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Article

Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives

by
Sergei Bobrovnikov
1,2,
Evgeny Gorlov
1,2,* and
Viktor Zharkov
1
1
V.E. Zuev Institute of Atmospheric Optics SB RAS, 634055 Tomsk, Russia
2
Faculty of Radiophysics, National Research Tomsk State University, 634050 Tomsk, Russia
*
Author to whom correspondence should be addressed.
Photonics 2024, 11(11), 1065; https://doi.org/10.3390/photonics11111065
Submission received: 25 September 2024 / Revised: 15 October 2024 / Accepted: 17 October 2024 / Published: 14 November 2024
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Abstract

:
This paper presents the results of an experimental study of the possibility of remote visualization of traces of some nitro-group-containing explosives (TNT, RDX, HMX, Composition-B, and Tetryl) on the surface of aluminum foil using the laser fragmentation/laser-induced fluorescence (LF/LIF) method. A tunable excimer KrF laser with a narrow generation line was used to fragment explosives and excite fluorescence of their NO fragments (nitric oxide molecules) from the second vibrationally excited state (v″ = 2). When recording optical responses, spectral selection of the γ(0, 0) fluorescence band of NO was carried out. The LF/LIF method is shown to be promising for creating scanning detectors that will allow remote detection of trace amounts of explosives with a concentration of up to 1 μg/cm2 on the surfaces of objects at a distance of several meters and simultaneously determine their location. The sensitivity of the one-color LF/LIF detection method can be increased by increasing the energy density of the probing radiation and/or by optimizing the LF/LIF excitation process.

1. Introduction

According to the basic principle of forensic science, formulated by the French criminologist E. Locard, each contact leaves a trace [1]. In this regard, it is obvious that the solution to the problem of successful and timely detection of a trace is the key to solving a crime or preventing an impending crime.
In the context of anti-terrorist activities, identifying any manifestation of an explosive device can be of decisive importance in preventing possible tragic consequences. That is why many countries continue to work on improving existing and developing new methods and means of detecting explosive devices, both by direct (vapors and traces of explosives) and indirect (elements of the structure) unmasking signs. The main direction of development of detection technologies is to increase the sensitivity of methods while maintaining high resistance to false alarms (selectivity). High sensitivity ensures fast and reliable detection and sufficient selectivity allows for the alarm signal to be identified against the background of many interfering factors and for the process of continuous monitoring of the facility’s safety to be carried out without being distracted by false alarm signals [2].
Of particular interest are technologies capable of remote, contactless detection of explosives or their carrier at a safe distance, thereby ensuring safety for personnel and other people. This is especially relevant for inspection points in scenarios, where an explosive device may be accidentally or intentionally activated. Among the promising control methods, it is worth highlighting optical (laser) technologies, which are fundamentally distinguished by the possibility of remote measurements [3,4,5,6]. Some of them (laser-induced breakdown spectroscopy, Raman spectroscopy) have been implemented in experimental devices and tested under real field conditions. In this context, they can be considered as the most advanced technologies.
The authors of this publication have been developing and researching a method for remote detection of vapors and traces of nitrogen-containing explosives using the laser fragmentation/laser-induced fluorescence (LF/LIF) spectroscopy for a number of years [7,8]. This paper presents some results of a study on the possibility of remote detection and visualization of traces (microscopic particles) of explosives on the surface of aluminum foil.
The combined use of the effects of laser fragmentation of large complex molecules and laser-induced fluorescence of their fragments (photolysis products) for in situ detection of non-fluorescent molecular compounds was first proposed by Rodgers et al. [9]. The essence of the method is to induce dissociation of the initial compound into simpler molecules (fragments) using a laser, and then, to carry out laser-induced fluorescence of the resulting fragments. The results of studies on the excitation features of the LF/LIF process for nitro compounds, including explosives, are presented in numerous works, for example, [10,11,12,13,14,15,16,17,18,19,20,21]. As a rule, the nitric oxide molecule NO is chosen as a characteristic fragment that unambiguously indicates the presence of the parent molecule of a nitro-group-containing explosive. Component analysis of the composition of nitro-group-containing explosives shows that the average content of N + O atoms in them is 76.95 ± 8.47% [2] and the appearance of NO fragments during photolysis is very likely. Studies of the composition of the photolysis products of nitro compounds have confirmed that NO fragments are the primary products of their dissociation in both the gas and solid phases [19,20,21,22,23,24,25,26].
Since the most intense absorption bands of nitro compounds and their NO fragments are located in the UV range, and the process of fragment formation takes a short time [23], one pulsed laser can be used for the simultaneous effective fragmentation of nitro compound molecules and excitation of NO fragment fluorescence. In this case, the choice of the laser radiation wavelength is determined by the resonance properties of the NO molecule and the method of excitation of NO fragment fluorescence.
It is known that NO photofragments of nitro compounds have a nonequilibrium distribution over vibrational levels, unlike the background nitrogen oxide molecules present in the atmosphere. It has been established that during TNT fragmentation, approximately 10% of NO fragments are generated in the second vibrational state, while the population of this level for background NO molecules is 7–8 orders of magnitude lower in accordance with the Boltzmann distribution [17]. This circumstance makes it possible to find a method for the selective excitation of NO fragment fluorescence during fragmentation of explosives without involving atmospheric NO in the interaction process, which makes it possible to lower the detection threshold by 7–8 orders of magnitude, respectively.
A significant positive factor in the excitation of fluorescence of the NO molecule from the excited vibrational state (v″ > 0) is the fact that a significant part of the excitation energy will be transferred to the anti-Stokes region of the fluorescence spectra, which, as is known, are shifted to the “blue” region of the spectrum relative to the line of the exciting laser radiation. In this case, the fluorescence spectra of possible atmospheric impurities excited from the ground state at a random coincidence of the absorption bands with the excitation line, will be shifted to the long-wave region of the spectrum. Thus, their influence on the detection threshold of NO fragments will be minimal. Obviously, the above-mentioned advantages of the LF/LIF method cause increased interest of researchers in this technology of remote detection of traces and vapors of nitro compounds and, despite a number of difficulties in technical implementation, attempts to assess the suitability of this method for practical application continue.
For example, the use of LF/LIF for detection and visualization of TNT traces with a concentration of ~2 μg/cm2 on a silicon wafer at a distance of 15 cm was demonstrated by Wynn et al. [21]. Fragmentation of TNT molecules and excitation of their NO fragments were carried out by pulsed laser radiation with a wavelength of 236.2 nm, corresponding to the γ(0, 1) absorption band of the NO molecule upon excitation from the first vibrational state [transition A2Σ+ (v′ = 0) ← X2Π (v″ = 1)]. This approach allows for an increase in excitation efficiency due to a higher population of the first vibrational state, but this requires sacrificing noise immunity, which leads to an increase in the detection threshold.
In order to ensure greater noise immunity of the method, in the present work, we used a method of excitation of NO fragments in the γ(0, 2) absorption band of the NO molecule [transition A2Σ+ (v′ = 0) ← X2Π (v″ = 2)]. This required the use of a tunable laser source with a tuning region in the vicinity of the 248 nm emission line, for which an excimer laser on a KrF medium with a narrow generation line was selected. The use of an excimer laser made it possible to increase the power of the exciting radiation and increase the detection range to 5 m. Along with the detection range, the surface scanning speed also increased, amounting to approximately 12 cm2/s at a detection threshold of 1 μg/cm2. It also became possible to expand the range of the studied nitro-group-containing explosives (TNT, RDX, HMX, Composition-B, and Tetryl). To conduct the experiments, an experimental setup was created and a research methodology was developed, the description of which is given below.

2. Experimental Setup

The experimental setup is a prototype of a remote scanning detector of traces of nitro-group-containing explosives. The prototype includes a laser radiation source, an optical system for forming a radiation beam, a multichannel system for spectral selection and photodetection, and a scanning system.
The general view of the prototype of the remote detector of traces (RDT) of explosives is shown in Figure 1. The source of probing radiation in the RDT is an excimer KrF laser (developed at the Institute of High Current Electronics SB RAS) with a narrow generation line (5 pm) tuned to a wavelength of 247.867 nm, corresponding to the edge of the branch P12 of the γ(0, 2) absorption band [transition A2Σ+ (v′ = 0) ← X2Π (v″ = 2)] of the NO molecule.
The laser parameters are presented in Table 1. The laser beam is directed into the transmitting optical system, where a horizontally oriented line (illumination line) with the required geometric and energy parameters in the plane of the object at a distance of 5 m is formed using anamorphic optical elements. A pyroelectric laser energy meter Gentec-EO QE50LP-H-MB-D0 (Gentec Electro-Optics, Inc., Québec, QC, Canada) was used to control the energy of the laser pulses.
The system of spectral selection and photodetection of the optical detection signal consists of an original double diffraction monochromator designed for the spectral selection of the γ(0, 0)-band of NO molecule fluorescence [transition A2Σ+ (v′ = 0) → X2Π (v″ = 0)] and suppression of the unshifted scattering line of the exciting radiation. The optical signal is received directly on the diffraction grating of the first monochromator (holographic concave grating, focal length 380 mm, f/3.2), which simultaneously functions as a receiving objective. The monochromator, supplemented with two custom interference bandpass filters, provides suppression of the unshifted scattering line by 12 orders of magnitude and carries out spectral selection of the γ(0, 0) fluorescence band of NO fragments of explosives with an efficiency of at least 25%.
Photodetection of optical signals is performed by a multichannel system based on a 32-anode PMT H7260-04 (Hamamatsu Photonics, Shizuoka, Japan). The multichannel spectral selection and photodetection system forms a horizontal array of 32 pixels measuring 1 × 1.2 cm2 at a distance of 5 m in the plane of the object location. The photodetection process in each pixel is performed in the photon counting mode with a time resolution of 5 ns. The field of view of the receiving optical system is aligned with the horizontal illumination line during the system adjustment. The object is scanned by mechanically rotating the spectral selection and photodetection system unit in the vertical plane in the angle range of ±5° from the horizontal. Laser radiation is transmitted through an optical hinge.
During scanning, photodetection data in the form of the number of photocounts in each pixel are fed to a PC-based data collection and processing system. The signal visualization program processes and restores the image in the scanning area in real time, highlighting pixels with an excess of signal over the background and displaying the response intensity in each pixel in accordance with the scale of conventional colors.
During the tests of the RDT prototype, a series of experiments were conducted to study the possibility of remote visualization of traces of TNT, RDX, HMX, Composition-B, and Tetryl on the surface of an object. The use of aluminum foil as a substrate was motivated by the desire to simulate unfavorable detection conditions (intense scattering of laser radiation in the direction of the detector) to evaluate the efficiency of the monochromator in suppressing unshifted scattering interference.
To prepare samples with traces of explosives, their solutions in acetone with a concentration of 1 g/L were used. The solution was prepared immediately before the start of the measurements. The solution was applied to a selected area of the aluminum foil substrate using a micropipette with an adjustable dosing volume of 2–20 μL (Thermo Fisher Scientific Inc., Waltham, MA, USA). The traces were applied to the foil surface in the form of inscriptions with the abbreviation of the corresponding compound (TNT, RDX, HMX, C-B, TET)—Figure 2. The volume of the applied solution was ~20 μL/symbol, the average mass of the substance was ~20 μg/symbol, and the inscription area was approximately 7 cm × 22 cm.
After drying, the substrate with traces of explosives was placed in the studied area of space at a distance of 5 m from the RDT prototype (Figure 3). Then, sequential scanning of the survey area was performed by moving the illumination line in the top-down direction by tilting the entire platform of the receiving and transmitting optical system in the vertical plane. The dimensions of the illumination line on the surface of the object were 10 × 0.3 cm2; the average energy density of the illumination line was ~14 ± 1 mJ/cm2; the repetition rate of laser pulses was 50 Hz. The scanning time for a surface measuring 10 × 48 cm2 was 40 s. Registration of optical responses from the samples was carried out in the photon counting mode with a signal accumulation duration of 50 laser pulses. All measurements, the results of which will be presented below, were carried out in laboratory conditions at normal atmospheric pressure and an air temperature of 25 °C.

3. Results and Discussion

The appearance of samples with traces of explosives and the results of experiments on their visualization are shown in Figure 4. The right side of the figures shows a scale of conventional colors showing the magnitude of the recorded signals in photocounts.
As can be seen from the figures, in all cases, the device was able to “read” the applied inscriptions, even under conditions of intense diffuse scattering of the probing laser radiation by the aluminum surface. The average background level from the surface containing no traces of explosives for all samples was estimated to be 1.4 ± 1.2 photocounts per pixel, which indicates a sufficient level of suppression with the monochromator of the interference from the laser radiation reflected from the surface with an acceptable overall transmission of the spectral selection system.
Judging by the scatter of signal values in the “trace” pixels, we can talk about the uneven application of explosive solutions on the surface of the substrates. For example, in the case of TNT (Figure 4b), a clear increase in signal is observed at the intersections of the lines forming the symbols. In the process of sample preparation, when applying the next element of the symbol, the solution flowing onto the neighboring element was indeed visually observed. A similar picture is observed for other substances. The results of the analysis of experimental data on the visualization of explosive traces (Figure 4) are presented in Table 2.
The average signal value per pixel was estimated based on the number of pixels corresponding to the surface elements of the object with traces of explosives and the total signal value per symbol. It is evident that this value is approximately the same for all substances except RDX. Under the same conditions, the signal from RDX is approximately twice as high as the signals from other explosives. This is most likely due to the manifestation of certain features of the photochemical and photophysical processes occurring in the RDX molecule under the action of UV laser radiation and leading to a higher yield of NO fragments.
Thermal decomposition of explosives in various aggregate states has been intensively studied for many decades, and one of the most difficult problems has turned out to be elucidation of the mechanism of their decomposition. The diversity of products and the sensitivity of the reaction to the conditions of its implementation indicate the existence of several competing channels of explosive decomposition, including nitro–nitrite rearrangement, elimination of HONO, heterolytic and homolytic dissociation of the N–NO2 bond, etc. The mechanism of thermal decomposition (from the ground electronic state) can differ significantly from the mechanism in laser photolysis (from the electronically excited state). In the context of this work, we are primarily interested in the channels of formation of NO molecules when solid explosives are exposed to UV laser radiation.
An analysis of the literature has shown that the range of explosives for which the relevant studies have been conducted is very limited. Nevertheless, we will present these data. For RDX, HMX, and PETN in the solid and/or gas phase, it has been established that NO molecules are the primary products of laser fragmentation of the parent molecules and are formed through the following decomposition channels: nitro–nitrite rearrangement [19,20,21,22,23,24,25,26], splitting off an oxygen atom from a nitro-group with subsequent breaking of the N–N bond [25], and homolytic dissociation of the N–NO2 bond [24]. Since the mechanisms of photodissociation of explosives vary depending on the molecular structure, wavelength of fragmenting radiation, energy and duration of the laser pulse, aggregate state of the substance, pressure and temperature, etc. [27], it is impossible to make adequate quantitative estimates of the NO yield for different explosives based on the fragmentary information available. The issue remains open and requires comprehensive studies using precision analytical methods.
When analyzing the literature data, an obvious conclusion was also made that the value of the recorded LIF of NO fragments will also be affected by the population distribution over the rotational levels of the vibrationally excited state v″ = 2 of the fragments, from which fluorescence is excited under the action of laser radiation. It is known that as a result of the fragmentation of nitro compounds, a nonequilibrium population distribution is observed both over vibrational and rotational states. The values of rotational temperature for different vibrational levels of NO fragments of some nitro compounds in the gas, liquid, and solid phases can be found, for example, in [14,15,16,19,20,22,23,26,28,29,30,31,32].
Summarizing the data of these works, it is impossible to make an unambiguous conclusion about the magnitude of the rotational temperature for target NO fragments in the vibrationally excited state v″ = 2 (or any other). However, it is obvious that it depends on many factors and can vary widely. Certainly, when assessing the relative LIF signal of NO fragments for different explosives, the rotational temperature is an important value and should be taken into account when assessing the potential capabilities of the LF/LIF method in detecting explosives. Estimates of the change in the absorption (excitation) intensity of NO fragments as a result of the A2Σ+ (v′ = 0) ← X2Π (v″ = 2) transition at different rotational temperatures, carried out using the LIFBASE software package [33], are presented in Figure 5. Figure 5b shows that when the rotational temperature changes from 300 K to 5000 K [19], the excitation efficiency of NO(v″ = 2)-fragments in the head of the P12 band can change by a factor of six. It is clear from Figure 5a that, regardless of the rotational temperature of the NO(v″ = 2)-fragments of the explosive, an increase in the sensitivity of the LF/LIF can be achieved by using laser radiation with a shorter wavelength, corresponding, for example, to the bandheads of branches (Q12 + P22), P11, and (Q11 + P21).
At a known pixel area (1 × 1.2 cm2), the area of explosive traces was determined for each symbol, and based on the explosive mass in one symbol (20 μg), the surface concentration of explosives was estimated to be about 1 μg/cm2. Judging by the demonstrated ability to visualize explosive traces (Figure 4) and the signal–background ratio greater than five, these concentrations are not the limit and can be reduced.
An experimental assessment of the limiting sensitivity of the LI/LIF method was not carried out in this study. However, the results of [21], which demonstrated the ability to detect TNT traces with a surface concentration of ~ng/cm2 at a distance of 15 cm, allow us to hope for a further multiple increase in the sensitivity of LF/LIF in remote measurements.

4. Conclusions

Thus, the capabilities of remote detection and visualization of traces of explosives on a metal surface (aluminum foil) using the LF/LIF method were demonstrated using a prototype of a remote scanning detector of traces of nitro-group-containing explosives based on an excimer KrF laser. The experiments showed that the setup can reliably detect a number of explosives at a distance of 5 m with a surface scanning speed of 12 cm2/s at an average surface concentration of trace of 1 μg/cm2. During the experiments, a twofold anomalous increase in the efficiency of RDX excitation was detected. Assumptions were made about the causes of this phenomenon.
An attempt was made to find an explanation for the interaction features associated with the abnormally low rotational temperature of NO fragments formed during RDX fragmentation. As calculations show, the difference in temperature can lead to a difference in the excitation efficiency of up to six times. Obviously, this issue requires additional research. The issue of the possibility of increasing the sensitivity and detection range also requires additional research. One of the obvious ways to increase the efficiency of the discussed one-color LF/LIF of nitro compounds is to increase the energy density of the probing radiation, leading to a nonlinear increase in the signal [19]. However, this will require the use of a more powerful laser. There are other ways to improve the LF/LIF method, the discussion of which is beyond the scope of this publication.
During the experiments, it was also revealed that the surface concentration was not uniformly distributed over the trace surface, which led to large errors in the sensitivity assessment. It is obvious that for an adequate sensitivity assessment, it is necessary to develop a special method for creating specified concentrations (~ng/cm2) of explosive traces on surfaces using precise methods with the availability of objective control means. In the near future, the authors plan to continue such studies.

Author Contributions

Conceptualization, S.B., E.G. and V.Z.; methodology, S.B., E.G. and V.Z.; validation, S.B., E.G. and V.Z.; formal analysis, S.B., E.G. and V.Z.; resources, S.B., E.G. and V.Z.; data curation, S.B., E.G. and V.Z.; writing—original draft preparation, E.G.; writing—review and editing, S.B.; visualization, E.G.; supervision, E.G.; project administration, S.B. All authors have read and agreed to the published version of the manuscript.

Funding

The research was carried out with the financial support of the Ministry of Science and Higher Education of the Russian Federation (Agreement No. 075-15-2024-557 dated 25 April 2024).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. External view of the prototype of a remote detector of traces of explosives.
Figure 1. External view of the prototype of a remote detector of traces of explosives.
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Figure 2. (a) Applying a solution of explosive in acetone to the surface of aluminum foil. (b) The sample with traces of Composition-B deposited in the form of the abbreviation “C-B”.
Figure 2. (a) Applying a solution of explosive in acetone to the surface of aluminum foil. (b) The sample with traces of Composition-B deposited in the form of the abbreviation “C-B”.
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Figure 3. Scheme of experiments on remote visualization of traces of explosives.
Figure 3. Scheme of experiments on remote visualization of traces of explosives.
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Figure 4. Samples with traces of explosives: (a) TNT; (c) RDX; (e) HMX; (g) Composition-B; (i) Tetryl. The results of remote visualization of explosives traces on the aluminum foil: (b) TNT; (d) RDX; (f) HMX; (h) Composition-B; (j) Tetryl.
Figure 4. Samples with traces of explosives: (a) TNT; (c) RDX; (e) HMX; (g) Composition-B; (i) Tetryl. The results of remote visualization of explosives traces on the aluminum foil: (b) TNT; (d) RDX; (f) HMX; (h) Composition-B; (j) Tetryl.
Photonics 11 01065 g004aPhotonics 11 01065 g004b
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Figure 5. The spectrum of the γ(0, 2) absorption band of NO simulated in LIFBASE for different rotational temperatures Trot: (a) the entire band; (b) a fragment of the band corresponding to the P12 branch [33].
Figure 5. The spectrum of the γ(0, 2) absorption band of NO simulated in LIFBASE for different rotational temperatures Trot: (a) the entire band; (b) a fragment of the band corresponding to the P12 branch [33].
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Table 1. Laser parameters.
Table 1. Laser parameters.
ParameterValue
Radiation wavelength, nm247.867
Maximum pulse energy, mJ200
Line width, pm5
Pulse repetition rate, Hz1–100
Pulse duration (τ0.5), ns20
Beam divergence, mrad1
Output beam size, mm18 × 9
Table 2. Results of the analysis of experimental data on the visualization of explosive traces.
Table 2. Results of the analysis of experimental data on the visualization of explosive traces.
ExplosivesSymbolNumber of PixelsSignal/Symbol (Photocounts)Average Signal/
Pixel (Photocounts)		Symbol Area (cm2)Average Concentration/
Symbol (μg/cm2)
TNTT141007.116.81.1
N181347.421.6
T151046.918.0
RDXR3361218.539.60.6
D3257918.138.4
X2747317.532.4
HMXH201708.5240.7
M302217.436
X231948.427.6
Composition-BC-312618.437.20.5
B342878.440.8
TetrylT171458.520.40.9
E282217.933.6
T161207.519.2
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Bobrovnikov, S.; Gorlov, E.; Zharkov, V. Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives. Photonics 2024, 11, 1065. https://doi.org/10.3390/photonics11111065

AMA Style

Bobrovnikov S, Gorlov E, Zharkov V. Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives. Photonics. 2024; 11(11):1065. https://doi.org/10.3390/photonics11111065

Chicago/Turabian Style

Bobrovnikov, Sergei, Evgeny Gorlov, and Viktor Zharkov. 2024. "Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives" Photonics 11, no. 11: 1065. https://doi.org/10.3390/photonics11111065

APA Style

Bobrovnikov, S., Gorlov, E., & Zharkov, V. (2024). Remote Detection and Visualization of Surface Traces of Nitro-Group-Containing Explosives. Photonics, 11(11), 1065. https://doi.org/10.3390/photonics11111065

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