above-described neutron source3. InFIG.2, the horizontal axis indicates energy (kinetic energy) of a neutron, and the vertical axis indicates the number of neutrons passing through a unit cross sectional area (one cm2) in unit time (one second). According to the above-described definition in the present application, inFIG.2, neutrons having energy in the range A are thermal neutrons, and neutrons having energy in the range B are fast neutrons.

The neutron beam made incident on the inspection target1by the neutron source3reacts with the target component in the inspection target1. This generates specific gamma rays deriving from the target component. In the first embodiment, a plurality of types of specific gamma rays are generated from the target component, with energy values of the specific gamma rays being different between a plurality of the types.

The gamma ray detection device5detects, as detection intensities, intensities of specific gamma rays belonging to a plurality of the types generated by incidence of the neutron beam on the inspection target1. The gamma ray detection device5includes a gamma ray detector5aand an intensity detection unit5b.

The gamma ray detector5adetects gamma rays for each value of energy (each wavelength) of gamma rays from the inspection target1, and inputs the detection data thereof to the intensity detection unit5b. The detection data may indicate a pulse height corresponding to energy of each detected gamma ray.

The intensity detection unit5bacquires an energy spectrum of gamma rays, based on each pulse height input from the gamma ray detector5a. This energy spectrum indicates, at each energy value of gamma rays, the number of times of detection of the gamma ray. In the present application, the detection intensity of gamma rays may be a value proportional to the number of times of detection of the gamma ray having the corresponding energy value. The number of times of detection may be the number of times of detection over predetermined measurement time in the first embodiment. The predetermined measurement time is a time period from a time point as the origin when a neutron beam is emitted to the inspection target1to a time point when a sufficient amount of gamma rays caused by the neutron beam are detected by the gamma ray detector5a. For example, the predetermined measurement time may be a time period of 100 seconds, 200 seconds, or 300 seconds from the above-mentioned origin, but is not limited to these time periods. In addition, the detection intensity of the gamma rays may be a count rate Rγdescribed later. The gamma ray detector5amay be constituted by a germanium detector, for example, but is not limited to this.

The intensity detection unit5bdetermines, as detection intensities, intensities of a plurality of types of the specific gamma rays (e.g., in the energy spectrum), based on the determined energy spectrum, and inputs these detection intensities to the ratio calculation unit7. On the assumption that among a plurality of types of the specific gamma rays, one type of the specific gamma ray is a first specific gamma ray, and another type of the specific gamma ray is a second specific gamma ray, the intensity detection unit5bmay detect intensities of the first specific gamma rays and the second specific gamma rays, as respective detection intensities, and may input these detection intensities to the depth detection unit11and the concentration evaluation unit13.

In an embodied example, a plurality of types of specific gamma rays deriving from35Cl as the target component include gamma rays having energy of 517 keV, 786 keV, 788 keV, 1165 keV, 1951 keV, and 6111 keV. In this case, for example, the first specific gamma ray may be the gamma ray having energy of 1951 keV, and the second specific gamma ray may be the gamma ray having energy of 517 keV. However, a combination of the first specific gamma ray and the second specific gamma ray is not limited to this. As an energy difference between the first specific gamma ray and the second specific gamma ray becomes larger, depth detection accuracy tends to become higher. However, use of a detection intensity of specific gamma rays having high energy (e.g., 6111 keV) may be avoided.

The ratio calculation unit7calculates a ratio between detection intensities of a plurality of types of the specific gamma rays input from the intensity detection unit5b. In an embodied example, the ratio calculation unit7calculates a ratio of a detection intensity of the second specific gamma ray to a detection intensity of the first specific gamma ray.

The depth data storage unit9astores depth data representing a relation between a depth at which the target component exists in the inspection target1and a ratio between detection intensities of a plurality of types of the specific gamma rays. The depth data may be acquired in advance, and may be acquired by an experiment, for example.

In this experiment, a plurality of specimens formed of the same material as that of the inspection target1are prepared. Depths at which the target component exists in a plurality of the specimens differ among these specimens. A ratio between detection intensities of a plurality of the specific gamma rays is determined for each specimen, using the above-described neutron source3and gamma ray detection device5. The above-described depth data are produced based on the depth of the target component in each of a plurality of the specimens and the above-described ratio for each of a plurality of the specimens. The thus-produced depth data are stored in advance in the depth data storage unit9a. In an embodied example, the depth data represent a relation between a depth at which the target component exists in the inspection target1and a ratio of an intensity of the second specific gamma rays to an intensity of the first specific gamma rays.

Note that the above-described experiment for acquiring the depth data and actual inspection on the inspection target1(the step S1described later) may be performed under the same conditions. The conditions include a neutron spectrum condition, a distance condition, and an orientation condition. The neutron spectrum condition is a condition that an energy spectrum of a neutron beam emitted from the neutron source3to the inspection target1(the specimen in the above-described experiment) is a set spectrum. The distance condition is a condition that a distance between the surface of the inspection target1(the specimen in the above-described experiment) and the detector5ais a set distance. The orientation condition is a condition that a relation (an incident angle) between an orientation of a neutron beam emission port (inFIG.1, an opening at a front end of the tubular shielding member3f) in the neutron source3and an orientation of the surface1aof the inspection target1(the specimen in the above-described experiment) is a set relation, and a relation between an orientation of the detector5aand an orientation of the surface1aof the inspection target1(detection angle) is a set relation (e.g., the incident angle is 90 degrees, and the detection angle is 45 degrees). The above-described “same conditions” may further include other conditions (e.g., a measurement time condition). The measurement time condition is a condition that the above-described measurement time is set time.

The depth detection unit11determines a depth at which the target component exists, based on the depth data stored in the depth data storage unit9aand a ratio calculated by the ratio calculation unit7. At this time, the depth detection unit11may apply the ratio to the depth data, and thereby determines a depth at which the target component exists. The depth detection unit11outputs the determined depth. The output depth may be stored in an appropriate storage medium, be displayed on a display, or be printed on a paper sheet.

Assuming that one of the first specific gamma ray and the second specific gamma ray is set as a selection gamma ray, the concentration data storage unit9bstores concentration data representing a relation between a detection intensity of the selection gamma ray and a concentration of the target component. The concentration data storage unit9bstores the concentration data for each depth in the inspection target1so as to be associated with the depth. The concentration data may be acquired in advance, and may be acquired by an experiment, for example.

In this experiment, the following steps (1) to (3) are performed.

(1) A specimen is prepared. The specimen (referred to as a known-concentration specimen) is formed of the same material as that of the inspection target1, and contains the target component at a known concentration.

(2) On the known-concentration specimen, a zero-concentration specimen is placed without a gap in a direction of thicknesses of both thereof. Here, the zero-concentration specimen is a specimen formed of the same material as that of the inspection target1and containing the target component at a concentration of zero. Each of the specimens has a rectangular parallelepiped shape.
(3) In the state of the step (2), the neutron source3emits a neutron beam such that the neutron beam passes through the zero-concentration specimen and the known-concentration specimen in this order, and a detection intensity of the thus-generated selection gamma rays is acquired by the gamma ray detection device5.

The above-described steps (1) to (3) are performed for each of a plurality of known-concentration specimens whose concentrations of the target component are different from each other. Thereby, the above-described concentration data are produced based on the concentration of the target component in each of a plurality of the known-concentration specimens and the detection intensity of the selection gamma rays for each of a plurality of the known-concentration specimens. Here, the detection intensity may be acquired by the following equation (A). Each symbol in the equation (A) is the same as that in the case of the equation (1) described later. The equation (A) is an equation when εγin the below-described equation (1) is eliminated, i.e., when εγis set as “1”.
Rγ={(A/t)/Iγ}/(Ip/50)  (A)

A distance to the known-concentration specimen from a surface that belongs to the zero-concentration specimen and on which the neutron beam is made incident corresponds to a depth (a depth from the surface1a) in the inspection target1. Thus, the concentration data acquired as described above for the one zero-concentration specimen (i.e., a thickness of this specimen) are data for one depth in the inspection target1. For this reason, the concentration data are acquired as described above for each of a plurality of zero-concentration specimens whose thicknesses are different from each other. Thereby, the concentration data are acquired for each depth in the inspection target1.

Alternatively, using a standard gamma ray source (e.g.,133Ba or152Eu) for example, detection efficiency εγ(described later) may be acquired in advance for each depth, and for each depth, the concentration data based on the detection efficiency for the depth may be acquired. In this case, at the above-described step (3), a detection intensity of the selection gamma rays is acquired by the below-described equation (1).

Note that the concentration data of depths (thicknesses of zero-concentration specimens) or the selection gamma ray (energy of the gamma ray) for which the experiment is not performed may be acquired by interpolation based on the concentration data or the detection efficiency for which the experiment is performed.

Note that the depth data storage unit9a, the concentration data storage unit9b, and the below-described detection efficiency storage unit8may be different storage areas in the same storage device such as a semiconductor memory, a hard disk, or a USB memory as illustrated inFIG.1, or may be separate storage devices.

Based on a depth determined by the depth detection unit11, the concentration data stored in the concentration data storage unit9band associated with this depth, and an input detection intensity of the selection gamma rays, the concentration evaluation unit13determines a concentration of the target component at this depth. At this time, the concentration evaluation unit13may apply the detection intensity of the selection gamma rays to the concentration data that are associated with the depth determined by the depth detection unit11and that are included in the concentration data associated with respective depths in the concentration data storage unit9b, and may thereby determine a concentration of the target component at this depth. The concentration evaluation unit13outputs the acquired concentration. The output concentration may be stored in an appropriate storage medium, be displayed on a display, or be printed on a paper sheet.

Note that the above-described experiment for acquiring the concentration data and the actual inspection (actual inspection at the time of acquiring a detection intensity of the selection gamma rays used at the below-described step S5(the below-described steps S105and S205in the second embodiment and the third embodiment)) is performed under the same conditions. The conditions include the above-described neutron spectrum condition, distance condition, and orientation condition. Note that the “same conditions” may further include another condition (e.g., the above-described measurement time condition). A beam diameter of a neutron beam emitted by the neutron source3in the above-described experiment is the same as that in the actual inspection of the inspection target1because of the configuration of the neutron source3(e.g., the tubular shielding member3f) for example.

(Detection Principle of Depth of Target Component)

<Gamma Rays Deriving from Target Component>

The detection principle of a depth of the target component according to the first embodiment is described in detail. When a neutron beam is made incident on the inspection target1, various elements existing in the inspection target1make reaction of capturing the neutrons, and become excited compound nuclei. The compound nuclei immediately transition from the excited state to a ground state, and at this time, emit gamma rays. Energy of the gamma rays and intensities of the gamma rays derive from the elements (nuclei) that emit the gamma rays.

<Detection Depth Range Based on Thermal Neutrons and Fast Neutrons>

Out of neutrons included in a neutron beam from the neutron source3, the thermal neutrons are captured by elements, but the fast neutrons are less likely to be captured by elements. Accordingly, at a high possibility, the thermal neutrons made incident on the inspection target1react with the target components that is in the inspection target1and that is in a range close to the surface1a. For example, this range is a range of several centimeters from the surface1awhen the inspection target1is a concrete structure. For this reason, the thermal neutrons are used to detect the target component in the range close to the surface1a.

Meanwhile, the fast neutrons made incident on the inspection target1hardly react with the target components in the range that is in the inspection target1and that is close to the surface1a, and the fast neutrons are repeatedly scattered in the inspection target1to become thermal neutrons. Thus, at a high possibility, the fast neutrons become thermal neutrons, and then react with the target components existing in a range deep from the surface1ain the inspection target1. For example, this range is a range of 10 cm to 30 cm from the surface1awhen the inspection target1is a concrete structure. For this reason, the fast neutrons are used to detect the target component in the range deep from the surface1a.

Therefore, by making neutron beams including both thermal neutrons and fast neutrons incident on the inspection target1, it is possible to handle detection of the target components both in a range close to the surface1aand in a range deep from the surface1a.

A plurality of mortar specimens formed of mortar were prepared, and the experiment was performed. Respective concentrations (hereinafter, referred to as chloride ion concentrations) of chloride ions as the target component in these mortar specimens were set as 0.3 kg/m3, 0.5 kg/m3, 1 kg/m3, 3 kg/m3, and 5 kg/m3. Each of the mortar specimens has a cubic shape, and has each edge of 40 mm.

For each of the mortar specimens, a neutron beam was made incident on the mortar specimen by the neutron source3, and an energy spectrum of the gamma rays thus generated in the mortar specimen was measured. For each of the mortar specimens, the experiment was performed under the same conditions. In other words, the conditions include the above-described neutron spectrum condition, distance condition, and orientation condition.

FIG.3illustrates a relation between a chloride ion concentration and a count rate Rγ(count/second) that is an intensity of the specific gamma rays detected by the gamma ray detector5ain the experiment.FIG.3illustrates measurement results of specific gamma rays having energy values of 517 keV, 786 keV, 788 keV, 1165 keV, and 1951 keV. Note that 786 keV and 788 keV are values close to each other, and for this reason, inFIG.3, the sum of detection intensities of the gamma rays of these two energy values is used as the count rate Rγof the specific gamma rays of one energy value 787 keV.

The count rate Rγindicates a total gamma-ray dose (gamma ray intensity) calculated from the number of times of detection of the gamma ray measured in unit time for each energy value. This total gamma-ray dose is a total dose of gamma rays radiated by35Cl that has captured neutrons. Specifically, the count rate Rγ(count/second) was determined by the following equation (1).
Rγ=[{(A/t)/εγ}/Iγ]/(Ip/50)  (1)

Here, A indicates the number of times of detection of the specific gamma ray for each energy.

The symbol εγindicates gamma ray detection efficiency (%/100), and is a value acquired in advance using a standard gamma ray source or the like. The gamma ray detection efficiency is a ratio of the number of times a gamma ray is detected by the gamma ray detector5ato a quantity of gamma rays from a gamma ray source (a position from which the gamma rays are emitted), is inversely proportional to energy of a gamma ray, and is inversely proportional to a distance between the gamma ray source and the gamma ray detector5a. In order to determine a depth of the target component, εγfor each energy value is set in the gamma ray detection device5on the assumption that a distance between the gamma ray source in the inspection target1and the gamma ray detector5ais a predetermined constant value (this εγis written also as εγSor εγd; the same applies to the second embodiment and third embodiment). Based on εγcorresponding to each of the types of the specific gamma rays, the gamma ray detector5determines, as a detection intensity of the specific gamma ray of the type, an integrated value of a count rate Rγover the above-described measurement time.

In the first embodiment, the gamma ray detection efficiency (i.e., the gamma ray detection efficiency used at the below-described step S2) concerning calculation of a ratio between detection intensities of a plurality of types of the specific gamma rays is a value in a state where anything other than air does not exists between the gamma ray source and the gamma ray detector5a.

Meanwhile, the gamma ray detection efficiency (e.g., the gamma ray detection efficiency used in the case of determining the above-described concentration data or used at the below-described step S5) concerning determination of a concentration of the target component may be a value depending on a material of the inspection target1or the specimen.

The symbol Iγindicates an intensity ratio (%/100) of the specific gamma ray when35Cl captures neutrons. In other words, Iγis a ratio representing the number of times of detection of each type of the specific gamma ray deriving from35Cl. For example, Iγrepresents the number of times of detection of each type of the specific gamma rays when35Cl captures 100 neutrons. In one example, when35Cl captures 100 neutrons, the number of the emitted specific gamma rays of 1165 keV and the number of the emitted specific gamma rays of 1951 keV are 26.82 and 19.05, respectively (accordingly, Iγ=0.2682 and Iγ=0.1905 are input).

The symbol t indicates the above-described measurement time (second).

The symbol Ipis an average current (μA) of the proton beam incident on the target3dat the time of measurement, and 50 indicates that the count rate Rγis normalized by 50 μA. This numerical value does not need to be 50, and may be 10 or 100. The count rate Rγindicates an intensity of gamma rays.

As understood fromFIG.3, for each value of a chloride ion concentration, a count rate Rγbecomes higher as energy of the specific gamma ray becomes higher. This indicates that a transmissivity of a gamma ray becomes higher as energy of a gamma ray becomes higher. The transmissivity represents a ratio of gamma rays that are among gamma rays generated in the mortar specimen or the inspection target1and that can pass through the surface of the mortar specimen or the inspection target1. In other words, when among the total amount of gamma rays generated at a position of a predetermined depth from the surface in the mortar specimen or the inspection target1, a certain amount of gamma rays pass through the surface, a ratio of the certain amount to the total amount is the transmissivity (the same applies to the following).

As understood fromFIG.3, a detection intensity of each type of the specific gamma rays becomes higher as a chloride ion concentration in the mortar specimen becomes higher. InFIG.3, a chloride ion concentration and a detection intensity of the specific gamma rays are in a substantially proportional relation. Accordingly, it can be said that an intensity ratio between a plurality of types of specific gamma rays does not depend on a concentration of the target component (chlorine).

Using a combination of a difference in transmissivity between a plurality of types of the specific gamma rays and a difference in intensity ratio between a plurality of types of the specific gamma rays enables a depth of the target component to be determined as described below.

FromFIG.3, it is understood that a chloride ion concentration can be evaluated even when a chloride ion concentration is as low as 0.3 kg/m3. Thus, by the concentration evaluation unit13, it can be detected whether chloride ion of a concentration causing corrosion of a reinforcing steel bar exists or not, since a lower limit value of a chloride ion concentration causing corrosion of a reinforcing steel bar is a value ranging from approximately 1.2 to 2.5 kg/m3.

FIG.4represents, as ratios, theoretical calculated values of transmissivities of a plurality of types of gamma rays to a concrete. InFIG.4, the horizontal axis indicates energy of a gamma ray, and the vertical axis indicates a transmissivity representing a ratio of gamma rays that are among gamma rays generated in the concrete and that pass through the surface of the concrete. In other words, assuming that a transmissivity of a gamma ray having energy of 2000 keV is 1, a ratio of a transmissivity of each type of gamma rays to this transmissivity is indicated by the vertical axis. Energy of the respective types of gamma ray is 500 keV and 1250 keV.

InFIG.4, the circle marks are calculated values for gamma rays generated at a depth of 1 cm from the surface of the concrete, the square marks are calculated values for gamma rays generated at a depth of 5 cm from the surface of the concrete, and the triangle marks are calculated values for gamma rays generated at a depth of 10 cm from the surface of concrete. As understood fromFIG.4, for each depth, a transmissivity becomes higher as energy of a gamma ray becomes higher.

FIG.5is a schematic view for illustrating the principle of depth detection of the target component according to the first embodiment.FIG.5illustrates the case where a neutron beam is made incident on a surface of a concrete as the inspection target1, the neutrons react with chlorine in the concrete, and the specific gamma rays are generated.

As illustrated inFIG.5, it is assumed that chlorine exists at a position whose depth from the surface is a first depth, a second depth, or a third depth in the concrete. It is assumed that when chlorine exists at the first depth, neutrons incident on the surface of the concrete react with the chlorine in the concrete, and thereby, a plurality of types of the specific gamma rays having energy E1, E2and E3are generated and emitted from the surface. Also in each of the cases where chlorine exists at the second depth and at the third depth, a plurality of types of the specific gamma rays having energy E1, E2and E3are similarly generated and emitted from the surface.

InFIG.5, when chlorine exists at the first depth, the specific gamma rays of energy E1, E2, and E3are generated at the first depth at intensities A1, A2, and A3, respectively, pass through the surface at transmissivities P1, P2, and P3, respectively, and are detected at intensities A1×P1, A2×P2, and A3×P3, respectively.

Similarly, when chlorine exists at the second depth, the specific gamma rays of energy E1, E2, and E3are generated at the second depth at intensities B1, B2, and B3, respectively, pass through the surface at transmissivities Q1, Q2, and Q3, respectively, and are detected at intensities B1×Q1, B2×Q2, and B3×Q3, respectively.

Similarly, when chlorine exists at the third depth, the specific gamma rays of energy E1, E2, and E3are generated at the third depth at intensities C1, C2, and C3, respectively, pass through the surface at transmissivities R1, R2, and R3, respectively, and are detected at intensities C1×R1, C2×R2, and C3×R3, respectively.

For the case of the first depth, a ratio between detection intensities of a plurality of types of the specific gamma rays that have passed through the surface is determined. For example, a ratio (A1×P1)/(A3×P3) of the detection intensity A1×P1to the detection intensity A3×P3is determined. This ratio does not depend on a chlorine concentration at the first depth. This is because A1and A3is each proportional to a chlorine concentration at the first depth, and accordingly, changes of A1and A3caused by a chlorine concentration cancel each other in A1/A3. Further, A1/A3does not depend on the first depth. This is because an intensity of a neutron beam (thermal neutrons) reaching a certain depth (e.g., the first depth) is proportional to a depth, an intensity of gamma rays generated at the depth is proportional to an intensity of the neutron beam (thermal neutrons) that has reaches the depth, and accordingly, changes of A1and A3caused by a depth cancel each other in A1/A3. Thus, in the ratio (A1×P1)/(A3×P3), A1/A3does not change depending on a concentration and an existence depth of chlorine, and is a value deriving from the target component. Meanwhile, transmissivities P1and P3are not proportional to the first depth, but are values that correspond to the first depth. Accordingly, an intensity ratio (A1×P1)/(A3×P3) is a value corresponding to the first depth.

Also in the case of the second depth, similarly, a ratio (B1×Q1)/(B3×Q3) of detection intensities is a value corresponding to the second depth. Also in the case of the third depth, a ratio of detection intensities (C1×R1)/(C3×R3) is a value corresponding to the third depth.

Accordingly, the above-described depth data representing a relation between such a ratio and a depth at which the target component (chlorine in this example) exists are acquired in advance, and based on the depth data and a ratio between detection intensities measured at the time of inspection, a depth at which the target component exists can be determined.

In the case where chlorine exists over a range from the surface to the third depth, a depth acquired by the above-described depth detection unit11is a rough value (e.g., an average depth) of a depth at which chlorine exists. Even in this case, from a depth output by the depth detection unit11, a rough value of a depth at which the chlorine exists can be grasped. For example, when a depth output from the depth detection unit11is close to a position of a reinforcing steel bar in a concrete structure as the inspection target1, it can be determined that the reinforcing steel bar may be corroded by chlorine (chloride ions). For the same inspection target1, repeatedly acquiring a depth of chlorine at predetermined inspection date intervals (e.g., monthly or yearly) enables a change in chlorine permeation depth in the inspection target1to be grasped.

FIG.6is a flowchart illustrating a nondestructive inspection method according to the first embodiment. The method may be performed using the above-described nondestructive inspection apparatus10. The method includes steps S1to S5.

At the step S1, the neutron source3emits a neutron beam to the surface1aof the inspection target1. Thereby, the neutron beam incident on the inspection target1reacts with the target component in the inspection target1, and a plurality of types of the specific gamma rays deriving from the target component are generated.

At the step S2, the gamma ray detection device5detects, as detection intensities, intensities of a plurality of the types of the specific gamma rays generated at the step S1. The step S2includes steps S21and S22. At the step S21, the gamma ray detector5adetects gamma rays of each energy values. At the step S22, the intensity detection unit5bgenerates an energy spectrum of gamma rays, based on the detection data (a pulse height corresponding to energy of each detected gamma ray) acquired at the step S21, and detects, as a detection intensity, an intensity of each type of the specific gamma rays, based on the acquired energy spectrum, in accordance with the above-described equation (1). A gamma ray detection efficiency εγused at the step S2is εγSdescribed above.

At the step S3, based on a result of the detection at the step S2, an index value indicating a depth at which the target component exists is generated. In other words, the ratio calculation unit7calculates, as the index value, a ratio between the detection intensities of a plurality of types of the specific gamma rays detected at the step S2. In an embodied example, this ratio is a ratio of the detection intensity of the above-described second specific gamma rays to the detection intensity of the above-described first specific gamma rays.

At the step S4, based on the ratio calculated at the step S3and the depth data in the depth data storage unit9a, the depth detection unit11determines a depth at which the target component exists.

At the step S5, based on the depth determined at the step S4, the concentration data concerning the determined depth and stored in the concentration data storage unit9b, and a detection intensity of the selection gamma rays, the concentration evaluation unit13determines a concentration of the target component at the determined depth.

In the case of using the concentration data acquired for each depth using the above-described equation (A), the step S5is performed as follows. Based on the depth determined at the step S4, the concentration data concerning the determined depth and stored in the concentration data storage unit9b, and the detection intensity of the selection gamma rays, the concentration evaluation unit13determines a concentration of the target component at the determined depth. In this case, the gamma ray detection device5(the intensity detection unit5b) determines the number A of times of detection of the selection gamma ray, based on the energy spectrum of the gamma rays acquired at the above-described step S2, and detects a detection intensity of the selection gamma rays, based on the number A of times of detection and the above-described equation (A). This detection intensity is input to the concentration evaluation unit13, and is used at the step S5by the concentration evaluation unit13. The detection intensity of the selection gamma rays used at this time may be newly extracted and acquired from the energy spectrum of the gamma rays acquired at the above-described step S2.

Meanwhile, when the concentration data for each depth are acquired using a gamma ray detection efficiency εγcorresponding to the depth, the step S5is performed as follows.

First, assuming that a gamma ray detection efficiency εγused at the step S2is εγS, and a gamma ray detection efficiency εγcorresponding to the depth determined at the step S4is εγd, the intensity detection unit5bdetects a detection intensity of the selection gamma rays, based on the equation (1) in which εγSis replaced with εγd, and based on the number A of times of detection of the selection gamma ray (e.g., already acquired or newly selected and acquired by the intensity detection unit5b, based on the energy spectrum of gamma rays acquired at the above-described step S2). In this case, as illustrated inFIG.1, the detection efficiency storage unit8stores detection efficiency data representing a gamma ray detection efficiency εγcorresponding to each depth in the inspection target1(i.e., a gamma ray detection efficiency εγused for acquiring concentration data for each depth and corresponding to each of these depths), and the intensity detection unit5bspecifies εγddescribed above, based on the detection efficiency data in the detection efficiency storage unit8and the depth (input from the depth detection unit11) determined at the step S4, and uses the equation (1) in which εγSis replaced with εγdas described above.

Next, based on the detection intensity detected by the intensity detection unit5b, the depth determined at the step S4, and the concentration data concerning the determined depth and stored in the concentration data storage unit9b, the concentration evaluation unit13determines a concentration of the target component at the determined depth.

Advantageous Effects of First Embodiment

Intensities of a plurality of types of the specific gamma rays generated by reaction between the target component and neutrons incident on the above-described inspection target1are detected. As described above, a ratio between the detection intensities of a plurality of the specific gamma rays is a value corresponding to a depth at which the target component exists. In other words, this ratio indicates the depth at which the target component exists. For this reason, acquiring such a ratio enables detection of a depth at which the target component exists. Thus, a depth of the target component in the inspection target1can be detected nondestructively. For example, without extracting a core from a concrete structure as the inspection target1, it is possible to detect a depth of a position of the target component existing in the inspection target1, and to evaluate a concentration of the target component at the depth.

A gamma ray detection efficiency (Fγin the calculation equation (1) of the above-described count rate Rγ) at a depth in a concrete in which the target component exists is acquired in advance by an experiment (i.e., detection efficiency data are acquired in advance for acquisition of concentration data, as described above), and thereby, it can be also detected what amount of chlorine exists at the earlier-determined depth of a position of the target component.

For example, 1 kg/m3stated in the concrete specifications for concrete structures as a chloride ion concentration (marginal concentration) at which a steel member inside a concrete starts to corrode is set as a lower limit, or a concentration smaller than 1 kg/m3is set as a lower limit, and for a range from the lower limit to an assumed high concentration (e.g., 10 kg/m3), the above-described concentration date are acquired (i.e., a calibration curve is drawn), and thereby, a concentration can be evaluated by comparing to each other data acquired at the time of actual measurement and the calibration curve (the concentration data). A chloride ion concentration at which a steel member inside a concrete starts to corrode varies depending on a type of concrete and a ratio of water and cement, and takes a value in a range from 1.2 to 2.5 kg/m3.

In the first embodiment, the neutron source3may be configured such that an angle made by a direction in which the proton beam is incident on the target3dand the direction of the neutron beam emission port in the neutron source3is 90 degrees. The neutron source3of this configuration emits, to the inspection target1, a neutron beam in which among fast neutrons and thermal neutrons, a fast neutron component is greatly reduced such that the neutron beam is constituted mainly by the thermal neutrons. Thereby, a depth of the target component in an area near the surface1aof the inspection target1can be detected accurately.

Meanwhile, in the first embodiment, when the below-described moderator3gis not provided, or when the moderator3gis provided, but a thermal neutron shielding material is installed on the surface1aof the inspection target1, and a neutron beam is made incident on the inspection target1via the thermal neutron shielding material, the neutron source3can irradiate the inspection target1with substantially only fast neutrons among thermal neutrons and fast neutrons. Thereby, a depth of the target component in an area deep from the surface1aof the inspection target1can be detected accurately.

Second Embodiment

FIG.7illustrates a configuration of a nondestructive inspection apparatus20according to a second embodiment of the present invention. The configuration of the nondestructive inspection apparatus20according to the second embodiment differs in the below-described matters from the configuration of the nondestructive inspection apparatus10according to the first embodiment. Concerning the second embodiment, the matters that are not described below may be the same as those in the case of the first embodiment. In an embodied example of the second embodiment, the inspection target1is a concrete structure, and a target component is chlorine, but the inspection target1and the target component are not limited to this combination.

The nondestructive inspection apparatus20according to the second embodiment includes a neutron source3, a gamma ray detection device5, a time-point specifying unit15, a depth data storage unit9c, a depth detection unit19, and a concentration data storage unit9b, and a concentration evaluation unit14.

In the second embodiment, the neutron source3emits a pulse neutron beam. Duration of a pulse of a proton beam for the pulse neutron emission is approximately 0.1 milliseconds or is shorter than 0.1 milliseconds, for example, but is not limited to this as long as detection of a depth of the target component is not hindered. Similarly, a repetition frequency of the proton beam pulse is approximately 100 Hz, for example, but is not limited to this as long as detection of a depth of the target component is not hindered.FIG.8Ais a schematic diagram for illustrating a pulse time width and a repetition period (an inverse number of the repetition frequency) of the proton beam in the neutron source3. InFIG.8A, the horizontal axis represents time, the vertical axis represents magnitude of a proton beam pulse signal (the synchronization signal), and the repetition period is equal to the repetition period of the proton beam.

A pulse neutron beam is emitted under a distance condition. The distance condition is a condition that a distance between a surface1aof an inspection target1(a specimen in the case of acquiring the below-described depth data) and an emission position of a pulse neutron beam in the neutron source3is a set distance. For example, this emission position may be a surface included in the target3dand on a side of the inspection target1. When the below-described moderator3gis provided, the emission position may be a surface included in the moderator3gand on a side of the inspection target1.

The neutron source3further includes the moderator3gthrough which neutrons generated in the target3dpass. The moderator3gis formed of a material (e.g., polyethylene) that decelerates fast neutrons passing therethrough to become thermal neutrons. Accordingly, neutrons generated in the target3dpass through the moderator3g, thereby partially becomes thermal neutrons, and are then incident on the inspection target1. Thus, the neutron source3can make thermal neutrons and fast neutrons incident on the inspection target1.

A pulse neutron beam from the neutron source3is made incident on the inspection target1, and reacts with the target component in the inspection target1. Thereby, gamma rays (specific gamma rays) deriving from the target component are generated.

The gamma ray detector5detects the specific gamma rays generated by a pulse neutron beam incident on the inspection target1. More specifically, the gamma ray detection device5detects an energy spectrum of gamma rays at each time point at and after a time point (i.e., a neutron generation time point as a reference time point) when the neutron source3makes a pulse proton beam incident on the target3d, and the gamma ray detection device5generates time-difference-to-spectrum data in which each time point (i.e., a time difference from the reference time point) with respect to the reference time is associated with the energy spectrum of the gamma rays detected at the time point concerned. The gamma ray detection device5includes a gamma ray detector5a, a data acquisition unit5c, and an intensity detection unit5b.

The gamma ray detector5adetects, at each time point, an intensity of gamma rays for each energy (each wavelength) of gamma rays from the inspection target1. In other words, the gamma ray detector5adetects an energy spectrum of the gamma rays at each time point, and for each time point, outputs the energy spectrum to the data acquisition unit5c.

The data acquisition unit5cgenerates the above-described time-difference-to-spectrum data representing an energy spectrum at each time point, based on an energy spectrum input from the gamma ray detector5aat each time point. The energy spectrum at each time point indicates the number of times of detection of a gamma ray at each energy, concerning gamma rays detected at the time point concerned. In the second embodiment, each time point of detecting the energy spectrum represents a time difference between a neutron generation time point (the reference time point) and a gamma ray detection time point. The gamma ray detection time point may be a time point when the gamma ray detector5adetects each gamma ray corresponding to the energy spectrum concerned.

FIG.9illustrates an outline of an embodied example of the time-difference-to-spectrum data generated by the data acquisition unit5c. InFIG.9, the horizontal axis indicates TOF (time of flight) that is a time difference between a neutron generation time point and a gamma ray detection time point, and the vertical axis indicates energy of a detected gamma ray. InFIG.9, A indicates a region where the number of times a gamma ray having the corresponding energy is detected at the corresponding time point is approximately equal to or larger than 1×103, B indicates a region where the number of times a gamma ray having the corresponding energy is detected at the corresponding time point is approximately equal to or larger than 4×102and equal to or smaller than 9×102, C indicates a region where the number of times a gamma ray having the corresponding energy is detected at the corresponding time point is approximately equal to or larger than 3×101and equal to or smaller than 2×102, D indicates a region where the number of times a gamma ray having the corresponding energy is detected at the corresponding time point is approximately equal to or larger than 1×101and equal to or smaller than 3×101, and E indicates a region where the number of times a gamma ray having the corresponding energy is detected at the corresponding time point is approximately equal to or smaller than 1×101.

The data acquisition unit5creceives, from the neutron source3, a synchronization signal indicating a proton beam incident time (neutron generation time), and generates the above-described time-difference-to-spectrum data, based on the synchronization signal. For example, the data acquisition unit5cmeasures time, assuming that a time point of receiving the synchronization signal is the origin of time, and generates the time-difference-to-spectrum data in which each time point with respect to the origin is associated with an energy spectrum detected by the gamma ray detector5aat the time point concerned.

The intensity detection unit5bacquires a detection intensity of the specific gamma rays, based on the time-difference-to-spectrum data acquired or generated by the data acquisition unit5c. Here, the specific gamma ray is a selection gamma ray concerning the below-described concentration data, and the detection intensity is a value proportional to the number of times the specific gamma ray is detected over the predetermined measurement time described above in the first embodiment.

The time-point specifying unit15specifies a time point when the specific gamma ray is detected, based on the time-difference-to-spectrum data acquired or generated by the gamma ray detection device5(data acquisition unit5c). For example, based on the time-difference-to-spectrum data, the time specifying unit15specifies a time point of detected energy of the specific gamma ray in the energy spectrums at respective time points detected by the gamma ray detector5a. In one example, the time-point specifying unit15extracts, from the above-described time-difference-to-spectrum data, data indicating the number of times of detection of the specific gamma ray at each time point, and specifies, based on the extracted data, a time point (i.e., a specific time point) when the specific gamma ray is detected, as a time point when energy of the specific gamma ray is detected.

When incidence of a pulse neutron beam on the inspection target1causes a plurality of types of the specific gamma rays to be radiated from the target component, the time specifying unit15may specify a time point when the pre-designated type of specific gamma ray (hereinafter, referred to also as a designated gamma ray) is detected.

The depth data storage unit9cstores the depth data representing a relation between a depth at which the target component exists in the inspection target1and a specific time point (a time point with respect to the reference time point) when the specific gamma ray (designated gamma ray) deriving from the target component is detected in the case where a pulse neutron beam is made incident on the inspection target1. The depth data may be acquired by an experiment, for example.

In this experiment, a plurality of specimens formed of the same material as the inspection target1are prepared. A depth at which the target component exists in each of a plurality of the specimens differs between these specimens. For each specimen, the above-described neutron source3makes a pulse neutron beam incident on a surface of the specimen, and the time-point specifying unit15specifies a detection time point of energy of the specific gamma ray (designated gamma ray), from energy spectrums of gamma rays at respective time points detected by the gamma ray detector5a. The depth of the target component in one specimen and the detection time point (specific time point) specified for this specimen are assumed to constitute one set of data, and based on a plurality of sets of data acquired for a plurality of the specimens, the above-described depth data are generated. The thus-generated depth data are stored in advance in the depth data storage unit9c.

The above-described experiment for acquiring the depth data and actual inspection (the below-described step S101) of the inspection target1is performed under the above-described distance condition. Further, the above-described experiment for acquiring the depth data and actual inspection (the below-described step S101) of the inspection target1may be performed under the above-described neutron spectrum condition and orientation condition.

The depth detection unit19determines a depth at which the target component exists, based on the depth data stored in the depth data storage unit9cand a time point specified by the time-point specifying unit15. At this time, the depth detection unit19may apply this time point to the depth data, thereby determining a depth at which the target component exists. The depth detection unit19outputs the determined depth. The output depth may be input to the concentration evaluation unit14, be stored in an appropriate storage medium, be displayed on a display, or be printed on a paper sheet.

For example, when the moderator3gis provided as described above, and thereby, the neutron source3makes both fast neutrons and thermal neutrons incident on the surface1aof the inspection target1, one or both of depth data for fast neutrons and depth data for thermal neutrons are acquired in advance as described above. Regarding this, the following description is made with reference toFIG.8B. InFIG.8B, the horizontal axis indicates time, and the vertical axis indicates the number of times of detection of the specific gamma ray. Time points ta, tb, tc, and tdinFIG.8Bcorrespond to time points ta, tb, tc, and tdinFIG.8A, respectively.

A distance between an emission position of the neutron source3and the surface1aof the inspection target1and a pulse width and a repetition frequency of a proton beam are set in advance by a simulation or an experiment such that when data representing the number of times of detection of the specific gamma ray at each time point are extracted from the above-described time-difference-to-spectrum data as described above, in the extracted data (e.g., data inFIG.8B), concerning a time point (specific time point) of detecting the specific gamma ray, a specific time point (e.g., the time point t1inFIG.8B) occurring due to fast neutrons emitted to the surface1aof the inspection target1by the neutron source3is shifted distinguishably from a specific time point (e.g., the time point t2inFIG.8B) occurring due to thermal neutrons emitted to the surface1aof the inspection target1by the neutron source3. This is enabled by the matter that the fast neutrons differ in a moving speed from the thermal neutrons. In other words, gamma rays caused by the emitted fast neutrons are detected at earlier time points, and gamma rays caused by the emitted thermal neutrons are detected at later time points. When a pulse width of a proton beam is large inFIG.8A, a width (time width) of a waveform representing the number of times of detection inFIG.8Bis widened. When a distance between an emission position of the neutron source3and the surface1aof the inspection target1is short, the time point t1and the time point t2inFIG.8Bbecome close to each other. In consideration of these, the distance, the pulse width, and the repetition frequency are set in advance.

In the present embodiment, the specific time point is a specific time point within a time range in which the number of times of detection of the specific gamma ray occurs, and for example, may be a time point when the number of times of detection becomes a peak, or may be a time point when the number of times of detection starts to occur.

When both the depth data for fast neutrons and the depth data for thermal neutrons are acquired, for example, the depth detection unit19extracts, from the above-described time-difference-to-spectrum data, data representing the number of times the specific gamma ray is detected at each time point, and determines a depth at which the target component exists, based on the depth data for fast neutrons and the earlier specific time point (t1) of the two specific time points (e.g., t1and t2inFIG.8B) of the number of times of detection of the specific gamma ray in the extracted data, or based on the depth data for thermal neutrons and the later specific time point (t2) of the two specific time points.

In the above description, the neutron source3is configured so as to irradiate the inspection target1with thermal neutrons and fast neutrons. In this case, it is possible to detect the target component existing in a range near the surface1aand in a range deep from the surface1ain the inspection target1.

Meanwhile, when the moderator3gis omitted, or when a thermal neutron shielding material4(FIG.7) is installed on the surface1aof the inspection target1in the configuration in which the moderator3gis provided, the neutron source3irradiates the inspection target1with substantially only fast neutrons among thermal neutrons and fast neutrons. In this case, it is possible to detect a depth of the target component existing in a range deep from the surface1ain the inspection target1. In this case, the depth data for thermal neutrons do not need to be acquired.

The concentration data storage unit9bin the second embodiment is the same as the concentration data storage unit9bin the first embodiment. In other words, the concentration data storage unit9bstores the concentration data representing a relation between a detection intensity of the selection gamma ray and a concentration of the target component. Here, the selection gamma ray may be the above-described designated gamma ray, or another type of the specific gamma ray.

Based on a depth determined by the depth detection unit19, the concentration data stored in the concentration data storage unit9band corresponding to the depth concerned, and an input detection intensity of the selection gamma ray, the concentration evaluation unit14determines a concentration of the target component at the depth concerned. The concentration evaluation unit14outputs the determined concentration. The output concentration may be stored in an appropriate storage medium, be displayed on a display, or be printed on a paper sheet.

FIG.10is a flowchart illustrating a nondestructive inspection method according to the second embodiment. This method may be performed using the above-described nondestructive inspection apparatus20. The method includes steps S101to S105.

At the step S101, the neutron source3emits a pulse neutron beam to the surface1aof the inspection target1. Thereby, the pulse neutron beam that has been made incident on the inspection target1reacts with the target component in the inspection target1, causing the specific gamma rays deriving from the target component to be generated.

At the step S102, the specific gamma rays that are among gamma rays generated at the step S101and that derive from the target component in the inspection target are detected, and a time point when the specific gamma rays (designated gamma rays) are detected is specified. In the present embodiment, the step S102may include steps S121and S122. At the step S121, the gamma ray detector5adetects an energy spectrum of the gamma rays at each time point. At the step S122, assuming that a time point when the above-described synchronization signal is received is the origin, time is measured, and meanwhile, time-difference-to-spectrum data in which the energy spectrum detected by the gamma ray detector5aat each measurement time point is associated with the time point concerned are generated.

At the step S103, based on a result of the detection at the step S102, the time-point specifying unit15specifies a time point when the specific gamma ray (designated gamma ray) is detected at or after the time point when the pulse neutron beam is emitted at the step S101. At this time, based on the time-difference-to-spectrum data generated at the step S122, the time-point specifying unit15may specify a time point when the specific gamma ray is detected.

At the step S104, the depth detection unit19determines a depth at which the target component exists, based on the time point specified at the step S103and the depth data in the depth data storage unit9c.

At the step S105, based on the depth determined at the step S104, the concentration data that are relevant to the depth concerned and that are stored in the concentration data storage unit9b, and a detection intensity of the selection gamma rays, the concentration evaluation unit14determines a concentration of the target component at the depth concerned.

When the concentration data acquired for each depth by using the above-described equation (A) are used, the step105is performed as follows.

First, based on the equation (A) and the number A of times of detection of the selection gamma ray, the intensity detection unit5bdetermines a detection intensity of the selection gamma rays. The number A of times of detection used at this time is based on a detection result that concerns the selection gamma rays and that is acquired (e.g., at the step S102, or by being newly selected at the step S102) for the inspection target1by the neutron source3and the gamma ray detection device5.

Next, based on the detection intensity of the selection gamma rays determined by the intensity detection unit5b, the depth determined at the step S104, and the concentration data that are relevant to the depth concerned and that are stored in the concentration data storage unit9b, the concentration evaluation unit14determines a concentration of the target component at the depth concerned.

Meanwhile, when the concentration data for each depth are acquired using a gamma ray detection efficiency εγcorresponding to the depth concerned, the step S105is performed as follows.

First, assuming that a gamma-ray detection efficiency εγcorresponding to the depth determined at the step S104is εγd, the intensity detection unit5bdetermines a detection intensity of the selection gamma rays, based on the above-described equation (1) and the number A of times of detection of the selection gamma ray. The number A of times of detection used at this time is based on a detection result that concerns the selection gamma rays and that is acquired (e.g., at the step S102, or by being newly selected at the step S102) for the inspection target1by the neutron source3and the gamma ray detection device5. The same detection efficiency data as that in the first embodiment are stored in the detection efficiency storage unit8as illustrated inFIG.7, and the intensity detection unit5bspecifies Era described above, based on the detection efficiency data in the detection efficiency storage unit8and the depth determined at the step S104, and uses the equation (1) as described above.

Next, based on the detection intensity of the selection gamma rays determined by the intensity detection unit5b, the depth (the depth input from the depth detection unit19) determined at the step S104, and the concentration data that are relevant to the depth concerned and that are stored in the concentration data storage unit9b, the concentration evaluation unit14determines a concentration of the target component at the depth concerned.

Advantageous Effect of Second Embodiment

The specific gamma rays generated by reaction between the target component and neutrons incident on the inspection target1are detected, and a time point when the specific gamma ray is detected is specified. The specified time point indicates a depth at which the target component exists. Accordingly, determining such a time point enables detection of a depth at which the target component exists. Thus, a depth of the target component in the inspection target1can be detected nondestructively. For example, without extracting a core from a concrete structure as the inspection target1, it is possible to detect a depth of a position of the target component existing in the inspection target1, and to evaluate a concentration of the target component at this depth.

Third Embodiment

Principle of Third Embodiment

FIG.11Aillustrates the detection principle according to a third embodiment. In the third embodiment, the gamma ray detection device5collimates gamma rays to be detected. In other words, the gamma ray detection device5detects gamma rays that are among gamma rays generated at a specific depth in the inspection target1and that have traveled in a direction within a specific range. More specifically, a neutron beam whose cross-sectional size is reduced is made incident on the inspection target1, and on the assumption that a position Pc (hereinafter, also referred simply as an intersection position Pc) is a position at which a reference straight line L of the gamma ray detector5aintersects with a straight line path of the neutron beam, gamma rays that are among gamma rays generated at the intersection position Pc and that have traveled along the reference straight line L are selectively made incident on the gamma ray detector5a, and the below-described gamma ray shielding portion5dprevents the gamma rays traveling in other directions from being incident on the gamma ray detector5a.

Thus, when the gamma ray detection device5detects the specific gamma rays deriving from the target component, it is understood that the target component exists at the intersection position Pc (depth). By changing a geometrical relation (a relation concerning a position and an orientation) between the reference straight line L and a neutron beam path, it can be inspected whether or not the target component exists at each intersection position Pc.

A neutron beam is made incident on the inspection target1in a state where a size of a cross section of the neutron beam is reduced so as to be equal to or smaller than an upper limit value. The upper limit value may be equal to or smaller than several tens of millimeters, and for example, is equal to or smaller than 50 millimeters or is equal to or smaller than 30 millimeters. A degree of size reduction of a neutron beam may be set in the neutron source3, depending on a required resolution of the intersection position Pc. When a cross section of a neutron beam is large, the number of times of detection of a gamma ray increases, but a resolution of the intersection position Pc is lowered. When a cross section of a neutron beam is small, the number of times of detection of a gamma ray decreases, but a resolution of the intersection position Pc is raised. A cross-sectional shape of a neutron beam may be a circular shape or a shape similar to a circular shape, for example, but is not limited to these, and may be an elliptical shape, a rectangular shape, or the like.

A degree of collimating gamma rays (an area of an opening21aof the below-described gamma ray shielding portion5d) is set depending on a degree of a spread of a range (intersection position Pc), with gamma rays from this range being to be detected. When an area of the opening21ais large, the number of times of detection of a gamma ray increases, but a resolution of the intersection position Pc is lowered. When an area of the opening21ais small, the number of times of detection of a gamma ray decreases, but a resolution of the intersection position Pc is raised.FIG.11Bis a partial enlarged view ofFIG.11A, and illustrates one example of shapes of the gamma ray detectors5aand the gamma ray shielding portion5d. As illustrated inFIG.11B, the gamma ray detector5aincludes a detection surface5a, and detects gamma rays incident on the detection surface5a. An area D1of the detection surface5a1is larger than a cross-sectional area D2of a gamma ray passage hole21of the gamma ray shielding portion5d, and is larger than an area D3of the opening21a. In other words, the gamma ray detector5aand the gamma ray shielding portion5dmay be formed such that in viewing in the direction of the center line L of the gamma ray passage hole21, the gamma ray passage hole21and the opening21aare smaller than the detection surface5a1as illustrated inFIG.11B, for example.

Configuration of Third Embodiment

FIG.12illustrates a configuration of a nondestructive inspection apparatus30according to a third embodiment of the present invention. The nondestructive inspection apparatus30includes the gamma ray detection device5and the neutron source3that is described in the first embodiment or the second embodiment. In the third embodiment, the gamma ray detection device5includes the below-described configuration. In the third embodiment, concerning the gamma ray detection device5, the matters that are not described below may be the same as those in the case of the first embodiment or the second embodiment described above.

In the third embodiment, the gamma ray detector5includes the gamma ray detector5a, an intensity detection unit5b, and the gamma ray shielding portion5d.

The gamma ray detector5adetects gamma rays for each energy of gamma rays generated in the inspection target1by the incident neutron beam, and inputs detection data thereof to the intensity detection unit5b. The detection data may be a pulse height corresponding to energy of each detected gamma ray. The gamma ray detector5amay be a germanium detector, for example, but is not limited to this.

The intensity detection unit5bacquires an energy spectrum of the gamma rays, based on pulse heights input from the gamma ray detector5a. Based on the energy spectrum, the intensity detection unit5bdetermines, as a detection intensity, an intensity of the specific gamma rays. When a plurality of types of the specific gamma rays are emitted from the target component by a neutron beam incident on the inspection target1, the intensity detection unit5bdetermines, as a detection intensity, an intensity of a designated type of the specific gamma rays. The intensity detection unit5boutputs the determined detection intensity of the specific gamma rays. The output detection intensity may be displayed on a display.

The gamma ray shielding portion5dis formed of a material (e.g., lead, tungsten, tantalum, or iron) having a high ability of shielding against gamma rays, and thereby substantially prevents gamma rays from passing therethrough. The gamma ray shielding portion5dforms the gamma ray passage hole21. The gamma ray passage hole21includes the opening21athrough which gamma rays are allowed to enter. The gamma ray detector5ais arranged at a position shifted from the opening21ato a deep side in the gamma ray passage hole21. The opening21aand the gamma ray detector5aare positioned on the reference straight line L. By such a gamma ray shielding portion5d, the gamma ray detector5adetects substantially only gamma rays entering along the reference straight line L from the opening21a. The reference straight line L may be the center line of the gamma ray passage hole21.

When the gamma ray detector5ais the germanium detector5a, a cooling device17(unillustrated inFIG.12, but illustrated inFIG.11AandFIG.11B) that cools the germanium detector5ais provided. The cooling device17may be provided outside the gamma ray shielding portion5d. In this case, the cooling device17may cool the germanium detector5athrough a hole18provided in the gamma ray shielding portion5don a side opposite to the opening21a.

The gamma ray shielding portion5dincludes a front end surface22on which the opening21ais formed. The reference straight line L may extend obliquely relative to the front end surface22. In other words, the front end surface22is formed such that the reference straight line L extends obliquely relative to the front end surface22. With this configuration, by performing inspection in a state where the front end surface22faces the surface1aof the inspection target1as illustrated inFIG.12, gamma rays from positions other than positions on an extension line of the reference straight line L can be more reliably prevented from leading to the gamma ray detector5a. As a result, a resolution of the intersection position Pc is improved. The front end surface22may be a plane, but is not limited to this.

FIG.13AandFIG.13Billustrate concrete examples of a shape of the gamma ray shielding portion5d.FIG.13AandFIG.13Bare each a view taken along the line XIII-XIII inFIG.11A, but illustrate the concrete examples different from each other.

In the case ofFIG.13A, the gamma ray shielding portion5dis formed so as to surround an entire circumference of the gamma ray passage hole21. In this case, the cross-sectional shape of the gamma ray passage hole21may be circular as illustrate inFIG.13A, or may be another shape.

In the case ofFIG.13B, the gamma ray shielding portion5dincludes two shielding blocks23and24that are separated by a gap from each other and that are formed of the above-described material so as to substantially prevent gamma rays from passing therethrough. The gap is the gamma ray passage hole21. The gap21extends along the reference straight line L. A size of the gap21in a first direction perpendicular to the reference straight line L is smaller than a size of the gap21in a second direction perpendicular to both the reference straight line L and the first direction. For example, a size of the gap21in the first direction is equal to or smaller than one half, one third, or one fifth of a size of the gap21in the second direction.

In the case ofFIG.13B, a size of the cross section of the neutron beam is set to be equal to or smaller than the above-described upper limit value in at least the first direction of the first and second directions.

Assuming that the gamma ray detector5a, the intensity detection unit5b, and the gamma ray shielding unit5dassociated with each other form one set, the gamma ray detection device5may include one set or a plurality of sets. InFIG.12, two sets are illustrated. The gamma ray shielding units5dassociated with the respective gamma ray detectors5ainFIG.12may each have the shape described based onFIG.13AorFIG.13B.

The concentration data storage unit9bin the third embodiment is the same as the concentration data storage unit9bin the first embodiment. In other words, the concentration data storage unit9bstores concentration data representing a relation between a detection intensity of the selection gamma ray and a concentration of the target component.

Based on a depth determined as described later, the concentration data stored in the data storage unit9band associated with the determined depth, and an input detection intensity of the selection gamma ray, the concentration evaluation unit16determines a concentration of the target component at the determined depth. The concentration evaluation unit16outputs the determined concentration. The output concentration may be stored in an appropriate storage medium, be displayed on a display, or be printed on a paper sheet.

FIG.14is a flowchart illustrating the non-destructive inspection method according to the third embodiment. This method may be performed using the above-described nondestructive inspection apparatus30described above. The method includes steps S201to S205.

At the step S201, the neutron source3and the gamma ray detection device5are arranged such that a path of a neutron beam emitted from the neutron source3and an extension line of the reference straight line L of the gamma ray shielding portion5dintersect with each other inside the inspection target1. In this arrangement, the gamma ray shielding portion5dand the gamma ray detector5amay be arranged such that the front end surface22of the gamma ray shielding portion5dfaces the surface1aof the inspection target1(e.g., the flat front end surface22is parallel to the flat surface1a). In this case, further, the front end surface22may contact with the surface1a, or a slight gap may be provided between the front end surface22and the surface1a. The incident neutron beam does not need to be perpendicular to the surface1aof the inspection target1, and the neutron beam may be made obliquely incident on the surface1a. Arrangement of the gamma ray detector5amay be changed depending on an angle between the incident neutron beam and the surface1a.

The step S202is performed in a state of the arrangement made at the step S201. At the step S202, by the neutron source3, a neutron beam is made incident on the surface1aof the inspection target1. At the step S202, the gamma ray detector5adetects gamma rays of respective values of energy generated by the incidence of the neutron beam, and inputs the detection data to the intensity detector5b. The neutron beam made incident on the surface1aat the step S202may be a pulse neutron beam as in the second embodiment, or a temporally continuous neutron beam.

At the step S203, based on the detection data (a pulse height corresponding to energy of each detected gamma ray) acquired at the step S202, the energy detection unit5bacquires an energy spectrum of the gamma rays, and based on the energy spectrum, the energy detection unit5bacquires, as a detection intensity, an intensity of the specific gamma rays (i.e., selection gamma rays) by the equation (A), for example. Further, at the step S203, the detection intensity is output from the intensity detection unit5b.

At the step S204, based on the detection intensity of the specific gamma rays output at the step S203, it is determined whether or not the target component exists at a depth (hereinafter, also referred to as an associated depth) that is in the inspection target1and that is associated with the reference straight line L at the step S201. The associated depth is a depth of the intersection position Pc surrounded by the broken-line circle inFIG.11AorFIG.12. In other words, the associated depth is a depth of an intersection position between a path of the neutron beam emitted from the neutron source3and the extension line of the reference straight line L.

The intersection position Pc (associated depth) can be determined at the step S204, based on a geometrical relation among the inspection target1and the neutron source3and the gamma ray detection device5arranged at the step S201. For example, the geometrical relation is detected by using an appropriate sensor or measurement device, a result of this detection is input to an appropriate computing device, and the computing device determines the intersection position Pc. Alternatively, based on the result of this detection, a person may determine the intersection position Pc by calculation. The thus-determined intersection position Pc includes the associated depth and a position in directions along the surface1a. An example of the geometrical relation may be a relation concerning a position and a direction among the path of the neutron beam, the reference straight line L, and the surface1aof the inspection target1.

The determination at the step S204may be performed by a person. For example, when the detection intensity of the specific gamma rays output at the step S203is displayed on a display, and the person looks at the displayed detection intensity, and determines that the target component exists at the associated depth, when the detection intensity is equal to or larger than a set lower limit value. The associated depth determined by the computing device as described above may be displayed on the display along with the detection intensity.

At the step S205, a concentration of the target component at the associated depth (intersection position Pc) determined at the step S204is determined. In this case, based on the above-described concentration data, the associated depth, and the detection intensity determined at the step S203, the concentration evaluation unit16determines a concentration of the target component at the associated depth. The associated depth used at this time is determined at the step S204as described above, and may be input to the concentration evaluation unit16and the intensity detection unit5bby the above-described computing device or by a person operating an appropriate operation unit.

In the case of using the concentration data acquired for each depth by using the equation (A), the step205is performed as follows.

First, based on the above-mentioned equation (A) and the number A of times of detection of the selection gamma ray, the intensity detection unit5bdetermines a detection intensity of the selection gamma rays. The number A of times of detection used at this time is based on a result of detection of the selection gamma rays acquired (at the step S202or by being newly selected at the step S202, for example) for the inspection target1by the neutron source3and the gamma ray detection device5under the same conditions as the above-described conditions including the orientation condition when the concentration data are acquired.

Next, based on the detection intensity of the selection gamma rays determined by the intensity detection unit5b, the associated depth determined at the step S204, and the concentration data for the associated depth stored in the concentration data storage unit9b, the concentration evaluation unit16determines a concentration of the target component at the associated depth.

Such concentration evaluation may be performed by a person. For example, the detection intensity, the concentration data for each depth, and the associated depth may be displayed on a display, and a person may look at these pieces of the displayed data and determine a concentration of the target component at the associated depth.

Meanwhile, when the concentration data for each depth are acquired by using the gamma ray detection efficiency εγcorresponding to the depth concerned, the step S205is performed as follows.

Assuming that the gamma ray detection efficiency εγcorresponding to the associated depth determined at the step S204is εγd, the intensity detection unit5bdetermines a detection intensity of the selection gamma rays, based on the above-described equation (1) and the number A of times of detection of the selection gamma ray. The number A of times of detection used at this time is based on a result of detection of the selection gamma rays acquired (at the step S202or by being newly selected at the step S202, for example) for the inspection target1by the neutron source3and the gamma ray detection device5under the same conditions as the above-described conditions including the orientation condition when the concentration data are acquired. The same detection efficiency data as that in the first embodiment are stored in the detection efficiency storage unit8as illustrated inFIG.12, and the intensity detection unit5bspecifies εγddescribed above, based on the detection efficiency data in the detection efficiency storage unit8and the associated depth determined at the step S204, and uses the equation (1) as described above.

Next, based on the detection intensity of the selection gamma rays determined by the intensity detection unit5b, the depth determined at the step S204, and the concentration data that are relevant to the depth concerned and that are stored in the concentration data storage unit9b, the concentration evaluation unit16determines a concentration of the target component at the depth concerned.

Such concentration evaluation may be performed by a person. For example, the detection intensity, the concentration data for each depth, the associated depth, and the detection efficiency data may be displayed on a display, and a person may look at these pieces of the displayed data and determine a concentration of the target component at the associated depth.

The following describes first to third examples as variations of the above-described nondestructive inspection method. The matters that are not described below are the same as those in the above-described nondestructive inspection method.

In the first example, a plurality of sets of the gamma ray detector5a, the intensity detection unit5b, and the gamma ray shielding portion5dare used. In other words, at the step S201, as inFIG.15A, a plurality of the gamma ray detectors5aand the neutron source3(not illustrated) are arranged such that the intersection positions Pc for a plurality of the gamma ray detectors5abelonging to the respective sets are different from each other. Then, the steps S203to S205are performed for detection data acquired by each of the gamma ray detectors5aat the step S202. Each time the steps S201to S205are thus performed, positions of a plurality of the gamma ray detectors5aare shifted in a direction along the surface1a(e.g., the direction of the arrow X inFIG.15A) at the re-started step S201, and the steps S202to S205are performed again, and thus, the steps S201to S205are repeated.

In this repetition, a neutron beam path may be fixed. In this case, in the repetition, a direction of the reference straight line L of each gamma ray detector5awith respect to the path of the neutron beam may be fixed, or may be changed.

In the second example, similarly to the first example, at the step S201, as inFIG.15B, a plurality of the gamma ray detectors5aand the neutron source3(not illustrated) are arranged such that the intersection positions Pc for a plurality of the gamma ray detectors5abelonging to the respective sets are different from each other. Then, the steps S203to S205are performed for detection data acquired by each of the gamma ray detectors5aat the step S202. Each time the steps S201to S205are thus performed, the neutron beam path is changed at the re-started step S201(for example, inFIG.15B, the path of the neutron beam is shifted in a direction of arrow X, or an incident angle θ of the neutron beam on surface1ais changed), and the steps S202to S205are performed again, and thus, the steps S201to S205are repeated. In this repetition, the positions and orientations of a plurality of the gamma ray detectors5awith respect to the surface1amay be fixed.

In the third example, as inFIG.15C, at the step S201, a plurality of the gamma ray detectors5aand the neutron source3(not illustrated) are arranged such that the intersection positions Pc for a plurality of the gamma ray detectors5abelonging to the respective sets are different from each other. Then, the steps S203to S205are performed for detection data acquired by each of the gamma ray detectors5aat the step S202. Each time the steps S201to S205are thus performed, an inclination of the reference straight line L of each gamma ray detector5arelative to the surface1ais changed at the re-started step S201, and the steps S202to S205are performed again, and thus, the steps S201to S205are repeated. In this repetition, the path of the neutron beam may be fixed.

The reference straight line L may be perpendicular to the front end face22as in the example ofFIG.15C. In such a case, the shape of the gamma ray shielding portion5ddescribed above with reference toFIG.13AorFIG.13Bmay be adopted. In the third example, one set of the gamma ray detector5aand others may be used instead of a plurality of sets, and in this case, the other matters are the same as those described above.

In the inspection according to the first to third examples, existence or absence and a concentration of the target component can be inspected over a wide range in the inspection target1.

The above-described third embodiment may be implemented in combination with the first embodiment or the second embodiment, or may be implemented independently of the first embodiment and the second embodiment.

Advantageous Effect of Third Embodiment

Since the gamma ray detector5ais arranged in the gamma ray passage hole21of the gamma ray shielding portion5d, the gamma ray detector5adetects substantially only gamma rays from the depth associated with the reference straight line L of the gamma ray shielding portion5d. For this reason, an orientation of the reference straight line L is changed, a detection intensity of the specific gamma rays is acquired for each of the orientations, and it can be determined that the target component exists at the depth associated with the orientation of the reference straight line L for which the detection intensity exceeds the set lower limit value. In this manner, a depth of the target component can be specified. When the concentration data for concentration evaluation are acquired in advance by an experiment, a concentration of the target component as well as a depth thereof can be acquired or evaluated.

The present invention is not limited to the above-described embodiment, and of course, various modifications can be made within the scope of the technical idea of the present invention. For example, each of the above-described advantageous effects does not necessarily limit the present invention. The present invention may be the invention achieving one of the advantageous effects indicated in the present specification, or may be the invention achieving another advantageous effect that can be grasped from the present specification. Any one of the following modified examples 1 to 3 may be adopted, or two or more of the modified examples 1 to 3 may be arbitrarily combined and adopted. In this case, the matters that are not described below may be the same as those described above.

Modified Example 1

In the first embodiment, the depth detection unit11may be omitted. In this case, based on a ratio determined at the above-described step S3and the depth data, a person may determine a depth of the target component. For example, a ratio and depth data determined at the step S3may be displayed on a display or be printed on a paper sheet, and a person may determine a depth of the target component while looking at the displayed or printed ratio and depth data.

Similarly, in the second embodiment, the depth detection unit19may be omitted. In this case, a person may determine a depth of the target component, based on a time point specified at the above-described step S103and the depth data. For example, the time point specified at the step S103and the depth data may be displayed on a display or be printed on a paper sheet, and a person may determine a depth of the target component while looking at the displayed or printed time point and depth data.

Modified Example 2

In the first embodiment, the concentration evaluation unit13may be omitted. In this case, based on a depth determined at the above-described step S4, a detection intensity of the selection gamma ray based on the gamma ray detection efficiency εγcorresponding to the depth, and the above-described concentration data, a person may determine a concentration of the target component at the depth. In other words, each piece of the data (e.g., a detection intensity of the selection gamma ray detected at the step S2, the above-described detection efficiency data, a depth determined at the step S4, and the above-described concentration data) used at the above-described step S5may be displayed on a display or be printed on a paper sheet, and a person may determine a concentration of the target component at the depth while looking at these pieces of the displayed or printed data.

Similarly, in the second embodiment, the concentration evaluation unit14may be omitted. In this case, based on a depth determined at the above-described step S104, a detection intensity of the selection gamma ray based on the gamma ray detection efficiency εγcorresponding to the depth, and the above-described concentration data, a person may determine a concentration of the target component at the depth. In other words, each piece of the data used at the above-described step S105may be displayed on a display or be printed on a paper sheet, and a person may determine a concentration of the target component at the depth while looking at these pieces of the displayed or printed data.

Modified Example 3

In the above-described first and third embodiments, the neutron source3is not limited to a neutron source using an accelerator including a pulse type as long as the neutron source3can make a neutron beam incident on the inspection target1. For example, the neutron source3may be an RI radiation source (e.g.,252Cf) or a DD or DT neutron generator tube that generates neutrons. When the RI radiation source is used, for example, inFIG.1andFIG.12, the RI radiation source is arranged at a position of the target3d, and the container3esurrounding the RI radiation source is provided, and the container3eis provided with the tubular shielding member3ffor the RI radiation source.

The present invention is not limited to the above-described embodiments, embodied examples, and modified examples, and can be widely applied without departing from the essence of the invention. For example, the above-described selection gamma ray is not limited to the above-described example as long as the selection gamma ray is the specific gamma ray.

REFERENCE SIGNS LIST