Detection of explosive objects: hardware for anti-terrorist services.

Апр 27, 2024
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Detection of explosive objects: hardware for anti-terrorist services

Detection of explosive objects: hardware for anti-terrorist services

A. V. Kikhtenko, K. V. Eliseev

ANDREY VLADIMIROVICH KIKHTENKO — Candidate of Chemical Sciences, head of the sector of the Siberian branch of the State Research and Production Association «Special Equipment and Communications» of the Ministry of Internal Affairs of Russia, major of internal service. Area of ​​scientific interests: forensics, operational technology.
KONSTANTIN VLADIMIROVICH ELISEEV — deputy head of the department of the State Research and Production Association «Special Equipment and Communications» of the Ministry of Internal Affairs of Russia, police colonel. Area of ​​scientific interests: forensic science, operational technology.

111024 Moscow, st. Pond Klyuchiki, 2, State Research and Production Association «Special Equipment and Communications» of the Ministry of Internal Affairs of Russia.

General approach to solving the problem of searching for explosive objects

Methods for detecting explosive devices can be divided into two main types: direct and indirect. Direct methods allow us to draw a conclusion about the presence (or absence) of an explosive in a suspected object. These methods are implemented in gas analyzers and other analytical instruments based on physicochemical principles (spectrophotometry, gas chromatography, nuclear quadrupole resonance, etc.). Biodetectors are also used for direct search, primarily dogs trained in mine detection service. Currently, this method is most widespread. Indirect search methods are no less widely used, with the help of which the possible presence of an explosive device is judged by indirect signs: by the presence of metal parts of the body, wires, fuse — metal detectors; according to the characteristic outlines of the device, wires, fuse — X-ray television installations; based on the presence of microcircuits and semiconductor devices — nonlinear locators.

Regarding the technical capabilities of detecting explosives, it should be said that among the arsenal of modern analytical methods and instruments there is no (and it is hardly possible to exist) a universal method of countering the terrorist threat. The tasks of searching for and neutralizing explosive objects are also varied. Therefore, solving these problems is possible only through the integrated use of methods and instruments.

Explosives as a search object

Almost all industrially produced high explosives are used to carry out terrorist attacks. These are, first of all, TNT (2,4,6-trinitrotoluene), hexogen, PETN (tetra-nitropentaerythritol) and compounds based on them. In addition, plastic substances are used for explosive actions — the so-called plastic explosive, which is a mixture of an explosive of normal or high power with filler substances that give the mixture special plasticine-   or   rubber-like   properties.   Contrary to popular belief, such a filler does not make the explosive more powerful. Its main purpose is to give the explosive a particular shape and/or ensure a tighter fit to the surface exposed to explosive action.
The vast majority of terrorist attacks are carried out with the use of TNT or TNT-containing substances, since they are widely used in military affairs when loading ammunition, as well as in carrying out explosive works for civilian purposes. The technology for producing TNT in large quantities that are significant from a practical point of view is quite complex and dangerous, so this substance is not produced in artisanal conditions.

Service dogs are specially trained using TNT. TNT is used to test the sensitivity of electronic detectors and explosive analyzers. This is also due to the fact that during production, transportation and storage, ammunition and other objects containing other types of explosives are usually in close proximity to TNT-containing materials and, therefore, can be contaminated with microparticles of trinitrotoluene, and therefore have the corresponding » odor.»

Gas analytical instruments for detecting explosives

The operation of gas analytical instruments used to detect explosives actually copies the principle implemented when using dogs — identifying an object containing an explosive by the presence of vapors or microparticles of this substance in the surrounding space. The best gas analyzers that currently exist are noticeably inferior in sensitivity and selectivity to a dog's sense of smell. But at the same time, gas analyzers are indispensable in solving a number of problems due to such properties and functions as the ability to work for an almost unlimited time, identify the type of explosive and document the detection result.

Let us recall that sensitivity is the minimum amount of a substance or the minimum concentration of vapors in the air to which a given analytical device is capable of responding. Another important characteristic of an analytical instrument is selectivity—the ability to reliably distinguish between chemical substances. If, with a low sensitivity of the gas analyzer, the probability of detection decreases, in other words, the frequency of “misses” increases, then insufficient selectivity leads to low noise immunity of the device and a high level of false alarms.

Contrary to the existing opinion that only sensitivity is important for detecting explosives , and selectivity is secondary, it should be noted that a high frequency of “false alarms” leads to discrediting the device and makes its use ineffective. A sensitive and highly selective device allows not only to detect the target substance, but also to approximately determine its type.

An important characteristic from the point of view of detecting an explosive by a gas analyzer is its saturated vapor pressure, or volatility. Explosives vary significantly in this parameter. Nitroglycerin and other nitroesters have high saturated vapor pressure. Dinitro- and trinitrotoluene (TNT), tetryl, and trinitrobenzene have less volatility, but sufficient for detection. Difficult to detect using a gas analyzer are heating elements, hexogen, and octogen. Plastic explosives based on these substances are the most difficult object to detect using gas analysis, since the particles of the low-volatile explosive are covered with a polymer binder.

Another important factor for detection is the conditions in which the explosive or the object containing it is located, and above all temperature. Thus, a decrease in ambient temperature by 5 °C leads to a twofold decrease in the pressure of saturated TNT vapor. Such a strong dependence of saturated vapor pressure on temperature is especially characteristic of the climatic conditions characteristic of Russia. And as a consequence, the long duration of low-temperature periods makes the use of gas analyzers problematic and leads to increased requirements for the sensitivity of devices. A drop in air temperature also reduces the efficiency of the “work” of service dogs.

From the point of view of detection sensitivity, it is important that if the explosive is unpackaged, then the vapor concentration above its surface reaches the saturated vapor pressure at a given temperature, but as it moves away from the object, the concentration quickly decreases due to the dilution of the vapor with air. At room temperature, the concentration of TNT vapor decreases to values ​​corresponding to the sensitivity threshold of modern gas analyzers, already at a distance of 15-20 cm. Different types of packaging, their permeability to explosive vapors and the time spent in it also have a significant impact on detection capabilities.

In currently used gas analyzers for detecting explosives, two main methods are implemented: ion mobility spectrometry (drift spectrometry) and gas chromatography. Gas analytical instruments on the market can be divided into two main groups: detectors and analyzers.
Detectors are portable instruments that detect the presence of explosive molecules in the air, usually without being assigned to a specific type.

Detection occurs in real time as air flows through the detector and is sampled in close proximity to the object. The detectors are effective in detecting highly volatile explosives such as nitroethers, nitroglycerin (dynamites) and TNT. To create devices of this type, the method of ion mobility spectrometry is used (a complex characteristic depending on the mass of the ion, its charge and structure). The probability and degree of ionization of molecules of a substance in the air directly taken from the object strongly depend on external conditions, for example, humidity. In this regard, the detector is characterized (and acceptable) by a slightly increased probability of false positives. By improving both the analytical unit and the mathematical processing of the received signals, it is possible to reduce the influence of negative factors and increase the selectivity of the device.

Unlike detectors, analyzers are capable of not only detecting explosives, but also determining their group membership in a specific type. To do this, computer processing of the analysis results is carried out and the corresponding data banks on explosives are used. As a rule, analyzers work with a preliminary accumulation of sample, so the analysis time is longer than in the case of detectors. Analyzers have significant dimensions and weight and higher power consumption. A significant advantage over detectors is the greater reliability of the information received by analyzers.

In terms of operating principle, explosives detection analyzers use both ion mobility spectrometry and gas chromatography. The technique for implementing the first method in the analyzer is somewhat different from that in the detector. A sample of the substance from the air is taken onto a special napkin (or cartridge), which makes it possible to concentrate the substance on the napkin. When examining the surface of documents on which microquantities of explosives may be applied, when inspecting locks and handles of luggage or the hands of the person being inspected, a sample is taken by wiping suspicious objects with a napkin. For this purpose, analyzers are usually equipped with a special sampling device, which is a portable vacuum cleaner in which a napkin is placed. The sample is taken remotely, just as it is done using a detector. A napkin with a selected sample is introduced into the analyzer, a stream of purified (possibly heated) air or other carrier gas removes the sample and transfers it to the analytical unit. This significantly reduces the possibility of external factors influencing the analysis. The ion mobility spectrometry method is fast, the analysis time is about 6-8 s.

Table 1. Gas analyzers for detecting explosives

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