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Detection and classification of liquid explosives using NMR
Abstract
In this work, we present a novel method for non-invasive identification of liquids, for instance to allow for the detection of liquid explosives at airports or border controls. The approach is based on a nuclear magnetic resonance technique with an inhomogeneous magnetic field, forming estimates of the liquid's spin-spin relaxation time, T2, and diffusion constant, D, thereby allowing for a unique classification of the liquid. The proposed detectors are evaluated using both simulated and measured data sets.
... A further development of the detector was made by the Aachen group, where a permanent Halbach magnet and a surface coil were used instead [10]. A theoretical analysis of signal processing of CPMG decays to extract (T 2 , D) pair for liquid discrimination was explored in [7]. In our approach, we have constructed a detector based on a permanent magnet that enables us to measure T 1 , T 2 and D in the same configuration without applying additional gradients -the inhomogeneity of the magnetic field suffices. ...
... All the samples were measured at the room temperature. Diffusion coefficients measured match the values from [7] within the experimental error. In our experimental setup (huge field gradient), for samples with longer T 2 (a couple of seconds), diffusion becomes the prevailing mechanism of echo amplitude decay even for the shortest possible values of τ . ...
... Since the sample is big enough to stretch over both coils, both measurements could in principle be performed without moving the sample. While T 2 and D measurements are fast (using appropriate numerical methods two scans with different τ are effectively sufficient [7] ), T 1 measurements are more time-consuming. Using the IR sequence, one needs to wait several T 1 -times for each partial measurement. ...
Liquid explosives pose a threat to security on airplanes and other public places, since they can easily be concealed as benign
liquids. A detector, able to quickly identify liquids, would increase the possibility to detect such threats and speed up
security checks. As a step towards a long-term goal to develop a liquid explosive detector, we have constructed an experimental
setup based on a low-cost 1.1T permanent magnet with huge static magnetic field gradient of 4.8T/m, which allows us to measure
proton relaxation times T
1 and T
2 and the self-diffusion coefficient D in liquid samples in a thin slice excited by radio-frequency pulses. We have developed a simple model in order to explain
diffusion-enhanced non-exponential magnetization recovery in inversion recovery T
1 experiment in this setup. Measuring a wide variety of liquid samples, we have demonstrated that it is possible to discriminate
between the liquids based solely on these parameters. We discuss further improvements to the detection method, among those
the choice of magnetic field, based on the fast field-cycling measurements.
... Similarly, Nuclear Magnetic Resonance (NMR) method was used to analyze unknown liquids together with infrared (IR) spectroscopy and classical chemical color tests in [5]. As given in [6].NMR method was also used to detect and classify liquid explosives. On the other hand, in [7] Raman spectroscopy method was proposed for non-contact detection of hazardous liquids stored in glass and plastic containers. ...
... According to the algorithm, the limit of the optimum hyperplane must be maximum. In this case, finding the most suitable hyperplane is possible with the solution of the limited optimization problem given in Eq. (6). Constraints due to this are as shown in Eq. (7) [24]. ...
Microwave spectroscopy method has become widespread in many applications including liquid classification. In this study, a microwave spectroscopy system that can classify liquids without opening the lid of their containers is proposed. Thus, the operators are prevented from being exposed to harmful substances and wasting time. Everyday liquids such as carbonated drinks, fruit juices, shampoo, cream and alcoholic beverages and hazardous liquids were characterized remotely by the microwave spectroscopy method in which spectroscopic signatures of a total of 52 liquids were used. In order to be able to classify liquids with the highest accuracy, it is also important to determine the most suitable measurement system as well as the correct selection of the classification algorithm and algorithm parameters that show the best performance. In this study, Support Vector Machines algorithm, which is a very successful algorithm in separating binary classes, is used. In addition, the effects of the algorithm on the classification performance have been examined using different kernel functions and cross-validation technique has been used for the performance analysis. As a result of the performance analysis, it is seen that up to 100% success can be achieved when linear or polynomial kernel functions have been preferred.
... The experimental results show that the use of three parameters allows identifying constituents of a liquid. Some authors proposed the use of two parameters to distinguish between substances: using relaxation times fT 1 , T 2 g [15] and using the transverse relaxation time and the molecular diffusion coefficient fT 2 , Dg [16]. However, the use of only two parameters makes the method less reliable. ...
In this paper we study the method of distinguishing the substances by measuring their nuclear paramagnetic longitudinal and transverse relaxation times and the diffusion coefficient of molecules. Experiments performed using a commercial high magnetic field NMR spectrometer show the possibility to use this method for reliable identification of liquids. Observables in these experiments rather often cannot be described by a single exponential function. In the article we discuss how to utilize the non-single exponential experimental dependences for a quantitative processing of the NMR experimental results.
... Classic NMR spectroscopy based on chemical shift is challenging for a security screening situation, because the magnetic fields generally have to be very high (> 1 T) and uniform (ppm) which is challenging to achieve in the presence of bottles that might be of any material or shape. However, numerous NMR-based methods relying on one or more MR relaxation parameters [1], [12], or NMR relaxation parameters with diffusion [13] have proven effective at distinguishing explosive from benign liquids in laboratory tests. Because of the complexity of the liquid detection problem, NMR methods continue to be explored as a promising approach for bottled liquid scanning. ...
Field Forensics, Inc. (FFI) has built a bottled liquids scanner utilizing ultra-low field NMR relaxometry. This device, called MagViz, is based upon a prototype developed at the Los Alamos National Laboratory (LANL) (Espy et al. Appl Supercond IEEE Trans 21(3): 530, 2011; Espy et al. Supercond Sci Technol 23:034023. doi:10.1088/0953-2048/23/3/034023, 2010) [1, 2]. Despite using conventional Faraday detection coils in lieu of SQUIDs, MagViz, has demonstrated sufficient sensitivity to identify a number of threat liquids of interest to the Department of Homeland Security (Matlashov et al. Appl Supercond IEEE Trans 21(3):465-468, 2011) [3]. By accurate measurement of T-1 and T-2, liquids contained in opaque bottles and even non-ferromagnetic metal containers can be reliably identified. Protons are aligned using a 50 mT pre-polarizing field. T-1 is determined in the pre-polarizing field, and T-2 relaxation time is typically measured at 2,048 Hz in a 48 mu T field. The coil assembly is contained within a table-top 0.79 m tall magnetically shielded enclosure. Although primarily intended for commercial and security applications, MagViz, works at Larmor frequencies that correspond to timescales that are characteristic of a host of interesting, slow, molecular dynamic processes like diffusion and intramolecular motion as well as biological processes such as protein folding, catalysis, and ligand binding and could conceivably serve as a COTS research instrument for fundamental studies in these areas.
... The proposed detectors are evaluated using both simulated and measured data sets. Chapter 7 is based on [52]: ...
The high volume and minimal screening of sea land cargo containers presents a vulnerability in which explosive devices may be smuggled across national borders. Fast neutrons are a strong candidate for use in container screening due to their high target penetration and ability to discriminate between materials of low atomic mass, such as explosives and non metallic container contents. An algorithm has been developed that uses flags, calculated from specific measurements of the reflected neutrons and photons produced during active neutron interrogation, to discern explosives hidden in cargo containers. Steps in algorithm development included Monte Carlo simulations for scatter characterization, identification of flags in idealized scenarios, refinement of flags in realistic scenarios, combining the flags into a detection algorithm, and evaluation of the algorithm and associated detection system. Simulations compared favorably with small scale neutron scatter measurements using the explosives surrogate, melamine. The detection algorithm included corrections for different types of cargo contents and cargo inhomogeneity, surrounding environment, and realistic neutron sources and radiation detectors. The proposed algorithm has two variations, one of which can be easily implemented with today???s technology. The proposed scanning system utilizes a shielded 14.1 MeV neutron generator, eleven large liquid scintillators neutron detectors, and several inorganic scintillators for photon spectroscopy. This system should cost less than $1M to install and dose estimates fall well within acceptable levels for both operators and smuggled persons. Algorithm performance has been quantified with various explosive sizes and positions, as well as heterogeneous cargo configurations, with typical minimum detectable amounts not exceeding 200 kg.
Sealed bottles of any opacity are automatically inspected for hazardous liquids in seconds using a novel Nuclear Magnetic Resonance method. Liquid explosives and explosive precursors such as hydrogen peroxide are detected using a multi-element Nuclear Magnetic Resonance protocol, solving the shortcomings of optical techniques. The Bottled Liquid Scanner is capable of inspecting multiple bottles in a single scan with unprecedented low false alarm rates, ensuring minimal or no disruption at security checkpoints.
Nuclear Quadrupole Resonance (NQR) and Nuclear Magnetic Resonance (NMR) are very prospective methods of the
bulk detection of explosives and illicit substances. Both methods are based on use of apparatus, which are very similar
technically and in some cases could be applied simultaneously. We report our experimental works on NQR/NMR
techniques for explosives detection. In addition of classical single-frequency NMR/NQR we also explored a potential of
double resonance (NMR/NQR) and multifrequency NQR approaches as well as magnetic resonance imaging (MRI)
techniques. Multifrequency (two/three) NQR technique involves various (two or three) transitions in the three energy
level system of 14N nuclei. It is shown that this kind of NQR technique allows filtering spurious signal after
radiofrequency pulses and increases the sensitivity of NQR detection. On the other hand, various liquids can be detected
using NMR. We shown that reliable discrimination among extended set of liquids reveal a need in use of additional
NMR parameters or complimentary techniques. It is demonstrated that MRI is also feasible method for detection of
explosive/illicit liquids.
A bottle screening method to detect smuggling drugs was developed based on a series of rapid and automated magnetic resonance measurements. Liquid alteration is detected by evaluating chemical and physical properties of the liquid, scanning the bottle with a compact, non-contact magnetic resonance probe. Chemical and molecular diffusion changes are detected by means of the magnetic resonance relaxation time. The inspection device automatically detects the presence of concealed powders dissolved in liquids in a second timescale. The non-ionizing, non-contact method effectively screens bottles, regardless of their shape, size, and color. The proposed system configuration does not require a large magnet generating uniform magnetic fields; rather it inspects the liquid using a compact probe positioned on the wall of the bottle. For a variety of samples tested, the screening does not require detailed information on the expected content of the bottle.
High-resolution nuclear magnetic resonance (NMR) provides detailed spectral information on the molecular level. As such, it has been a principal tool of chemists since the early 1950s and used to probe the molecular structure, configuration and composition of liquids. However, high-resolution NMR techniques require very homogeneous DC and RF magnetic fields and are thus inappropriate as the basis for a practical liquid explosives screening system. These field requirements are relaxed for low-resolution NMR, but the unique spectral features of the high-resolution NMR signal are obscured. In spite of this, low-resolution NMR does provide substantial chemical information regarding liquids. Specific parameters available from low- resolution NMR include the signal amplitude (A0), the spin-lattice relaxation time (T1), the spin-spin relaxation time (T2), the diffusion constant (D), and the spin-spin coupling constant (J). Sequences of RF pulses can be designed to respond to one or more of these parameters and, therefore, unique NMR signatures for various liquids can be defined. General considerations of the relative importance of each of these parameters, along with practical considerations regarding allowable scanning and data processing times, will in large part determine the nature of signal processing methods to be used in the NMR Liquid Explosives Screening System.
Nuclear resonance techniques involving free precession are examined, and, in particular, a convenient variation of Hahn's spin-echo method is described. This variation employs a combination of pulses of different intensity or duration ("90-degree" and "180-degree" pulses). Measurements of the transverse relaxation time ${T}_{2}$ in fluids are often severely compromised by molecular diffusion. Hahn's analysis of the effect of diffusion is reformulated and extended, and a new scheme for measuring ${T}_{2}$ is described which, as predicted by the extended theory, largely circumvents the diffusion effect. On the other hand, the free precession technique, applied in a different way, permits a direct measurement of the molecular self-diffusion constant in suitable fluids. A measurement of the self-diffusion constant of water at 25\ifmmode^\circ\else\textdegree\fi{}C is described which yields $D=2.5(\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{$-${}5}$ ${\mathrm{cm}}^{2}$/sec, in good agreement with previous determinations. An analysis of the effect of convection on free precession is also given. A null method for measuring the longitudinal relaxation time ${T}_{1}$, based on the unequal-pulse technique, is described.
A spin echo method adapted to the measurement of long nuclear relaxation times (T 2 ) in liquids is described. The pulse sequence is identical to the one proposed by Carr and Purcell, but the rf of the successive pulses is coherent, and a phase shift of 90° is introduced in the first pulse. Very long T 2 values can be measured without appreciable effect of diffusion.
The detection of explosives and illicit material for the purposes of aviation security is an important area for preventing terrorism and smuggling. A number of different methods of explosive detection have been developed in the past that can detect such material from a very small up to a very large quantity. For the purposes of aviation security, the checks are performed on passengers, their carry on luggage, checked baggage, and cargo containers. Similar technology is used in post-offices for detecting dangerous substances in mail. In this paper we review some of these technologies and in particular discuss the application of computers for the analysis of data and images generated from security equipment.
This paper is a tutorial which describes "main stream" sonar digital signal processing functions along with the associated implementation considerations. The attempt is to promote further cross-fertilization of ideas among digital signal processing applications in sonar, radar, speech, communications, seismology, and other related fields.