JP2009236635A - Method and apparatus for detecting nitrogen-containing substance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method and an apparatus for detecting a nitrogen-containing substance. <P>SOLUTION: In the method and apparatus for detecting a nitrogen-containing substance in an area to be detected, gamma rays is radiated from nitrogen element contained in the nitrogen-containing substance present in the area by irradiating the area to be detected with neutrons; and annihilation gamma rays radiated when positrons generated by electron pair creation of the gamma rays undergo electron pair annihilation, is measured. Thus, it is possible to provide the method and apparatus for detecting a nitrogen-containing substance, small and simple, and suitable for mine detection in an area like a mountain area or slope land where a large crane cannot go into. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a method and a detection apparatus for detecting a nitrogen-rich substance, for example, an explosive by detecting gamma rays emitted from a nitrogen element irradiated with neutrons. More specifically, the present invention relates to a method for detecting neutrons. By measuring the gamma rays emitted when the positrons generated by the generation of electron pairs of gamma rays emitted from nitrogen-containing materials are annihilated, it is determined whether or not nitrogen-containing materials exist in the region irradiated with neutrons. The present invention relates to a detection method and a detection apparatus for a nitrogen-containing substance to be determined.

  Conventionally, as a landmine detection method, a metal detection method or a radar detection method is known. In the metal detection method, there is a problem that there are many false detections in a place where a lot of metals are in the soil, and in addition, recent antipersonnel mines are often covered with plastic or wooden containers, so the detection accuracy is extremely low. The value is shown. Usually, the detection rate of landmines by metal detection method is about 500 to 1000 times, and it is said that detection by manned can only search about 1 square meter per day at most.

  The radar detection method is performed by a radar exploration device that transmits radio waves to the ground and receives reflected waves of the transmitted radio waves to search for an embedded object (for example, see Patent Document 1). However, the radar detection method has problems that it is difficult to distinguish landmines and foreign objects when stones or foreign objects similar to antipersonnel landmines are buried, and that they are easily affected by moisture in the soil. . There has also been proposed a detector that improves the detection capability by using both metal detection and radar detection (see Patent Document 2).

  In recent years, a mine detection method has been proposed in which the explosive component of a mine itself is identified as soil by using neutrons, which have a strong material permeability. This is a detection method using the phenomenon that nitrogen elements do not exist at high density in the soil other than substances such as the explosive component of landmines in nature. By irradiating neutrons and irradiating nitrogen elements contained in mine explosives, 10.8 MeV gamma rays emitted from the explosives by the N (n, γ) reaction are measured to detect underground mine. Is. This method is a detection method using the fact that the energy of gamma rays emitted by the neutron capture reaction is determined by the element (see Non-Patent Document 1). That is, it is performed by irradiating the nitrogen element contained in the explosive with neutrons, and measuring 10.8 MeV gamma rays generated by the neutron capture reaction represented by the following general formula [I] with a gamma ray detector. .

Recently, with the development of a portable and powerful DD fusion neutron source, the landmine detection method based on this new principle (hereinafter referred to as “neutron detection method”) is approaching the realization stage. FIG. 1 is a conceptual diagram of a landmine detection method using a portable neutron source. According to the neutron detection method, it is possible to accurately grasp the position of a landmine. For example, Patent Documents 3 and 4 disclose fusion neutron sources capable of obtaining a neutron flux suitable for landmine detection. Incidentally, at present, it is realized to generate 2.45 MeV neutrons of 10 ⁸ n / s.

  Since the elemental content of explosives and prohibited chemicals (for example, carbon, oxygen, nitrogen, etc.) can be clearly differentiated from other substances, measuring the content of these elements allows explosives and An example of a portable device that detects forbidden drugs is disclosed in Patent Document 5. In the neutron detection method, gamma rays are also generated from elements other than nitrogen in the soil. Therefore, in order to measure the position of dangerous substances (nitrogen-containing substances), gamma rays from elements other than nitrogen are reduced or eliminated. It becomes important. However, with conventional technology, it is difficult to reduce or eliminate gamma rays from elements other than nitrogen. To achieve this, it is necessary to collect a huge amount of data and perform long-term analysis work. There was a problem.

  As a gamma ray detector using an electron pair production reaction, a Compton scintillation detector and a LAT detector mounted on the artificial satellite GLAST are known. As a Compton-type scintillation detector, a detector that detects gamma rays by a neutron capture reaction using a Compton-type detector in which gamma rays are formed by stacking BGO scintillators on a rod has been proposed. However, in the Compton method, it is very difficult in principle to solve conical incident axis ambiguity, multiple scattering at the detector, removal of background from other elements, and the like. Furthermore, in the conventional Compton scintillation detector, since it was only possible to detect that the gamma ray was emitted from the range of the conical region, in order to extract and detect only the gamma ray caused by nitrogen with a reliable accuracy, It was necessary to collect a huge amount of data.

  The LAT detector can detect high-energy gamma rays of 10 GeV to 100 GeV, and its charged particle track measurement unit is a silicon detector (silicon strip detector) in which a large number of 18-layer thin electrodes of several hundred microns width are arranged. ) And tungsten sheets, and the electromagnetic calorimeter for measuring energy is composed of fine blocks of cesium iodide scintillator. However, since the LAT detector cannot measure the incident direction of gamma rays of several to several tens MeV, it cannot detect gamma rays emitted from nitrogen elements.

By the way, in a field where charged particles are mixed, a detector for measuring a neutron spectrum is disclosed in Non-Patent Document 2, for example.
Further, as a vehicle-mounted landmine detector for military use, there is one disclosed in Non-Patent Document 3, for example.

JP 2006-250451 A JP 2002-303680 A JP 2004-311152 A Japanese Patent Laid-Open No. 2005-291853 JP 2005-337764 A M. A Lone, R. A. Leavitt, D.A. Harrison: "Prompt Gamma Rays from Thermal Neutron Captire", Atomic Data and Nuclear Data Tables, 26, 511 (1981). M. Takada, et al .: "Characteristics of a phoswich detector to measure the neutron spectrum in a mixed field of neutrons and charged particles", Nuclear Instruments and Methods in Physics Research A 476 (2002) 332-336 E.T.H.Clifford, et al .: "A militarily fielded thermal neutron activation sensor for landmine detection", Nuclear Instruments and Methods in Physics Research A 579 (2007) 418-425

  The present inventors pay attention to the fact that the pair production reaction is more dominant than the Compton reaction above a specific energy (at least more than 50%), and the energy spectrum is measured for each event as in the past. In addition, it captures the tracks of electrons and positrons generated by the electron-pair generation reaction, thereby estimating the incident gamma-ray energy and flight direction, making it possible to more accurately search for the presence and location of explosives. A substance detection method and a detection apparatus have been proposed (Japanese Patent Application No. 2007-214103).

  That is, the energy to be measured can be detected using nitrogen-containing electron-pair production of 10.8 MeV gamma rays generated by nitrogen element by neutron capture reaction. This proposal reduces or eliminates background gamma rays from elements other than nitrogen, so that nitrogen-containing substances can be detected with high accuracy, and the incidence direction of gamma rays emitted by nitrogen elements is calculated, and nitrogen-containing substances are calculated. It has become possible to measure the exact position of, for example, three-dimensional coordinates.

  The nitrogen-containing substance detection device includes a charged particle two-dimensional position detector including a plurality of gas drift chambers connected to the first scintillator S1, and a second scintillator including a low-density scintillator and a photomultiplier tube It is a wire-type landmine detector composed of S2. In the drift chamber, He gas is used as a drift gas in order to reduce the influence of scattering of electrons and positrons by the gas. A conceptual diagram of this wire type detector is shown in FIG. This detector uses an electron pair production reaction. By measuring the flight direction of 10.8 MeV gamma rays emitted by the neutron capture reaction of the nitrogen element in the explosive, the mine (nitrogen It became possible to detect the position of the contained substances. In addition, this detection device has an advantage that it can search a relatively large area, for example, 1 m square at a time, and can also specify the buried depth of landmines and the like.

  However, this detector has a problem that it requires a high-power neutron source and is not suitable for landmine exploration in areas where large cranes cannot enter, such as mountainous areas, sloping terrain and narrow spaces. .

  As a result of intensive research aimed at solving the problems of the above-described technology, the present invention detects gamma rays emitted from nitrogen-containing materials irradiated with neutrons. For example, when detecting explosives, the electron pair production reaction by 10.8 MeV gamma rays emitted from the nitrogen element contained in the nitrogen-containing material irradiated with neutrons, and the emission when the positrons produced by this pair production reaction annihilate The present inventors have found that the presence of a nitrogen-containing substance can be detected by measuring 0.5 MeV gamma rays that have been produced, and have completed the present invention.

An object of the present invention is to provide a detection method and a detection apparatus that can reduce or eliminate gamma rays from elements other than nitrogen in detection of nitrogen-containing substances by neutron detection.
Another object of the present invention is to generate an electron pair by a 10.8 MeV gamma ray emitted from a nitrogen-containing material irradiated with neutrons, and a 0.5 MeV gamma ray when the positron generated by the pair generation reaction is annihilated. Is used to detect the presence of nitrogen-containing substances.
Another object of the present invention is to provide a nitrogen-containing substance detection device suitable for landmine exploration in areas where large cranes such as mountainous areas and sloping lands do not enter, which do not require a high-power neutron source. is there.
Moreover, the objective of this invention is providing the detection method and detection apparatus of a nitrogen-containing substance which are small and simple.

The present invention provides a small and simple method and apparatus for detecting a nitrogen-containing substance, which is suitable for landmine exploration in areas where large cranes such as mountainous areas and sloping terrain cannot enter, which do not require a high-power neutron source. It consists of the following technical means.
(1) A method for detecting a nitrogen-containing substance existing in a detection area, and by irradiating the detection area with neutrons, gamma rays are emitted from a nitrogen element contained in the nitrogen-containing substance existing in the area, A method for detecting a nitrogen-containing substance present in a region to be detected, comprising measuring annihilation gamma rays emitted when positrons generated by electron pair generation of gamma rays are annihilated.
(2) A method for detecting a nitrogen-containing substance in a detected region, wherein irradiating the detected region with neutrons causes gamma rays to be emitted from a nitrogen element contained in the nitrogen-containing substance existing in the region, and Gamma rays introduced into the first low-density scintillator and gamma rays emitted when the positrons generated by the generation of electron pairs of gamma rays annihilate are measured by the second high-density scintillator. Method for detecting nitrogen-containing substances present in the region.
(3) When luminescence from the first scintillator and luminescence from the second scintillator are detected at the same time, positrons generated by electron pair generation by gamma rays emitted from nitrogen elements contained in the nitrogen-containing material are annihilated by electron pairs. The method for detecting a nitrogen-containing substance present in the detection area according to (1) or (2) above, wherein the presence of the nitrogen-containing substance is confirmed by determining that the substance has been detected.
(4) The nitrogen-containing substance existing in the detection region according to any one of (1) to (3) above, wherein the gamma ray from the nitrogen-containing substance to be measured is 10.8 MeV and the annihilation gamma ray is 0.5 MeV. Detection method.
(5) Any of (2) to (4) above, wherein the outer surface of the first scintillator other than the surface connected to the gamma ray incident surface and the photomultiplier tube is optically connected to the second scintillator. A method for detecting a nitrogen-containing substance existing in the detection region described in 1.
(6) The method for detecting a nitrogen-containing substance present in a detection region according to any one of (1) to (5), wherein the nitrogen-containing substance is an explosive.

(7) A neutron irradiation means, first and second scintillators, a photomultiplier tube that receives light emitted from both scintillators, and a data processing unit that processes data from the photomultiplier tube, and neutron irradiation A device for detecting a nitrogen-containing substance present in a detected region for detecting the presence of a nitrogen-containing substance based on gamma rays emitted from nitrogen elements contained in the nitrogen-containing substance present in the detected region,
(A) a first scintillator made of a low-density substance and a second scintillator made of a high-density substance are optically coupled;
(B) A detected region having a data processing unit that determines that a nitrogen-containing substance is present in the detected region when the first and second scintillators detect gamma ray radiation simultaneously. Detection device for nitrogen-containing substances present.
(8) In the first scintillator, the positron generated by the generation of the electron pair of the incident gamma ray emitted by the nitrogen element is annihilated and the annihilation gamma ray is emitted, and the second scintillator measures the annihilation gamma ray. The detection apparatus of the nitrogen containing substance which exists in the to-be-detected area | region as described in (7).
(9) The outer surface of the first scintillator other than the gamma ray introduction surface and the surface connected to the photomultiplier tube is optically connected to the second scintillator, as described in (7) or (8) above. For detecting nitrogen-containing substances in the detected area of
(10) The detection of the nitrogen-containing substance present in the detection region according to (9), wherein the first scintillator has a cylindrical shape and the second scintillator is provided concentrically around the first scintillator. apparatus.
(11) The nitrogen-containing substance present in the detected region according to any one of (7) to (10), wherein the gamma ray from the nitrogen-containing substance to be measured is 10.8 MeV and the annihilation gamma ray is 0.5 MeV. Detection device.
(12) The apparatus for detecting a nitrogen-containing substance present in a detection region according to any one of (7) to (11), wherein the nitrogen-containing substance is an explosive.

The present invention has the following effects.
(1) It is possible to provide a detection method and apparatus capable of easily and accurately detecting a nitrogen-containing compound by a neutron detection method capable of reducing or removing gamma rays from elements other than nitrogen.
(2) The detector of the present invention having a phoswich structure can accurately measure only the presence of 10.8 MeV gamma rays (for example, whether or not landmines are buried) in a short time and contains nitrogen even when a low-power neutron source is used. It becomes possible to detect the existence of a substance.
(3) It is possible to provide a small and simple method and apparatus for detecting a nitrogen-containing substance that is suitable for landmine exploration in areas where large cranes such as mountainous areas and slopes cannot enter.
(4) It is possible to provide a landmine detection device and a baggage inspection device that are small, simple and capable of accurate measurement.

The present invention in the best mode relates to a detection method and a detection apparatus for detecting a substance containing a high concentration of elemental nitrogen, particularly explosives embedded in soil. Examples of explosives include TNT (molecular formula: C ₇ H ₅ N ₃ O ₆ ), RDX (molecular formula: C ₃ H ₆ N ₆ O ₆ ), pen slit (molecular formula: C (CH ₂ ONO ₂ ) ₄ ), and the like. Are known, but all are substances containing nitrogen (N) at a high rate. Table 1 shows the nitrogen content in the main explosives.

  In order to detect explosives buried in soil, it is important to reduce or eliminate gamma rays emitted from elements other than nitrogen contained in the soil and explosives. Since soil contains a large amount of hydrogen and oxygen, nitrogen cannot be accurately identified without reducing the gamma rays emitted from these elements. As described above, the energy of gamma rays emitted by the neutron capture reaction is determined by the element. Table 2 lists the energy of gamma rays that are described in Non-Patent Document 1 and that are emitted by the main elements in the neutron capture reaction.

  The present invention makes it possible to remove or reduce other background gamma rays by capturing only 10.8 MeV gamma rays emitted from elemental nitrogen. There are a plurality of gamma-ray energies emitted from nitrogen as shown in Table 2. The reason for capturing only 10.8 MeV gamma-rays is as follows. That is, the gamma rays irradiated to the scintillator have an energy loss of about 10%, but it is assumed that a loss of about 10% occurs when a gamma ray other than 10.8 MeV (for example, 5.562 MeV gamma ray) is captured. Then, it may be impossible to determine whether the gamma rays are caused by nitrogen elements or carbon. In the current technology, it is difficult to specify a gamma ray having only a difference of about several MeV from the gamma rays shown in Table 2. Furthermore, if it is 10.8 MeV gamma ray, even if a loss of 10% occurs, it is possible to identify the gamma ray caused by the nitrogen element.

  In addition, there is an advantage that the reaction rate of electron pair generation increases as the energy of gamma rays increases. In particular, gamma rays of 10 MeV or more have an advantage that the electron pair production reaction rate is higher than that of other low energy gamma rays because the electron pair production reaction is more dominant than Compton scattering. The present invention utilizes the fact that positrons generated by the electron pair generation reaction of 10.8 MeV gamma rays emit 0.5 MeV annihilation gamma rays when annihilation of electron pairs occurs. The presence of the nitrogen-containing substance can be confirmed by detecting the emitted 0.5 MeV gamma ray by the second scintillator.

As the neutron source, for example, neutrons of 10 ⁸ n / sec or more are irradiated. Examples of neutron sources for irradiating neutrons include, but are not limited to, a DD fusion reactor, 252Cf, and 241AM-Be. Landmines are usually buried several tens of centimeters from the surface of the earth, but if they are deep enough, they can be sufficiently heated by scattering with the soil, so the intensity of energy has little to do with the neutron capture reaction, It may be a DT fusion reactor.

  The present invention relates to a nitrogen-containing substance detection device using a detector having a Phoswich structure, and FIG. 3 conceptually shows an example of the detection unit. The detection device of the present invention includes a plastic scintillator S3, a BGO scintillator S4, and a photomultiplier tube 7. The plastic scintillator S3 is made of a low-density plastic, and generates an electron pair with respect to incident gamma rays. The energy imparted on the track by the charged particles of both electrons and positrons is converted into light. The phenomenon of light emission occurs. The decay time of light emitted from the plastic scintillator S3 is about 10 nsec. In the plastic scintillator S3, the positron is stopped and the electron pair annihilates. That is, the positron is combined with a nearby electron and emits two opposing annihilation gamma rays. This annihilation gamma ray passes through the plastic scintillator S3 and is stopped by the denser BGO scintillator S4. Therefore, scintillation occurs and light is emitted. The decay time of light emission in the BGO scintillator S4 is about 50 nsec. When the light emission in each of the plastic scintillator S3 and the BGO scintillator S4 is detected simultaneously, it is determined that the positron from the nitrogen element of the nitrogen-containing substance has been detected, and a signal is output. Even if low-energy gamma rays, which are background noise, enter the detector, only one of the two types of scintillators emits light, so that it can be determined as noise and removed.

  That is, positrons generated by electron pair generation from 10.8 MeV gamma rays radiated by the neutron capture reaction of nitrogen element cause pair annihilation immediately before stopping, and two positrons having sufficient permeability to the plastic scintillator S3. Of opposite annihilation gamma rays of 0.5 MeV. This gamma ray is emitted by the high-density BGO scintillator S4. At this time, since light emitted from the scintillators S3 and S4 has different temporal characteristics, it is possible to determine an event in which positrons are incident on the scintillator S3 by performing waveform discrimination. In the waveform discrimination performed here, the light emitted from the detector is converted into an electrical signal by a photomultiplier tube, and this data is analyzed by a data processing unit. The electrical signal obtained at that time is branched into several lines, filtered, and each voltage is analyzed using a digital waveform analyzer to determine the type of radiation incident on the detector and the amount of light emitted. The measurement is performed by using a conversion operator from the light emission amount to the dose for each radiation. An example of the relationship between the intensity of light emission obtained from scintillators S3 and S4 after waveform discrimination and the elapsed time is shown in FIG.

  The electronic circuit of the phoswich detector of the detection device of the present invention is appropriately constituted by a high-pressure amplifier, a wave height discriminator, a waveform discriminator, a delay circuit, an integration circuit, and the like. An example of a phoswich detector electronic circuit unit is shown in FIG. The analog output electric signal from the photomultiplier tube of the phoswich detector is divided into two or more by a signal divider that is impedance matched. Each signal is input to a different product filtering module and divided into two signal systems. As shown in the figure, one side is subjected to a micro product process of about 50 ns and an analog signal system (hereinafter referred to as a “Fast system”) with sufficient amplification, and the other is subjected to a micro product process of about 1 μs and an analog signal with sufficient amplification. This is a signal system (hereinafter, Slow system). By inputting each of these signals to the constant fraction discriminator module, a logic pulse is generated at an accurate timing when a voltage exceeding a certain threshold is sensed. By inputting these signals to each channel of the coincidence module, synchronous measurement is performed and a gate signal is generated. In order to capture each pulse, the gate is sufficiently opened with respect to the pulse width. The waveform is integrated and measured by a charge integration type ADC.

In order to detect the presence of nitrogen element without using a waveform discriminator, for example, nitrogen element is detected as follows.
The analog signal output from the scintillator is input to the electronic circuit unit and amplified by the voltage multiplier. The amplified analog signal is input to a wave height discriminating circuit to discriminate whether the signal is detected by the plastic scintillator S3. At this time, in order to discriminate the signal from the signal from the BGO scintillator S4, the threshold value at the time of wave height discrimination is set to be equal to or higher than the wave height of the annihilation gamma ray (0.5 MeV) detected by the BGO scintillator. When the signal exceeds the threshold by the wave height discriminator, a digital signal is output and the delay circuit is activated. In the delay circuit, the integration circuit is activated by delaying the signal detected by the plastic scintillator S3 by a time (about 10 nsec) until the signal is sufficiently attenuated. The integration circuit receives the signal from the delay circuit and starts integrating the analog signal output from the output from the voltage amplifier. The integration time in the integration circuit is set to an integration time equal to or longer than the decay time (about 50 nsec) of the BGO scintillator S4 so that the extinction gamma ray detected by the BGO scintillator S4 can be sufficiently measured. When the integration is completed, the pulse height classifier determines whether the signal is an annihilation gamma ray or higher signal, and if it is an annihilation gamma ray or higher signal, a digital signal is output. Further, the result obtained by discriminating the wave height of the signal from the plastic scintillator S3 performed first and the signal obtained by integrating the signal from the BGO scintillator S4 are input to the AND circuit. In the AND circuit, a positron is determined only when both results are “true”, and noise is determined otherwise.

  In this way, the energy of the incident gamma ray is measured from the amount of light emitted from the scintillator. The detector having the phoswich structure of the present invention can accurately measure only the presence of 10.8 MeV gamma rays (for example, whether or not landmines are buried) in a short time, and the presence of nitrogen-containing substances even when using a low-power neutron source Can be detected.

  The detector of the present invention having a phoswich structure was simulated using the electromagnetic shower Monte Carlo calculation code EGS4. As a result, it was found that the positron generated two pair annihilation gamma rays according to the detection principle, and this pair annihilation gamma ray passed through the plastic scintillator and stopped inside the BOG scintillator. Then, the energy of the annihilation gamma ray of the positron is all converted into light in the BGO scintillator and measured by the phototube. The result of simulation by EGS4 is shown in FIG.

  Next, the conceptual diagram of the structure of the phoswich detector in the detection apparatus of the nitrogen containing substance of this invention is shown by FIG. The phoswich detector of the present invention is, for example, a combination of two different scintillators optically combined with a single photoelectric conversion element, and a low-density substance such as NE102A (manufactured by NE Technology). The first scintillator S3 made of BC400 (manufactured by Bikeron) and the second scintillator S4 made of a high-density substance such as BGO (bismuth germanate) and NaI (Tl) are optically coupled. The first scintillator has, for example, a cylindrical shape, and the second scintillator is optically bonded and integrated so as to wrap around the outer periphery of the first scintillator, and is connected to the gamma ray incident surface and the photomultiplier tube in the first scintillator. It is preferable that an outer surface other than the surface to be optically connected to the second scintillator. For example, a double structure having a first scintillator as a core and a second scintillator as an outer skin is formed. The cross-sectional structure of the scintillator of the present invention preferably has a small inner diameter of S4 in order to reduce the influence from the background γ rays. Also, a photomultiplier tube is connected to one end of the integrated cylindrical scintillator via a light guide, and the light emitted from the first and second scintillators is measured. Yes. The total length of the phoswich detector of the present invention is about 10 cm, excluding the photomultiplier tube, and is extremely miniaturized. In the phoswich detector having this structure, since the light from the first and second scintillators is simultaneously detected by the photomultiplier tube, it is desirable to obtain data converted into a waveform obtained by separating both by the waveform discrimination circuit. Further, as described above, measurement is possible even without a waveform discrimination circuit.

  In the phoswich detector composed of the first and second scintillators, a plurality of photomultiplier tubes may be used. For example, by providing a photomultiplier tube that receives only the light of the first scintillator and a photomultiplier tube that directly receives only the light from the second scintillator, a detector provided with the above-described waveform analysis circuit; Although equivalent measurement results can be obtained, there are no problems in the detection itself although there are problems such as the need to provide a plurality of photomultiplier tubes and the complexity of the apparatus and the inability to make it compact.

  Further, a phoswich detector in which the first and second scintillators and the photomultiplier tube are continuously provided on one line has been proposed (see, for example, JP-A-6-123777). Electrons and positrons generated by electron pair generation in one scintillator may be emitted outside the detector, and positrons incident on the second scintillator may be reduced, and accurate measurement cannot be expected.

  Hereinafter, details of the present invention will be described by way of examples, but the present invention is not limited to the examples.

The present embodiment relates to a mine detector using an annihilation gamma ray radiated by a pair annihilation reaction that occurs following an electron pair production reaction by a 10.8 MeV gamma ray.
As shown in FIG. 1, the landmine detector of the present embodiment is composed of a portable neutron source 1 and a gamma ray detector 6. For example, the portable neutron source 1 and the gamma ray detector 6 are provided with wheels and the like. However, a known technique can be applied to the housing.

[1] Neutron generating means As the neutron generating means 1 used in this embodiment, a known neutron source can be used. Here, a DD fusion reactor disclosed in Patent Document 3 is used. The neutron generating means 1 installs a grid-like spherical cathode at the center of the spherical anode, converges ions generated by the discharge between the electrodes to the center by an electric field, and collides the ions to obtain a fusion reaction. At that time, high-energy neutrons are generated by the reaction shown in the following formula 2.

[2] Gamma ray detector 6
FIG. 3 is a diagram showing the main components of the gamma ray detector of the present embodiment.
<< Scintillator S3, S4 >>
The scintillator S3 is a plastic scintillator composed of a transparent plastic called polyvinyl toluene having a columnar shape of about 10 cm in length and a diameter of 5 cm and a slight mixture of two kinds of fluorescent substances. The reason why the length of the scintillator S3 is relatively long is that it is necessary to ensure the emission of annihilation gamma rays from the positrons generated by generating the incident gamma rays as electron pairs. In order to generate electron pairs in the scintillator S3, a scintillator such as BGO having a high density is usually suitable. However, there are many events in which annihilation γ rays are absorbed in S3. Therefore, it is preferable to use a plastic scintillator having a low density, for example, trade name NE102, Pilot U (density 1.032 to 1.25 g / cm ³ ) manufactured by Applied Koken Kogyo Co., Ltd. is preferable. In the scintillator S3, electrons and positrons are generated by generating electron pairs inside the scintillator, and at the same time, light emission proportional to the energy applied by the charged particles occurs.

  The scintillator S4 is configured by a BGO scintillator. The scintillator S4 is made of a material having a high density because positrons need to capture all gamma rays emitted by annihilation of electron pairs. In the present embodiment, BGO having a thickness of 3 cm is optically coupled to the outside of the first scintillator S3 formed in a cylindrical shape. The BGO layer having this thickness can completely capture the annihilation gamma rays incident from the first scintillator S3. The scintillators S3 and S4 are connected to the photomultiplier tube 8 through a light guide, and convert light into an electrical signal.

<Data processing section>
The data processing unit includes a time recording module, a simultaneous measurement module, a wave height recording module, a waveform discriminator, and a storage device. The photomultiplier tube receives light from the scintillators S3 and S4. The received light is converted into an electric signal by a photomultiplier tube and discriminated into a light waveform from the first scintillator S1 and a light waveform from the second scintillator S4 using a digital waveform analyzer. For example, the received light waveform of the first scintillator S3 having an attenuation time of about 10 ns or less shown in FIG. 4 is discriminated from the received light waveform of the fourth scintillator S4 having an attenuation time of about 50 ns or less. When it was measured that the light received by the first scintillator S3 and the second scintillator S4 was detected at the same time, it was determined that the gamma rays from the nitrogen element contained in the nitrogen-containing material were received, and the nitrogen-containing material was detected. Display.

  The gamma rays emitted from elements other than the nitrogen element contained in the nitrogen-containing substance are all low in energy compared to the 10.8 MeV gamma ray of the nitrogen element to be measured, so the probability of causing an electron pair generation reaction is low. Gamma rays are rarely measured. In the present invention, since only the data of the electron pair production reaction is used, background events due to gamma rays emitted from other elements can be greatly reduced.

  As described above, the landmine detector of the present embodiment is a novel detector that uses the electron pair generation reaction and the electron pair annihilation reaction without using the Compton type detector, and the complexity of the analysis by multiple scattering is complicated. It is possible to significantly reduce the background.

  The detection method or detection apparatus for a nitrogen-containing substance of the present invention can accurately detect the presence of a substance having a relatively high nitrogen content by detecting positrons with a phoswich detector. Therefore, the detection device of the present invention is suitable for detecting explosives and forbidden drugs at airports, ports, etc., and monitoring their carry-on and transfer to vehicles. Further, since the explosive is detected by detecting the nitrogen element, it is possible to provide a detection means effective for possessing and bringing in a liquid-mixed explosive used in recent years by terrorists and the like. The new landmine exploration device provided by the present invention is of a type that scans the surface of the ground while irradiating the ground with neutrons and can be designed to be relatively small so that large devices such as mountainous areas and inclined surfaces do not enter. Suitable for local mine exploration. Moreover, this invention is applicable also to the nitrogen distribution measurement contained in a plant, for example. Nitrogen has a very important role for plants, but measuring the behavior of nitrogen in plants can provide an effective measurement means in plant research.

A schematic diagram of a landmine detection method using a portable neutron source is shown. A schematic diagram of a wire-type landmine detection method is shown. The conceptual diagram of the phoswich detector of this invention is shown. The output waveform from the photomultiplier tube of this invention is shown. It is a block diagram which shows an example of the phoswich detector electronic circuit unit of this invention. The simulation result of the movement of the positron in the detector of this invention is shown.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Portable type neutron source 2 Soil 3 Thermal neutron 4 Gamma ray 5 Mine 6 Gamma ray detector 7 Photomultiplier tube S1, S2 Plastic scintillator S3 Plastic scintillator S4 of this invention BGO scintillator of this invention

Claims (12)

  A method for detecting a nitrogen-containing substance existing in a detection area, wherein irradiating the detection area with neutrons causes gamma rays to be emitted from nitrogen elements contained in the nitrogen-containing substance existing in the area, and A method for detecting a nitrogen-containing substance present in a detection region, wherein annihilation gamma rays emitted when a positron generated by electron pair generation annihilates an electron pair are measured.   A method for detecting a nitrogen-containing substance existing in a detection area, wherein irradiating the detection area with neutrons causes gamma rays to be emitted from nitrogen elements contained in the nitrogen-containing substance existing in the area, and A gamma ray that is emitted when a positron that is introduced into the first low-density scintillator and generated by generating a gamma-ray electron pair annihilates is measured with a second high-density scintillator. Detection method for nitrogen-containing substances present.   The positrons generated by electron pair generation by gamma rays emitted from the nitrogen element contained in the nitrogen-containing material when the light emission from the first scintillator and the light emission from the second scintillator are detected at the same time; The method for detecting a nitrogen-containing substance present in a detection area according to claim 1 or 2, wherein the presence of the nitrogen-containing substance is confirmed by determination.   The gamma ray from the nitrogen-containing substance to be measured is 10.8 MeV, and the annihilation gamma ray is 0.5 MeV. The method for detecting a nitrogen-containing substance existing in the detection region according to claim 1.   The detected area according to any one of claims 2 to 4, wherein an outer surface of the first scintillator other than a surface connected to the gamma ray incident surface and the photomultiplier tube is optically connected to the second scintillator. For detecting nitrogen-containing substances present in   The method for detecting a nitrogen-containing substance present in a detection region according to any one of claims 1 to 5, wherein the nitrogen-containing substance is an explosive. A neutron irradiation means, a first and second scintillator, a photomultiplier tube that receives light emitted from both scintillators, and a data processing unit that processes data from the photomultiplier tube; A device for detecting a nitrogen-containing substance present in a detection area for detecting the presence of a nitrogen-containing substance based on gamma rays emitted from a nitrogen element contained in the nitrogen-containing substance existing in the detection area,
(A) a first scintillator made of a low-density substance and a second scintillator made of a high-density substance are optically coupled;
(B) A detected region having a data processing unit that determines that a nitrogen-containing substance is present in the detected region when the first and second scintillators detect gamma ray radiation simultaneously. Detection device for nitrogen-containing substances present.   8. In the first scintillator, positrons generated by generation of electron pairs of incident gamma rays radiated by nitrogen element annihilate electron pairs and annihilation gamma rays are emitted, and in the second scintillator, the annihilation gamma rays are measured. An apparatus for detecting a nitrogen-containing substance existing in the described detection area.   9. The detected region according to claim 7, wherein an outer surface other than a gamma ray incident surface and a surface connected to the photomultiplier tube in the first scintillator is optically connected to the second scintillator. Nitrogen-containing substance detection device.   The detection apparatus of the nitrogen-containing substance which exists in the to-be-detected area | region of Claim 9 with which the 1st scintillator consists of a column shape and the 2nd scintillator is provided concentrically centering on the 1st scintillator.   The gamma ray from the nitrogen-containing substance to be measured is 10.8 MeV, and the annihilation gamma ray is 0.5 MeV, The detection apparatus for the nitrogen-containing substance existing in the detection region according to claim 7.   The nitrogen-containing substance detection device according to any one of claims 7 to 11, wherein the nitrogen-containing substance is an explosive.