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

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 KIHTENKO — Candidate of Chemical Sciences, Head of Sector of the Siberian Branch of the State Institution Scientific and Production Association «Special Equipment and Communications» of the Ministry of Internal Affairs of Russia, Major of the Internal Service. Research interests: forensics, operational technology.
KONSTANTIN VLADIMIROVICH ELISEEV — Deputy Head of Department of the State Institution Scientific and Production Association «Special Equipment and Communications» of the Ministry of Internal Affairs of Russia, Police Colonel. Research interests: forensics, operational technology.

111024 Moscow, Prud Klyuchiki St., 2, State Institution Scientific 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 of 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 substance in a suspected object. These methods are implemented in gas analyzers and other analytical devices based on physical and chemical principles (spectrophotometry, gas chromatography, nuclear quadrupole resonance, etc.). Biodetectors are also used for direct search, primarily dogs trained in mine detection services. Currently, this method has become the most widespread. No less widely used are indirect search methods, 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 case, wires, detonator — metal detectors; by the characteristic outlines of the device, wires, detonator — X-ray television installations; by the presence of microcircuits, semiconductor devices — nonlinear locators.

Regarding the technical capabilities of detecting explosives, it should be said that among the arsenal of modern analytical methods and devices there is no (and it is hardly possible for it to exist) universal method of counteracting the terrorist threat. The tasks of searching for and rendering harmless explosive objects are also varied. Therefore, the solution of these problems is possible only through the complex application of methods and devices.

Explosives as an object of search

Almost all industrially produced high explosives are used to carry out terrorist attacks. These include, 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 explosives, which are 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 the explosive effect.The vast majority of terrorist attacks are carried out using TNT or TNT-containing substances, since they are widely used in military affairs for equipping ammunition, as well as for civil blasting operations. The technology for producing TNT in large, practical quantities is quite complex and dangerous, so this substance is not produced in makeshift conditions.

Special training of service dogs is conducted using TNT. The sensitivity of electronic detectors and analyzers of explosives is tested using TNT. This is also due to the fact that during production, transportation and storage, ammunition and other objects containing explosives of other types are usually in close proximity to TNT-containing materials and, therefore, can be contaminated with microparticles of trinitrotoluene, and therefore have a corresponding «smell».

Gas analysis devices for detecting explosives

The operation of gas analysis devices used to detect explosives essentially 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 of the currently existing gas analyzers are significantly 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 a virtually 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 that a given analytical device can respond to. Another important characteristic of an analytical device is selectivity, the ability to reliably distinguish chemical substances. If low sensitivity of a gas analyzer reduces the probability of detection, in other words, the frequency of «gaps» 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» discredits the device, making 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 substance with a gas analyzer is its saturated vapor pressure, or volatility. Explosives vary significantly in this parameter. Nitroglycerin and other nitroesters have a high saturated vapor pressure. Dinitro- and trinitrotoluene (TNT), tetryl, and trinitrobenzene have a lower volatility, but are sufficient for detection. PETN, hexogen, and octogen are difficult to detect with a gas analyzer. Plastic explosives based on these substances are the most difficult 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 the temperature. Thus, a decrease in the ambient temperature by 5 °C leads to a twofold decrease in the pressure of saturated vapors of TNT. Such a strong dependence of the pressure of saturated vapors on temperature is especially characteristic of the climatic conditions typical for 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 the devices. A drop in air temperature also reduces the effectiveness of the «work» of service dogs.

From the point of view of detection sensitivity, it is important that if the explosive is not packaged, 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 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.

There are two main methods used in gas analyzers for detecting explosives: ion mobility spectrometry (drift spectrometry) and gas chromatography. Gas analysis devices available on the market can be divided into two main groups: detectors and analyzers.
Detectors are portable devices that detect the presence of explosive molecules in the air, usually without assigning them to a specific type.

Detection occurs in real time as an air flow sampled in the immediate vicinity of the object passes through the detector. The detectors are effective in searching for volatile explosives such as nitro esters, nitroglycerin (dynamites) and TNT. The method of ion mobility spectrometry (a complex characteristic depending on the mass of the ion, its charge and structure) is used to create devices of this type. The probability and degree of ionization of the substance molecules in the air directly sampled from the object strongly depend on external conditions, such as humidity. In this regard, the detector is characterized (and acceptable) by a slightly increased probability of false alarms. 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 affiliation to a specific type. For this purpose, computer processing of the analysis results is performed and the corresponding data banks on explosives are used. As a rule, analyzers work with preliminary accumulation of the sample, therefore the duration of the analysis is longer than in the case of detectors. Analyzers have significant dimensions and weight and higher energy consumption. A significant advantage over detectors is the greater reliability of the information obtained by analyzers.

As for the operating principle, explosives analyzers use both ion mobility spectrometry and gas chromatography. The technique for implementing the first method in an analyzer is slightly different from that in a detector. A sample of the substance from the air is collected on a special napkin (or cartridge), which makes it possible to concentrate the substance on the napkin. When examining the surface of documents that may contain microscopic amounts of explosives, when inspecting locks and handles of luggage or the hands of the person being inspected, the sample is collected 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 collected remotely, similar to how it is done with a detector. The napkin with the collected sample is introduced into the analyzer, a flow of purified (possibly heated) air or another 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