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Material characterization in cemented radioactive waste with the
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associated particle technique
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C. Carasco*a, B. Perota, A. Mariania, W. El Kanawatia, V. Valkovicb, D. Sudacc, J. Obhodasc.
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a
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Durance, France
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b
A.C.T. d.o.o., Prilesje 4, Zagreb, Croatia
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c
Institute Ruder Boskovic, 54 Bijenicka c. 10000 Zagreb, Croatia
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Abstract
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The elemental characterization of materials constituting radioactive waste is of great
CEA, DEN, Cadarache, Nuclear Measurement Laboratory, F-13108 Saint-Paul-lez-
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importance for the management of storage and repository facilities. To complement the
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information brought by gamma or X-ray imaging, the performances of a fast neutron
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interrogation system based on the Associated Particle Technique (APT) have been
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investigated by using MCNP simulations and by performing proof-of-principle experiments.
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APT provide a 3D localisation of the emission of fast neutron induced gamma rays, which
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spectroscopic analysis allows to identify the elements present in specific volumes of interest
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in the waste package. Monte Carlo calculations show that it is possible to identify materials
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enclosed behind the thick outer envelop of a 870 litres cemented waste drum, provided that
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the excited nuclei emit gamma rays with a sufficient energy to limit photon attenuation.
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Neutron attenuation and scattering are also predominant effects that reduce the sensitivity and
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spatial selectivity of APT, but it is still possible to localise items in the waste by neutron
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time-of-flight and gamma-ray spectroscopy. Experimental tests confirm that the elemental
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characterisation is possible across thick mortar slabs.
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Pacs: 28.41.Kw, 25.40.Fq, 24.10.Lx, 89.20.Bb
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Keywords: associated particle technique, fast neutron inspection, radioactive waste
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characterization, Monte Carlo simulation.
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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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The management of radioactive waste is a great concern in countries involved with
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electronuclear programs [1]. In view of waste repository, the amount of toxic or reactive
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chemicals must be controlled, Though the production of recent waste includes quality control
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allowing to know the composition of the drums. However, the content of older wastes
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remains uncertain. Nuclear non destructive techniques can be used to characterize the
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radiological content of historical waste, such as passive gamma-ray spectroscopy and neutron
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measurements [2]. The physical and chemical properties of the waste can be controlled by
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gamma or X-ray imaging [4], which provide the position, shape and density of the materials,
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and by “neutron in – gamma out” techniques bringing complementary information about their
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chemical composition [5,6]. Fast neutron interrogation is a promising technique for large and
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dense objects [7,8], APT [9] being one of the most sensitive methods. Tagged Neutron
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Inspection Systems (TNIS) have been recently developed for security applications [11, 12].
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During the last decades, CEA has produced a great amount of 870 litres drums filled with
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radioactive waste blocked in cement. The use of APT to characterize waste materials is a real
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challenge due to a density close to 2 g.cm-3, much larger than in cargo containers where the
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average density is around 0.2 g.cm-3 [13]. This paper presents the calculated performances of
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a TNIS dedicated to 870 litres drums, as well as proof-of-principle experiments to test APT
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neutron interrogation across thick mortar slabs.
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2. Numerical simulation
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The radioactive waste are mixed with cement and placed in an iron drum, an external cement
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ring blocking this embedded waste, see Fig. 1. Due to aluminium-mortar reactions producing
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hydrogen, which could weaken the waste matrix, the control of this element is of importance
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[14]. Fig. 1 shows a 870 litres drum containing a 8 kg aluminium block with a 2.7g.cm-3
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density surrounded by a 2.35 g.cm-3 concrete matrix. The neutron source is shielded with
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paraffin and lead with an aperture for the tagged beam. Fig. 1 shows that the neutron tagged
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beam is spread when entering into the drum due to its high density and hydrogen content.
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Calculation has been preformed with MCNPX and the ENDF-B-VII library. Type 5 tallies
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estimate the time-energy dependent photon flux at the level of the NaI(Tl) scintillation
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detectors. In order to obtain realistic spectra, MCNP results are smeared with a 2.1 ns FWHM
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normal distribution and the 5”×5”×10” NaI(Tl) energy response function [15]. Time spectra
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associated to the 135° tally are presented in Fig.2. The aluminium density being similar to the
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one of mortar, the block cannot be distinguished in the time of flight (TOF) spectrum.
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However, the TOF area of interest can be selected on the basis of γ- or X-ray imaging. By
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selecting the time region between 31.5 ns and 35 ns geometrically corresponding to the block,
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the 2.211 MeV aluminium peak is visible in the gamma-ray spectrum, whereas it is not
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present in the adjacent slice.
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Another calculation has been performed with a 20cm×20cm×20cm iron box with 0.5 cm thick
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walls instead of the aluminium block. It is filled with 5 litres of 0.9 g.cm-3 density CH2 to
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simulate the possible presence of a motor filled with oil in the waste. The presence of organic
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compounds is indeed a critical issue in alpha bearing waste drums due to hydrogen
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production [16]. The smeared TOF and energy spectra for the 90° tally are reported in Fig. 3.
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Oil, which density is smaller than concrete, corresponds to the hollow centred at 36 ns. The
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associated energy spectrum reveals the presence of carbon, which is not seen in the adjacent
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slice.
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3. Experiment
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To illustrate APT capability, experimental tests have been performed with the EURITRACK
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system developed within the EU 6th Framework Program [17]. The cemented waste package
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is simulated with 6 cm thick mortar slabs made with calcium carbonate. For practical reasons,
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the waste mock-up is placed inside a cargo container and sixteen 5”×5”×10” NaI(Tl) gamma-
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ray detectors located 186 cm above the package are used. This setup, which is not optimal,
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nevertheless illustrates the ability of the TNIS to detect materials across concrete. A graphite
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block is placed between the mortar slabs with a 12 cm additional mortar layer to simulate an
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inspection deep inside a waste package.
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The 30 min. measurement presented in Fig.4 has been calculated with a 4.107 n.s-1 total
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neutron emission, from which the tagged neutron beam represents about 1%. On the alpha-
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gamma coincidence time spectrum, the peak at 13 ns corresponds to materials surrounding
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the neutron source. The two large peaks located between 20 and 25 ns correspond to the 6 cm
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and 12 cm mortar layers. The peak at 33 ns is related to the graphite block, followed by the
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rear mortar wall at 38 ns. The large bump at the end of the spectrum is due to neutrons
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scattered towards the detectors and to materials located behind the mortar slabs. The net
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energy spectra corresponding to the hatched areas of the time spectrum have been obtained
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after subtraction of the random background estimated in negative times and they have been
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unfolded on the basis of a database formed by the signatures of pure elements [18], allowing
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the main elements identification. Carbon indicates the presence of the graphite block. The
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EURITRACK energy threshold was 1.35 MeV when these data were taken. Thus, elements
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having no characteristic peaks above 1.35 MeV, such as zinc or lead found in Fig. 4, are
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incorrectly found by the algorithm though they are not present. Such errors have been
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suppressed since by reducing the threshold to 0.6 MeV and by constructing the corresponding
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elemental signature database [19]. Nevertheless, low energy photons will be greatly
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attenuated by the external cement layer of the 870 litres waste drum, thus preventing the
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detection of elements with only low energy gamma rays.
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Another test has been performed with aluminium blocks, see Fig. 5. Aluminium is identified
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on the TOF spectrum between 26 ns and 35 ns, and its gamma rays clearly appear on the
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associated energy spectrum. As previously, the unattended presence of sodium in the
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unfolding result is a consequence of the 1.35 MeV threshold. Other elements and materials
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relevant for radioactive waste management, such as chlorine (PVC waste), magnesium,
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wood, water, and boron have also been investigated, showing other possible applications
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[20].
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4. Conclusion
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Monte Carlo simulations show that APT allows to identify and locate elements such as
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carbon and aluminium in a 870 litres drum filled with cemented waste. Preliminary
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experiments also illustrate the ability of APT to detect materials across thick mortar slabs.
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Further investigations have to be performed by numerical simulation to study the random
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background brought by the radiological emission of the waste, as well as counting statistics in
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a limited acquisition time. Material identification in the middle of the drum will also be
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studied because neutron attenuation and scattering are expected to significantly deteriorate
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APT sensitivity and spatial selectivity. On the other hand, the thick concrete envelope
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surrounding the waste is critical for the detection of low energy gamma rays. Thus, the
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application of APT to large and dense waste packages can only be envisaged for elements
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with high energy gamma signatures such as the 2.211 MeV and 4.439 MeV lines of
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aluminium and carbon, respectively.
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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 in the development and commissioning of the
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system, namely CEA LIST, INFN, JRC, CAEN, SODERN, IPJ, and KTH, as well as Rijeka
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Seaport and Custom authorities, who allowed that the system be implemented and the
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measurements be performed on their grounds.
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Figure Captions
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Figure 1. Numerical model of the neutron inspection interrogation system for 870 L
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radioactive waste drum. The position of MCNP tallies is represented by the black points
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placed in front of the NaI(Tl) scintillators. The angles are relative to the tagged neutron beam
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axis. The dots represent the positions of tagged neutron interactions.
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Figure 2. TOF spectra associated to the 135° tally without energy cut (full line, top left) and
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with an energy cut between 2.20 MeV and 2.22 MeV to select the 2.221 MeV aluminium
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gamma-ray (dashed line, top right), thus revealing the position of the aluminium block
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between 31.5 ns and 35 ns. After time and energy smearing of the data issued by the 135°
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tally, TOF spectrum with an energy cut between 2.10 MeV and 2.36 MeV (middle) and
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energy spectrum obtained with a TOF cut between 31.5 ns and 35 ns (right).
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Figure 3. TOF spectrum obtained after smearing the time data associated to the 90° tally with
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a normal distribution having a standard deviation of 0.9 ns (left) , energy spectra obtained
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when selecting the region between 33 ns and 38 ns (middle) and the region between 29.8 ns
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and 32.5 ns (right), both time regions being represented in grey area and diagonally hatched
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lines in the TOF spectrum, respectively.
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Figure 4. TOF spectrum (top left) associated to the top right setup, showing the mortar front
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wall (diagonal hatches), graphite block (horizontal hatches), and mortar rear wall (vertical
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hatches). The corresponding energy spectra (from left to right) and unfolding results highlight
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the presence of carbon between the mortar blocks.
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Figure 5. TOF spectrum related to the measurement with 42 kg aluminium placed inside the
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mortar container (left) showing the TOF area associated to the aluminium blocks (horizontal
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hatched area) and energy spectrum corresponding to the aluminium region (left).
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Fig 1
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Fig 2
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9
1
2
Fig 3
mortar
mortar
mortar
C, 4.439 MeV
MeV4.439M
EV
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Fig 4
Al, 1.810 MeV
Al, 2.211 MeV
MeV4.439M
MeV4.439M
EV
EV
Al, 3.002 MeV
MeV4.439M
EV
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Fig. 5
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