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Landmine detection: the problem and the challenge
Applied Radiation and Isotopes 53 (2000) 557±563
www.elsevier.com/locate/apradiso
Landmine detection: the problem and the challenge E.M.A. Hussein*, E.J. Waller Laboratory for Threat Material Detection, Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3
Abstract This paper explores the role of radiation methods in addressing the problem of detecting landmines. The application of neutron activation analysis, with an isotopic source or a pulsed neutron generator, is discussed. The use of neutron moderation as an indicator of the presence of a landmine is also explored. In addition, information provided by measuring scattered photons (gamma- and X-rays) is examined. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Landmines; Explosives; Radiation methods
1. Introduction Anti-tank and anti-personnel mines come in all shapes and sizes, and can be encased in metal, plastic, wood or nothing at all. Their fusing mechanism varies from simple pressure triggers to trip wires, tilt rods, acoustic and seismic fuses, or even light- or magneticin¯uenced fuses. They can be embedded in a ®eld cluttered with various materials and objects, buried underground at various depths, scattered on the surface, planted within buildings, or covered by plant overgrowth. Anti-personnel (AP) landmines and booby traps are typically shaped in the form of a disk or a cylinder, with diameters from 20 to 125 mm, length from 50 to 100 mm, and can weigh as little as 30 g. TNT, Tetryl, and Comp B are the common types of explosives used. APs are usually shallow buried, at a range from ¯ush
* Corresponding author. Tel.: +1-506-447-3105; fax: +1506-447-3380. E-mail address: [email protected] (E.M.A. Hussein).
to the surface to a maximum depth of about 50 mm; as they do reduced damage if buried any deeper (MineFacts, 1995). In contrast, Anti-tank or Anti-vehicular (AT) landmines often have the shape of truncated cylinders or squares with round corners, with a largest dimension from 150 to 300 mm, and a thickness of 50±90 mm. The explosive material is typically TNT, Comp B, or RDX. ATs are buried at various depths from near-surface to greater than 150 mm (MineFacts, 1995). AT mines are usually associated with warfare and con®ned to battle®elds, which can be corridored, thus minimizing the risk to the general public. AP mines pose the greater danger to civilians, as they are often used in civil and guerrilla wars and placed in areas accessible to the public. The requirements of civilian demining (mine clearance) are quite dierent from those of military demining; and this aects the detection problem. During a military countermine operation, the objective is to breach a mine®eld as fast as possible, often using brute force. Civilian demining, on the other hand, is more dicult and dangerous than military demining, as it requires complete removal of all mines. Normal trac
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and use of land must be re-established and therefore absolutely no explosive material can be left unremoved. It is also necessary to restore public con®dence, since even rumours can lend an entire ®eld or road useless. Post-cleaning of cleared roads or ®elds, also called proo®ng, is essential to rebuild the public's con®dence. This process involves the detection and removal of any mines that may remain undetected. Mines left undetected in the ®rst phase of demining are likely those that were more deeply buried and can cause future problems if left undetected. These aspects make the detection process even more challenging, and may require special proo®ng detection technology. The purpose of this paper is to present the challenges facing the landmine detection problem caused by the variety of mines, detection requirements and terrain. The paper also assesses the suitability of dierent methods for mine detection and the role radiation and radioisotopes can play in helping humanity get rid of this scourge. First, however, a brief description of the detection problems is given. 2. The detection problem 2.1. Requirements A landmine detection system should be able to detect mines regardless of the type of explosives used, since mines are made of a variety of explosive materials. Mines come in a variety of shapes and in various types of casings, and therefore a detection system should be either insensitive to the geometrical shape of the mine and the type of casing material, or preferably provide imaging information. This latter feature will enable the system to better distinguish mines from background clutter, such as rocks, metal shreds, etc. This, in turn, will reduce the false-positive alarm rate and the time wasted in trying to clear an innocuous object thought to be a mine. On the other hand, it is vital that the detection system not miss a genuine mine; that is, near zero false-negative alarms need to be achieved. Since mines can be buried at dierent depths under the ground surface, the detection system should not be overly sensitive to the depth of burial. The operator of a detection system should be able to avoid close proximity to the position of the mine to minimize the possibility of inadvertent triggering of the mine. Detection should also be performed at a reasonable operational speed, and at not too prohibitive a cost. In summary, the ideal system must be accurate, not too slow and not too expensive. In addition to the detection requirements, practical considerations dictate that a detection system should be easily deployable in the ®eld and be usable by tech-
nically unsophisticated people. In other words, the system should not represent a logistical burden by requiring complex machines and operation. 2.2. Approach A landmine is a foreign object in an otherwise benign medium, the ground. Therefore, the ®rst natural step in mine detection is to sense the presence of an anomaly on or near the surface of the ground, by detecting the existence of an unexpected object. However, this alone is not sucient to provide a de®nite indication of mine, as depending on the detection method the identi®ed anomaly could be some other innocuous object. Removing an identi®ed anomaly, with all the care and attention given to a landmine, to discover in vain that the eort was directed towards clearing a harmless object, is a time consuming and costly process (estimates range upwards of US $1000 per mine cleared). The major problem in demining is to discriminate between a dummy object and a landmine (US Department of State, 1998). Therefore, it is important to characterize a detected anomaly as containing an explosive or non-explosive material. Most common and emerging techniques for landmine detection focus on the detection of anomalies in the ground, without providing a de®nite indication that an explosive material is present. It is in material characterization that radiation-based techniques can play a vital role in mine detection (IAEA, 1997). 3. Common detection methods Currently, four common yet primitive mine detection techniques are widely used: (i) simple visual inspection, (ii) hand-held metal detectors, (iii) classical mine prodders and (iv) biological sniers. The limitation of each of these methods is discussed below. In visual inspection, one looks for disturbed dirt, apparent mines, etc. Although this is an anomaly identi®cation method, familiarity with the shape and look of a class of landmines can help in characterizing the observed anomaly as likely to contain an explosive charge. The limitations of super®cial visual inspection are obvious. Metal detectors attempt to obtain information on buried mines by emitting into the soil a time-varying magnetic ®eld to induce an eddy current in metallic objects; which in turn generates a detectable magnetic ®eld. However, landmines typically contain a small amount of metal in the ®ring pin while many others contain no metal at all. Increasing the sensitivity of a metal detector to detect a smaller amount of metal makes it also very susceptible to metal shreds that are often found in mine infected areas. Although the use
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of pulsed waveform and monitoring of multi-frequency emissions may improve the capabilities of electromagnetic induction probes, they will remain unsuited for use in magnetic and heavily mineralized soil. Metal detectors, even when successful, can only succeed in identifying the presence of an anomaly, without providing information on whether explosive material is present or not. Mine prodders enable subsurface inspection by employing bayonets or hand-held probes (about 250 mm long) to poke the ground, inch by inch, to sense the presence of a hard (solid) object in the soil. It is, therefore, another anomaly identi®cation technique that provides no material characterization information. Prodding is done at an angle to avoid causing detonation if the mine is pushed from the top, where the primary trigger usually is. Aside from the inability of this primitive method to distinguish between a landmine and any other solid object that can be present in the soil, such as a rock, it is a dangerous operation. Biological sning by dogs is also used. Dogs have greater olfactory senses compared to humans, especially for trace quantities, and can be trained to detect the presence of explosives. This is, in eect, a material characterization process as dogs are sning the vapours emitted from the explosive material. This technique requires, however, extensive training, and the dogs' limited attention span makes it dicult to maintain continuous operation. Electronic chemical sniers can also be used, though they are not as sophisticated as dogs in terms of their detection abilities. Moreover, mine®elds are usually saturated with residual vapour emissions from recently detonated explosives, which may add to the chemical clutter of the area, thereby confusing the dogs' senses. Overall, the detection eort by any of the above widely-used means is tedious and requires disciplined, well-trained personnel. This has motivated international eorts to develop remote detection and turnkey techniques, some of which are discussed in the following section. 4. Emerging techniques A number of mine detection techniques are emerging as alternatives to the currently used methods that have not appreciably changed since World War II. The dierence in the thermal capacitance between soil and mine aects their heating/cooling rates and therefore their associated infrared emissions (Ashley, 1996). Infrared cameras are used to map heat leakage patterns from the ground. The technique essentially measures the thermal emissivity of the ground and interprets changes in emissivity as being caused by the presence of a foreign object; therefore, material charac-
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terization information is not provided. However, this technology has the advantages of being passive, can be performed remotely, by aerial search, and can cover a large area in a short time. Infrared thermography is best suited for locating mine®elds rather than searching for individual mines. It cannot however work when the soil and mine are in thermal equilibrium, and therefore is generally limited for use either at sunset or sunrise where a temperature gradient can be established at the ground surface. The dierence in the re¯ectance and polarization of soil when disturbed by laser energy may be used to identify the presence of an anomaly (Ashley, 1996). This requires a powerful laser, complex data interpretation and provides no material characterization information. Since eddy current can be generated only in conducting materials, such as metals, and microwaves are completely re¯ected o metallic surfaces, metal encased landmines can be detected by both pulse-induction metallic detectors and microwaves (ground penetrating radar). Unfortunately, however, not all mines are metallic. Nevertheless, microwaves are also scattered, though to a lesser extent, by non-metallic objects and characteristic re¯ection signatures can be related to material type, and hence can be used to identify explosives. This approach has signi®cant diculties because of the propagation losses in the soil, the low contrast between target and soil, and the large variety of echoes from the rough surface and other shallow contrasts such as rocks, tree roots, etc. (Peters et al., 1994). The discrimination of mine from clutter under a wide variety of surface and soil conditions remains very dicult. In addition, water has a high anity to absorbing microwaves, making it dicult to operate groundpenetrating radar under wet conditions. This is an anomaly identi®cation method, with no material characterization ability. Although the above methods do not provide material characterization information, experience and familiarity with these methods may enable the reduction of the false-positive detection rate arising from innocuous objects. However, one mistake in such assessment can be fatal and at the end one would have to deal with each identifying anomaly as being a landmine and remove it as such; which is a tedious endeavor. It is therefore desirable to have a material characterization method that can determine whether the identi®ed anomaly contains an explosive material or not. Radiation based techniques can be useful in this regard as discussed below. 5. Penetrating radiation Penetrating radiation (neutron and photon) oers
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some attractive features that can be utilized in landmine detection, particularly for material characterization. However, unlike conventional radiographic or tomographic methods, one cannot rely on the radiation transmission modality, as it requires access to two opposing sides of an object; a situation not attainable with landmines. Therefore, one has to rely on secondary radiation emissions (activation) or radiation scattering. The main reason penetrating radiation would be used in landmine detection, in spite of its radiological shielding requirements, is to provide material characterization information. It is therefore useful to closely scrutinize the composition of explosive materials. The most common explosive material found in landmines is most likely TNT; although RDX and other plasticized explosives are also used (Ashley, 1996). These explosives are rich in nitrogen, which serves as a bonding agent. However, the amount of nitrogen alone is not sucient to de®nitely distinguish an explosive material from other innocuous materials (Hussein, 1992). Explosives are also rich in oxygen (the oxidizing agent). Therefore, knowing the nitrogen content together with the oxygen content provides a more unambiguous identi®er of an explosive material. Hydrogen and carbon are also present in most explosives and their relative elemental content may be also used to characterize a detected anomaly as likely to contain an explosive material. The detection of hydrogen and carbon alone is not, however, as a de®nite indicator of an explosive material as that of the detection of nitrogen and oxygen. The composition of soil in which a landmine is embedded varies from dry sand to wet fertile soil. However, the Earth's crust consists mainly of eight basic elements: oxygen (49.52%), silicon (25.75%), aluminum (7.51%), iron (4.7%), calcium (3.39%), sodium (2.64%), potassium (2.40%) and magnesium (1.94%), while all other elements constitute about 2.15% by weight (Christopher, 1981). Therefore, the only element in the Earth's crust that also exists in an explosive material is oxygen. However, other objects such as vegetation (e.g. tree roots) and plastic scrap may be also buried in soil. These and other hydrocarbon materials contain elements similar to those found in explosives, and may confuse a material characterization method. Some of the ambiguities in composition indication can be reduced by taking advantage of the fact that explosives have a density that is higher than that of most common organic materials, but less than that of metals and many types of soil (Hussein, 1992). Therefore, combining elemental composition information with material density information can be useful in providing de®nite characterization of an anomaly as containing an explosive material. With these aspects in
mind, the use of photons and neutrons in detection is assessed below. 5.1. Photons For photons (x- or gamma-rays) to be utilized in landmine detection, they must be used in the scattering or secondary emission modalities. Photons can be emitted as a result of neutron activation, which is discussed in the following section. However, absorption of incident photons can lead to the generation of characteristic X-rays (photopeaks) due to re-arrangement of electrons in the shells of the aected atom. These X-rays are usually re-absorbed within the medium. Bremsstrahlung radiation can also be seen as a secondary emission, generated by the recoil electrons in photon±electron interactions. This process is mainly important in materials with high atomic number, a few of which are abundant in soil, and none of which are present in explosive materials. The third possible photon reaction that leads to secondary emission is the pair production process, in which the electron and positron pair produced annihilate each other generating two 511 keV photons. This reaction is only dominant at high photon energies (greater than about 4 MeV) and for high atomic number materials. Disadvantages of this latter reaction are that it requires the presence of heavy elements which are not abundant in soil and landmines, and high energy photons are required from expensive accelerators that are not easily mountable in ®eld conditions and require special skill and care to operate. The second approach to employing photons in the landmine detection process, excluding photon transmission, is photon scattering. Photon scattering can occur either coherently or incoherently with the electrons of the matter. Coherent (Rayleigh) scattering is a small angle scattering (i.e. forward biased) and would therefore, like transmission, require access to both sides of an interrogated object. It is therefore not a reaction one would anticipate to use for the one-side inspection problem of landmine detection. On the other hand, Compton scattering is dominant in most materials and at almost all photon energies. In this process, photons incoherently collide (Compton scatter) with the atomic electrons with a probability that is dependent on the electron density, and consequently the mass density, of the medium. Therefore, Compton scattering can provide a density map, which can be used as an indicator that an anomaly has a density in the range of explosive materials. This is the essence of the X-ray backscattering system of Campbell and Jacobs (1992). Gamma-rays can also provide similar information, as was shown by Roder (1975). As the scattered photons travel back towards the detector they are removed by further scattering or
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absorption, with the photo-absorption probability being strongly dependent on the atomic number. This approach can provide atomic number (or more precisely eective atomic number) information that can be used for further characterizing a material as being an explosive material. The unique elemental composition of explosives gives them a certain range of eective atomic number. A combination of density and atomic number can then be used to characterize the presence of explosives. This is similar to the approach utilized in commercially available dual energy transmission Xrays systems. The technique is used in radiography and litho-density oil well logging (Serra, 1984), and has been proposed for luggage inspection (Hussein, 1994). Localization of the detected object may prove, however, to be dicult when applying this approach, since photons need to propagate over some distance so that the photon energy is suciently lowered to the range in which the photoelectric eect becomes dominant (for obtaining atomic number information). Alternatively, a dual energy photon source may be employed, where one energy is higher so that photoelectric eect is not dominant, while the other energy is lower to allow for some photo-absorption while permitting the photons to Compton scatter back into the detector. For more information on these possibilities, advantages and limitations of photon techniques, the reader can consult the review paper by Hussein and Waller (1998). 5.2. Neutrons The advantage of neutrons over photons is that they can provide elemental information. This information can be used for detecting explosives through the characteristic photon emissions produced by neutron activation. Since explosives used in landmines are rich in nitrogen content, activation of nitrogen by neutron capture can be used for its detection. Absorption of low-energy (thermal) neutrons results in the emission of 10.8 MeV gamma rays, which are easily distinguishable. This is the basis of the thermal neutron activation analysis system, termed ``MineSCANS'', developed by SAIC Canada (Waller, 1998). The portable isotopic neutron spectroscopy system, named ``PINS'' (Carey et al., 1992), developed by the Idaho National Engineering & Environmental Laboratory (INEEL), which employs a 252Cf neutron source (whose neutrons are moderated by the interrogated object) for assaying chemical weapons, may be also tailored for use in landmine detection. Thermal neutrons can also be used to activate hydrogen and silicon (producing 2.22 and 3.54 MeV dominant photons, respectively). All military explosives contain from 2 to 3% (by weight) hydrogen, while the ground may contain various concentrations of hydro-
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gen, depending on its water content. Silicon, however, is absent in explosive materials and is present only in soil. Therefore, a thermal neutron activation system may be used to monitor the hydrogen±silicon ratio to search for an anomaly (IAEA, 1997). However, since anomalies can be identi®ed by other non-nuclear means, such an approach will not be helpful in characterizing an anomaly as a landmine. Thermal neutron activation requires the employment of a bulky moderating material to slow down fast neutrons emitted from either an isotopic source or neutron generator, since there are no portable means of directly generating thermal neutrons. The soil itself can be used as a moderating material, but then the amount of activation will depend on the type of soil (in particular, its hydrogen content). Since the activation probability (cross section) is relatively low, a strong neutron source is required. This causes some diculties in radiological shielding and handling which aects the portability of the device. Moreover, nitrogen is present in fertile soil and tree roots. Under such conditions it becomes dicult to detect mines based on nitrogen content alone. Moreover, as mentioned earlier, nitrogen and oxygen are present in vegetation, such as tree roots, and therefore the detection of their presence may not be sucient to characterize an explosive material. Fast neutron activation, mainly through the inelastic scattering of neutrons, can be used to provide characteristic gamma rays to identify other elements present in explosive materials. The cross sections of two of the major elements in explosives, nitrogen and oxygen, have characteristic resonance's in the energy range from 1 to 3 MeV. Nitrogen has resonance peaks at 1.116, 1.184, 1.593, 1.783 MeV, while oxygen has peaks at 1.312, 1.651, 1.832, 1.907 MeV and a dip at 2.360 MeV (Gomberg and Kushner, 1991). The use of fast neutrons to provide elemental composition is not a new concept (Hussein, 1992) and fast neutron activation has been suggested for elemental identi®cation of explosives (Sawa and Gozani, 1991). However, the activation cross section (interaction probability) of fast neutrons is low for the elements of interest, thus necessitating a high intensity neutron source. The PELAN (Pulsed Elemental Analysis with Neutrons) technique (Vourvopoulos et al., 1997) uses fast neutron activation to detect the presence of hydrogen, carbon, nitrogen and oxygen. The technique utilizes a 14 MeV neutron pulse from a portable generator to activate the carbon and oxygen present in the material, since the activation cross section at this neutron energy is relatively high. When the pulse is stopped, the fast neutrons will continue to diuse within the interrogated medium and its surroundings, and depending on the size and nature of the material encountered, may be slowed to a suciently low neutron energy (below
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1 eV) where they can be readily captured by hydrogen and nitrogen. The gamma-rays emitted during and after the neutron pulse can be used to detect the four basic elements of explosives provided the soil has sucient hydrogen content to slow the neutrons down. However, as the pulsed neutrons diuse from the soil, the number of slow neutrons bombarding an interrogated anomaly may be too low to allow for a de®nite post-pulse activation analysis. The neutron scattering cross section is generally higher than that of activation. Moreover, the backscattering cross section is in general higher than the average scattering probability, at all angles (Gomberg and Kushner, 1991). This makes resonance backscattering an attractive candidate for a single-side access application such as mine detection. By measuring the energy spectrum of backscattered neutrons and monitoring the number of neutrons appearing at the above resonance energies, one can determine whether a large concentration of both nitrogen and oxygen is present in the soil. A number of detectors, surrounding a single source, can then be used to provide an image of the interrogated area. Gomberg and Kushner (1991) took advantage of these resonances in the scattering cross section to distinguish explosive from other innocuous materials in luggage. A similar approach has been used by Brooks et al. (1998) for the detection of explosives in small (0.2±1 kg) samples. Time-of-¯ight measurements were used by these workers to determine the neutron energy, and required an accelerator-based neutron source. Such sophisticated techniques are dicult to apply in the ®eld conditions of mine detection. However, the use of backscattering for material identi®cation is particularly suited for mine detection, and further eort should be made to develop less complex systems that take advantage of backscattering resonances without the need to perform time-of-¯ight measurements, thus enabling the use of common radioisotopic sources and conventional neutron detectors. Total removal (absorption and scattering) of neutrons oers the advantage of incorporating all interaction modalities, and hence can provide a stronger signal than activation and angle/energy-dependent scattering, for the same source strength. Neutron removal is the process employed in transmission techniques. Although transmission is impossible to apply in the one-side inspection problem of landmines, the concept of measuring the overall eect on an incident signal should also be explored. For example, it may be possible to observe the eect of the resonances in the cross sections of three of the four main elements of explosives (C, N, O) in the energy range of 1±3 MeV from energy spectra of isotopic sources, such as 252Cf and 241Am/Be. Cf-252 has an average energy of about 2.14 MeV and spans an energy range from about 0.1± 6 MeV. For 241Am/Be, the average energy is about 4.5
MeV and its energy spectrum spans a range from 2 to 10 MeV. By measuring the energy spectrum of backscattered neutrons from one of these sources, and observing perturbations in the neutron spectrum due to the scattering resonances in the cross sections of the elements of interest, one may be able to produce a ``®ngerprint'' characteristic signature of an explosive. This is similar to the neutron transmission technique used by Gokhale and Hussein (1997) and is currently being investigated by the authors. One other detection approach is to simply detect the hydrogen content in soil by neutron slowing down. Although, hydrogen is not a unique indicator of explosives, use can be made of the fact that the level of hydrogen content in soil, landmine and the environment (moisture, vegetation, tree roots, etc.) is quite dierent. Brooks (1998) measured the change in hydrogen in soil by measuring the intensity of low energy neutrons re¯ected back from soil exposed to 252Cf neutrons. Among the radiation interrogation approaches, neutrons are the most suitable candidates for providing composition information non-intrusively, as they directly interact with the nucleus. Neutron-based devices tend, however, to be bulky due to radiological shielding or in some cases moderation requirements. They also tend to be relatively expensive. However, given their potential, eorts should continue to develop compact and inexpensive devices. For information on the potential application of neutron methods, see Hussein and Waller (1998).
6. Summary The challenge of landmine detection is not only to develop techniques that can meet this demanding problem, but to also tailor such techniques to local conditions. It is dicult to apply any one technique unless the nature of the mine, soil and background clutter is well known, and it is inconceivable that a single detection technology will be able to meet all needs. A fusion of systems is needed as a solution to the landmine detection problem. Radiation-based techniques have the unique ability to identify the presence of an explosive material in an anomaly detected by other methods.
References Ashley, S., 1996. Searching for land mines. Mechanical Engineering 118 (4), 62±67. Brooks, F.D., 1998. Hydrogen Detectors for Landmines, personal communication. Brooks, F.D., Buer, A., Allie, M.S., Bharuthram, K.,
E.M.A. Hussein, E.J. Waller / Applied Radiation and Isotopes 53 (2000) 557±563 Nchodu, M.R., Simpson, B.R.S., 1998. Determination of HCNO concentrations by fast-neutron scattering analysis. Nuclear Instruments & Methods in Physics Research, Section A 410, 319±328. Carey, A.G., Cole, J.D., Gehrke, R.J., Greenwood, R.C., 1992. Chemical agent and high explosive identi®cation by spectroscopy of neutron-induced gamma rays. IEEE Transactions on Nuclear Science 39, 1422±1426. Campbell, J.C., Jacobs, A.M., 1992. Detection of buried land mines by backscatter imaging. Nuclear Science and Engineering 110, 417±424. Christopher, C.M., 1981. Engineering geology. Merrill, Columbus, OH. Gokhale, P., Hussein, E.M.A., 1997. A californium-252 neutron transmission technique for detection of explosives. Applied Radiation and Isotopes 48, 973±979. Gomberg, H., Kushner, B., 1991. Neutron elastic scatter (NES) for explosive detection systems (EDS). In: Khan, S. (Ed.), Proc. First Int. Symposium on Explosive Detection Technology, DOT/FAA/CT-92/11, FAA Technical Center, Atlantic City, NJ, p. 123. Hussein, E.M.A., 1992. Detection of explosive materials using nuclear radiation: a critical review. In: Makky, W.H. (Ed.), Proc. Conf. on The Aviation Security Problem and Related Technologies, SPIE, CR42, pp. 126±136. Hussein, E.M.A., 1994. Inspection of luggage using gamma radiation. In: Lawrence, A.H. (Ed.). Proc. Cargo Inspection Technologies, The International Society for Optical Engineering, San Diego, CA, 2276, pp. 321± 325. Hussein, E.M.A., Waller, E.J., 1998. Review of one-side approaches to radiographic imaging for detection of explo-
563
sives and narcotics. Radiation Measurements 26, 581± 591. IAEA, 1997. Detection of explosives (in particular landmines) by low cost methods, IAEA/PS/AGM97-3, International Atomic Energy Agency, Vienna, Austria. MineFacts, 1995. MineFacts CD-Rom Version 1.2, National Ground Intelligence Center, Charlottesville, VA. Peters, L., Daniels, J.J., Young, J.D., 1994. Ground penetrating radar as a subsurface environmental sensing tool. Proc. IEEE 82, 1802±1822. Roder, F.L., 1975. Theory and application of X-ray and gamma-ray backscatter to landmine detection, Report No. 2134. U.A. Army Mobility Equipment Research and Development Center, Fort Belvoir, VA. Sawa, Z.P., Gozani, T., 1991. PFNA technique for the detection of explosives. In: Khan, S. (Ed.), Proc. First Int. Symposium on Explosive Detection Technology, DOT/ FAA/CT-92/11, FAA Technical Center, Atlantic City, NJ, p. 83. Serra, O., 1984. Fundamentals of Well-log Interpretation, 1. The Acquisition of Logging Data. Elsevier, Amsterdam Chapter 12. US Department of State., 1998. Hidden killer, the global land mine crisis, U.S. Department of State Publication 10575. Vourvopoulos, G., Dep, L., Paschal, J., Spichiger, G., 1997. PELAN Ð a transportable, neutron-based UXO identi®cation technique. In: Proceedings of UXO Forum'97, Nashville, TN, pp. 342±349. Waller, E.J., 1998. MineSCANS Ð mine system countermeasures by analysis of nitrogen signature, presented at the 1998 International Advanced Studies Institute Science and Technology Series, Monterey, CA.
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Landmine detection: the problem and the challenge E.M.A. Hussein*, E.J. Waller Laboratory for Threat Material Detection, Department of Mechanical Engineering, University of New Brunswick, P.O. Box 4400, Fredericton, NB, Canada E3B 5A3
Abstract This paper explores the role of radiation methods in addressing the problem of detecting landmines. The application of neutron activation analysis, with an isotopic source or a pulsed neutron generator, is discussed. The use of neutron moderation as an indicator of the presence of a landmine is also explored. In addition, information provided by measuring scattered photons (gamma- and X-rays) is examined. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: Landmines; Explosives; Radiation methods
1. Introduction Anti-tank and anti-personnel mines come in all shapes and sizes, and can be encased in metal, plastic, wood or nothing at all. Their fusing mechanism varies from simple pressure triggers to trip wires, tilt rods, acoustic and seismic fuses, or even light- or magneticin¯uenced fuses. They can be embedded in a ®eld cluttered with various materials and objects, buried underground at various depths, scattered on the surface, planted within buildings, or covered by plant overgrowth. Anti-personnel (AP) landmines and booby traps are typically shaped in the form of a disk or a cylinder, with diameters from 20 to 125 mm, length from 50 to 100 mm, and can weigh as little as 30 g. TNT, Tetryl, and Comp B are the common types of explosives used. APs are usually shallow buried, at a range from ¯ush
* Corresponding author. Tel.: +1-506-447-3105; fax: +1506-447-3380. E-mail address: [email protected] (E.M.A. Hussein).
to the surface to a maximum depth of about 50 mm; as they do reduced damage if buried any deeper (MineFacts, 1995). In contrast, Anti-tank or Anti-vehicular (AT) landmines often have the shape of truncated cylinders or squares with round corners, with a largest dimension from 150 to 300 mm, and a thickness of 50±90 mm. The explosive material is typically TNT, Comp B, or RDX. ATs are buried at various depths from near-surface to greater than 150 mm (MineFacts, 1995). AT mines are usually associated with warfare and con®ned to battle®elds, which can be corridored, thus minimizing the risk to the general public. AP mines pose the greater danger to civilians, as they are often used in civil and guerrilla wars and placed in areas accessible to the public. The requirements of civilian demining (mine clearance) are quite dierent from those of military demining; and this aects the detection problem. During a military countermine operation, the objective is to breach a mine®eld as fast as possible, often using brute force. Civilian demining, on the other hand, is more dicult and dangerous than military demining, as it requires complete removal of all mines. Normal trac
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and use of land must be re-established and therefore absolutely no explosive material can be left unremoved. It is also necessary to restore public con®dence, since even rumours can lend an entire ®eld or road useless. Post-cleaning of cleared roads or ®elds, also called proo®ng, is essential to rebuild the public's con®dence. This process involves the detection and removal of any mines that may remain undetected. Mines left undetected in the ®rst phase of demining are likely those that were more deeply buried and can cause future problems if left undetected. These aspects make the detection process even more challenging, and may require special proo®ng detection technology. The purpose of this paper is to present the challenges facing the landmine detection problem caused by the variety of mines, detection requirements and terrain. The paper also assesses the suitability of dierent methods for mine detection and the role radiation and radioisotopes can play in helping humanity get rid of this scourge. First, however, a brief description of the detection problems is given. 2. The detection problem 2.1. Requirements A landmine detection system should be able to detect mines regardless of the type of explosives used, since mines are made of a variety of explosive materials. Mines come in a variety of shapes and in various types of casings, and therefore a detection system should be either insensitive to the geometrical shape of the mine and the type of casing material, or preferably provide imaging information. This latter feature will enable the system to better distinguish mines from background clutter, such as rocks, metal shreds, etc. This, in turn, will reduce the false-positive alarm rate and the time wasted in trying to clear an innocuous object thought to be a mine. On the other hand, it is vital that the detection system not miss a genuine mine; that is, near zero false-negative alarms need to be achieved. Since mines can be buried at dierent depths under the ground surface, the detection system should not be overly sensitive to the depth of burial. The operator of a detection system should be able to avoid close proximity to the position of the mine to minimize the possibility of inadvertent triggering of the mine. Detection should also be performed at a reasonable operational speed, and at not too prohibitive a cost. In summary, the ideal system must be accurate, not too slow and not too expensive. In addition to the detection requirements, practical considerations dictate that a detection system should be easily deployable in the ®eld and be usable by tech-
nically unsophisticated people. In other words, the system should not represent a logistical burden by requiring complex machines and operation. 2.2. Approach A landmine is a foreign object in an otherwise benign medium, the ground. Therefore, the ®rst natural step in mine detection is to sense the presence of an anomaly on or near the surface of the ground, by detecting the existence of an unexpected object. However, this alone is not sucient to provide a de®nite indication of mine, as depending on the detection method the identi®ed anomaly could be some other innocuous object. Removing an identi®ed anomaly, with all the care and attention given to a landmine, to discover in vain that the eort was directed towards clearing a harmless object, is a time consuming and costly process (estimates range upwards of US $1000 per mine cleared). The major problem in demining is to discriminate between a dummy object and a landmine (US Department of State, 1998). Therefore, it is important to characterize a detected anomaly as containing an explosive or non-explosive material. Most common and emerging techniques for landmine detection focus on the detection of anomalies in the ground, without providing a de®nite indication that an explosive material is present. It is in material characterization that radiation-based techniques can play a vital role in mine detection (IAEA, 1997). 3. Common detection methods Currently, four common yet primitive mine detection techniques are widely used: (i) simple visual inspection, (ii) hand-held metal detectors, (iii) classical mine prodders and (iv) biological sniers. The limitation of each of these methods is discussed below. In visual inspection, one looks for disturbed dirt, apparent mines, etc. Although this is an anomaly identi®cation method, familiarity with the shape and look of a class of landmines can help in characterizing the observed anomaly as likely to contain an explosive charge. The limitations of super®cial visual inspection are obvious. Metal detectors attempt to obtain information on buried mines by emitting into the soil a time-varying magnetic ®eld to induce an eddy current in metallic objects; which in turn generates a detectable magnetic ®eld. However, landmines typically contain a small amount of metal in the ®ring pin while many others contain no metal at all. Increasing the sensitivity of a metal detector to detect a smaller amount of metal makes it also very susceptible to metal shreds that are often found in mine infected areas. Although the use
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of pulsed waveform and monitoring of multi-frequency emissions may improve the capabilities of electromagnetic induction probes, they will remain unsuited for use in magnetic and heavily mineralized soil. Metal detectors, even when successful, can only succeed in identifying the presence of an anomaly, without providing information on whether explosive material is present or not. Mine prodders enable subsurface inspection by employing bayonets or hand-held probes (about 250 mm long) to poke the ground, inch by inch, to sense the presence of a hard (solid) object in the soil. It is, therefore, another anomaly identi®cation technique that provides no material characterization information. Prodding is done at an angle to avoid causing detonation if the mine is pushed from the top, where the primary trigger usually is. Aside from the inability of this primitive method to distinguish between a landmine and any other solid object that can be present in the soil, such as a rock, it is a dangerous operation. Biological sning by dogs is also used. Dogs have greater olfactory senses compared to humans, especially for trace quantities, and can be trained to detect the presence of explosives. This is, in eect, a material characterization process as dogs are sning the vapours emitted from the explosive material. This technique requires, however, extensive training, and the dogs' limited attention span makes it dicult to maintain continuous operation. Electronic chemical sniers can also be used, though they are not as sophisticated as dogs in terms of their detection abilities. Moreover, mine®elds are usually saturated with residual vapour emissions from recently detonated explosives, which may add to the chemical clutter of the area, thereby confusing the dogs' senses. Overall, the detection eort by any of the above widely-used means is tedious and requires disciplined, well-trained personnel. This has motivated international eorts to develop remote detection and turnkey techniques, some of which are discussed in the following section. 4. Emerging techniques A number of mine detection techniques are emerging as alternatives to the currently used methods that have not appreciably changed since World War II. The dierence in the thermal capacitance between soil and mine aects their heating/cooling rates and therefore their associated infrared emissions (Ashley, 1996). Infrared cameras are used to map heat leakage patterns from the ground. The technique essentially measures the thermal emissivity of the ground and interprets changes in emissivity as being caused by the presence of a foreign object; therefore, material charac-
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terization information is not provided. However, this technology has the advantages of being passive, can be performed remotely, by aerial search, and can cover a large area in a short time. Infrared thermography is best suited for locating mine®elds rather than searching for individual mines. It cannot however work when the soil and mine are in thermal equilibrium, and therefore is generally limited for use either at sunset or sunrise where a temperature gradient can be established at the ground surface. The dierence in the re¯ectance and polarization of soil when disturbed by laser energy may be used to identify the presence of an anomaly (Ashley, 1996). This requires a powerful laser, complex data interpretation and provides no material characterization information. Since eddy current can be generated only in conducting materials, such as metals, and microwaves are completely re¯ected o metallic surfaces, metal encased landmines can be detected by both pulse-induction metallic detectors and microwaves (ground penetrating radar). Unfortunately, however, not all mines are metallic. Nevertheless, microwaves are also scattered, though to a lesser extent, by non-metallic objects and characteristic re¯ection signatures can be related to material type, and hence can be used to identify explosives. This approach has signi®cant diculties because of the propagation losses in the soil, the low contrast between target and soil, and the large variety of echoes from the rough surface and other shallow contrasts such as rocks, tree roots, etc. (Peters et al., 1994). The discrimination of mine from clutter under a wide variety of surface and soil conditions remains very dicult. In addition, water has a high anity to absorbing microwaves, making it dicult to operate groundpenetrating radar under wet conditions. This is an anomaly identi®cation method, with no material characterization ability. Although the above methods do not provide material characterization information, experience and familiarity with these methods may enable the reduction of the false-positive detection rate arising from innocuous objects. However, one mistake in such assessment can be fatal and at the end one would have to deal with each identifying anomaly as being a landmine and remove it as such; which is a tedious endeavor. It is therefore desirable to have a material characterization method that can determine whether the identi®ed anomaly contains an explosive material or not. Radiation based techniques can be useful in this regard as discussed below. 5. Penetrating radiation Penetrating radiation (neutron and photon) oers
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some attractive features that can be utilized in landmine detection, particularly for material characterization. However, unlike conventional radiographic or tomographic methods, one cannot rely on the radiation transmission modality, as it requires access to two opposing sides of an object; a situation not attainable with landmines. Therefore, one has to rely on secondary radiation emissions (activation) or radiation scattering. The main reason penetrating radiation would be used in landmine detection, in spite of its radiological shielding requirements, is to provide material characterization information. It is therefore useful to closely scrutinize the composition of explosive materials. The most common explosive material found in landmines is most likely TNT; although RDX and other plasticized explosives are also used (Ashley, 1996). These explosives are rich in nitrogen, which serves as a bonding agent. However, the amount of nitrogen alone is not sucient to de®nitely distinguish an explosive material from other innocuous materials (Hussein, 1992). Explosives are also rich in oxygen (the oxidizing agent). Therefore, knowing the nitrogen content together with the oxygen content provides a more unambiguous identi®er of an explosive material. Hydrogen and carbon are also present in most explosives and their relative elemental content may be also used to characterize a detected anomaly as likely to contain an explosive material. The detection of hydrogen and carbon alone is not, however, as a de®nite indicator of an explosive material as that of the detection of nitrogen and oxygen. The composition of soil in which a landmine is embedded varies from dry sand to wet fertile soil. However, the Earth's crust consists mainly of eight basic elements: oxygen (49.52%), silicon (25.75%), aluminum (7.51%), iron (4.7%), calcium (3.39%), sodium (2.64%), potassium (2.40%) and magnesium (1.94%), while all other elements constitute about 2.15% by weight (Christopher, 1981). Therefore, the only element in the Earth's crust that also exists in an explosive material is oxygen. However, other objects such as vegetation (e.g. tree roots) and plastic scrap may be also buried in soil. These and other hydrocarbon materials contain elements similar to those found in explosives, and may confuse a material characterization method. Some of the ambiguities in composition indication can be reduced by taking advantage of the fact that explosives have a density that is higher than that of most common organic materials, but less than that of metals and many types of soil (Hussein, 1992). Therefore, combining elemental composition information with material density information can be useful in providing de®nite characterization of an anomaly as containing an explosive material. With these aspects in
mind, the use of photons and neutrons in detection is assessed below. 5.1. Photons For photons (x- or gamma-rays) to be utilized in landmine detection, they must be used in the scattering or secondary emission modalities. Photons can be emitted as a result of neutron activation, which is discussed in the following section. However, absorption of incident photons can lead to the generation of characteristic X-rays (photopeaks) due to re-arrangement of electrons in the shells of the aected atom. These X-rays are usually re-absorbed within the medium. Bremsstrahlung radiation can also be seen as a secondary emission, generated by the recoil electrons in photon±electron interactions. This process is mainly important in materials with high atomic number, a few of which are abundant in soil, and none of which are present in explosive materials. The third possible photon reaction that leads to secondary emission is the pair production process, in which the electron and positron pair produced annihilate each other generating two 511 keV photons. This reaction is only dominant at high photon energies (greater than about 4 MeV) and for high atomic number materials. Disadvantages of this latter reaction are that it requires the presence of heavy elements which are not abundant in soil and landmines, and high energy photons are required from expensive accelerators that are not easily mountable in ®eld conditions and require special skill and care to operate. The second approach to employing photons in the landmine detection process, excluding photon transmission, is photon scattering. Photon scattering can occur either coherently or incoherently with the electrons of the matter. Coherent (Rayleigh) scattering is a small angle scattering (i.e. forward biased) and would therefore, like transmission, require access to both sides of an interrogated object. It is therefore not a reaction one would anticipate to use for the one-side inspection problem of landmine detection. On the other hand, Compton scattering is dominant in most materials and at almost all photon energies. In this process, photons incoherently collide (Compton scatter) with the atomic electrons with a probability that is dependent on the electron density, and consequently the mass density, of the medium. Therefore, Compton scattering can provide a density map, which can be used as an indicator that an anomaly has a density in the range of explosive materials. This is the essence of the X-ray backscattering system of Campbell and Jacobs (1992). Gamma-rays can also provide similar information, as was shown by Roder (1975). As the scattered photons travel back towards the detector they are removed by further scattering or
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absorption, with the photo-absorption probability being strongly dependent on the atomic number. This approach can provide atomic number (or more precisely eective atomic number) information that can be used for further characterizing a material as being an explosive material. The unique elemental composition of explosives gives them a certain range of eective atomic number. A combination of density and atomic number can then be used to characterize the presence of explosives. This is similar to the approach utilized in commercially available dual energy transmission Xrays systems. The technique is used in radiography and litho-density oil well logging (Serra, 1984), and has been proposed for luggage inspection (Hussein, 1994). Localization of the detected object may prove, however, to be dicult when applying this approach, since photons need to propagate over some distance so that the photon energy is suciently lowered to the range in which the photoelectric eect becomes dominant (for obtaining atomic number information). Alternatively, a dual energy photon source may be employed, where one energy is higher so that photoelectric eect is not dominant, while the other energy is lower to allow for some photo-absorption while permitting the photons to Compton scatter back into the detector. For more information on these possibilities, advantages and limitations of photon techniques, the reader can consult the review paper by Hussein and Waller (1998). 5.2. Neutrons The advantage of neutrons over photons is that they can provide elemental information. This information can be used for detecting explosives through the characteristic photon emissions produced by neutron activation. Since explosives used in landmines are rich in nitrogen content, activation of nitrogen by neutron capture can be used for its detection. Absorption of low-energy (thermal) neutrons results in the emission of 10.8 MeV gamma rays, which are easily distinguishable. This is the basis of the thermal neutron activation analysis system, termed ``MineSCANS'', developed by SAIC Canada (Waller, 1998). The portable isotopic neutron spectroscopy system, named ``PINS'' (Carey et al., 1992), developed by the Idaho National Engineering & Environmental Laboratory (INEEL), which employs a 252Cf neutron source (whose neutrons are moderated by the interrogated object) for assaying chemical weapons, may be also tailored for use in landmine detection. Thermal neutrons can also be used to activate hydrogen and silicon (producing 2.22 and 3.54 MeV dominant photons, respectively). All military explosives contain from 2 to 3% (by weight) hydrogen, while the ground may contain various concentrations of hydro-
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gen, depending on its water content. Silicon, however, is absent in explosive materials and is present only in soil. Therefore, a thermal neutron activation system may be used to monitor the hydrogen±silicon ratio to search for an anomaly (IAEA, 1997). However, since anomalies can be identi®ed by other non-nuclear means, such an approach will not be helpful in characterizing an anomaly as a landmine. Thermal neutron activation requires the employment of a bulky moderating material to slow down fast neutrons emitted from either an isotopic source or neutron generator, since there are no portable means of directly generating thermal neutrons. The soil itself can be used as a moderating material, but then the amount of activation will depend on the type of soil (in particular, its hydrogen content). Since the activation probability (cross section) is relatively low, a strong neutron source is required. This causes some diculties in radiological shielding and handling which aects the portability of the device. Moreover, nitrogen is present in fertile soil and tree roots. Under such conditions it becomes dicult to detect mines based on nitrogen content alone. Moreover, as mentioned earlier, nitrogen and oxygen are present in vegetation, such as tree roots, and therefore the detection of their presence may not be sucient to characterize an explosive material. Fast neutron activation, mainly through the inelastic scattering of neutrons, can be used to provide characteristic gamma rays to identify other elements present in explosive materials. The cross sections of two of the major elements in explosives, nitrogen and oxygen, have characteristic resonance's in the energy range from 1 to 3 MeV. Nitrogen has resonance peaks at 1.116, 1.184, 1.593, 1.783 MeV, while oxygen has peaks at 1.312, 1.651, 1.832, 1.907 MeV and a dip at 2.360 MeV (Gomberg and Kushner, 1991). The use of fast neutrons to provide elemental composition is not a new concept (Hussein, 1992) and fast neutron activation has been suggested for elemental identi®cation of explosives (Sawa and Gozani, 1991). However, the activation cross section (interaction probability) of fast neutrons is low for the elements of interest, thus necessitating a high intensity neutron source. The PELAN (Pulsed Elemental Analysis with Neutrons) technique (Vourvopoulos et al., 1997) uses fast neutron activation to detect the presence of hydrogen, carbon, nitrogen and oxygen. The technique utilizes a 14 MeV neutron pulse from a portable generator to activate the carbon and oxygen present in the material, since the activation cross section at this neutron energy is relatively high. When the pulse is stopped, the fast neutrons will continue to diuse within the interrogated medium and its surroundings, and depending on the size and nature of the material encountered, may be slowed to a suciently low neutron energy (below
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1 eV) where they can be readily captured by hydrogen and nitrogen. The gamma-rays emitted during and after the neutron pulse can be used to detect the four basic elements of explosives provided the soil has sucient hydrogen content to slow the neutrons down. However, as the pulsed neutrons diuse from the soil, the number of slow neutrons bombarding an interrogated anomaly may be too low to allow for a de®nite post-pulse activation analysis. The neutron scattering cross section is generally higher than that of activation. Moreover, the backscattering cross section is in general higher than the average scattering probability, at all angles (Gomberg and Kushner, 1991). This makes resonance backscattering an attractive candidate for a single-side access application such as mine detection. By measuring the energy spectrum of backscattered neutrons and monitoring the number of neutrons appearing at the above resonance energies, one can determine whether a large concentration of both nitrogen and oxygen is present in the soil. A number of detectors, surrounding a single source, can then be used to provide an image of the interrogated area. Gomberg and Kushner (1991) took advantage of these resonances in the scattering cross section to distinguish explosive from other innocuous materials in luggage. A similar approach has been used by Brooks et al. (1998) for the detection of explosives in small (0.2±1 kg) samples. Time-of-¯ight measurements were used by these workers to determine the neutron energy, and required an accelerator-based neutron source. Such sophisticated techniques are dicult to apply in the ®eld conditions of mine detection. However, the use of backscattering for material identi®cation is particularly suited for mine detection, and further eort should be made to develop less complex systems that take advantage of backscattering resonances without the need to perform time-of-¯ight measurements, thus enabling the use of common radioisotopic sources and conventional neutron detectors. Total removal (absorption and scattering) of neutrons oers the advantage of incorporating all interaction modalities, and hence can provide a stronger signal than activation and angle/energy-dependent scattering, for the same source strength. Neutron removal is the process employed in transmission techniques. Although transmission is impossible to apply in the one-side inspection problem of landmines, the concept of measuring the overall eect on an incident signal should also be explored. For example, it may be possible to observe the eect of the resonances in the cross sections of three of the four main elements of explosives (C, N, O) in the energy range of 1±3 MeV from energy spectra of isotopic sources, such as 252Cf and 241Am/Be. Cf-252 has an average energy of about 2.14 MeV and spans an energy range from about 0.1± 6 MeV. For 241Am/Be, the average energy is about 4.5
MeV and its energy spectrum spans a range from 2 to 10 MeV. By measuring the energy spectrum of backscattered neutrons from one of these sources, and observing perturbations in the neutron spectrum due to the scattering resonances in the cross sections of the elements of interest, one may be able to produce a ``®ngerprint'' characteristic signature of an explosive. This is similar to the neutron transmission technique used by Gokhale and Hussein (1997) and is currently being investigated by the authors. One other detection approach is to simply detect the hydrogen content in soil by neutron slowing down. Although, hydrogen is not a unique indicator of explosives, use can be made of the fact that the level of hydrogen content in soil, landmine and the environment (moisture, vegetation, tree roots, etc.) is quite dierent. Brooks (1998) measured the change in hydrogen in soil by measuring the intensity of low energy neutrons re¯ected back from soil exposed to 252Cf neutrons. Among the radiation interrogation approaches, neutrons are the most suitable candidates for providing composition information non-intrusively, as they directly interact with the nucleus. Neutron-based devices tend, however, to be bulky due to radiological shielding or in some cases moderation requirements. They also tend to be relatively expensive. However, given their potential, eorts should continue to develop compact and inexpensive devices. For information on the potential application of neutron methods, see Hussein and Waller (1998).
6. Summary The challenge of landmine detection is not only to develop techniques that can meet this demanding problem, but to also tailor such techniques to local conditions. It is dicult to apply any one technique unless the nature of the mine, soil and background clutter is well known, and it is inconceivable that a single detection technology will be able to meet all needs. A fusion of systems is needed as a solution to the landmine detection problem. Radiation-based techniques have the unique ability to identify the presence of an explosive material in an anomaly detected by other methods.
References Ashley, S., 1996. Searching for land mines. Mechanical Engineering 118 (4), 62±67. Brooks, F.D., 1998. Hydrogen Detectors for Landmines, personal communication. Brooks, F.D., Buer, A., Allie, M.S., Bharuthram, K.,
E.M.A. Hussein, E.J. Waller / Applied Radiation and Isotopes 53 (2000) 557±563 Nchodu, M.R., Simpson, B.R.S., 1998. Determination of HCNO concentrations by fast-neutron scattering analysis. Nuclear Instruments & Methods in Physics Research, Section A 410, 319±328. Carey, A.G., Cole, J.D., Gehrke, R.J., Greenwood, R.C., 1992. Chemical agent and high explosive identi®cation by spectroscopy of neutron-induced gamma rays. IEEE Transactions on Nuclear Science 39, 1422±1426. Campbell, J.C., Jacobs, A.M., 1992. Detection of buried land mines by backscatter imaging. Nuclear Science and Engineering 110, 417±424. Christopher, C.M., 1981. Engineering geology. Merrill, Columbus, OH. Gokhale, P., Hussein, E.M.A., 1997. A californium-252 neutron transmission technique for detection of explosives. Applied Radiation and Isotopes 48, 973±979. Gomberg, H., Kushner, B., 1991. Neutron elastic scatter (NES) for explosive detection systems (EDS). In: Khan, S. (Ed.), Proc. First Int. Symposium on Explosive Detection Technology, DOT/FAA/CT-92/11, FAA Technical Center, Atlantic City, NJ, p. 123. Hussein, E.M.A., 1992. Detection of explosive materials using nuclear radiation: a critical review. In: Makky, W.H. (Ed.), Proc. Conf. on The Aviation Security Problem and Related Technologies, SPIE, CR42, pp. 126±136. Hussein, E.M.A., 1994. Inspection of luggage using gamma radiation. In: Lawrence, A.H. (Ed.). Proc. Cargo Inspection Technologies, The International Society for Optical Engineering, San Diego, CA, 2276, pp. 321± 325. Hussein, E.M.A., Waller, E.J., 1998. Review of one-side approaches to radiographic imaging for detection of explo-
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sives and narcotics. Radiation Measurements 26, 581± 591. IAEA, 1997. Detection of explosives (in particular landmines) by low cost methods, IAEA/PS/AGM97-3, International Atomic Energy Agency, Vienna, Austria. MineFacts, 1995. MineFacts CD-Rom Version 1.2, National Ground Intelligence Center, Charlottesville, VA. Peters, L., Daniels, J.J., Young, J.D., 1994. Ground penetrating radar as a subsurface environmental sensing tool. Proc. IEEE 82, 1802±1822. Roder, F.L., 1975. Theory and application of X-ray and gamma-ray backscatter to landmine detection, Report No. 2134. U.A. Army Mobility Equipment Research and Development Center, Fort Belvoir, VA. Sawa, Z.P., Gozani, T., 1991. PFNA technique for the detection of explosives. In: Khan, S. (Ed.), Proc. First Int. Symposium on Explosive Detection Technology, DOT/ FAA/CT-92/11, FAA Technical Center, Atlantic City, NJ, p. 83. Serra, O., 1984. Fundamentals of Well-log Interpretation, 1. The Acquisition of Logging Data. Elsevier, Amsterdam Chapter 12. US Department of State., 1998. Hidden killer, the global land mine crisis, U.S. Department of State Publication 10575. Vourvopoulos, G., Dep, L., Paschal, J., Spichiger, G., 1997. PELAN Ð a transportable, neutron-based UXO identi®cation technique. In: Proceedings of UXO Forum'97, Nashville, TN, pp. 342±349. Waller, E.J., 1998. MineSCANS Ð mine system countermeasures by analysis of nitrogen signature, presented at the 1998 International Advanced Studies Institute Science and Technology Series, Monterey, CA.