Hyperspectral imaging and remote trace detection of cis-1,3,4,6-tetranitrooctahydroimidazo-[4,5 d] imidazole (BCHMX) compared with traditional explosives using laser induced fluorescence

Hny S.Ayou ,Ashrf F.El-Sherif ,Ahmed Eleih

a Department of Physics,Faculty of Science,Cairo University,Egypt

b Laser Photonics Research Center,Engineering Physics Department,Military Technical College,Cairo,Egypt

c Explosives Department,Military Technical College,Kobry Elkobbah,Cairo,Egypt

Keywords: Hyperspectral imaging Remote trace detection BCHMX Laser induced fluorescence

ABSTRACT cis-1,3,4,6-Tetranitrooctahydroimidazo-[4,5 d] imidazole (BCHMX) is an advanced energetic compound that expected to spread worldwide in the near future.Since,no approved remote detection methods were reported in current literature for this material,we performed hyper-spectral imaging and laser induced fluorescence (LIF) to a BCHMX sample under low laser fluence for determining the optimum laser wavelength used in any future BCHMX-LIF based remote detection systems.For this purpose,an experimental setup consisted of a sun spectrum lamp and hyper-spectral camera was built to illuminate and image white powder samples of BCHMX in comparison with the traditional explosives,HMX(1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane),RDX (1,3,5-trinitro-1,3,5-triazacyclohexane),PETN (2,2-Bis[(nitroxy)methyl]propane-1,3-diyldinitrate).The imaging reveals strong BCHMX sample absorption contrast among other samples at wavelength ranging from 400 to 410 nm.When light source was replaced by a 405 nm laser diode illuminator,a strong BCHMX sample LIF at the spectral range from 425 to 700 nm was observed under low laser fluence condition of 0.1 mJ/cm2.Finally,we demonstrated successfully the ability of the 405 nm LIF and the hyperspectral imaging technique to detect finger print traces of BCHMX on white cellulose fabric from a distance of 15 m and a detection limit of 1 μg/cm2.? 2020 China Ordnance Society.Publishing services by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

The growth in the explosives researches is to find out high performance low sensitive explosives [1-3].Due to its superior performance,BCHMX (cis-1,3,4,6-Tetranitrooctahydroimidazo-[4,5 d] imidazole),is considered as one of the most advanced energetic materials [4,5].BCHMX was easily prepared by two step method for its synthesis [6].It has been studied as a plastic explosive for replacement of Composition C-4 [7].In addition,its application as energetic filler for different shaped charges was also studied [8,9].Sensitivity,stability and detonation properties were sufficiently presented in different publications [10-13].Its melt cast composition with TNT (2,4,6-Trinitrotoluene) was studied in comparison with Composition B(60%RDX/40%TNT by wt.)[14,15].The results of the performance and the thermal stability proved that BCHMX-TNT could be a candidate to replace composition B in different applications [14].A similar security measures update in airports and similar critical entities should take place by including remote trace detection facilities for this material [16-18].Several publications reported the different methods used for detection of high energy materials.As reported by Wang [19],X-ray Fluorescence can be used to identify and detect explosive materials.Gulia et al.discussed the importance of Raman spectroscopy to detect traces of explosive at distance reached 5 m[20].In addition,Klapec et al.summarized the different detection techniques and explosive residue characterization in his review manuscript [21].Unfortunately,no specific non-contact remote detection method approved for BCHMX has been reported in literatures.Hence,this study presents the efficiency of hyperspectral imaging and LIF in trace detection of BCHMX as non-destructive,non-contact remote detection technique.Moreover,the optimum excitation and detection LIF wavelength were observed and the trace detection limit of such technique was determined.In this context,hyperspectral imaging technology has various applications[22],it makes possible to perform spectrometry on remote target image,such that a cube hyperspectral image file format contains a cluster of photos[22],each one shows target luminance and irradiance features at narrow spectral bandwidth within 5 nm.Merging between hyperspectral imaging and laser target illumination allows remote detection of target fluorescence (LIF) and irradiance at low laser fluence condition to improve the detection of energetic materials residues and traces in addition to prevent bulk fragments from photodecomposition.The hyperspectral detection of trace energetic materials was studied in previous works [23].Also LIF of nitramines was treated by several publications using deep ultraviolet above photodecomposition level [24,25].But there is on information in literature about merging the two techniques for trace detection in the same case study.For the sake of comparison,It has been decided to perform our tests on BCHMX,which is a white powder material,and simultaneously on similar color traditional energetic materials namely,HMX (1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane) [26],PETN (penta-erythritol tetranitrate)[27] and RDX (1,3,5-trinitro-1,3,5-triazacyclohexane) [28].As shown in Fig.1,these materials are different in their molecular structure.BCHMX is bicyclic aliphatic nitramine molecule with four nitro groups,RDX and HMX are monocyclic aliphatic nitramine molecules with different nitro groups,while PETN is a nitrate ester explosive which is used mainly as explosive filler in the detonating cords and several plastic explosives.

One remarkable advantage of hyperspectral imaging and spectroscopy beside the low detection level,is the fact that there is no need for extensive sample preparation,such as dissolving the material in a proper solvent that may quench any relevant fluorescence[29,30].Furthermore,the hyperspectral camera can perform remote stand-off detection of trace and residues from distance ranges from 1 m up to 30 m depending on the used optical zooming system[16,18].In this work,we decided to identify the best laser wavelength suitable for inducing the BCHMX sample to fluorescence,first by illuminating the samples using broad spectrum halogen lamp,imaging these samples by hyperspectral camera and using comparative contrast visualization over different wavelength narrow band pass digital filter to detect the spectral absorption bands of the sample.The sample was illuminated by a low fluence laser beam which has a wavelength close to that of the BCHMX absorption and it was possible to induce sample fluorescence with a better yield and without driving the samples to photodecomposition.

2.Experimental samples preparation

PETN,HMX and RDX are products of Heliopolis Company,Cairo,Egypt.These explosives were recrystallized in our laboratories.BCHMX sample was prepared according to the method presented in patent [31],using a two stage synthetic process.Table 1 simplifies samples physical properties.

Table 1 Physical properties of the samples.

1 g of each sample was placed in a clear glass cuvette with high optical transmission grade.As shown in Fig.2,the four sample cuvettes were mounted next to each other to a holder with a highly reflective white Lambertian background material,made of polytetrafluoroethytene (PTFE),to enhance samples-background contrast differentiation.

Fig.1.BCHMX molecular structure compared to RDX,HMX and PETN.

Fig.2.Experimental samples,(a) Holder montage illustration (b) Samples visual appearance.

3.Experimental work

Experimental tests on the prepared samples were performed on three steps,the first was to identify the absorption wavelengths of BCHMX over the visible-near infrared spectrum (in comparison with other samples) using hyperspectral imaging and wide spectrum illumination from a minimum distance of only 1 m.The second step was illuminating the samples with a laser beam,having a wavelength that lies in the absorption bandwidth,and to detect any yielding LIF using hyperspectral imaging from the same distance as in the first step.The last step was testing of the detection possibility of BCHMX finger print trace,using hyperspectral camera with optical zooming system,and to find out the sensitivity of the technique as well as its detection limit.

4.Results and discussion

4.1.Identification of BCHMX characteristic absorption

The setup of this test consists of a hyperspectral camera type SOC710,with resolution of 696 lines per cube at 520 pixels per line,equipped with Scheneider-Kreuznach (1.9/35) CCTV lens,interfaced to windows XP (service pack 2) compatible SOC710 Acquisition Software,hyperspectral analysis toolkit v3.0 software for cube image analysis as shown in Fig.3.

The hyperspectral camera was placed 1 m apart from the samples.A halogen lamp illuminator type Newport 780 was placed oblique to the sample to avoid camera saturation with direct reflections.The lamp temperature was adjusted to 3200 K to provide a broad spectrum illumination in the range from 200 nm to 3.5 μm.The diaphragm of the lamp head was controlled to change sample irradiance.We performed hyperspectral imaging for the samples subjected to 50 lux of light intensity as measured by luxmetre.

As shown in Fig.4,the analysis of resultant cube image reveals strong absorption of BCHMX sample in the spectral range from 400 to 410 nm,but no fluorescence was seen due to strong samples irradiance under broad spectrum lighting.Unlike BCHMX,other samples didn’t show any absorption at this wavelength band as shown in Fig.5,neither in the rest of the imaging spectral range of the camera that is limited between 375 nm and 1050 nm.As a result of this finding,we decided to proceed in LIF testing using a laser source with wavelength of 405 nm which is the median of the characteristic absorption spectrum of BCHMX sample.

4.2.Identification of BCHMX characteristic LIF

Fig.3.Experimental setup to test visual contrast differentiation of BCHMX under broad spectrum illumination,(a) setup illustration (b) experimental setup.

Fig.4.Selected frames from the hyperspectral cube image of samples under broad spectrum illumination.

Fig.5.Visual dark contrast differentiation of BCHMX sample using broad spectrum halogen lamp illumination at hyperspectral filter in the range from 400 to 410 nm.

Fig.6.Experimental setup to test visual contrast differentiation of BCHMX using 405 nm LIF,(a) setup illustration (b) experimental setup.

In this test,the halogen lamp has been replaced by 10 W,405 nm laser diode module manufactured by Changchun New Industries Optoelectronics Tech Co.Ltd.A 10x Thorlabs BE10M-A beam expander was placed in front of the laser diode to control the divergence of the laser beam as shown in Fig.6.The laser fluence was adjusted at the samples side such that it does not exceed 0.1 mJ/cm2,which 1/500 of the photodecomposition threshold of nitramines.For this purpose we used energy meter type Gentec-EO.We performed hyperspectral imaging for the samples under laser irradiance.

Fig.7.Selected frames from the hyperspectral cube image of samples performing 405 nm LIF.

By analyzing the resultant cube image as shown in Fig.7,BCHMX sample reveals strong absorption for the 405 nm laser radiations(Fig.8a),with strong fluorescence starting from 425 nm,peaking at 580 nm and ending at 700 nm.No fluorescence zone was detected at the wavelength range from 410 nm to near 425 nm(Fig.8b).As expected,the other samples did not show any significant fluorescence (Fig.8c).

As a result of the previous observation,the third step was proceeded to investigate the capability of hyperspectral camera to perform BCHMX trace stand-of detection.

4.3.BCHMX remote trace detection

To perform remote detection of BCHMX traces,the hyperspectral camera and the 405 nm laser projection system are aligned in front of 5 × 7 cm white cellulose fabric,at variable distances ranging from 1 to 20 m as shown in Fig.9a.1 μg BCHMX was deposed on the cellulose fabric using fingertip to create a white finger print trace that is invisible relative to the hosting surface(only a minimum of 1 μg BCHMX powder was sufficient for mechanical deposition by fingertip to form a detectable hyperspectral image fingerprint trace).We replaced the 35 mm hyperspectral camera zoom lens with a 117 mm Fujinon D16×7.3B-S41 TV-Z lens and equipped with a Fujion remote control box type CRD-2A zoom control system (Fig.9b).As recommended by manufacturer,the hyperspectral camera was subject to three steps standard calibration procedure including dark level correction,spectral mapping and pixels gain uniformity correction(The calibration procedure is explained in details in the SOC-710 and HSAnalysis2XL user’s manual).We adjusted the laser fluence at each hyperspectral shot at every imaging distance using the same method as in the second test.We perform imaging in low lux ambient conditions ranging from 0.01 lux to 1 lux at daylight illumination (Fig.10a).

After several imaging trials with variable optical zoom (at constant pixels number),wherein we have changed the distance to the target from 1 m to 20 m,we succeeded in detecting the BCHMX trace finger print,at a maximum distance of 15 m with good contrast at mesopic illumination level(0.001-1 m-2).We enhanced the image contrast using DADiSP digital image processing software version 4.1,by superimposing the irradiance image (Fig.10b) and the fluorescence image (Fig.10c),then performing edge detection on the superimposed image (Fig.10d).At distances greater than 15 m,the contrast of the trace was visible,but not as sharp as required to perform digital enhancement.

By comparing the results of all the studied samples,it is obvious that BCHMX has high LIF intensity at different wavelengths as shown in Fig.11.In addition,RDX and HMX have very close LIF intensities in comparison with each other.The reason of this difference is the molecular structure of the studied samples which leads to different LIF behavior.The stretching modes of NO2lead to the characteristic absorption frequencies which are regular to the organic explosives containing nitro-group.Each sample has variable bond lengths and vibrancy states.According to Refs.[24],RDX and HMX has no conjugated π orbital system,even the π orbitals of the nitro group are so close in energy to those of σ,n orbitals in the aliphatic nitraimine molecules,so that no photoluminescence can be observed before photodecomposition [33].In the case of PETN,the situation is different because the aliphatic nitrate ester molecule has a strong π π* transition state and a characteristicn,π*triplet level that may cause phosphorescence rather than fluorescence.BCHMX molecule showed a strong fluorescence in the visible band because it possess both conjugated π orbital system and σ,n*orbital transition.The presence of carbon-carbon bond in common position between the two cycles might lead to hyperspectral differentiation of BCHMX among other nitramines in analogy with the case of slight and extensive aromatic systems of nitroarene [33].Also Klasovity et al.studied the NMR spectra of BCHMX and proved that its molecule is isochronous and has dihedral inter-nuclear angle in its molecule [6].BCHMX has the longest nitrogennitrogen bond length compared with the other nitramines which caused increasing of its sensitivity to the limit of PETN[34,35].Also BCHMX has non-binding interatomic distances of oxygen atoms in all of the nitro groups in its molecule which are shorter than those corresponding to the intermolecular contact radii for oxygen in carbonyl or other nitro groups [6].According to the mentioned data,it was predicted that BCHMX could have different LIF spectra compared with the studied nitramines.On the other side,BCHMX has heat of detonation and impact sensitivity very close to PETN[34]which might be the reason of some similarity on the behavior of both of them.

Fig.8.Visual contrast differentiation of BCHMX sample using 405 nm LIF with different filters,(a) BCHMX sample appears dark at 405 nn filter due to absorbtion (b) all samples appears dark at 410-425 nm filter,(c) BCHMX sample appears of bright contrast due to fluorescence at 425-700 nm filter.

Fig.9.Experimental setup to test visual contrast differentiation of BCHMX using 405 nm LIF,(a) setup illustration (b) experimental setup.

Fig.10.Hyperspectral visualization of BCHMX finger print trace on a white cellulose fabric using 405 nm LIF from a distance of 15 m,(a) visual image (b) 405 nm hyperspectral irradiance filter (c) 580 nm hyperspectral fluorescence filter (d) Digitally enhaced edge detected irradiance-fluorescence superimposed image.

Fig.11.405 nm LIF spectra of the experimental samples as acquired from the hyperspectral cube images.

5.Conclusion

BCHMX is a new energetic material belongs to the nitramines family.It has several distinct spectroscopic characteristics other than traditional monocyclic aliphatic nitramines similar to HMX and RDX,but it shows some relevance to the behavior of PETN as non-cyclic aliphatic nitrate ester.Remote detection of BCHMX trace was successfully achieved using a novel method based on hyperspectral imaging and LIF techniques.BCHMX traces subjected to 405 nm laser illumination showed a strong photoemission in the spectral range from 425 nm to 700 nm.The trace detection limit of BCHMX using this technique was found to be 1 μg/cm2at laser fluence of 0.1 mJ/cm2and a distance of 15 m we expected that the detection limit of BCHMX may decrease according to hyperspectral camera sensitivity and optical collimation system specifications.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The authors are very grateful to the members of Physics Department,Faculty of Science,Cairo University and Laser Research Technology Center,Engineering Physics Department,Military Technical College,Cairo,Egypt for their encouragement,support and helpful suggestions.