On a Demining Device Based on the Technique of Neutron Back Scattering in Combination with a Metal Detector

On a Demining Device Based on the Technique of Neutron Back Scattering in Combination with a Metal Detector Victor Bom a a Delft University of Technology, Mekelweg 15, 2629 JB Delft, the Netherlands The neutron backscattering (NBS) technique is a well established method to find hydrogen in objects. The NBS technique used in demining takes advantage of the fact that land mines contain much more hydrogen atoms than the dry sand in which they may be buried. An advantage of the NBS method is the high speed of operation. A test site has recently become operational in Delft where a NBS imaging detector can be moved over land mines buried in dry sand at speeds up to 1 m/s. First results indicate that anti-tank mines can be found at depths of 20 cm at a speed of 30 mm/s using a Cf source of 5 10 4 n/s. By applying a stronger source or a neutron generator scan speeds of up to 1 m/s are envisaged. Since the speed of operation of a NBS device compares to that of a metal detector the combination of such devices seems obvious. The choice of construction materials however presents problems. 1. Introduction The neutron backscattering (NBS) technique is a well established method to show the presence of hydrogen in objects. For instance, it is in use to monitor water levels in large containers [1,2] and to determine the moisture level in building materials [3]. By the end of the 20th century an international effort was made to introduce nuclear techniques into the search for buried land mines and in 1999 NBS was first introduced for demining purposes by Brooks [4]. The NBS demining technique [5 8] takes advantage of the fact that land mines contain much more hydrogen atoms than the dry sand in which they may be buried. The hydrogen in the mine is present in the explosive chemicals and in the plastic casing. To find a land mine the soil is irradiated with fast neutrons. The neutrons lose energy by scattering and become thermal. A thermal neutron detector monitors the slow neutrons coming back from the soil. Hydrogen is a very effective moderator and the thermal neutron flux will show an increase above a mine. An advantage of the NBS method is the high speed of operation. Mine detection times of the order of a second are feasible when a sufficiently strong neutron source is used. Mines with metal content as well as metal free mines may be found. Additional advantages of NBS based devices over metal detectors or ground penetrating radar devices are the insensitivities for rocks and stones, metal objects and underground holes such as burrows. The main limitation of the NBS method lies in the sensitivity to soil moisture. The hydrogen content of a land mine is comparable to that of sand with a 10% moisture content [9 12], resulting in a loss of contrast between the mine and its surroundings. The NBS method can therefore be applied most advantageously in arid countries. NBS imaging uses a two dimensional position sensitive slow neutron detector. A two dimensional distribution or image is obtained of the slow neutron radiation which is scattered back from the soil and the mine. A concentration of hydrogen shows up as a hot spot : a more or less circular area with a higher neutron radiation intensity than the surroundings. The advantage of using a two dimensional image is that the sensitivity for mine detection is greatly enhanced in comparison to only monitoring the overall count rate. The mine response is not a real image of the mine because scattering of the neutrons in the soil blurs the image and the hot spots always are circular. The size of the spot has a weak depen- 1

2 V. Bom Figure 1: The response of an anti tank mine (left, around x = 200) and an anti personnel mine (right, around x = 480). dence on the mine size but is strongly dependent on the mine depth. Deeper mines will give hot spots with larger diameters because the neutrons that have been thermalized in the mine diffuse over larger distances, not only vertically towards the surface but also horizontally. The high speed of operation of a NBS device makes it possible to use it for land mine scanning. At the Reactor and Neutron Physics Department at the Nuclear Research Center of the Egyptian Atomic Energy Authority a scanning NBS device is being developed with the help of the Delft University of Technology and supported by the IAEA. This Egyptian device is based on the Delft DUNBID land mine detector system [13,14], which was tested in Egypt [15]. The combination of a NBS device and a metal detector presents constructional difficulties while giving few advantages. Hydrogen containing materials (plastics) can not be used in the construction of a combined device because of the sensitivity of the NBS device to hydrogen. On the other hand, metallic construction parts will decrease the sensitivity of the metal detector. Also, slight movements or vibrations will induce a signal in the metal detector, necessitating a very rigid construction. The advantage of a combined device is said to be the capability to filter out the hits of a metal detector on metallic non land mine objects such as shells or cartridges: the NBS detector then would indicate the hit to be a mine or not. Such

Demining Device Based on the Technique of Neutron Back Scattering 3 objects often far outnumber the land mines in former combat areas. The NBS system could also cover the possibility of low metallic mines. This however comes down to always following the indication of the NBS system with little added value from the metal detector. The goal of the test described in this paper was to investigate the scanning properties of a NBS system and to determine the maximum scanning speed in relation the depth of the mines. 2. Experimental Setup 2.1. The Delft Test Lane Facility The Delft test lane facility consists of a box of 3 8 1 m 3 containing 35 tons of dry sand. The sand is heated from the bottom to keep it as dry as possible and to simulated a desert condition. The box is placed inside a heated Quinset hut. This hut has thin aluminum walls and therefore the scattering of neutrons from the surroundings is low. The NBS detector and the associated electronics are mounted on a frame which can be moved by a stepping motor over the sand surface with a speed between 1 mm/s and 1 m/s. 2.2. ESCALAD: The Egypt SCAnning LAnd mine Detector The Egypt SCAnning LAnd mine Detector (ESCALAD) consists of 16 3 He proportional counter tubes with resistive wires, mounted parallel in a flat encasing of 142 117 15 cm 3. The specifications of the counter tubes are: effective length 100 cm, diameter 2.5 cm and 3 He pressure 10 bar. The sensitive detector area is 100 66 cm 2. The direction of scanning is perpendicular to the tubes. The system is being constructed at the Delft University. First tests are done in the new Delft test lane. The position of a neutron event with respect to a soil related reference frame in the scan direction is determined from the position of the tube being hit in the encasing and the position of the encasing with respect to the soil. The position along the tubes is determined by charge division 1. 1 Mesytec Inc. electronics were used for position determination Counts are stored in a 32 256 array of pixels according to the measured positions. This yields a 2D image of the intensity of the back scattered slow neutrons with a pixel size of about 3 3 cm 2. 3. Measurements The fast neutrons were obtained from a variety of sources: a radioactive 252 Cf source 5 10 5 n/s, a DD neutron generator 2 10 6 n/s, and a radioactive PuBe source with a strength of about 5 10 7 n/s. The source was placed in the detector encasing, the point of neutron emission being around 12 cm from the ground. The standoff distance between the bottom of the detector encasing and the sand surface was roughly 10 cm. The depth of the buried mines is the distance between the soil surface and the top of the mine. A moisture level below 0.5% was determined on a sand sample by comparison of the weight before and after heating to 110 C for 15 hours. The fact that mines could be detected is an indication that the moisture level was sufficiently low. Figure 1 shows the results for a dummy anti tank mine (23 7 cm 2, PVC shell, 2.3 kg melamine filling) and the DLM2 [16] using the Cf source. The mines were buried at respectively 10 cm and 5 cm. We did not bother to level the sand or to get an even sand surface. A scan speed of 1.8 m/s was used. The top image shows the raw pixel counts as obtained by the data acquisition system. The central line of high intensity is caused by fast neutrons which come from the source directly. Since the source is very close to the tubes these fast neutrons cause a high rate in the central pixels of the detector in spite of their low detection efficiency. This device specific rate can be easily corrected for. In the raw image the mines are not easily distinguishable. The background in the image is in real situations not known because of the impossibility to scan the surface without the mine. The background is therefore removed by analysis of the statistical fluctuations of the pixel contents.

4 V. Bom The image shown in the middle is obtained after removal of the background and applying a Gaussian filter. The two mines are now clearly visible at positions 210 (AT mine) and 475 (DLM2) along the image. The shape of the mine images is approximately circular and has an extension of about a meter. The bottom image of Fig. 1 is obtained by changing the intensity scale to enhance the view of he DLM2 mine. The image also shows structures around positions 600 and at position 500 just above the DLM2. These structures are likely due to variations in the density of the sand or to small hills and valleys of the sand surface. 4. Conclusions The image of a mine must be distinguishable from the background fluctuations to be detected. The size and intensity of these background structures on a given stretch of soil determine the observational limit of the least size of mine and the greatest detection depth. The fluctuations are determined by the soil moisture level, by the level of homogeneity of the soil and by counting statistics. The counting statistics in a scanning NBS system become poor at high scan speeds and improve by applying stronger sources. Figure 1 shows that with a relatively small neutron flux the scan speed can already be in the meter/minute range. While the influence of counting statistics can be minimized by employing stronger sources, the size of background fluctuations due to soil inhomogeneities is independent of source strength. First results indicate that in the Delft test lane anti-tank mines can be found at depths of 20 cm at a speed of 30 mm/s using a Cf source of 5 10 4 n/s. This limit may be different when the detector is used on other soils. Tests are planned in Egypt on desert soils where ESCALAD will be tried with real land mines. Acknowledgments Our thanks goes to the IAEA for supporting the research into humanitarian demining at the Nuclear Research Center of the Egyptian Atomic Energy Authority. REFERENCES 1. PLANT ASSESSMENT TECHNOLOGY (PAT) GROUP, Industrial Technology Division. (2004) Levscan (neutron backscattered techniques). Spot level and interface measurements. [Online]. Available: http://www.mint.gov.my/ PRODUCTS/BTI/PAT/LEVSCAN.htm 2. Nuclear Scanning Services, Inc. (1997) Level and interface measurement. [Online]. Available: http://www.nuclearscanning.com/ neutron.htm 3. NORDTEST. (1998) Building structures, moisture content: Neutron method. NT BUILD 346. [Online]. Available: http://www.nordicinnovation.net/ nordtestfiler/build346.pdf 4. F.D. Brooks, A. Buffler, and S.M. Allie, Detection of plastic land mines by neutron back scattering, in 6th International Conference on Applications of Neutron Science, Crete, Greece, jun 1999. 5. C. Datema, V.R. Bom, and C.W.E. van Eijk, Land mine detection with the neutron back scattering method, IEEE Trans. Nucl. Sci., vol. 48, no. 4, pp. 1087 1091, aug 2001. 6.. (2002, apr) DUNBLAD, the Delft University Neutron Backscattering LAndmine Detector. Proceedings of the Fifth International Symposium on Technology and the Mine Problem. [Online]. Available: http://www.demine.org/scot/ Papers/pdfpapers.html 7. G. Nebbia et al., DIAMINE (detection and imaging of anti-personnel landmines by neutron backscattering tewchnique), in Proceedings of the Fifth International Symposium on Technology and the Mine Problem, apr 2002. [Online]. Available: http://www. demine.org/scot/papers/pdfpapers.html 8. J. Csikai, B. Király, and R. Dóczi, Application of neutrons to plastic landmines detection, IAEA/PS/RC-799-2, pp. 46 54, sep 2001, cd available from IAEA, Vienna. 9. C. Datema, V.R. Bom, and C.W.E. van

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