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Use of the associated particle technique to characterize materials in
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radioactive waste
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C. Carasco*a, B. Perota, V. Valkovicb, D. Sudacb.
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a
Commissariat à l’Energie Atomique, 13108 St Paul-lez-Durance, France
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b
Institute Ruder Boskovic, 54 Bijenicka c. 10000 Zagreb, Croatia
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Abstract
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Test measurements have been performed in order to study the capability of the associated
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particle technique to characterize the elemental composition of materials constituting
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radioactive waste enclosed in cemented packages. The tests have been performed with the
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EURITRACK Tagged Neutron Inspection System, which was originally designed to
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complement X-ray scanners in the detection of explosives and other illicit materials hidden in
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cargo containers. In order to simulate cemented radioactive waste, a mortar cube filled with
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various materials has been build and interrogated by a 14-MeV tagged neutron beam.
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Inspections with the TNIS yield information about the position and chemical composition of
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the materials. Inspection results are reported and discussed.
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Pacs: 25.40.Fq, 28.20.-v, 29.25.Dz, 89.20.Bb
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Keywords: associated particle technique, fast neutron inspection, radioactive waste
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characterization.
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*Corresponding author: Tel: +33 442256130, Fax: +33 442252367, cedric.carasco@cea.fr
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1. Introduction
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Great improvements have been achieved during the last few years in the development and
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optimisation of Tagged Neutron Inspection Systems (TNIS), in order to identify explosives
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hidden in containers or in wallets [1,2]. The EURopean Illicit TRAfficking Countermeasures
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Kit (EURITRACK) system [3,4], developed within the EU 6th Framework Program, has been
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assembled and commissioned in the Croatian seaport of Rijeka where it has been used
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successfully in a test campaign with real cargo containers [5]. The use of the Associated
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Particle Technique (APT) implemented in the EURITRACK TNIS has been investigated for
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radioactive waste characterization. The objective is to identify and determine the relative
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proportions of the elements constituting the waste materials, in view to optimize waste
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management. This paper presents the system and the tests performed to demonstrate the
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ability of the APT to identify materials of interest in radioactive waste, because they are
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prohibited, or limited, or with a significant effect on non-destructive assay, etc.
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2. Setup presentation.
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The system uses a neutron generator based on the d+t→α+n fusion reaction, where a 14-MeV
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neutron and an alpha particle are emitted almost back to back. The particularity of the
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associated particle technique [6] is to detect the alpha particle in coincidence with a gamma-
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ray produced after non elastic reactions. The detection of the alpha particle by a 64-element
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YAP:Ce scintillation array coupled to a multi-anode photomultiplier [7,8] allows to define
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the direction of the neutron. The detection of the gamma ray by NaI(Tl) scintillators [9] in
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coincidence with the YAP:Ce assembly with a dedicated electronics [10] permits to build a
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time-of-flight (TOF) spectrum from which the flight path of the neutron is inferred, providing
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in this way an in-depth information. Since the gamma-ray energy is specific to the element
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with which the neutron interacted [10], it is also possible to get information about the
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chemical composition the inspected objects. The gamma-ray energy spectrum obtained from
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a given area of the TOF spectrum is a mixture of the signatures of the nuclei from the
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corresponding region of the inspected object. To get a chemical insight into the inspected
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area, the measured energy spectrum is unfolded between 1.35 and 8 MeV on the basis of the
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signature of elements from a database [12]. Finally, by using appropriate correction factors
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[13], it is possible to recover the chemical proportions of the detected elements.
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The system which has been used for the tests is drawn in Fig.1. The radioactive waste
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package has been simulated with a 6-cm thick mortar container. For practical reasons, the
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mortar container has been build inside a 3-mm iron cargo container on top of which a set 16
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5”×5”×10” NaI(Tl) shielded detectors have been placed. Put together, the mortar and iron
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layers mimic iron containers in which radioactive waste are blocked by cement.
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The mortar composition itself has been inspected with the TNIS. The energy spectrum
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associated to the mortar, shown in Fig.2, revealed no signature from silicon (e.g. the 1.779-
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MeV characteristic peak) while SiO2-based mortar was expected. The spectrum rather
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indicated that it is mainly composed of carbon and oxygen, with a carbon-to-oxygen ratio of
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3.2±0.3 that agrees with the chemical composition of calcium carbonate (CaCO3). The mortar
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manufacturer confirmed this information a posteriori. After determining its chemical
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composition, the mortar cubic package has been filled with materials of interest, which are
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prohibited or stringently controlled in cemented radioactive waste. Detection tests have been
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performed with samples of graphite, wood, grease, water, magnesium, aluminum, PVC,
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mercury and boron in the mortar container.
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3. Detection tests.
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The results presented in the following have been obtained after 30-min acquisitions with a
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4.107 n.s-1 total neutron flux. Due to the limited size of the YAP:Ce detector and to its
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distance to the neutron source, the tagged neutron beam flux represents only 1% of the total
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neutron flux.
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In the first series of tests, a 15-kg graphite block has been placed inside the mortar container.
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The setup is shown in Fig.3 together with the associated position spectrum, which is a
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histogram of the distances between the neutron source (i.e. the tritium target inside the
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generator) and the neutron interaction point inside the setup. The energy spectra
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corresponding to well-identified areas of the setup (hatched areas on the position spectrum)
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are also represented in Fig. 3. The position spectrum also shows a flat region at negative
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positions that corresponds to the random background, which energy spectrum is shown in
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Fig.4. This spectrum shows no specific feature (no peaks) but it has been subtracted to obtain
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the net energy spectra of Fig. 3. The first peak on the position spectrum of Fig. 3, located at
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300-mm, is related to the walls of the set containing the neutron generator. The large peak at
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800-mm corresponds to the front wall of the mortar container, the one at 1400-mm is related
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to the graphite block and the rear wall of the mortar setup is represented by the peak at 1600-
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mm. The large bump at the end of the spectrum is due to scattered neutrons and the
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transmission set of the EURITRACK system, which was not used here. The net energy
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spectra of Fig. 3 have been unfolded on the basis of a database formed by the signatures of
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several pure elements [12]. The unfolding fits are shown in dashed lines in Fig.3 . The main
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elements contributing to a given energy spectrum have been identified by the unfolding
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algorithm and their relative contributions indicated. These contributions represent the relative
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number of counts in the energy spectrum associated to each element. The graphite is clearly
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indicated by a strong enhancement of the carbon fraction in the energy spectrum
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corresponding to the inner part of the mortar container. The presence of zinc or lead is due to
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the lack of characteristic peaks in the corresponding gamma-ray inelastic energy spectra [12].
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For the same reason, metals, heavy elements and neutrons, which have also been incorporated
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in the energy spectra database, are difficult to recognize by the unfolding algorithm between
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1.35- and 8-MeV. However, since it is known that some of these elements show fast neutron
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induced gamma rays at lower energy [11], a database built with a low-energy threshold of
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about 600-keV would lead to a better elemental identification. A second test consisted in
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keeping the 15-kg graphite block inside the mortar container and adding a 12-cm mortar layer
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behind the 6-cm thick front mortar wall, as shown in Fig.5. It is possible to identify the two
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front layers of mortar in the position spectrum and the peak associated to the graphite block is
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still distinguishable. The net energy spectra obtained after subtracting the random background
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show that in spite of the 18-cm thick mortar layer in front, the graphite block is still clearly
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indicated in the middle slice by a strong enhancement of the carbon fraction.
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A measurement has then been performed by filling the mortar container with 67 kg of wood.
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The setup is shown in Fig.6 with the corresponding position and energy spectra. As for
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graphite, the presence of wood is indicated by an enhancement of the carbon fraction in the
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middle slice. In a general way, any organic compound will be identified by a strong carbon
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contribution in the energy spectrum or by a change of the carbon-to-oxygen ratio compared to
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the one of mortar. Fig. 7 shows a 8-L motor oil drum mixed with about 30- to 40-kg iron junk
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and in this case, oil is indicated by a decrease of the oxygen fraction in the middle slice. The
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capacity to detect water in a radioactive waste package is shown in Fig. 8, which presents the
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detection test of a 5-L water bottle located inside the mortar container. The bottle is here
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identified in the middle slice due to an increase of the oxygen fraction in the corresponding
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energy spectrum, but this increase is small because the calcium carbonate mortar already
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contains a high fraction of oxygen (CaCO3). It is worth noting that the mortar contribution is
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not negligible in the energy spectrum corresponding to the water time window in Fig. 8,
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because of an overlapping of the time spectrum peaks. Consequently, water is difficult to
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detect when surrounded by materials already containing high oxygen fractions like CaCO3- or
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SiO2-based mortars. On the other hand, it should be possible to detect water in metallic waste
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through the presence of oxygen. Fig.9 shows test results obtained with pure magnesium and
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aluminium samples, and with PVC (C2H3Cl), placed in the mortar container. Magnesium is
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detected by the presence of its 1.369-MeV characteristic peak (though partially cut by the
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low-energy threshold) and aluminium by the 1.809-, 2.211- and 3.004-MeV gamma rays.
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The presence of PVC is indicated by an increase of the carbon fraction in the energy
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spectrum and by the presence of chlorine with the 1.727- and 1.763-MeV overlapping peaks,
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and the 2.127-MeV peak. On the other hand, boron proved to be difficult to identify as it is
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shown in Fig. 10, which presents the energy spectrum associated to a 5-Kg sample of boric
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acid (B(OH)3) compared to the one corresponding to pure oxygen (pure water measurement).
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Fig. 10 shows that the boric acid energy spectrum is dominated by oxygen. Since counting
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statistics is poor, it is envisaged to reproduce this measurement with an increased acquisition
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time during a next series of tests. However, it seems that boron could be difficult to detect in
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radioactive waste because it represents only a small fraction (less than 10%) of the elements
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presents in the waste. Eventually, Fig.10 shows that it is not possible to detect mercury with
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the APT, since no characteristic peak can be seen in the fast neutron induced gamma-ray
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spectrum between 1.35- and 8-MeV.
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5. Conclusion
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Measurements performed with the EURITRACK Tagged Neutron Inspection System (TNIS)
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show the ability of the Associated Particle Technique to detect materials that are forbidden or
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strictly controlled in cemented radioactive waste packages. The TNIS allows to identify and
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locate, via gamma-ray spectroscopy and neutron time-of-flight, chemical elements inside the
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waste package. An element is identified by its characteristic gamma rays with an unfolding
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algorithm. The TNIS succeeded in detecting graphite, organic materials, PVC, aluminium
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and magnesium inside a mortar container simulating a cemented radioactive waste package.
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However, the system showed difficulties to detect water and boron and seems not to be able
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to identify mercury. Improvements can be brought to the TNIS in order to increase its
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performances. For instance, the database used to unfold the energy spectra can be modified to
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take advantage of low-energy gamma rays, which are strongly needed to identify metals and
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to increase the discrimination power of the unfolding algorithm. This last can also be
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improved. Finally, the geometry of the setup is not optimized. Indeed, the EURITRACK
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system has been designed for the inspection of cargo containers, which shape and size are
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completely different to those of radioactive waste. A dedicated setup would greatly improve
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the accuracy of the system.
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Acknowledgments
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The development and implementation of the EURITRACK inspection system has been
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supported by the European Union through the EURopean Illicit TRAfficking
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Countermeasures Kit project (FP6-2003-IST-2 Proposal/Contract 511471). We would like to
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thank the EURITRACK partners who took part to the development and commissioning of the
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system: CEA LIST, INFN, JRC, CAEN, SODERN, IPJ, KTH.
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Special thanks to the Rijeka seaport authorities, which allowed the measurements to be
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performed.
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Figure Captions
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Figure 1. Drawing of the setup which has been used for the detection test measurements.
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Figure 2. Energy spectrum associated to the mortar showing the main elements which have
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been identified by the unfolding algorithm. The numbers indicate the counts associated to the
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elements in the total energy spectrum.
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Figure 3. Position spectrum (top left) corresponding to the measurement with a graphite
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block inside the mortar container (top right), with the energy spectra corresponding to the
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front mortar wall (bottom left and diagonally hatched area in the position spectrum), the
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graphite block (bottom middle and horizontally hatched area in the position spectrum) and
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the rear mortar wall (bottom right and vertically hatched area in the position spectrum). Note
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that the neutron source is located on the left of the picture and that the tagged beam is going
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from left to right. The grey area of the position spectrum corresponds to the random
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background and the dashed lines of the energy spectra show the unfolding fits. The numbers
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indicate the counts associated to the elements in each energy spectrum given by the unfolding
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algorithm.
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Figure 4 Energy spectrum associated to the random background.
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Figure 5. Position spectrum (top left) corresponding to the measurement with a graphite
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block inside the mortar container in which a 12-cm layer has been added (top right), with the
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1
energy spectra corresponding to the front mortar walls (bottom left and diagonally hatched
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area in the position spectrum), the graphite block (bottom middle and horizontally hatched
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area in the position spectrum) and the rear mortar wall (bottom right and vertically hatched
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area in the position spectrum).
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Figure 6. Position spectrum (top left) corresponding to the measurement with wood filling
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the mortar container (top right), with the energy spectra corresponding to the front mortar
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wall (bottom left and diagonally hatched area in the position spectrum), the wood (bottom
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middle and horizontally hatched area in the position spectrum) and the rear mortar wall
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(bottom right and vertically hatched area in the position spectrum).
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Figure 7. Position spectrum (top left) corresponding to the measurement with a 8-L motor oil
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drum mixed with metal junk placed inside the mortar container (top right), with the energy
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spectra corresponding to the front mortar wall (bottom left and diagonally hatched area in the
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position spectrum), the oil and the metal junk (bottom middle and horizontally hatched area
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in the position spectrum) and the rear mortar wall (bottom right and vertically hatched area in
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the position spectrum).
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Figure 8. Position spectrum (top left) corresponding to the measurement with a 5-Lwater
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bottle placed inside the mortar container (top right), with the energy spectra corresponding to
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the front mortar wall (bottom left and diagonally hatched area in the position spectrum), the
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water (bottom middle and horizontally hatched area in the position spectrum) and the rear
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mortar wall (bottom right and vertically hatched area in the position spectrum).
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Figure 9. Position spectra (left) and energy spectra (right) associated to the horizontally
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hatched area of the position spectrum, for measurements where magnesium (top), aluminium
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(middle) and PVC (bottom) have been placed inside the mortar container.
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Figure 10. Energy spectra associated to boron (top, full line) oxygen (top, dashed line) and
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Mercury (bottom).
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