Concept of a Combined Device for Localization and Identification of Explosives

Transcription

1 Concept of a Combined Device for Localization and Identification of Explosives A.V. Kuznetsov a, V.P. Averianov a, A.V. Evsenin a, I.Yu. Gorshkov a, O.I. Osetrov a, and D.N. Vakhtin a a V.G. Khlopin Radium Institute, 28, 2 nd Murinsky pr., , Saint-Petersburg, Russia Speed and reliability of detection of explosive substances are contradicting requirements, which cannot be met within the framework of a single approach. Modern methods of ES detection are based on using a minimum of two complementary methods: a fast method of localization of suspicious anomalies, and a slower method of identification of these anomalies. Recent work at Radium Institute has led to formulation of a new approach to the detection of explosive substances. Such approach will be briefly illustrate here. 1. Introduction Localization of suspicious anomalies is done by the analysis of scattered ultra-high frequency electromagnetic waves (microwaves), which provides a 3D image of the internal structure of the inspected volume, and can categorize the objects contained in this volume both by their shape and by their dielectric characteristics. These anomalies are then identified by Nanosecond Neutron Analysis/Associated Particle Technique (NNA/APT), which uses a portable neutron generator with built-in detector of associated alphaparticles, and one or several gamma-ray detectors. 2. Localization of Anomalies by a Microwave Sensor 2.1. Detection of Buried Explosive Substances The microwave sensor (Modulated Continuous- Wave Radar) is based on continuous electromagnetic microwave radiation in the range 2 8 GHz. It has some important advantages over conventional pulsed radars: it is easy to achieve a broad band of frequencies with fixed borders, which translates into superior spatial resolution; it is possible to adjust the amplitudefrequency and phase-frequency characteristics off-line; Figure 1: Prototype modulated continuous-waver radar during laboratory tests. Technical characteristics of the existing prototype are listed in Table reftab:one. requirements concerning emitting and receiving channels and degree of decoupling of antennae are much less strict. Unlike radars that use pulsed broadband radiation, continuous radiation with stepped frequency change allows one to measure properties of the scattered field with pinpoint accuracy, which is needed for image reconstruction. Besides, the method also provides data about dielectric characteristics of the found object, and thus allows preliminary identification of the anomaly. 1

2 2 A.V. Kuznetsov for dry sand with humidity 0.1% by weight 50 cm; for wet sand with humidity 16% by weight 5 cm; for concrete 20 cm. The maximum scanning speed is 20 cm/s. Figure 2 shows results of scanning over a Teflon cylinder (dielectric constant ε = 2.1, height 40 mm, diameter 70 mm) located under 40 mm of sand with humidity 10% by weight. The surface of the sand was made rough with typical roughness size of 2 3 cm. Horizontal axis (X) corresponds to the direction of scanning; vertical axis (Y) corresponds to depth. Top part of Fig. 2 shows the original on-line image. Double horizontal line at about Y = 7 m is the image of the rough surface of the sand. Bottom part of Fig. 2 shows the result of processing the raw data, which yields the transversal dimension of the hidden object and its electrical length (product of its length and square root of dielectric constant). Presence of both front and back surfaces of the hidden object on the image tells that it is a dielectric. Table 1: Technical characteristics of the existing prototype Figure 2: Top: image obtained on-line during the scan. Bottom: result of the analysis. The prototype portable microwave sensor created at Radium Institute for detection of explosive substances in homogeneous environments is shown in Fig. 1, the main characteristics are listed in Table 1. Spatial resolution of the prototype is 5 cm in the scanning plane, and 2.5 cm in-depth. Maximum depth of scanning a homogeneous medium is: Parameter Value Sweeping range 2 8 GHz Minimal frequency changing step 1.5 MHz Minimal time of measurement of a single cycle at frequency changing step 200 MHz 100 ms Power of the emitted signal 1 mw Sensitivity of the receiver 120 db/w Dynamical range 50 db Relative measurement error within one sweeping cycle 0.1 db Spatial resolution in air in-depth 2.5 cm longitudinal 5 cm transversal 4 cm

3 Concept of a Combined Device for Localization and Identification of Explosives 3 Figure 3: The inspected suitcase. Figure 4: Distribution of transmission coefficient (red - transmission 1, blue - 0) Luggage Inspection All methods used for detection of explosive substances (ES) in luggage are based on indirect detection. If one deals with non-industrial ( improvised ) explosive substances the situation becomes very complicated, since a large variety of substances and mixtures can be used in homemade explosives. Another problem is that, due to a huge number of luggage to be inspected, the time for each individual analysis has to be very short. Also, wide availability of information about characteristics of inspection systems makes it easy to mask explosives to prevent their detection. Masking can be done by hermetically packing ES, changing their density and effective charge by using mixtures, using non-nitrogenous ES, etc. To increase the probability of detection of ES new detection methods should be developed on new principles. We have developed a new concept of luggage inspection system based on a microwave sensor. This system takes into account properties of interaction between microwave radiation and substance, and makes possible: To obtain a 3D image of concealed objects; automatic classification of the found objects as conductor/dielectric; automatic determination of the dielectric constant of the material of the object; automatic determination of the equivalent mass of the detected object. The system is intended for inspection of passengers luggage moving on the conveyor belt at about 50 cm/s. It can be either used independently as an inexpensive sensor, or as a complimentary device to an x-ray and/or QR system. The principles behind the system are: irradiation of the inspected luggage with microwaves; detection of the scattered and transmitted electromagnetic field; image reconstruction; image analysis. The device consists of several (from 2 to 4) linear antenna arrays arranged in an arch around the conveyor belt. While the luggage is moving at constant speed on the belt, the system detects EM field scattered by the luggage in a broad band of frequencies. This information is then digitized, analyzed, and a 3D image is produced. Such a scheme was implemented in a device working in the frequency range 2 8 GHz. In the horizontal direction the aperture was synthesized by moving the inspected luggage; in the vertical direction by moving the transmitting antennae. Figure 3 shows an example of the inspected suitcase filled with various common items, and containing an imitator of plastic explosive (wax), which has the value of the dielectric constant close to that of some of the explosives. The measured

4 4 A.V. Kuznetsov Figure 5: Distribution of the thickness of dielectrics inside the suitcase (mm), assuming ε = 3. metal) in a suitcase, and estimate their thickness in the equivalent thickness of the explosive. As a result, one can automatically estimate the threat level associated with the suitcase. Measurements of the scattered field allows one to build a 3D image of the objects inside the suitcase, and estimate their dielectric constant and thickness. This may help a conventional x-ray system to make a transition from a surface density to the real volume density of the concealed objects. If the dielectric is shielded by a metallic foil, which can in some cases (like QR) be used to mask explosives, the method can automatically: determine the presence of the foil; if the dielectric is shielded from all sides: determine the shape of the object (including its dimensions in the direction, normal to the projection plane in x-ray systems) if the dielectric is shielded from one side: to determine electrical length and equivalent thickness of the explosive, which can be used in image analysis by x-ray systems. Figure 6: One of the cross-sections of the suitcase (at the bottom of the picture is a metallic bar, on the top - photo camera, in the center - a battery.) characteristics of the EM field allow one to obtain the following information: distribution of the transmission coefficient across the suitcase (see Fig. 4, which shows transmission coefficient with resolution 4 cm 4 cm); distribution of the thickness of dielectrics inside the suitcase evaluated in millimeters of equivalent thickness of TNT (see Fig. 5, resolution 4 4 cm 2 ); two-dimensional cross-sections of the suitcase (see Fig. 6, resolution 2 2 cm 2 ); 2.3. Summary of Microwave Results Measurements of the transmission coefficient allows one to detect dielectrics (unshielded by 3. Identification of Anomalies using NNA/APT Method 3.1. Nanosecond Neutron Analysis/Associated Particle Technique Identification of the discovered anomaly is done by irradiating it with fast neutrons and detecting secondary gamma-rays produced by neutrons in the material of the anomaly. Detection is done within very narrow (nanosecond) time intervals counted from the moment of detection of an alpha-particle, which accompanies neutron emission in the d+t reaction. Position-sensitive detector of accompanying alpha-particles provides spatial resolution of the device along the scanning plane, while nanosecond timing provides spatial resolution in-depth (see Fig. 7). Thus, NNA/APT provides a three-dimensional elemental image of the inspected volume, which can be used to identify the constituent substances. Compared to the existing methods, NNA/APT allows significant (in some cases more than by two orders of magnitude) suppression of the background in measured gamma-spectra, and thus leads to a

5 Concept of a Combined Device for Localization and Identification of Explosives 5 Figure 7: Principle of getting position information in NNA/APT. drastic reduction of identification time Portable Device for Detection of Explosives: SENNA At the heart of the portable NNA/APT device is a neutron generator with built-in detector of associated alpha-particles. Semiconductor detectors were chosen for this role, and several types of segmented detectors were built and installed into portable neutron generators produced by VNIIA, Moscow. Several prototypes of a portable devices for explosives detection (SENNA) based on VNIIA s NG-27 neutron generators were built. The latest SENNA version #3 is shown in Fig. 8. It has two BGO-based gamma-detectors, digital spectrometer and DAQ, and is located in a single suitcase. The fast digital spectrometer and DAQ allow one to determine the detection time of each gamma-quantum relative to the detection time of an alpha-particle in a 9-segment alpha-detector with accuracy < 2 ns, which is equal to a position resolution (in-depth) of about 10 cm. Spectrometer and DAQ work at counting rates over s 1 per gamma-detector without degradation of the spectrum quality. They also allow one to increase the number of gamma-detectors almost Figure 8: SENNA version #3 prototype (NG-27, two gamma-detectors, digital spectrometer, battery power, with remote control). without limitations in order to reduce the detection time. The mass and dimensions of SENNA version #3 are: 30 kg and cm 3. Figure 9 shows an example of spectra obtained with SENNA during identification of chemical imitator DLM2 1 of TNT buried under 2 cm of soil. Another series of tests was performed with a 300 g TNT imitator placed inside a suitcase, filled with common items (see top part of Fig. 10). Even with such a complicated filling, the automatic identification program succeeded in detection of ES by correlating concentrations of main chemical elements in different sectors of suitcase (see bottom part of Fig. 10) Summary of the NNA/APT Results Different possibilities of using NNA/APT method for detection and identification of explosives have been studied. Portable sensor SENNA based on NNA/APT was built. It allows one to localize and identify different kinds of explo- 1 DLM2 was prepared according to the recipe proposed by George Vourvopoulos, and provided by Frank Brooks (University of Capetown) and Ulf Rosengard (IAEA).

6 6 A.V. Kuznetsov Figure 9: Energy spectrum of gamma-rays from TNT imitator (DLM2): 100 g of TNT imitator g plastic box) and its decomposition into contributions from C, N, and O. sives. SENNA can be expanded to include more gamma-detectors to allow inspection of larger volumes, including passengers luggage and containers. Acknowledgments This work was funded from the following sources: ISTC project #1050, CRDF project RP2-564, IAEA contract # 10986, and APSTEC Ltd. (Saint-Petersburg, Russia). REFERENCES 1. Advanced Technology for Contraband Detection, Compiled and Edited by Tsahi Gozani, SAIC, March [2] Associated particle imaging (API), Report of Bechtel Nevada (BN) Special Technologies Laboratory (STL), USA, DOE/NV , UC-700, May A.V. Kuznetsov, et al., Portable multisensor for detection and identification of explosive substances. Proc. of the International Conference EUDEM2-SCOT: Requirements and Technologies for Demining and the Removal/ Neutralization of Unexploded Ordnance, September, 2003, Vrije Uni- Figure 10: Top: suitcase containing cotton, wool, books, soap, CDs, alcohol, and a 300 g TNT imitator (under soap). Bottom: results of the analysis of 9 areas of the suitcase corresponding to 9 segments of the associated particle alpha-detector. An explosive was detected in the central area with probability 100% (statistical estimation). versiteit Brussel, Bruxelles, Belgium..Sahli, A.M. Bottoms, J.Cornelis (Eds.), pp D.N. Vakhtin et al., Decision-taking procedure for explosives detection by nuclear technique // H.Schubert, A. Kuznetsov (eds.), Proc. of the NATO ARW # Detection of explosives and land mines: methods and field experience, St.-Petersburg, Russia,

7 Concept of a Combined Device for Localization and Identification of Explosives September 2001, Kluwer Academic Publishers, pp (2002). 5. A.V. Kuznetsov, A.V. Evsenin, I.Yu. Gorshkov, O.I. Osetrov, D.N. Vakhtin, Detection of buried explosives using portable neutron sources with nanosecond timing, Applied Radiation and Isotopes, Vol. 61, Issue 1, pp (2004). 6. A.V. Evsenin, A.V. Kuznetsov, O.I. Osetrov, D.N. Vakhtin Detection of hidden explosives by nanosecond neutron analysis technique // H. Schubert, A. Kuznetsov (eds.), Proc. of the NATO ARW # Detection of bulk explosives: advanced techniques against terrorism, St.-Petersburg, Russia, June Kluwer Academic Publishers, pp (2004). 7. V.P. Averianov, I.Yu. Gorshkov, A.V. Kuznetsov, A.S. Vishnevetski, Detection of explosives using continuous microwaves, Proc. of the NATO ARW # Detection of bulk explosives: advanced techniques against terrorism, St.-Petersburg, Russia, Kluwer Academic Publishers, NATO Science Series, Series II: Mathematics, Physics and Chemistry - Vol.138, 2004, pp V.P. Averianov, et al., Microwave system for inspection of luggage and people - detection of explosives on human body. Proc. of the NATO ARW Detection and Disposal of Improvised Explosives, St.-Petersburg, Russia, 7-9 September 2005, H. Schubert, A. Kuznetsov (eds.), Springer, p.43 (2006) 9. D.N. Vakhtin, et al., SENNA - portable sensor for explosives detection based on Nanosecond Neutron Analysis, ibid., p.87