RU2185614C1 - Detector of hidden explosives - Google Patents

Abstract

FIELD: detection of hidden explosives, mines included, demining of territories within the bounds of humanitarian actions. SUBSTANCE: detector includes source of pulse ionizing radiation generating gamma- quanta beam with maximal energy of gamma-quanta exceeding 31.0 MeV, detector of secondary radiation and analyzer of signals of detector. Given units are positioned on mobile platform. Availability of source of gamma-quanta with energy above 31.0 MeV in mix of detector makes it possible to use gamma-activation analysis for identification of hidden explosives. Technical result of invention lies in mine-field reconnaissance with velocity not less than 8.0 sq m/min and in detection of explosives hidden in ground at depth up to 50 cm. EFFECT: diminished time of search and raised reliability of detection. 2 cl, 4 dwg

Description

 The invention relates to the field of detection of hidden explosives, including mines, and can be used, for example, when clearing territories as part of humanitarian actions.

 Known devices for the detection of hidden explosives (UHV) can be divided into two main groups: devices using indirect methods by which the characteristics inherent in a product containing an explosive (BB) are detected, such as the material of the case or fuse, the shape of the object, its temperature contrast with the environment, and many others, and devices that detect the actual explosives or their constituent components.

 The most widely used devices that implement indirect methods, such as induction, magnetometric or radio wave, and detecting objects that have metal parts in their structures, or objects that differ from the environment in their dielectric constant. By virtue of their operating principle, devices based on indirect methods, in real conditions of their use, have two significant drawbacks. Firstly, with their help it is impossible to detect an explosive object (for example, using a metal detector it is impossible to detect an object if it does not contain metal parts). Secondly, the operation of these devices is accompanied by a large number of false signals, which can reach 100-1000 per mine found, which leads to a sharp decrease in the rate of clearance, quick fatigue of sappers, and, as a result, the probability of missing explosive objects increases [1].

 Devices using direct methods for detecting explosives can be free from these disadvantages. The most famous of these are nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) methods, gas analytical and nuclear physical. Without going into a detailed discussion of the physical foundations that determine the essence of these methods, but based on the level of their elaboration and the results of the practical application of the technical means created on their basis, we can say the following.

 Devices using the NMR method can actually be used only in stationary conditions and are able to provide control of relatively small volumes of the medium (up to several liters) placed inside the zone in which a powerful magnetic field is created.

 Devices using the NQR method can have great search capabilities, however, the region of the spectrum in which the resonant frequencies of such widely used explosives as trotyl and hexogen are located lies in the range of the most intense industrial radio interference, which can impede the operation of these devices.

 Devices using gas analysis methods require the creation of a sedentary gaseous medium in which explosive molecules accumulate in an amount sufficient for detection. Therefore, the use of these devices, for example, in open areas has a very low efficiency.

 Devices using nuclear-physical methods for the detection of UHF allow one to identify explosives by the increased concentration of atoms of certain elements, in particular nitrogen, in the studied zone, since nitrogen is a part of all explosives used in practice.

A device is known according to the US patent [2], using the gamma-activation method to search for hidden explosives. The device consists of a gamma-ray source, a device that scans the irradiation zone over the surface of the object being examined, and two detectors detecting gamma-ray anihillation quanta, which are the end products of the ¹⁴ N (γ, 2n) ¹² N photonuclear reaction. The device has high sensitivity and speed However, it cannot be used to search for explosives located on the surface of the earth or underground, since the controlled volume must be between the detectors.

 From this drawback is free device for searching for UHV according to US patent [3], which is closest to the proposed (prototype).

The device is a mobile platform on which, in particular, one or more devices for searching for UHF, such as a metal detector, a thermal imager and a subsoil radar, are provided that provide primary reconnaissance of the territory. For the final identification of UHF, a source of ionizing radiation — a thermal neutron flux, a secondary radiation detector — gamma quanta, and a detector signal analyzer — are used. The device operates as follows. After primary reconnaissance, a neutron source is installed above the site of the proposed UHF location, and the soil is irradiated with thermal neutrons. If there are atoms of the natural ¹⁴ N nitrogen isotope in the examined area, then these nuclei capture neutrons, become excited, and then emit gamma rays with an energy of 10.8 MeV. The presence of gamma rays with this energy in the detected radiation is a signal for the detection of explosives. The nitrogen detection method used in the device, known as the TNA method, has a significant drawback due to the fact that upon irradiation of the soil with neutrons various side processes are initiated, accompanied by the emission of gamma rays having an almost continuous energy spectrum in the energy range 9-11 MeV, the spectral the density of this radiation is highly dependent on the chemical composition of the soil. Therefore, the process of recording radiation associated with the presence of nitrogen in the examined zone proceeds with an extremely unfavorable signal-to-noise ratio, which is why, in order to achieve an acceptable level of reliability, a large number of events must be recorded, which increases the time required for the identification of UHF. In addition, the thermal neutron flux density rapidly decreases with depth, which also limits the capabilities of the device. From the data given in the description of the patent it follows that the device is capable of detecting with a probability of 93% explosive charges with a mass of about 4 kg to a depth of 20 cm and charges with an explosive mass of 300 g (which is much more than the mass of the charge of most anti-personnel mines) when they lie practically on surfaces, and exposures longer than 4 minutes are necessary for their identification. Thus, it should be recognized that this device can hardly be used as the main search tool in humanitarian demining actions, since UN standards [4] require the removal of all explosive objects to a depth of 20 cm with a probability of 99.6%.

 The problem solved by the invention is the expansion of the arsenal of technical means used for the detection of SOW, while reducing search time and increasing the reliability of detection.

 The problem is solved by the proposed device containing an ionizing radiation source, a secondary radiation detector, and a detector signal analyzer located on a moving platform that uses an ionizing radiation source that generates a pulsed beam of bremsstrahlung with a maximum gamma-ray energy of more than 31 MeV. The presence of a gamma-ray source with an energy of more than 31 MeV in the device is an essential distinguishing feature, because, firstly, it allows the use of a gamma-ray probe as a probe instead of a neutron beam and, secondly, the gamma-activation method is used to identify UHV analysis.

 An electron accelerator with an energy of at least 31 MeV and a target in which the energy of accelerated electrons are transformed into a bremsstrahlung beam are used as a source of pulsed gamma radiation. In addition, it was proposed to perform an electronic accelerator according to the split microtron scheme. The overall dimensions of the device make it possible to use a ground or air vehicle (helicopter, aerostat or airship) as a mobile platform.

Using the inventive device allows
1. detect all types of mines and explosive objects, regardless of their design and packaging;
2. detect UHV with a charge mass of at least 40 g;
3. detect UHV in the soil at a depth of up to 0.5 m;
4. detect UHV in a time of 5-20 ms;
5. detect UHV with a probability of at least 99.6%;
6. conduct reconnaissance at a speed of at least 8 m ² / min;
7. conduct reconnaissance of the area covered with shrubbery.

Figure 1 shows a General view of the device, figure 2 is a variant of its placement on the tracked chassis in the working position, side view; figure 3 is the same, a top view; figure 4 is the same, in the stowed position, top view. The following notation is used in the figures and in the text:
1 - detector analyzer
2 - electronic accelerator
3 - electromagnet
4 - electromagnet
5 - target
6 - secondary radiation detector
7 - movable platform
8 - telescopic rod
9 - tower
10 - autonomous generator
11 - pulse modulator klystron
12 - cooling device
The UHV detection device consists of an electronic accelerator 2 (Fig. 1), a target 5, a secondary radiation detector 6 and a signal analyzer of the detector 1 located on the platform 7. As an electronic accelerator 2, a pulsed split microtron with an energy of 50-70 MeV can be used, since this type of accelerator due to its overall dimensions is most suitable for installation on mobile media. Target 5 is a plate of heavy metal (lead, tungsten, tantalum, uranium, etc.) with a thickness of 0.1-2 mm. The secondary radiation detector 6 consists of scintillators and photodetectors, for example photoelectronic multipliers, scanning the sensitive volume of the detector. The signal analyzer of the detector 1 generates a signal about the detection of CERs. Between the accelerator 2 and the target 5, electromagnets 3 and 4 can be installed, deflecting the electron beam in two perpendicular planes.

Выбор этих процессов в качестве реперных обеспечивает высокую селективность обнаружения ВВ, т. к. при облучении любых других химических элементов гамма-пучком с энергией меньше 100 МэВ не образуются никакие другие изотопы с периодом полураспада в диапазоне от 1 до 100 мс. Изотопы ¹²В и ¹²N являются β -активными и в процессе распада испускают электроны и позитроны с максимальной энергией ~ 13 МэВ и ~17 МэВ, которые, двигаясь в веществе, в свою очередь индуцируют гамма-кванты. Эти гамма-кванты, так же как и анигилляционные с энергией 511 КэВ, вместе с электронами и позитронами составляют вторичные продукты распада и могут быть зарегистрированы детектором. Следовательно, если облучить обследуемый объект импульсом гамма излучения с энергией гамма-квантов выше пороговых значений E_γ для реакций 1-3, то в последующем за ним временном интервале 1-20 мс он откликнется, при наличии в нем достаточной концентрации азота и/или углерода, потоком вторичных частиц от распада изотопов ¹²В и ¹²N, в противном случае этого потока в измеряемый промежуток времени не будет. Это обстоятельство обеспечивает высокую чувствительность устройства и достоверность обнаружения СВВ.The work of the proposed device is based on the gamma-activation method proposed in [5], the essence of which is that the identification of explosives uses the decay products of short-lived isotopes ¹² V (boron-12) and ¹² N (nitrogen-12), having half-lives of 20.2 and 11.0 ms, respectively. These isotopes are produced as a result of photonuclear reactions on nitrogen ( ¹⁴ N) and carbon ( ¹³ C) (an admixture of the ¹³ C isotope in natural carbon is 1.107%), the chemical elements that form the basis of all modern explosives when irradiated with gamma rays with higher energies threshold value E _γ

The choice of these processes as reference provides high selectivity for the detection of explosives, because when irradiating any other chemical elements with a gamma beam with an energy of less than 100 MeV, no other isotopes are formed with a half-life in the range from 1 to 100 ms. The ¹² V and ¹² N isotopes are β-active and in the process of decay emit electrons and positrons with a maximum energy of ~ 13 MeV and ~ 17 MeV, which, when moving in matter, in turn induce gamma rays. These gamma quanta, as well as 511 keV anigillation ones, together with electrons and positrons make up the secondary decay products and can be detected by a detector. Therefore, if the object under investigation is irradiated with a gamma radiation pulse with gamma-ray energy above the threshold values E _γ for reactions 1-3, then in the subsequent time interval of 1–20 ms, it will respond if there is a sufficient concentration of nitrogen and / or carbon in it , a stream of secondary particles from the decay of isotopes ¹² V and ¹² N, otherwise this stream will not be in the measured period of time. This circumstance provides high sensitivity of the device and reliability of detection of UHV.

 The proposed detection device UHV works as follows. An electron beam from accelerator 2 with an energy of more than 31 MeV is deflected in magnets 3 and 4 and directed to a brake target 5, in which it is converted into a narrow beam of bremsstrahlung directed along the axis of the primary electron beam with a maximum gamma-ray energy greater than 31 MeV. During the duration of the current pulse of the accelerator 2, the beam of bremsstrahlung irradiates a limited area of the surface being examined, penetrating the soil to a certain depth. After 0.1-1.0 ms after the end of the current pulse, the secondary radiation detector 6 operates for 1-20 ms, the data from which is processed by the analyzer 1, which generates a signal about the detection of explosives. If no explosive is detected, then the field in magnets 3 and / or 4 changes, respectively, the direction of the electron beam incident on the brake target changes, the irradiation region shifts, and the search cycle for the explosive resumes.

 The high selectivity of the device in the detection of explosives is due to the fact that in explosives (TNT, RDX, HMX, TEN, tetryl), nitrogen and carbon are (17-38)% and (24-50)% by mass, respectively, while Earth's crust, their concentration is significantly less than 1%. Therefore, the main sources of false signals can be either soil sections with a high concentration of nitrogen fertilizers or organic inclusions. In order to separate these signals from signals received from the explosive, it is possible to carry out additional irradiation of the suspicious area with gamma rays with energies in the range from 18 to 23 MeV. In this case, the detector will record only the products of the photonuclear reaction on carbon. Comparing the detector readings obtained with these two irradiations with different gamma-ray energies, we can draw a conclusion about the chemical composition of the examined object.

 The invention is illustrated by the following examples.

 Example 1

A variant of a device for detecting UHV on a tracked chassis in the operating position is shown in FIG. 2 (side view) and in FIG. 3 (top view). In tower 9, a split microtron with an energy of 50 MeV, electromagnets for deflecting the beam, a target, biological protection, and a klystron — a source of microwave power for the microtron — are installed. Outside the tower, a klystron pulse modulator 11, a cooling device 12, and an autonomous electric generator 10 are fixed. The secondary radiation detector 6 is mounted on a telescopic rod 8 and is placed in front of the test surface in the operating position in the forward direction. In the stowed position shown in FIG. 3 (top view), the secondary radiation detector 6 is mounted on the vehicle. Estimates show that the total mass of equipment installed on the carrier can not exceed 10 tons, and the overall dimensions in the stowed position, including the tracked chassis, are (L • W • H) 6100 • 3800 • 2600 mm ³ .

Exploration of the terrain is carried out from a fixed chassis, while the device sequentially irradiates in front of itself equally spaced sections of the surface. After the examination of a certain area, the device moves forward and begins the examination of the next surface area. If we assume that the distance between adjacent irradiated areas is 50 mm and the repetition frequency of the current pulses of the microtron is 50 pulses / s, then the device will need a time of about 1 min to examine an area of 4000 • 2000 mm ² in size.

 Example 2

The following are the results of computer simulations of the proposed device for the detection of UHF. For the model experiment, the following initial data were used:
The energy of accelerated electrons is 50 MeV.

 Surge current 50 mA.

 The duration of the current pulse is 6 μs.

 The target is a lead plate 0.5 mm thick.

 The height of the target of bremsstrahlung above the ground is 2 m.

The average angle of entry of the gamma ray beam into the soil surface is 45 ^o .

 The height of the detector above the controlled surface is 1.5 m.

The sensitive area of the detector is 0.5 • 0.5 m ² .

 The efficiency of detecting gamma rays with a 0.98% detector.

 Duration of registration after the end of the radiation pulse 18 ms.

 The explosive is TNT.

 Soil parameters - tabular data for concrete were used to describe the physical characteristics of the soil.

 During the simulation, physical processes taking place with accelerated electrons in the inhibitory target and gamma rays formed in air, in soil, in an object with explosive, and also the interaction of electrons and positrons - products of beta decays from photonuclear reactions (1- 3) - with the substance as they move into the aperture of the detector.

 During the simulation, the number of background events recorded by the device due to natural cosmic radiation, as well as the natural and induced radioactivity of the soil, was also calculated.

 An analysis of the results of computer simulation shows that the proposed UHV detection device is capable of detecting explosive charges with a mass of 40 g to a depth of 20 cm and a mass of 3000 g to a depth of 40 cm, with a detection probability of at least 99.6%. Taking into account that these characteristics of the device were obtained by removing the target and the detector from the ground at a distance of more than 1 m, we can conclude that the device is able to conduct reconnaissance of the area covered with shrubbery.

Sources of information
1. Hearts N.I. and others // Strategic stability. - 2000. - 2. - S. 33-40.

 2. US patent 4756866, 376/157, CL. G 21 G 1/12 publ. 07/12/1988.

 3. US patent 6026135, 376/159, CL. G 21 G 1/06 publ. 02/15/2000.

 4. International standards for conducting demining operations in the framework of humanitarian actions under the auspices of the UN // UN. - 1996. - 75 p.

 5. Trower W.P. // Nuclear Instruments and Methods, B 79 (1993) 589.

Claims (3)

 1. A device for detecting hidden explosives, including an ionizing radiation source, a secondary radiation detector and a detector signal analyzer located on a movable platform, characterized in that the ionizing radiation source is used, which generates a pulsed gamma-ray beam with a maximum gamma-ray energy of more than 31 MeV  2. The device according to claim 1, characterized in that the ionizing radiation source consists of a pulsed electron accelerator with an energy of more than 31 MeV and a target.  3. The device according to p. 2, characterized in that the electron accelerator is made according to the scheme of a split microtron.

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