The EURITRACK Project: Status of a Tagged Neutron Inspection System for Cargo Containers

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1 The EURITRACK Project: Status of a Tagged Neutron Inspection System for Cargo Containers B. Perot a, G.Perret a, A. Mariani a, J.-L. Ma a, J.-L. Szabo b, E. Mercier b, G. Sannie b, G. Viesti c, G. Nebbia c, S. Pesente c, M. Lunardon c, P. Formisano c, S. Moretto c, D. Fabris c, A. Zenoni d, G. Bonomi d, A. Donzella d, A. Fontana e, G. Boghen e, V. Valkovic f, D. Sudac f, M. Moszynski g, T. Batsch g, M. Gierlik g, D. Woski g, W. Klamra h, P. Isaksson h, P. Le Tourneur i, M. Lhuissier i, A. Colonna j, C.Tintori j, P. Peerani k, V. Sequeira k, M. Salvato k a Commissariat à l Energie Atomique, St Paul-lez-Durance Cedex, France b Commissariat à l Energie Atomique, Gif-Sur-Yvette Cedex, France c INFN and Dipartimento di Fisica Università di Padova, I Padova, Italy; d INFN and Dipartimento di Ingegneria Meccanica Università di Brescia, Brescia, Italy e INFN and Dipartimento di Fisica Nucleare e Teorica Università di Pavia, Pavia, Italy f Institute Ruder Boskovic, Zagreb, Croatia g Soltan Institute for Nuclear Studies, PL Otwock-Swierk, Poland h Department of Physics Alba Nova of Royal Institute of Technology, Stockholm, Sweden i EADS-SODERN, 20 Av. Descartes Limeil-Brévannes Cedex, France j CAEN S.p.A., Viareggio, Italy k European Commission Joint Research Centre IPSC CCR, I Ispra, Italy The EURopean Illicit TRAfficing Countermeasures Kit project is part of the 6th EU Framework program, and aims at developing a neutron inspection system for detecting threat materials in cargo containers. Neutron interaction in the container produces specific gamma-rays used to determine the chemical composition of the inspected material. The use of D+T tagged neutrons allows the inspection of a suspect voxel identified by previous x-ray scan. The EURITRACK project consists in developing: a transportable deuterium-tritium neutron generator including a position sensitive alpha detector (8 8 matrix of YAP:Ce crystals), fast neutron and gamma-ray detectors, front-end electronics to perform coincidence and spectroscopic measurements, and an integrated software which manage detector positioning, data acquisition, processing and analysis. All components have been developed and tested in Laboratory condition with neutron beams, their integration in the final portal will start in September 2006, whereas a field demonstration in a European seaport is planned for The provisional performances of the system were assessed by Monte Carlo calculations validated by experiments in which elemental or complex target were detected in cargo containers filled with different matrices (Fe-based or wooden). The status of the EURITRACK project will be presented. 1. Introduction Non-intrusive inspection of cargo containers has become a key issue in the fight against terrorism. Todays inspections are mainly based on imaging with x-ray or gamma-ray scanners. Fast neutrons can be additionally employed to deduce information about elemental composition of the 1

2 2 G. Viesti Figure 1: General view of the EURITRACK Tagged Neutron Inspection System. transported goods [1]. Gamma-rays emitted in fast neutron induced reactions characterize carbon, oxygen and nitrogen, which are the major components of explosives or narcotics. In addition, the associated particle technique [2] makes it possible to inspect a specific region of the container with an improved the signal-to-noise ratio. The D+T fusion reaction is used to produce a 14 MeV neutron and an alpha particle, which are emitted nearly back to back, direction and timeof-flight of the neutron being then deduced from the measurement of the associated alpha particle. The Tagged Neutron Inspection System (TNIS) concept has been studied in Europe during the last few years [3 5]. The EURITRACK project aims at developing a second generation TNIS for cargo containers to be used in connection with existing x-ray scanners [6]. It is proposed that the x-ray scan would determine the suspect voxel inside the cargo container to be inspected by the TNIS. Consequently it is not planned to scan the entire container with neutrons. A general view of the system is presented in Fig. 1. The portable sealed-tube neutron generator includes a 8 8 matrix of YAP:Ce alpha particle detectors coupled to a multianode photomultiplier [7]. High volume NaI(Tl) detectors are located around the cargo container to detect the neutron-induced gammarays with good efciciency. The neutron attenuation across the container is monitored by a 5 5 BC501A liquid scintillation detector. Detectors are equipped with fast photomultiplier tubes to achieve nanosecond time resolution [8]. A dedicated front-end electronics is used to verify coincidences between any alpha and gamma-ray detectors [9]. The EURITRACK TNIS components were developed during year 2005 [6 8]. Their integration into a first version of the TNIS was completed at Institute Ruder Boskovic (IRB) in Zagreb, Croatia, and followed by detection tests. The design and expected performances of the system were previously studied using Monte Carlo simulation [10,11], showing in particular that the TNIS can detect, in 10 minutes, a 100 kg block of TNT explosive hidden in a container fully filled with iron freight of 0.2 g/cm 3 mean density, which is the reference case of the EURITRACK project. In the following, we present the laboratory verification of the expected sensitivity.

3 The EURITRACK Project 3 Figure 2: The mixed target, including a liquid nitrogen flask, graphite blocks and water bottles, placed in the centre of a container fully filled with iron boxes. Boxes are contain wire balls to achieve a 0.2 g/cm 3 mean iron density. 2. Experimental Results Figure 2 shows a picture of the experimental tests at IRB. The presence of the explosive was simulated in this particular experiment by an equivalent mixed target composed of liquid nitrogen, graphite and water, whereas the container is loaded with iron boxes filled with wire balls. More experimental tests were also performed by using paper that exhibits the same C/O ratio of TNT. Figure 3 shows the alpha-gamma coincidence time spectrum and the random-backgroundsubtracted energy spectrum of the target, for long and short measurement times. One can clearly identify on the time spectrum the main features of the setup. The first peak, which is used as the zero time reference, corresponds to the neutron generator itself. Events at negative time are due to random coincidences. The second peak at about 7 8 ns is related to the first walls of both the cargo container and the iron box located in front of the target. Then the signal of the wire balls filling can be observed, up to a small peak near ns due to the second wall of the box. The main peak centred at ns corresponds to the nitrogen flask whereas the smaller peak at 34 ns comes from graphite and water, which appear in the same time window. Note that with a real 100 kg TNT block, nitrogen, carbon and oxygen would produce a single time peak with different relative γ-ray yields in the energy spectra. The present experimental setup enhances, indeed, the signal of the nitrogen flask, which is closer to the neutron source and shields graphite and water from the tagged neutron beam. Finally, after another small peak at ns, which is related to the first wall of the iron box located after the target, the last peak near ns marks the second walls of this box and of the cargo container. The full-energy peaks of nitrogen (2.3, 3.7, 4.4, 5.1, and 7.0 MeV), carbon (4.4 MeV) and oxygen (2.7, 3.7, 6.1, and 7.1 MeV) and associated escape peaks are either clearly visible or can be guessed in the long measurement energy spectrum. The peaks of aluminium (1.8, 2.2, and 3.0 MeV), which mainly constitutes the liquid nitrogen dewar, are also present. An unfolding algorithm, using a library of pure elements energy spectra, is used to separate the relative contributions of all elements. Despite statistical uncertainties, it is possible to discern these signatures also in the 10-minutes energy spectrum of Fig. 3. Given that the random background increases with the square of the total neutron emission rate, whereas the useful signal increases linearly, the signal-to-randombackground ratio is logically 4 times smaller than in the long measurement being the rate about 4 times higher: neutrons/s Vs neutrons/s. Note that at neutrons/s, the total count rate of all the gamma-ray detectors is close to 105 Hz, when the low energy threshold of the discriminators is set at about 2 MeV. 3. Conclusions The tests of the EURITRACK TNIS performed at IRB Zagreb demonstrate its ability to detect, in 10 minutes, a multi-element sample hidden in a container fully-filled with iron goods of 0.2 g/cm 3 mean density. The alpha-gamma coincidence time spectrum reveals a strong heterogeneity in the centre of the container, where car-

4 4 G. Viesti Figure 3: Alpha-gamma coincidence time spectra (left column) and random-background-subtracted energy spectra (right column) of the mixed target in Fig. 2. In the time spectra, the investigated part of the container is shown in grey whereas the hatched area corresponds to the random background region. A 760 minutes long measurement at neutrons/s total neutron emission is presented to ease peaks recognition (top row) and a 10 minutes long measurement at neutrons/s illustrates the TNIS capability to detect the illicit item in a realistic time (bottom row). bon, oxygen and nitrogen have been identified in the energy spectrum. Energy spectra of the other regions of the container, which are filled with iron, show no significant peak above 2 MeV. This confirms the presence of the hidden goods in the cargo. An unfolding algorithm determines the relative contributions of the elements of interest (C, N, O, Al, Fe, ), allowing material identification. Several tests with other cargo configurations have been performed at IRB. Finally, the TNIS has been integrated at CEA Saclay (France) at the end of 2006, on a mechanical portal that will allow a fine positioning of the neutron generator and detectors around the inspected truck. This has been the last laboratory step before the final demonstration, planned during 2007, at the Rijeka seaport in Croatia. Acknowledgments This work is supported by the European Union through the EURopean Illicit TRAfficking Countermeasures Kit project (FP IST-2 Proposal/Contract ). REFERENCES 1. T. Gozani, Nucl. Instr. Meth. B213 (2004) V. Valkovic et al., Nucl. Instr. Meth. 76 (1969) G. Viesti et al., Nucl. Instr. Meth. B241 (2005) G. Nebbia et al., Nucl. Instr. Meth. A533 (2004) S. Pesente et al., Nucl. Instr. Meth. A531 (2004) B. Perot et al., Proc. SPIE Vol. 6213, Non- Intrusive Inspection Technologies; G. Vour-

5 The EURITRACK Project 5 vopoulos and F. P. Doty Eds., p G. Nebbia et al., Proc. IEEE Nuclear Science Symposium and Medical Imaging Conference, Puerto Rico M. Gierlik et al., IEEE Trans. Nucl. Science submitted. 9. M. Lunardon et al., this proceeding. 10. G. Perret et al., Journal of Physics: Conference Series 41 (2006) A. Donzella et al., Journal of Physics: Conference Series 41 (2006) 233.