Neutron-resonance capture as a tool to analyse the internal compositions of objects non-destructively

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1 Neutron-resonance capture as a tool to analyse the internal compositions of objects non-destructively H. Postma, Delft University of Technology, Mekelweg 15, 2629 JB Delft, the Netherlands P. Schillebeeckx, EC-JRC IRMM, Retieseweg, Geel, Belgium Abstract Neutron resonance capture analysis (NRCA) is a non-destructive method for analysing the bulk composition of materials and objects. It has been developed at the GELI- NA facility of the Institute for Reference Materials and Measurements (IRMM) in Geel (B) as a joint project with the Delft University of Technology. In this paper some features of NRCA are discussed on the basis of results obtained in the field of archaeology. Introduction Neutrons are used in various ways to analyse the elemental compositions, crystallographic structures and texture of archaeological and art objects, and for radiography and imaging of objects. One of the analytical methods is based on the occurrence of resonances in neutron capture cross sections. Since resonances occur at neutron energies specific for each nuclide, they can be used to recognize elements. In addition, the areas of resonance peaks provide information about elemental amounts. Resonance capture can be observed easily by detecting the prompt gamma rays emitted after neutron capture. The energies of captured neutrons can be determined with the time-of-flight (TOF) method with neutrons travelling a known distance. This method requires a pulsed neutron source, which provides the start pulse for the TOF measurement. The stop pulse is generated by detection of the prompt gamma rays. It is not necessary to know the energy of the prompt gamma rays with a high resolution. Therefore, large scintillation detectors, not necessary with good energy resolution, but with a good time resolution can be used. In addition, gamma rays can be accepted over a wide energy range, typically from about 300 KeV up to the neutron binding energy. Together with the fact that capture events are followed by several gamma-ray transitions in cascade, it is possible to obtain large detection efficiencies for capture events. Resonance capture as a function of neutron energy is the basis of the analytical method Neutron Resonance Capture Analysis (NRCA). NRCA has been explored at the GELINA facility of the IRMM in Geel (B) in a joint project with the Delft Univer- Figure 1. A schematic drawing showing the TOF system at the GELINA facility. 14

2 sity of Technology in the Netherlands with applications in archaeology, the nuclear field and for the characterisation of reference materials. In this paper mainly the use of NRCA for archaeological and art objects is considered. Experimental facility The basic element of the GELINA facility is a linear accelerator producing very short pulses of electrons with energies from 70 to 150 MeV, with a repetition rate up to 800 Hz and a maximum power of 10 kw. Stopping these electrons in uranium produces Bremsstrahlung, which in turn generates neutrons by photonuclear and photofission processes. Moderation in two water containing Betanks close to the uranium target produces a white spectrum of neutrons. Above about 1 ev the neutron flux is in first approximation proportional with the inverse of neutron energy. Details of the GELINA facility and a general review of the neutron-nuclear research carried out at this facility are discussed in another contribution to this issue [1]. Figure 1 shows a schematic presentation of the linac, neutron production target and TOF system. In the more recent NRCA experiments two C 6 -based liquid scintillation detectors (7.5 cm thick and 10 cm in diameter) are used as capture detectors. They are placed at a distance of about 7 cm from the centre line of the neutron beam. The objects can be positioned at a distance of about 15 or 30 m from the pulsed neutron source. The beam diameter at the sample position is about 7.5 cm. Figure 2 shows a set-up with two C 6 detectors viewing from opposite sides a prehistoric bronze axis. Shielding such detectors against neutrons scattered from these detectors as much as possible. Another advantage of C 6 is its short decay time, resulting in a time resolution better than 1 ns. In addition, their response is well understood and they have sufficient energy resolution to control the selected energy range (0.3 to 10 MeV). Normally a cadmium filter (0.75 mm thick) is placed early in the beam to remove so-called overlap neutrons below 0.7 ev. To avoid overloading of the detectors by Bremsstrahlung flashes, filters of lead (1.5 cm thick), or bismuth (1.5 cm thick) and lately also sulphur (5.0 cm thick) are used. Features of NRCA In figure 3 an example of a TOF capture spectrum with a prehistoric bronze axe obtained at the 29 m measurement station is shown. The spectrum shows the detector efficiency of the capture detection system as a function of the TOF of the neutron creating the capture event. (NB: this is not a gamma-ray spectrum as some people tend to think.) The time-of-flight of the neutron, T n, can be converted to the neutron energy, E n, using the relation: E m L 2 1 n = n 2 Tn where L is the flight path length and mn the neutron mass. At the top of the figure the energy scale is indicated. Some of the most important resonances are marked in this figure, for copper at 230, 578, and 994 ev, and for (1) Figure 2. Two C 6 detectors opposite to each other with respect to the object, located at the centre of the neutron beam. The object is a socketed bronze axe. Figure 3. Example of a TOF-capture spectrum obtained for a prehistoric bronze axe with flight path of m. Some resonance peaks are indicated. the object is not necessary because of their very low neutron sensitivity. The only neutron-related background is due to capture in the surrounding materials producing a gamma-ray background. Therefore, it is important to keep material away from tin at 38.8 and 111 ev. Other marked peaks are from silver and antimony. This spectrum illustrates that many sharp resonance peaks occur which can be distinguished even in the kev region. This is due to the high-energy resolution of the GELINA facility. Vol. 11 n. 2 July 2006 NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE 15

3 Figure 4. A parametric fit of a part of the TOF spectrum between 42 and 60 ev. The spectrum in figure 3 also reveals that often TOF regions for minor or trace elements can be selected such that their detection is not hampered by the presence of other elements. For instance the minor elements Sb and Ag in bronze objects have resonances at 21.4 and 6.22 ev, respectively 5.19 ev (see figure 3) which are not influenced by the response due to the presence of resonances of other components, notably the main elements copper and tin. Since in addition the detection efficiency at the lower energies is very large minor components like Sb and Ag can be detected in the ppm range. A qualitative analysis based on recognizing resonances can be done on-line during data acquisition and may already give a quick impression about the elemental composition. For a quantitative analysis the areas under resonance peaks must be determined. Many of the peaks in a spectrum like the one in figure 3 are sufficiently well separated to determine their areas by summing over the channels and subtracting the background determined from adjacent regions. For a better accuracy peak fitting is necessary. This is certainly the case for closely lying, partly overlapping peaks. Resonance peaks may show on their high energy sides rather broad structures or even a bump due to neutron scattering in the object followed by neutron capture. In fact the possibility of potentional and resonance scattering requires a detailed analysis. Fitting can be done applying a resonance shape analysis using codes such as REFIT developed by Moxon [2]. This code was primarily developed to determine resonance parameters from TOF-spectra of well-characterised samples of simpler forms. The code determines the full response of the detector system starting from first principles and accounts for the self-shielding and multiple scattering effect, the Doppler broadening, the time resolution of the TOF-facility, and other effects such as the neutron sensitivity of the detector system. Since NRCA is not intended to determine resonance parameters the simpler approach of parametric fitting of resonance peaks has mostly been used. This is demonstrated in figure 4 for the 47-eV antimony resonance with some neighbouring smaller peaks. The 47-eV peak is fitted as a Gaussian broadened Lorentz line, the other peaks and the high energy shoulder of the antimony peak, due to scattering plus capture, are fitted with Gaussian functions. This approach has shown to work satisfactorily. In the case of saturated resonance peaks more complicated line shapes should be taken into account. There are two ways to derive the composition of an object. Absolute amounts of elements can in principle be determined if the neutron flux, the detection efficiency and other details determining the time resolution and peak shapes are well known. This approach using the REFIT code has been applied successfully in Ref. [3] for the determination of the amount of Gd in U-samples and for the determination of impurities in reference materials. Another way is to determine relative amounts of elements with respect to a major element on the basis of ratios of peak areas and compare them with those from calibration samples. In the case of bronzes relative amounts of the elements are determined with respect to copper. The weight ratio of two elements (I and II) can be determined with: W(I) W(II) = C A(I,E I ) A(II, E II) Figure 5. The ratio of the 230 and 578 ev peak areas as a function of thickness in gr.cu/cm 2 used to determine the thickness of a fragment of a bronze cauldron. (2) 16

4 where A(j,E j ) denotes the observed area of the resonance at energy E j for elements j. The factor C can be determined from a calibration sample with a known ratio of the two elements I and II. There is an important aspect to be taken into account in the quantitative analysis; namely the self-shielding effect of resonances. That is, the number of neutrons at resonance energies diminish during passage of the beam through the sample by resonance capture and scattering. Especially for strong resonances self-shielding is an important phenomenon. The effect of selfshielding can be calculated as a function of penetration depth using the Doppler broadened total resonance cross section. The two resonance areas in eq.2 must be corrected for self-shielding, if necessary in a recurrent manner. Of course this also holds for calibration samples. This self-shielding effect might be seen as a drawback, however, it can be turned into an advantage. Due to self-shielding the area ratio of two resonances of different strengths varies with sample thickness. As a demonstration figure 5 shows the ratio of the areas of the 230- and 578-eV copper resonances. This ratio varies considerably with thickness. It has been used to estimate the Cu thickness of a fragment from a bronze cauldron (7 th cbc and excavated in Satricum) as a check on the correctness of the analysis [4]. If elements show up with several resonances in a capture spectrum, the weight ratio can be determined on the basis of a number of resonance pairs. For bronzes the Sn/Cu weight ratio is obtained from six pairs of resonances using three for copper (at 230, 650 and 994 ev) and two for tin (38.8 and 111 ev). Figure 6 shows the result in case of an Etruscan votive with the data uncorrected and corrected for self-shielding [5]. The resulting values of the Sn/Cu weight ratio are in excellent mutual agreement after applying the self-shielding correction. Inconsistent weight ratios indicate an inhomogeneous composition. The occurrence of resonances of different strengths is an additional powerful feature of NRCA. Strong resonances are very suitable for detecting minor and trace elements often in the ppm range. The weaker resonances are perfect to investigate major elemental components of an object. If a strong resonance of a major element is investigated, it provides information of this element only in a surface layer of the object. With the excellent time resolution of the GELINA facility high-energy resonances can be included in the analyses. The upper energy limit depends on the complexity of Figure 6. The Sn/Cu weight ratio determined for 6 pairs of resonances uncorrected and corrected for self-shielding for an Etruscan statuette. Figure 7. Three Cu-resonances and one Pb-resonance in the region of ev used to determine the Pb/Cu weight ratio of an Etruscan votive. Figure 8. Two bronze statuettes, one is a genuine Etruscan artefact and the other a later imitation (probably from the Renaissance period). Vol. 11 n. 2 July 2006 NOTIZIARIO NEUTRONI E LUCE DI SINCROTRONE 17

5 the TOF spectrum and thus on the kind of material. For materials like marble resonances up to about 30 kev can be used [6]. Since capture spectra of ancient bronze objects are often complicated mainly due to the minors As, Sb and Ag, the upper limit is at about 10 kev. That means that lead, which has its lowest useful resonances at about 3 kev, can be included in the analysis. Lead is an important element in bronze objects since it is often added in considerable quantities. In bronze objects lead can be detected with concentrations above about 1 %. Figure 7 shows a section of the TOF spectrum between 3250 and 3650 ev showing the 3357-eV resonance of 206 Pb and three Cu-resonances at 3310, 3503 and 3588 ev. Since these four resonances are weak the Pb/Cu weight ratio can be obtained with at most small self-shielding corrections. Short review of applications NRCA has been applied to a considerable number of artefacts, so far mainly bronze objects. A series of Etruscan statuettes from a collection originally owned by earl Corazzi of Cortona (I), but bought by the Dutch government in 1826 and now at the National Museum of Antiquities in Leiden (NL) was studied. It turned out that it was possible to distinguish suspected fakes from genuine statuettes on the basis of the elemental compositions [5]. The conclusion was based on the observation that in some of these statuettes several per cents of zinc occur. With the smelting technique available to the Etruscan smiths, not more than a fraction of a per cent of zinc can enter into bronze. Two Etruscan objects, one genuine and one false, are shown in figure 8. A number of NRCA measurements at GELINA concerned single objects. One of them was an antique commemorative plaque from the West-African country Benin, of which the originality was questioned. The determination of the composition was helpful and suggested that it is indeed genuine and by comparison datable to the period of 1725 to 1897 [7]. A recent project concerns the elemental compositions of bronze prehistoric axes from north-west Europe. This may be helpful in understanding production methods, trade relations in this part of Europe and the usage of these axes. For example one type of axe has such thin walls that it is unsuitable for cutting. It is suggested that it may have been used for ceremonial purposes. There are remarkable differences in the compositions of prehistoric axes, which might be related to differences in local techniques and availability of metals. There are other fields in which NRCA can be applied. One concerns the study of the "poison" Gd in nuclear fuel material. NRCA can be used to detect other isotopes produced in nuclear reactor processes and for the characterisation of reference material [3]. Conclusion The above examples show that even in a restricted field like bronze artefacts a lot of interesting results can be obtained. Probable even more so if NRCA is combined with results from other neutron based techniques as PGAA and TOF neutron diffraction, and the imaging method to be developed in the Ancient Charm project. The latter is important for inhomogeneous objects. A comparison of NRCA and PGAA as elemental analysis techniques is discussed in Ref. [8]. In conclusion the following statements can be made: NRCA is fully non-destructive, it is not necessary to take samples from an object or to remove some of the patina, The residual activation is negligible, The existence of weak and strong resonances provides an interesting flexibility for the analysis, and The high penetrability of neutrons is an important feature of neutron based analytical techniques. Acknowledgements We like to sincerely thank the National Museum of Antiquities in Leiden (NL) for the loan of several bronze artefacts. References 1. W. Mondelaers and P. Schillebeeckx, GELINA, a neutron time-of-flight facility for high-resolution neutron data measurement, in this issue. 2. M. Moxon, REFIT2: A least Squares Fitting Program for Resonance Analysis of neutron Transmission and capture Data, NEA-0914/02 (1989) 3. P. Schillebeeckx, A. Borella, A. Moens, R. Wynants, M. Moxon, H. Postma, C.W.E. Van Eijk, "The use of Neutron Resonance Capture Analysis to determine the elemental and isotopic composition of nuclear material", Proceedings of the 27th Annual Symposium on Safeguards and Nuclear Material Management, London, United Kingdom, May 10-12, (2005), Proc. ISBN H. Postma, M. Blaauw, P. Schillebeeckx, G. Lobo, R. Halbertsma and A.J. Nijboer, "Non-destructive elemental analysis of copper-alloy artefacts with epithermal neutron-resonance capture", Czech. J. of Physics 53 (2003), A H. Postma, P. Schillebeeckx and R. Halbertsma, "Neutron Resonance Capture Analysis of some genuine and fake Etruscan Copper-alloy statuettes", Archaeometry 46 (4) (2004), R.C. Perego, H. Postma, M. Blaauw, P.Schillebeeckx, A. Borella, "Neutron Resonance Capture Analysis: improvements of the technique for resonances above 3 kev and new applications", Proceedings MTAA11 conference, Univ. of Sussex, Guildford (UK) June 2004, to be published in J. of Radioanal. Nucl. Chem. 7. M. Blaauw, H. Postma and P. Mutti, "An attempt to date an antique Benin bronze using neutron resonance capture analysis", Applied Radiation and Isotopes 62 (2005) H. Postma and P. Schillebeeckx, "Non-destructive analysis of objects using neutron resonance capture", J. of Radioanal. Nucl. Chem. 265 (2005)