- Email: [email protected]
SP and its potential to investigate ceramic objects from the Brazilian cultural heritage
Applied Radiation and Isotopes 75 (2013) 6–10
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
The neutron tomography facility of IPEN-CNEN/SP and its potential to investigate ceramic objects from the Brazilian cultural heritage M.A. Stanojev Pereira a,n, R. Schoueri b, C. Domienikan b, F. de Toledo b, M.L.G. Andrade b, R. Pugliesi b a b
´gico e Nuclear (IST/CTN), Estrada Nacional 10, 2686-953 Sacave´m, Portugal Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa–Campus Tecnolo ~ Paulo, Brazil ´ria, 05508-000 Sao Instituto de Pesquisas Energe´ticas e Nucleares, Centro do Reator de Pesquisas, Av. Lineu Prestes 2242, Cidade Universita
H I G H L I G H T S c c c c
A neutron tomography facility was installed at a 5 MW nuclear research reactor. Tomography is obtained in 4000 s with a spatial resolution of 347 mm. A restored ceramic object was imaged and cracks and discontinuities could be seen. The facility will be employed to study objects left by the Brazilian Indians.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2012 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 31 January 2013
A neutron tomography (NT) facility was installed at the IEA-R1 nuclear research reactor of the Nuclear and Energy Research Institute IPEN-CNEN/SP. According to the determined operational characteristics, the time spent to obtain a complete tomography is 4000 s at a neutron flux of 1 106 n cm 2 s 1 and the best achievable spatial resolution in the image is 347 mm. The main objectives of this paper are to describe the facility as well as to demonstrate its potential to investigate ceramic objects from the Brazilian cultural heritage left by Indians. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Neutron tomography Neutron radiography Scintillator Archeology
1. Introduction Neutron imaging is a set of techniques which make use of neutrons as penetrating radiation to investigate the internal structure of an object. Because of the characteristics of the neutron–matter interaction these techniques are mainly employed to investigate hydrogenous substances such as water, water-logged ceramics, organic fibers, wood, seeds, rubber, resins, glue, etc., even wrapped by thick metal layers. Therefore they are ideal to investigate cultural heritage objects left by the Brazilian Indians, such as vessels, statues, musical instruments, ornaments, weapons, etc. A neutron image is obtained by irradiating the object in a uniform neutron beam and a converter screen transforms the transmitted neutron intensity into ionizing radiation which is
n
Corresponding author. Tel.: þ351 551131339993. E-mail addresses: [email protected], [email protected] (M.A.S. Pereira).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.01.030
able to sensitize certain media, forming the image. For radiography, films and track detectors can be used and for real-time a scintillator is used and the image is captured by a video camera. In both cases, the images are two-dimensional (2D) projections of the internal structure of the object. On the other hand the tomography technique provides a three-dimensional (3D) visualization of its internal structure (Hughes, 1957; Berger, 1965; Bjoern, 2006; Nikolay et al., 2011). Usually a tomography is obtained as shown in Fig. 1 (IAEA, 2008; Koerner et al., 2000; Banhart, 2008): the object to be inspected is positioned on a rotating table and is irradiated in the neutron beam; the neutrons transmitted by the object sensitize a scintillator, forming a 2D image which is captured by a video camera; the object is rotated by a small angular step in the beam and another image is captured. The file containing all the images is processed by the software Octopus to perform image reconstruction, and the software VGStudio provides a threedimensional view of the internal structure of the object. In order to avoid direct exposure of the camera’s CCD chip to radiation, a
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
7
Table 1 Main characteristics of the NT facility. Gamma filter-length Mean energy of neutron spectra Beam diameter Neutron flux at irradiation position(a) L/D ratio(b) Rotating table Voxel size(c) a b c
Bismuth 20 cm 7 meV Max. 15 cm 1 106 n cm 2 s 1 90 Angular step 0.91 0.291 mm
Au-foil method. L—source to object distance; D—inlet aperture of the collimator. provided by software Octopus.
Fig. 1. Main parts of a NT facility.
Fig. 3. Inside view of the NT shielding and the free space for irradiation.
Fig. 2. Top view of the IEA-R1 reactor operating at 4.5 MW.
plane mirror reflects the image to the camera which is usually installed at 901 with respect to the radiation beam. Because of the low light intensity generated, the scintillator, the mirror and the camera are installed inside a light tight box. The objectives of this work are to describe the NT facility of IPEN-CNEN/SP, determine its operational characteristics, namely the irradiation time to obtain a tomography and the spatial resolution in the image, and demonstrate its potential to investigate the internal structure of archeological objects of the Brazilian cultural heritage, left by Indians. Improvements to the facility are also discussed.
2. Description of the NT facility of IPEN-CNEN/SP The neutron imaging group of IPEN-CNEN/SP has installed a neutron tomography facility at the 5 MW pool type IEA-R1 nuclear research reactor, which became operational in 2010. The neutrons are driven to the NT facility through one of the irradiation channels or beam-holes (BH), inserted in the biological shielding of the reactor. Fig. 2 is a top view of the IEA-R1 reactor showing the core as well as the BH-08, where the facility is installed. The main characteristics of the NT facility are summarized in the Table 1. In order to minimize gamma radiation at the irradiation position, a 20 cm bismuth filter is installed within the BH-08. The images are obtained by using a scintillator coupled to a CCD video camera. The scintillator is the NE-426 (0.4 mm, 18 cm 24 cm) 6LiF/ZnS, with peak emission at 450 nm and the camera is an ANDOR model ikon-M, with CCD having 1024
1024 pixels (12 12 mm2 size) and provides 16 bit images. The camera has also a Peltier cooling device able to decrease the CCD temperature down to 90 1C. The lens used is a Nikon 58 mm/f1.2. The irradiations are performed inside the facility’s shielding in a free space with 2 m 2 m 1.2 m (width) (see Fig. 3). Some details of the irradiation position are shown in Fig. 4. In order to determine the operational characteristics of the facility, during the capturing process the camera’s CCD is kept at a constant temperature of 20 1C, the CCD chip array was fixed at 512 512 pixels, and the reactor power was 4.5 MW [IAEA, 2008]. For the present facility, the sample is rotated by 3601 and the irradiation time to acquire each one of the 400 images for the tomography is about 10 s. It was determined by means of the curve that relates gray level—GL as functions of the irradiation time—Ti. The curve, shown in Fig. 5, was obtained by performing irradiations in the direct beam (without object) and plotting the gray level intensity corresponding to each image, as functions of the irradiation time, varying in the range 2 oTio15 s. A total of 7 points has been plotted and the intensity of each point was evaluated by averaging the gray level intensities of about 5000 individual pixels. The standard deviations of the intensities are inserted in the points and they ranged from 0.1% to 2% of the plotted value. A straight line was drawn between the points to guide the eye and the behavior of ‘‘GL vs. Ti’’ is linear until about 12 s after which the pixels saturation begins. For reconstruction, the images must be free of saturated pixels; therefore they must be acquired in the linear region (Octopus, 2011). For the present data, a safe irradiation time that satisfies such requirement is about 10 s, that is, 80% of the maximal irradiation time that limits the linear region.
8
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 4. Details of the NT facility. Fig. 6. Gray level distribution and the fitted function.
the experimental data, are shown in Fig. 6. ESF ¼ p1þ p2 atanðp3ðxp4ÞÞ
ð2Þ
where p1, p2, p3 and p4 are free parameters and x is the scanning coordinate. The obtained value was Ui¼(347 726) mm and it corresponds to the full width at half maximum (FWHM) of the Line Spread function associated to (2) and is given by (3) [Wade et al., 1987] Ut ¼ 2=p3
Fig. 5. Behavior of the gray level as functions of the irradiation time.
Another important parameter of the NT facility is the spatial resolution, defined as the minimal distance between objects in such a way that they can be distinguished from each other. Usually it is quoted in terms of the total unsharpness—Ut, consisting of two components: the geometrical unsharpness— Ug from the angular divergence of the neutron beam responsible for the penumbra in the image, and the intrinsic unsharpness—Ui from the detection system (scintillator, detection geometry, and camera focus condition) and they are related by (Wade et al., 1987; Harms et al., 1972) Ut n ¼ Uin þ Ug n
ð1Þ
where 1 ono3. Since for the present facility L/D 90 (see Table 1), taking into account the irregular shape of the object (see Fig. 7), the geometrical unsharpness—Ug in the image varies between 347 and 555 mm (Berger, 1965). The intrinsic unsharpness was evaluated by scanning the gray level distribution in the image of a neutron opaque object, here a knife-edge gadolinium foil 127 mm in thickness, irradiated for 8 s in tight contact to the scintillator. In this case, the penumbra effect in the image is negligible (Ug 0) and according to (1), Ut Ui. The obtained distribution and the ‘‘Edge Spread Function’’—ESF , fitted to
ð3Þ
This value can be explained as follows. The first contribution comes from the scintillator which, because of the thickness of its scintillation layer of 400 mm, limits the resolution to 200 mm; the second comes from the detection geometry that limits the resolution to the effective pixel size of 254 mm, evaluated for the camera– scintillator distance of 60 cm2, a camera’s field of view of 13 13 cm (provided by the NIKON lens mentioned in Section (2), and a CCD chip array of 512 512 pixels; the third important contribution comes from the imperfect and subjective focus of the camera lens since presently it depends on the visual acuity of the technician ¨ (IAEA, 2008; Grunzweig et al., 2007; Kardilov et al., 2005).
3. Neutron images In order to demonstrate the potentiality of the facility to investigate archeological objects of the Brazilian cultural heritage, a contemporary ceramic vessel, shown in Fig. 7, was imaged. This vessel was selected because the ceramic represents one of the most usual material found in the archeological objects left by the Brazilian Indians, and because like the archeological ceramic, the contemporary one also consists of elements which are fairly transparent for neutrons, but has in its structure a ‘‘hydrogen rich substance, namely water’’, making it ideal for the neutron imaging investigation (Rant, et al. 2005; Munita et al., 2011). The vessel was damaged and, the induced damage resulted in an oblique crack which divided the vessel into two parts. By using organic glue, another hydrogenous rich substance, the parts were glued by a restorer and then it was imaged. The obtained images are shown in the figures below. Fig. 8 is an example of 3D image, in which the outermost part of the ceramic was removed to make the track of the glue used in the restoration procedure visible. In Fig. 9 the glue was separated
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 10. 3D image: thickness wall.
9
horizontal
slice
showing
the
measurement
of
the
Fig. 7. Photography: ceramic vessel restored by using organic glue.
Fig. 8. 3D image: external view—vessel and glue. Fig. 11. 3D image: track of glue enlarged and the measurement of its thickness.
Table 2 Comparison between the measurements performed by using the VGStudio software and an ordinary caliper.
Fig. 9. 3D image: the track of the glue separated from the ceramic material.
from the ceramic material, showing in detail how the glue was used to restore the vessel. Figs. 10 and 11 demonstrate the possibilities to perform some quantitative measurements. Fig. 10 is a horizontal slice of the vessel, and the thickness of its wall in the indicated position was measured, by using the ‘‘caliper tool’’ of the software VGStudio. In Fig. 11 the track of glue is enlarged and its thickness in the indicated position is measured, by using the same ‘‘caliper tool’’
Position in vessel
VGStudio (7 0.1 mm)
Caliper (7 0.05 mm)
Neck—outer diameter Neck—inner diameter Body outer diameter Body inner diameter Wall thickness Glue thickness
16.8 11.2 57.4 49.5 5.8 4.7
16.5 11.4 58 Not measurable Not measurable Not measurable
previously used. Some other measurements were performed and Table 2 shows a comparison between them and the ones performed by using an ordinary caliper. As can be seen the results are very close to each other. It is important to mention that in some cases it was not possible to perform the measurement by using the ordinary caliper. Fig. 12 (left) is a slice of the reconstructed image file, in which a discontinuity in the ceramic material, here a void inside the vessel wall, indicated by the arrow can be seen. In the figure to the right, the void was enlarged to make its visualization easier.
4. Concluding remarks The operational parameters as well as the potential of the tomography facility installed at IPEN-CNEN/SP to inspect objects
10
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 12. Left: void in the ceramic material, indicated by the arrow. Right: void enlarged to make its visualization easier.
of the Brazilian Indian culture are presented. The obtained results with this contemporary object show the possibility of visualizing cracks, details of the restoration process, track of glues, and discontinuities in the ceramic, and allow quantification of internal details. This kind of analysis can be applied to archeological ceramic objects or even other kinds of materials, like clay, organic fibers, wood, seeds, rubber, and resins, because all of them contain hydrogen in their structures, that is ideal for the neutron imaging investigation [Hughes 1957; Kardilov et al., 2005; Rant et al., 2005]. With respect to the facility, some particular aspects must be emphasized:
Some tests have been carried out and, depending on the technician, differences in the resolution value on the order of 20% and 30% are possible.
Acknowledgments The authors are indebted to ‘‘Sa~ o Paulo Research Foundation’’ – FAPESP – Brazil, for the partial financial support to this project, under Grant 09/50261-0.
References The neutron beam was designed in such a way that the mean energy of the neutron spectrum is close to the cold region, 7 meV. This is very desirable because, the cross-section for the hydrogen nucleus is 60% higher when compared to the value for the original thermal spectrum (peak energy 25 meV), leading to a higher sensitivity to hydrogenous substances. The spatial resolution is relatively poor when compared to some of the best tomography facilities [IAEA, 2008]. In spite of this limitation, the crack and the glue as well as the void in the ceramic material, were fairly visible. However, there are some alternatives to improve the spatial resolution and therefore the image quality: (i) by using the original architecture of 1024 1024 pixels in the CCD chip, instead of the presently employed 512 512, the irradiation time is four times greater, but the resolution would be improved because the effective pixel size would be 127 mm, instead of 254 mm. (ii) By using new scintillators, manufactured with 6LiF(ZnS) layers in the range of 25–300 mm. In spite of their smaller light output, leading to higher irradiation times, they provide an intrinsic unsharpness between 100 and 150 mm [Banhart, 2008; ¨ Grunzweig et al., 2007]. (iii) By employing a standard object, commercially available, to optimize and standardize the camera focus procedure to substitute the present one which is subjective and depends on the visual acuity of the technician.
Banhart, J., 2008. Advanced Tomographic Methods in Materials Research and Engineering. Oxford University PressHahn-Meitner-Institute Berlin Germany. Berger, H., 1965. Neutron Radiography, Methods, Capabilities and Applications. Elsevier, New York. Bjoern, W., 2006. Reviews in Mineralogy and Geochemistry 63, 459–471. ¨ ¨ Grunzweig, C., Frei, G., Lehmann, E., Kuhne, G., David, C., 2007. Highly absorbing gadolinium test device to characterize the performance of neutron imaging detector systems. Rev. of Sci. Instrum., 78. Harms, A.A., Garside, B.K., Chan, P.S.W., 1972. Edge spread function in neutron radiography. App. Phys. 43 (9), 3863–3865. Hughes, D.J., 1957. Neutron Cross Sections (barn). Pergamon, London. IAEA Neutron imaging: A Non-destructive Tool for Materials Testing. IAEA– TECDOC, 2008–1064(ISBN:978-92-0-110308-6). Kardilov, N., Hilger, A., Manke, I., Strobl, M., Treimer, W., Banhart, J., 2005. Nuclear industrial applications at the new cold neutron radiography and tomography facility of the HMI. Instrum. methods Phys. Res. A 542, 16–21. Koerner, S., Lehmann, E., Vontobel, P., 2000. Design and optimization of a CCDneutron radiography detector. Nucl. Instrum. Methods Phys. Res. A 454, 158–164. Munita, C.S., Toyota, R.G., Neves, E.G., Demartin, C.C., Schaam, D.P., Oliveira, P.M.S., 2011. Chemical characterization of Marajoara pottery. IAEA Radiation Technology Series (2) /http://www.iaea.org/Publications/index.htmlS. Nikolay, K., Ingo, M., Andre´, H., Markus, S., John, B., 2011. Neutron imaging in materials science. Mater. Today 14 (6), 248–256. Octopus, 2011. Octopus 8, 4 /http://www.inct.be/en/software/octopusS. Rant, J., Milic, Z., Turk, P., Lengar, I., 2005.In: Proceedings of the 8th International Conference of the Slovenian Society for Non-Destructive Testing. Application of Contemporary Non-Destructive Testing in Engineering. Portoroz, Slovenia, pp. 181–188. Wade, J.R., Howard, A.L., Radiography, 1987. Experiments at Argonne National Laboratory. Nucl. Technol. 176, 408–419.
Contents lists available at SciVerse ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
The neutron tomography facility of IPEN-CNEN/SP and its potential to investigate ceramic objects from the Brazilian cultural heritage M.A. Stanojev Pereira a,n, R. Schoueri b, C. Domienikan b, F. de Toledo b, M.L.G. Andrade b, R. Pugliesi b a b
´gico e Nuclear (IST/CTN), Estrada Nacional 10, 2686-953 Sacave´m, Portugal Instituto Superior Te´cnico, Universidade Te´cnica de Lisboa–Campus Tecnolo ~ Paulo, Brazil ´ria, 05508-000 Sao Instituto de Pesquisas Energe´ticas e Nucleares, Centro do Reator de Pesquisas, Av. Lineu Prestes 2242, Cidade Universita
H I G H L I G H T S c c c c
A neutron tomography facility was installed at a 5 MW nuclear research reactor. Tomography is obtained in 4000 s with a spatial resolution of 347 mm. A restored ceramic object was imaged and cracks and discontinuities could be seen. The facility will be employed to study objects left by the Brazilian Indians.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2012 Received in revised form 21 January 2013 Accepted 22 January 2013 Available online 31 January 2013
A neutron tomography (NT) facility was installed at the IEA-R1 nuclear research reactor of the Nuclear and Energy Research Institute IPEN-CNEN/SP. According to the determined operational characteristics, the time spent to obtain a complete tomography is 4000 s at a neutron flux of 1 106 n cm 2 s 1 and the best achievable spatial resolution in the image is 347 mm. The main objectives of this paper are to describe the facility as well as to demonstrate its potential to investigate ceramic objects from the Brazilian cultural heritage left by Indians. & 2013 Elsevier Ltd. All rights reserved.
Keywords: Neutron tomography Neutron radiography Scintillator Archeology
1. Introduction Neutron imaging is a set of techniques which make use of neutrons as penetrating radiation to investigate the internal structure of an object. Because of the characteristics of the neutron–matter interaction these techniques are mainly employed to investigate hydrogenous substances such as water, water-logged ceramics, organic fibers, wood, seeds, rubber, resins, glue, etc., even wrapped by thick metal layers. Therefore they are ideal to investigate cultural heritage objects left by the Brazilian Indians, such as vessels, statues, musical instruments, ornaments, weapons, etc. A neutron image is obtained by irradiating the object in a uniform neutron beam and a converter screen transforms the transmitted neutron intensity into ionizing radiation which is
n
Corresponding author. Tel.: þ351 551131339993. E-mail addresses: [email protected], [email protected] (M.A.S. Pereira).
0969-8043/$ - see front matter & 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apradiso.2013.01.030
able to sensitize certain media, forming the image. For radiography, films and track detectors can be used and for real-time a scintillator is used and the image is captured by a video camera. In both cases, the images are two-dimensional (2D) projections of the internal structure of the object. On the other hand the tomography technique provides a three-dimensional (3D) visualization of its internal structure (Hughes, 1957; Berger, 1965; Bjoern, 2006; Nikolay et al., 2011). Usually a tomography is obtained as shown in Fig. 1 (IAEA, 2008; Koerner et al., 2000; Banhart, 2008): the object to be inspected is positioned on a rotating table and is irradiated in the neutron beam; the neutrons transmitted by the object sensitize a scintillator, forming a 2D image which is captured by a video camera; the object is rotated by a small angular step in the beam and another image is captured. The file containing all the images is processed by the software Octopus to perform image reconstruction, and the software VGStudio provides a threedimensional view of the internal structure of the object. In order to avoid direct exposure of the camera’s CCD chip to radiation, a
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
7
Table 1 Main characteristics of the NT facility. Gamma filter-length Mean energy of neutron spectra Beam diameter Neutron flux at irradiation position(a) L/D ratio(b) Rotating table Voxel size(c) a b c
Bismuth 20 cm 7 meV Max. 15 cm 1 106 n cm 2 s 1 90 Angular step 0.91 0.291 mm
Au-foil method. L—source to object distance; D—inlet aperture of the collimator. provided by software Octopus.
Fig. 1. Main parts of a NT facility.
Fig. 3. Inside view of the NT shielding and the free space for irradiation.
Fig. 2. Top view of the IEA-R1 reactor operating at 4.5 MW.
plane mirror reflects the image to the camera which is usually installed at 901 with respect to the radiation beam. Because of the low light intensity generated, the scintillator, the mirror and the camera are installed inside a light tight box. The objectives of this work are to describe the NT facility of IPEN-CNEN/SP, determine its operational characteristics, namely the irradiation time to obtain a tomography and the spatial resolution in the image, and demonstrate its potential to investigate the internal structure of archeological objects of the Brazilian cultural heritage, left by Indians. Improvements to the facility are also discussed.
2. Description of the NT facility of IPEN-CNEN/SP The neutron imaging group of IPEN-CNEN/SP has installed a neutron tomography facility at the 5 MW pool type IEA-R1 nuclear research reactor, which became operational in 2010. The neutrons are driven to the NT facility through one of the irradiation channels or beam-holes (BH), inserted in the biological shielding of the reactor. Fig. 2 is a top view of the IEA-R1 reactor showing the core as well as the BH-08, where the facility is installed. The main characteristics of the NT facility are summarized in the Table 1. In order to minimize gamma radiation at the irradiation position, a 20 cm bismuth filter is installed within the BH-08. The images are obtained by using a scintillator coupled to a CCD video camera. The scintillator is the NE-426 (0.4 mm, 18 cm 24 cm) 6LiF/ZnS, with peak emission at 450 nm and the camera is an ANDOR model ikon-M, with CCD having 1024
1024 pixels (12 12 mm2 size) and provides 16 bit images. The camera has also a Peltier cooling device able to decrease the CCD temperature down to 90 1C. The lens used is a Nikon 58 mm/f1.2. The irradiations are performed inside the facility’s shielding in a free space with 2 m 2 m 1.2 m (width) (see Fig. 3). Some details of the irradiation position are shown in Fig. 4. In order to determine the operational characteristics of the facility, during the capturing process the camera’s CCD is kept at a constant temperature of 20 1C, the CCD chip array was fixed at 512 512 pixels, and the reactor power was 4.5 MW [IAEA, 2008]. For the present facility, the sample is rotated by 3601 and the irradiation time to acquire each one of the 400 images for the tomography is about 10 s. It was determined by means of the curve that relates gray level—GL as functions of the irradiation time—Ti. The curve, shown in Fig. 5, was obtained by performing irradiations in the direct beam (without object) and plotting the gray level intensity corresponding to each image, as functions of the irradiation time, varying in the range 2 oTio15 s. A total of 7 points has been plotted and the intensity of each point was evaluated by averaging the gray level intensities of about 5000 individual pixels. The standard deviations of the intensities are inserted in the points and they ranged from 0.1% to 2% of the plotted value. A straight line was drawn between the points to guide the eye and the behavior of ‘‘GL vs. Ti’’ is linear until about 12 s after which the pixels saturation begins. For reconstruction, the images must be free of saturated pixels; therefore they must be acquired in the linear region (Octopus, 2011). For the present data, a safe irradiation time that satisfies such requirement is about 10 s, that is, 80% of the maximal irradiation time that limits the linear region.
8
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 4. Details of the NT facility. Fig. 6. Gray level distribution and the fitted function.
the experimental data, are shown in Fig. 6. ESF ¼ p1þ p2 atanðp3ðxp4ÞÞ
ð2Þ
where p1, p2, p3 and p4 are free parameters and x is the scanning coordinate. The obtained value was Ui¼(347 726) mm and it corresponds to the full width at half maximum (FWHM) of the Line Spread function associated to (2) and is given by (3) [Wade et al., 1987] Ut ¼ 2=p3
Fig. 5. Behavior of the gray level as functions of the irradiation time.
Another important parameter of the NT facility is the spatial resolution, defined as the minimal distance between objects in such a way that they can be distinguished from each other. Usually it is quoted in terms of the total unsharpness—Ut, consisting of two components: the geometrical unsharpness— Ug from the angular divergence of the neutron beam responsible for the penumbra in the image, and the intrinsic unsharpness—Ui from the detection system (scintillator, detection geometry, and camera focus condition) and they are related by (Wade et al., 1987; Harms et al., 1972) Ut n ¼ Uin þ Ug n
ð1Þ
where 1 ono3. Since for the present facility L/D 90 (see Table 1), taking into account the irregular shape of the object (see Fig. 7), the geometrical unsharpness—Ug in the image varies between 347 and 555 mm (Berger, 1965). The intrinsic unsharpness was evaluated by scanning the gray level distribution in the image of a neutron opaque object, here a knife-edge gadolinium foil 127 mm in thickness, irradiated for 8 s in tight contact to the scintillator. In this case, the penumbra effect in the image is negligible (Ug 0) and according to (1), Ut Ui. The obtained distribution and the ‘‘Edge Spread Function’’—ESF , fitted to
ð3Þ
This value can be explained as follows. The first contribution comes from the scintillator which, because of the thickness of its scintillation layer of 400 mm, limits the resolution to 200 mm; the second comes from the detection geometry that limits the resolution to the effective pixel size of 254 mm, evaluated for the camera– scintillator distance of 60 cm2, a camera’s field of view of 13 13 cm (provided by the NIKON lens mentioned in Section (2), and a CCD chip array of 512 512 pixels; the third important contribution comes from the imperfect and subjective focus of the camera lens since presently it depends on the visual acuity of the technician ¨ (IAEA, 2008; Grunzweig et al., 2007; Kardilov et al., 2005).
3. Neutron images In order to demonstrate the potentiality of the facility to investigate archeological objects of the Brazilian cultural heritage, a contemporary ceramic vessel, shown in Fig. 7, was imaged. This vessel was selected because the ceramic represents one of the most usual material found in the archeological objects left by the Brazilian Indians, and because like the archeological ceramic, the contemporary one also consists of elements which are fairly transparent for neutrons, but has in its structure a ‘‘hydrogen rich substance, namely water’’, making it ideal for the neutron imaging investigation (Rant, et al. 2005; Munita et al., 2011). The vessel was damaged and, the induced damage resulted in an oblique crack which divided the vessel into two parts. By using organic glue, another hydrogenous rich substance, the parts were glued by a restorer and then it was imaged. The obtained images are shown in the figures below. Fig. 8 is an example of 3D image, in which the outermost part of the ceramic was removed to make the track of the glue used in the restoration procedure visible. In Fig. 9 the glue was separated
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 10. 3D image: thickness wall.
9
horizontal
slice
showing
the
measurement
of
the
Fig. 7. Photography: ceramic vessel restored by using organic glue.
Fig. 8. 3D image: external view—vessel and glue. Fig. 11. 3D image: track of glue enlarged and the measurement of its thickness.
Table 2 Comparison between the measurements performed by using the VGStudio software and an ordinary caliper.
Fig. 9. 3D image: the track of the glue separated from the ceramic material.
from the ceramic material, showing in detail how the glue was used to restore the vessel. Figs. 10 and 11 demonstrate the possibilities to perform some quantitative measurements. Fig. 10 is a horizontal slice of the vessel, and the thickness of its wall in the indicated position was measured, by using the ‘‘caliper tool’’ of the software VGStudio. In Fig. 11 the track of glue is enlarged and its thickness in the indicated position is measured, by using the same ‘‘caliper tool’’
Position in vessel
VGStudio (7 0.1 mm)
Caliper (7 0.05 mm)
Neck—outer diameter Neck—inner diameter Body outer diameter Body inner diameter Wall thickness Glue thickness
16.8 11.2 57.4 49.5 5.8 4.7
16.5 11.4 58 Not measurable Not measurable Not measurable
previously used. Some other measurements were performed and Table 2 shows a comparison between them and the ones performed by using an ordinary caliper. As can be seen the results are very close to each other. It is important to mention that in some cases it was not possible to perform the measurement by using the ordinary caliper. Fig. 12 (left) is a slice of the reconstructed image file, in which a discontinuity in the ceramic material, here a void inside the vessel wall, indicated by the arrow can be seen. In the figure to the right, the void was enlarged to make its visualization easier.
4. Concluding remarks The operational parameters as well as the potential of the tomography facility installed at IPEN-CNEN/SP to inspect objects
10
M.A.S. Pereira et al. / Applied Radiation and Isotopes 75 (2013) 6–10
Fig. 12. Left: void in the ceramic material, indicated by the arrow. Right: void enlarged to make its visualization easier.
of the Brazilian Indian culture are presented. The obtained results with this contemporary object show the possibility of visualizing cracks, details of the restoration process, track of glues, and discontinuities in the ceramic, and allow quantification of internal details. This kind of analysis can be applied to archeological ceramic objects or even other kinds of materials, like clay, organic fibers, wood, seeds, rubber, and resins, because all of them contain hydrogen in their structures, that is ideal for the neutron imaging investigation [Hughes 1957; Kardilov et al., 2005; Rant et al., 2005]. With respect to the facility, some particular aspects must be emphasized:
Some tests have been carried out and, depending on the technician, differences in the resolution value on the order of 20% and 30% are possible.
Acknowledgments The authors are indebted to ‘‘Sa~ o Paulo Research Foundation’’ – FAPESP – Brazil, for the partial financial support to this project, under Grant 09/50261-0.
References The neutron beam was designed in such a way that the mean energy of the neutron spectrum is close to the cold region, 7 meV. This is very desirable because, the cross-section for the hydrogen nucleus is 60% higher when compared to the value for the original thermal spectrum (peak energy 25 meV), leading to a higher sensitivity to hydrogenous substances. The spatial resolution is relatively poor when compared to some of the best tomography facilities [IAEA, 2008]. In spite of this limitation, the crack and the glue as well as the void in the ceramic material, were fairly visible. However, there are some alternatives to improve the spatial resolution and therefore the image quality: (i) by using the original architecture of 1024 1024 pixels in the CCD chip, instead of the presently employed 512 512, the irradiation time is four times greater, but the resolution would be improved because the effective pixel size would be 127 mm, instead of 254 mm. (ii) By using new scintillators, manufactured with 6LiF(ZnS) layers in the range of 25–300 mm. In spite of their smaller light output, leading to higher irradiation times, they provide an intrinsic unsharpness between 100 and 150 mm [Banhart, 2008; ¨ Grunzweig et al., 2007]. (iii) By employing a standard object, commercially available, to optimize and standardize the camera focus procedure to substitute the present one which is subjective and depends on the visual acuity of the technician.
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