FIELD OF THE INVENTION

[0001] The present invention relates to the field of radiographic and neutron radiation analysis of materials and, specifically, to methods for detecting nitrogen-containing explosives present in various objects.

[0002] The present invention can be used by security services in fighting against terrorism and organized crime on passenger and freight transport, at public and post offices in order to detect explosives concealed in objects under inspection, first of all, in mailings, passengers' baggage and hand luggage without the opening and examination thereof.

PRIOR ART

[0003] Counteracting illegal circulation of explosives and terrorist acts performed with their used has become one of the main tasks in activities of security services of civilized countries in the fight against terrorism and organized crime. One of the lines of this counteraction is connected with organizing, whenever necessary, control at airports, governmental and diplomatic establishments, nuclear power stations, post offices over objects such as briefcases, bags, suitcases, luggage packages, electronic equipment, computers, mobile phones and the like, as well as mailings, since these objects are most frequently used by terrorists and criminals to conceal explosives when committing terrorist acts or illegally transporting explosives.

[0004] The necessity to control large flows of mailings or inspect hand luggage and baggage of passengers, primarily at airports, when checking and examination time is limited, requires using methods and equipment, which would not require opening and visually inspecting each object under investigation, but would ensure rapid detection of explosives with a high probability of correct detection and low number of false alarms.

[0005] Among numerous prior art methods of detecting explosives in objects under investigation the following three groups of methods have found practical use today.

[0006] The first group comprises various methods using X-rays to detect explosives. These methods involve the exposure of an object under investigation to X-rays with an energy of 100 to 150 keV, recording X-rays that have passed through the object under investigation and identification of an explosive on the basis of X-ray attenuation by materials contained in the object under investigation depending on the atomic numbers of chemical elements contained by said materials (Patrick Flanagan, “Technology vs. Terror”, EUSA, 1989, No. 7, p.p. 46-49, 51; advertising brochures and datasheets of EG&G Astrophysics Research Corporation, Long-Beach, Calif., USA, and In Vision Technologies, Foster City, Calif., USA).

[0007] Since modern explosives, particularly plastic ones, predominantly consist of chemical elements of a low atomic number, they display only a weak absorption of 100-150 keV X-rays. This makes it difficult to effectively detect explosives made in unconventional configurations or masked by way of placing them, for instance, into bars of soap or electronic devices. The practice of applying detection methods based on the X-rays has shown that explosives can be found with their use in no more than 40-60% of cases.

[0008] Methods of the second group are based upon the fact that most explosives belong to the class of highly volatile organic compounds characterized by a high vapor pressure. These methods provide for the use of gas chromatography and ion mobility spectrometry in making a chemical analysis of vapors emitted by an explosive and/or particles thereof, which may be present on the surface of the object under investigation. By the results of this analysis a decision is made about the presence of an explosive in the object under investigation (Hughes D. Thermedics Begins Production of Bomb Detection Unit, Aviation Week & Space Technology, Jun. 19, 1989). Such method has been realized, for instance, in the explosive and drug detection system IONSCAN 350 produced by Barringer Instruments, Ltd., Canada (Intersec. The Journal of International Security, Vol. 3, No. 6, November 1993).

[0009] Technical means implementing the aforementioned methods have a high sensitivity, but are incapable of detecting most powerful plastic explosives due to their very low vapor pressure. Moreover, these methods do not guarantee detecting all types of explosives if the latter are placed in a fluid-tight packing or specially treated packing.

[0010] Explosive detection methods belonging to the third group are based on the determination of the presence and concentration of major chemical elements present in the explosive composition by means of a neutron radiation analysis. The prior art explosive detection methods of this kind and units implementing thereof (U.S. Pat. No. 5,114,662, 1992; U.S. Pat. No. 5,144,140, 1992; U.S. Pat. No. 5,388,128, 1995; EP 0295429, 1992; EP 0297249, 1993) comprise steps of positioning of an object under investigation into a radiation-protected chamber; exposing it to thermal neutrons; recording the secondary gamma radiation of 10.8 MeV energy; finding, on the basis of the results of secondary gamma radiation recording, the distribution of nitrogen concentration within the object under investigation and determining the presence of an explosive in said object by the finding an increased nitrogen concentration.

[0011] All modem explosives are known to contain a fairly large quantity of nitrogen corresponding to 9 to 35 mass percent when density of an explosive is in the range 1.25 to 2.00 g/cm ³ . The thermal neutron irradiation of an explosive at about 0.025 eV brings about a radiation capture of thermal neutrons by nuclei of nitrogen-14 atoms, which results in the formation of a nucleus of nitrogen-15 atoms in exited state. During the transition from the exited state to the ground state about 14% of nuclei of nitrogen-15 atoms on average emit gamma-ray quanta of 10.8 MeV.

[0012] However, during radiation capture of thermal neutrons not only explosives emit 10.8 MeV gamma-ray quanta, but also all other nitrogen-containing materials present in the object under investigation, such as wool, leather, nylon, silk, as well as the air filling and surrounding the object under investigation. Compared with explosives, most non-explosive nitrogen-containing materials have a considerably larger volume, the nitrogen content being about equal for all such materials. This is why the information about the distribution of nitrogen concentration within an object under investigation allows to distinguish an explosive with a high nitrogen concentration from other nitrogen-containing materials that do not belong to explosives.

[0013] At the same time, the aforementioned prior art methods make it possible to obtain only a rather approximate distribution of nitrogen concentration within an object under investigation for the reason that each gamma-ray detector registers gamma-ray quanta emitted from the entire volume of the object under investigation being exposed to thermal neutrons, including gamma-ray quanta from the nitrogen of ambient air also exposed to thermal neutrons. Furthermore, while an object under investigation is continuously transported by the conveyor through the radiation-protected chamber, the recorded number of gamma-ray quanta may be determined by their emission from all nitrogen-containing materials exposed, including non-explosives distributed along the length of the object under investigation. First, this hampers locating the explosive in the object under investigation and, second, it increases the probability of false alarms. Moreover, thresholds levels in detection of explosives by said prior art methods appears to be inadmissibly high since, on the average, only about 14% of nuclei of nitrogen atoms contained in the explosive emit the above described gamma radiation.

[0014] All explosives are known to have not only a high nitrogen density, but high oxygen density as well, said value lying for major types of explosives within the range of 0.80 to 1.15 g/cm ³ . Practically all known materials that are not explosive, but cause false alarms due to the high nitrogen density, e.g. nylon, silk, wool, leather, melamine, etc., have a low oxygen density. In this connection, an attempt has been made to use the ongoing nuclear interactions, for instance, between fast neutrons of 14 MeV energy and oxygen nuclei, in order to detect secondary gamma radiation of 6.1 MeV and thus to obtain additional information about oxygen density in an object under investigation. By combining the obtained results on intensity of secondary gamma radiation from nitrogen and oxygen it becomes possible to make an assessment of the presence or absence of an explosive in the object under investigation.

[0015] Therefore, a number of other prior art explosive detection methods based on the neutron radiation analysis of materials and units for their implementation (U.S. Pat. No. 5,080,856, 1992; U.S. Pat. No. 5,200,626, 1993) provide for positioning an object under investigation in the radiation-protected chamber; simultaneously exposing the object under investigation to thermal neutrons at 0.025 eV and fast neutrons at about 14 MeV produced with the use of controlled pulsed sources of fast neutrons; recording the instantaneous rates of 10.8 MeV gamma radiation emitted due to the radiation capture of thermal neutrons by nuclei of nitrogen-14 atoms, and 6.1 MeV gamma radiation emitted by nuclei of oxygen-16 atoms in exited state resulting from the interaction between fast neutrons at around 14 MeV and stable nuclei of oxygen-16 atoms; using the results of said gamma radiation recording to obtain information on the concentration of nitrogen and oxygen in the object under investigation; and making a decision on the presence of an explosive in said object by finding an increased concentration of nitrogen and oxygen and a value of the nitrogen/oxygen ratio lying in a predetermined range.

[0016] An additional recording 6.1 MeV gamma radiation increases information content of the method and, consequently, a probability of correct explosive detection inherent to said method. However, recording gamma rays of said quantum energy without cutting off the pulsed source of fast neutrons is complicated due to the presence of gamma radiation having quantum energies within the range of 5 to 7 MeV emitted during nuclear interactions of thermal and fast neutrons with nuclei of atoms of a number of other chemical elements, such as chlorine, manganese, sodium or iron, since this makes it necessary to use detectors of a very high resolution. This disadvantage is eliminated by the disconnection of the controlled pulsed source of fast neutrons or removal of the object under investigation from the radiation-protected chamber for a time commensurable to the oxygen-16 halftime. Such measures however lead to a longer overall screening time and require a greater sophistication in the design of units realizing prior art methods of the discussed type.

[0017] Furthermore, exposing the object under investigation to fast neutrons requires additional radiation protection for the chamber in order to comply with the current national requirements for protection of the personnel and population against the effect of ionizing radiation, which leads to an increase in the mass and dimensions of a unit implementing the prior art method. Also, an increase of the radiation dose absorbed by the object under investigation takes place, which can cause a deterioration of consumer properties of industrial articles contained therein, e.g. electronic equipment and photographic materials.

[0018] For the above reasons, the most effective in explosive detection are those methods equipment units (U.S. Pat. No. 5,078,952, 1992; U.S. Pat. No. 5,153,439, 1992; U.S. Pat. No. 5,200,626, 1993; EP 0336634, 1993; WO 91/14938 ) that are based on the combined use of X-ray radiography and neutron radiation analysis using thermal neutrons or thermal and fast neutrons simultaneously. All prior art methods of explosive detection of the above-discussed type include the initial positioning of an object under investigation into an X-ray unit; exposing the object under investigation to X-rays; recording the X-rays that have passed through the object under investigation to form one or more X-ray images of the object under investigation; next positioning the object under investigation into the radiation-protected chamber; exposing the object under investigation to thermal neutrons or to thermal and fast neutrons simultaneously; recording the secondary 10.8 MeV gamma radiation and emitted by the object under investigation during radiation capture of thermal neutrons by nuclei of nitrogen-14 atoms, or recording the secondary 10.8 MeV gamma rays and secondary 6.1 MeV gamma rays emitted by nuclei of oxygen-16 atoms in exited state formed as a result of the interaction between fast neutrons and stable nuclei of oxygen-16 atoms; and making a decision about the presence of an explosive in the object under investigation on the basis of an analysis of both the X-ray image of the object under investigation and the distribution of nitrogen or nitrogen and oxygen therein.

[0019] The combined use of X-ray radiography and neutron radiation analysis provides greater probability of correct explosive detection; however, it does not relieve the mentioned prior art methods from the above disadvantages inherent to each of these groups of methods individually.

[0020] Besides, the use of all above described prior art methods of explosive detection that are based on the neutron radiation analysis of materials provides for the irradiation by neutrons of the entire volume of the object under investigation. This requires, first, the use of a source with a large flow of fast neutrons, so that the predetermined detection characteristics could be attained. This leads to an increase in the mass and dimensions of the unit implementing the method, due to the necessity of additional radiation protection measures, and increases the cost of the unit. Second, this makes it necessary to expose to neutrons the areas of the object under investigation that do not contain an explosive, but may contain non-explosive nitrogen- and oxygen-containing materials, which materials, after the exposure to thermal and fast neutrons, will emit gamma rays. As a result, the probability of false alarms will increase. Third, the unnecessary neutron irradiation of areas of the object under investigation that do not contain an explosive increases the radiation dose absorbed by articles within the object under investigation, e.g. electronic equipment and photographic materials, which can cause an impairment of their consumer qualities.

[0021] The closest prior art to the proposed invention is constituted by a method of detecting arms and explosives in objects under investigation (RU 2065156, 1996), said method being based on the combined use of X-ray radiography and neutron radiation analysis. The prior art method of detecting arms and explosives in objects under investigation includes the following steps:

[0022] exposing an object under investigation to X-rays;

[0023] recording X-rays that have passed through the object under investigation;

[0024] using an X-ray image of the object under investigation to detect areas with a density of inorganic and organic materials exceeding pre-established threshold values;

[0025] determining in these areas a geometric shape of articles with a density of inorganic materials exceeding the established threshold value;

[0026] further exposing the areas of the object under investigation with a density of organic materials exceeding the established threshold value to thermal neutrons produced by a thermal neutron radiator in the form of a radionuclide californium-252-based source;

[0027] recording the secondary 10.8 MeV gamma radiation emitted by the exposed areas of the object under investigation and using, as informative parameters for arms and explosive detection, the value of X-ray attenuation by the object under investigation depending on the atomic numbers of chemical elements present in the composition of materials within the object under investigation and on the X-ray wavelength, geometrical shape of areas with a density of inorganic materials exceeding the established threshold value, and the intensity of the secondary gamma radiation recorded.

[0028] As noted above, during the transition from an exited state to the ground state only an average of 14 nuclei of nitrogen atoms in 100 nuclei of atoms that have captured thermal neutrons emit a 10.8 MeV gamma-ray quantum, whereas the rest of said nuclei emit two or more gamma-ray quanta with lower energies, i.e. only about 14% of the mass of nitrogen contained in an explosive emits gamma rays used as one of the basic informative parameters. This circumstance leads, on the one hand, to an increase in the explosive detection threshold when using this or other prior art methods. On the other hand, this makes it necessary to establish lower threshold value for the intensity of secondary 10.8 MeV gamma rays in order to provide the required probability of correct explosive detection, which, in its turn, leads to a greater probability of a false alarm due to the presence in the object under investigation of other, non-explosive nitrogen-containing materials.

[0029] Therefore, the described method, like all other prior art methods using X-ray radiography and the neutron radiation analysis in which the areas of an object under investigation with a density of organic materials higher than the established threshold value are exposed to thermal neutrons, does not allow ensuring the required low probability of a false alarm, since the presence of an explosive is only judged by the value of intensity of 10.8 MeV gamma rays during radiation capture of thermal neutrons by nuclei of nitrogen-14 atoms. In this case, the presence in the object under investigation of materials that are not explosive, but contain nitrogen in quantities similar to its content in explosives may lead to wrong decisions.

[0030] Since the number of recorded gamma-ray quanta greatly depends on the position of the neutron-exposed area of the object under investigation emitting secondary gamma rays relative to the gamma-ray detector, primarily on the distance between them, the results of recording gamma-ray quanta are much affected by the location of that area within the volume of the object under investigation. Ignoring their relative position in the prior art methods leads to a distortion of results of gamma radiation recording, which may cause omission of the explosive, thus reducing the probability of correct detection.

[0031] The use in the state-of-the-art methods of an once-established invariable threshold value for overall intensity of gamma rays, which value stays constant in a fairly wide range of change in possible masses of explosives being detected, requires using a very low threshold value sufficient enough to detect an explosive, including that having a minimal mass, with the predetermined probability of correct detection. Using such a low threshold value corresponding to the minimal mass of an explosive will considerably increase probability of false alarms caused by gamma rays emitted by non-explosive nitrogen-containing materials and by nitrogen of air.

[0032] Moreover, in practicing any of all existing methods based on the neutron radiation analysis the use of a thermal neutron radiator that has an invariable directional pattern, with its emitting surface forming a rather wide divergent beam of thermal neutrons, does not make it possible to expose to thermal neutrons a small-size area having a density of organic materials greater than the established threshold value. This leads to the inevitable neutron irradiation of adjacent areas of the object under investigation that may contain non-explosive nitrogen-containing materials as well as air. As a result, gamma rays are emitted by the non-explosive nitrogen-containing materials and the nitrogen of air contained in areas of the object under investigation adjacent to the area being exposed, which also increases the probability of a false alarm.

[0033] Furthermore, the prior art methods do not ensure a high level of radiation safety for the reason that, as a rule, they make use of a radionuclide neutron source, which may lead to radioactive contamination of the environment in emergency situations caused by an explosion or fire, as well as to irradiation of the population, should the source be embezzled. Irradiating many areas of an object under investigation with a density of organic materials higher that the established threshold value also leads to an unjustified increase of the radiation dose absorbed by articles contained in objects under investigation, which may result in an impairment thereof.

[0034] Where a check is required on the presence of small masses of explosive in small-size objects under investigation, e.g. mail wrappers and hand luggage of passengers such as bags, briefcases, video and photographic cameras, portable computers, mobile phones and the like, these objects are positioned for neutron exposure in a radiation-protected chamber with dimensions meant for checking the luggage or mailings of significantly larger dimensions. Consequently, the cavity can contain a considerable amount of air not replaced by the object under investigation. When exposed to thermal neutrons, air will also emit gamma rays which, given the appreciable volume of remaining air, either will preclude a small mass of explosive from being detected or may cause a false alarm due to the high background component of gamma rays from nitrogen of air present in the radiation-protected chamber.

[0035] In addition, virtually all known state-of-the-art methods do not include a step of the identification, by its X-ray image, of an article contained in the area of the object under investigation and having a density of organic materials higher than the established threshold value. The absence of such information about the presence of such compact article in the area of the object under investigation being exposed to thermal neutrons decreases a probability of correct explosive detection and does not make it possible to reduce the probability of a false alarm.

[0036] For the same reason, the existing methods provide during the neutron radiation analysis for the exposure to thermal neutrons of all areas of the object under investigation with a density of organic materials exceeding the established value, which greatly extends the entire explosive detection time.

[0037] As shown by the above-presented analysis of the existing explosive detection methods, still unresolved to date remain major problems stemming from an insufficiently high probability of correct detection, a high probability of false alarms, an increased detection threshold of an explosive being detected, a prolonged time of explosive detection, an insufficient level of radiation safety, and a high probability of deterioration of consumer properties of certain industrial goods contained in an object under investigation as a result of checking. This accounts for the absence in the arsenal of security services of modern facilities ensuring a highly efficient and rapid explosive detection in an object under investigation.

DISCLOSURE OF THE INVENTION

[0038] The primary object of the present invention is to increase the probability of correct explosive detection.

[0039] A further object of the present invention is to reduce the probability of a false alarm when detecting an explosive in an object under investigation.

[0040] A still further object of the present invention is to decrease, by implementing the method of the invention, a detection threshold for an explosive in an object under investigation.

[0041] The object of the present invention is also to reduce the overall time of detecting an explosive in an object under investigation.

[0042] In addition, the object of the present invention is to heighten the level of radiation safety and decrease a danger of impairment of consumer properties of the object or of articles contained therein as a consequence of its exposure to neutrons.

[0043] Other objects and advantages of the present invention will be made clear below in examining the preferred embodiment of the invention and graphic materials illustrating it.

[0044] According to the present invention, the set objects are achieved by the following steps included in the proposed method of detecting an explosive in an object under investigation:

[0045] exposing the object under investigation to X-rays;

[0046] recording the X-rays that have passed through the object under investigation and finding, on a basis of a rate of X-ray attenuation, areas of the object under investigation with a density of organic materials exceeding a pre-established value;

[0047] identifying, on a basis of an analysis of the recorded X-rays, articles present in the aforementioned found areas of the object under investigation with said density of organic materials exceeding the pre-established value, and selecting, among said areas, an area containing an unidentified article;

[0048] determining, on a basis of an analysis of the recorded X-rays, dimensions and a position of said unidentified article within the object under investigation;

[0049] evaluating a mass of explosive being detected, on a basis of the dimensions of the unidentified article and, for instance, the average density of existing explosives;

[0050] determining and forming a directional pattern of a thermal neutron radiator corresponding to the dimensions of the unidentified article and exposing the selected area of the object under investigation to thermal neutrons from said neutron radiator preferably arranged as a controlled thermal neutron radiator with variable directional pattern based on the deuterium-deuterium reaction and equipped with a neutron moderator;

[0051] recording, using at least two gamma-ray detectors, gamma-ray quanta with the energy of 10.8 MeV and pairs of simultaneously emitted cascade gamma-ray quanta, preferably, with the energy of 5.534 MeV and 5.266 MeV from said selected area;

[0052] determining an overall intensity of gamma rays emitted by the object under investigation, preferably, by summing up numbers of 10.8 MeV gamma-ray quanta recorded by each of the gamma-ray detectors, said numbers multiplied by a weight factor of each corresponding detector readings, and numbers of pairs of cascade gamma-ray quanta, simultaneously recorded by each of pairs of gamma-ray detectors, multiplied by the weight factor of said pair of detectors, wherein the weight factor of a detector is determined, e.g., experimentally, taking into account the position of the unidentified article relative to the given gamma-ray detector, and the weight factor of said pair of detectors is determined, e.g., as the arithmetical mean of weight factors of detectors comprising that pair;

[0053] determining a threshold value for the overall intensity of gamma rays emitted by the object under investigation, basing on the supposed mass of explosive being detected and making a decision about presence of the explosive within the object under investigation if the threshold value is exceeded by overall intensity of the gamma rays emitted by the object under investigation.

[0054] Optionally, when detecting an explosive in a small-size object under investigation, the mass of air surrounding the object under investigation is reduced before its exposure to thermal neutrons, preferably by replacing it by a gaseous medium free from nitrogen.

[0055] The recording not only 10.8 MeV gamma-ray quanta emitted by the irradiated area of the object under investigation, but also 5.534 MeV and 5.266 MeV cascade gamma-ray quanta with the use of at least two gamma-ray detectors, the count of pairs of cascade gamma-ray quanta recorded simultaneously, the determination of the overall gamma-ray intensity and comparison of the latter with the threshold value yield a higher probability of correct explosive detection or a lower minimal mass of explosive detectable by the method of the invention, which is confirmed by the following reasoning.

[0056] As noted above, during the transition from exited state to ground state an average of only 14% of nuclei of nitrogen atoms that have captured thermal neutrons emit gamma-ray quanta having the energy of 10.8 MeV. However, another 19% of nuclei of nitrogen atoms, on the average, make this transition in two steps through an intermediate exited state lasting not longer than 10 ⁻¹⁵ s. During such two-step transition to the ground state a nucleus of nitrogen atom, due to negligible time of staying in intermediate exited state, emits in different directions and virtually simultaneously two cascade gamma-ray quanta with the energy of 5.534 MeV and 5.266 MeV, respectively. Since cascade gamma-ray quanta composing a pair are emitted in different directions, their recording requires, as a rule, use of two gamma-ray detectors.

[0057] An additional recording 5.534 MeV and 5.266 MeV cascade gamma-ray quanta makes it possible to capture gamma rays emitted, on the average, not by 14%, but by 33% of nuclei of nitrogen atoms that have captured thermal neutrons, As a results, an increase in the overall intensity of recorded of and, thus, in the informative value of the method are attained. In other words, the combined recording 10.8 MeV gamma-ray quanta and cascade gamma-ray quanta increases the probability of correct detection at preset values of the probability of false alarms and minimal mass of an explosive to be detected.

[0058] However, in difference to 10.8 MeV gamma-rays, a direct recording 5.534 MeV and 5.266 MeV cascade gamma-ray quanta is practically unfeasible owing to the presence of gamma rays with quantum energies within a range of 5-6 MeV emitted during the interaction of thermal neutrons with nuclei of atoms of a number of other chemical elements, e.g., sulfur, chlorine and sodium. Hence, in case of direct recording, presence of additional cascade gamma-ray quanta from said elements can lead to false alarms.

[0059] This is why the methods of the present invention provides for checking the coincidence in time of the moments of receiving two gamma-ray quanta with the energies of 5.534 MeV and 5.266 MeV by two gamma-ray detectors in order to ascertain that the aforementioned two gamma-ray quanta constitute a pair of cascade gamma-ray quanta emitted by a single nucleus of a nitrogen atom as a result of the two-step transition from the exited state to the ground state, but not by nuclei of atoms of other chemical elements.

[0060] Locating the area of the object under investigation containing an unidentified article and having a density of organic materials exceeding the established value; determining the dimensions and position of the unidentified article within the object under investigation; determining and forming a directional pattern of the thermal neutron radiator corresponding to the dimensions of the unidentified article; exposing to thermal neutrons only said located area; recording gamma-ray quanta by several, at least by two, gamma-ray detectors positioned at different distances from the area being irradiated; and determining the overall intensity of gamma rays emitted by the object under investigation with taking into account the weight factor of readings of each of the detectors and those of pair(s) of detectors, results in providing more information for use in detecting an explosive and, thus, increases the probability of correct detection.

[0061] Determining the position of the unidentified article within the object under investigation on the basis of an analysis of X-rays recorded makes it possible to establish the weight factor of readings of each gamma-ray detector on the basis of relative position of the unidentified article and gamma-ray detector and so to determine the overall gamma-ray intensity by summing up, with appropriate weight factors, numbers of 10.8 MeV gamma-ray quanta and numbers of pairs of cascade gamma-ray quanta simultaneously recorded by each of pairs of gamma-ray detectors.

[0062] Since the proposed method provides for recording gamma-ray quanta by several, at least by two, gamma-ray detectors, the result of recording gamma-ray quanta by each of them will have its own weight determined by the position of each gamma-ray detector relative to the area exposed to thermal neutrons.

[0063] Therefore, the proposed method provides for experimentally determining, during the calibration, weight factors of readings of detectors which allow for the distance between a gamma-ray detector and the area of the object under investigation exposed to thermal neutrons, as well as for inherent efficiency and sensitivity anisotropy of each gamma-ray detector, the scattering of gamma-ray quanta, the dimensions and mass of an explosive used for calibration. This also ensures a greater probability of correct explosive detection.

[0064] Use of the steps of determining the dimensions of an unidentified article inside the object under investigation on the basis of an analysis of X-rays recorded; assessing the supposed mass of an explosive being detected on the basis of the dimensions of the unidentified article and the average density of existing explosives; determining the threshold value for overall gamma-ray intensity on the basis of the supposed mass of the explosive being detected; and making a decision on the presence of the explosive in the object under investigation, should that threshold value be exceeded by overall gamma-ray intensity ensures a rational choice of a threshold value for overall gamma-ray intensity in accordance with the supposed mass of an explosive being detected. This, in its turn, leads to a greater probability of correct explosive detection and a lower probability of false alarms.

[0065] The proposed method makes it possible, on the basis of the dimensions of an unidentified article obtained from the analysis of X-rays recorded and the average density of existing explosives, to approximately estimate the mass of the unidentified article in the instance where the latter is an explosive. Obtaining an estimate of the supposed mass of an explosive being detected on the basis of results of X-ray radiography monitoring and using the estimate as a priori information for the operation of neutron radiation analysis of the object under investigation in accordance with the present invention makes it possible to most efficiently determine a threshold value for overall gamma-ray intensity that allows for the supposed mass of an explosive being detected, which mass corresponds to the dimensions of the unidentified article, and yields the highest value of correct detection probability ever possible under these conditions.

[0066] Besides, using an invariable threshold value for overall gamma-ray intensity established without regard to the a priori information about the supposed mass of an explosive being detected would require choosing a fairly low threshold value sufficient enough to detect the minimal mass of explosive with the preset probability of correct detection. Using such a low threshold value corresponding to the minimal mass of explosive will lead to false alarms caused by gamma rays emitted by non-explosive nitrogen-containing materials and nitrogen of air.

[0067] In the instance where the results of X-ray radiography monitoring show that the supposed mass of an explosive being detected is considerably higher than the minimal mass, the proposed method makes it possible to increase the threshold value for overall gamma-ray intensity in accordance with the value of supposed explosive mass without noticeably reducing correct detection probability, but rather leading to a significantly lower probability of a said false alarms.

[0068] Finding the area of the object under investigation that contains an unidentified article and has a density of organic materials higher than the established value; determining the dimensions and position of the unidentified article within the object under investigation; determining and forming a directional pattern of thermal neutron radiator corresponding to the dimensions of the unidentified article; and exposing to thermal neutrons only the area of the object under investigation containing the unidentified article make it possible, on the one hand, to expose the whole volume of the unidentified article to thermal neutrons and, on the other, not to expose to thermal neutrons areas of the object under investigation that are adjacent to the unidentified article and may contain non-explosive nitrogen-containing materials. The first circumstance makes the whole volume of the unidentified article emit gamma rays, thus increasing the overall intensity of gamma rays recorded and leading to a higher probability of correct explosive detection. The second circumstance accounts for a lesser gamma radiation emitted by non-explosive nitrogen-containing materials present in the areas of the object under investigation adjacent to the unidentified article, which means a lower probability of a false alarm.

[0069] In addition, exposing to thermal neutrons revealing only the area containing an unidentified article reduces the overall time of explosive detection thanks to exclusion from neutron radiation analysis the areas with a density of organic materials greater than the established value with all articles contained therein being identified.

[0070] Irradiating by thermal neutrons only a limited area of the object under investigation and using for this purpose a controlled thermal neutron radiator with a variable directional pattern, which radiator is based on the deuterium-deuterium reaction and is equipped with a neutron moderator, precludes exposing to thermal neutrons other areas of the object under investigation known to contain no explosive.

[0071] First, this increases the level of radiation safety; second, decreases the probability of impairing consumer properties of such goods as photographic materials and electronic equipment due to exposure to neutrons; third, prevents non-explosive nitrogen-containing materials from thermal neutron exposure, which areas will not in this case emit secondary gamma rays that may lead to a false alarm.

[0072] Reducing, before thermal neutron exposure, the mass of air surrounding the object under investigation through replacing air by, e.g., a gaseous nitrogen-free medium lowers the probability of a false alarm that may be caused by gamma rays emitted by nitrogen contained in the air, or diminishes the minimum explosive mass detected over invariable gamma radiation recording time with the preset probabilities of correct detection and false alarm, thanks to a lower background component of nitrogen in the air inside the radiation-protected chamber.

[0073] This is particularly important in the instance where a check is required on the presence of an explosive in small-size objects under investigation, e.g., mail wrappers and passenger hand luggage such as bags, briefcases, video and photographic cameras, portable computers, mobile phones and the like. When small objects are placed for thermal neutron exposure in the radiation-protected chamber having dimensions designed for checking considerably larger objects, its cavity will inevitably contain a substantial volume of air not replaced by the object under investigation. When exposed to thermal neutrons, the remaining air will also emit gamma rays. The volume of said air being great enough, this can cause a false alarm or prevent a small explosive mass from being detected owing to the significant background component of gamma rays of nitrogen in the air.

[0074] The aforementioned circumstances confirm that the declared objects and technical result of the present invention are achieved thanks to the fact that the proposed invention has characteristic features listed above.

[0075] The aforementioned objects and other objects, as well as advantages of the invention, will become more obvious from the below description of the preferable invention embodiment and accompanying drawings.