Proceedings of the 39th International Symposium for Archaeometry, Leuven (2012) 180-185

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Investigating the influence of Instrumental Neutron Activation Analysis on European Trade Glass Beads

A. Bonneau1-2, J.-F. Moreau3, R.G.V. Hancock4, R. Auger2 and B. Emard5

1. Géotop, Earth and Atmospheric department, University of Quebec in Montreal, 201, avenue du Président-Kennedy,

Montréal, H2X 3Y7, Canada, bonneau.adelphine@courrier.uqam.ca

2. Historical Archaeology laboratory, Laval University, 3, rue de la Vieille-Université, Quebec City, G1R 5K2, Canada, Reginald.Auger@celat.ulaval.ca

3. Archaeology laboratory, University of Quebec in Chicoutimi, 555 Boulevard Université, Chicoutimi, G7H 2B1, Canada, Jean-Francois_Moreau@uqac.ca

4. Department of Medical Physics and Applied Radiation Sciences and Department of Anthropology, McMaster University, Hamilton, L8S 4K1, Canada,

ronhancock@ca.inter.net 5. Archaeologist, Hydro-Québec, Division Équipement et Services partagés, 855 rue Sainte Catherine est 18e, Montréal, H2L

4P5, Canada, Emard.Bertrand@hydro.qc.ca

ABSTRACT Intensive chemical analyses of trade glass beads discovered in North America have been performed since the 1990’s using instrumental neutron activation analysis (INAA). However, new instrumentations such as Raman spectroscopy proved to be good and inexpensive for glass characterization, which is a compelling interests of archaeologists (Bonneau et al. 2013). Since INAA turns a tiny fraction of atoms into radioactive isotopes, their decay rays may interfere in the glass structure and thus influence Raman spectroscopy results. In this study, preliminary results on ten white trade glass beads from an Amerindian site, ClFi-10, are presented. The main influence of INAA was recorded in the “Qn model” results with signs of atom rearrangements which interfere with sub-group divisions of the glass. Ip results and characterization remain mostly the same. KEYWORDS Glass degradation, Instrumental Neutron Activation Analysis, Raman spectroscopy, Trade glass beads. Introduction Intensive chemical analyses of trade glass beads discovered in North America have been performed since the 1990’s using instrumental neutron activation analysis (INAA) (e.g. Hancock et al. 1996, 1997, Moreau et al. 2002, Moreau and Hancock 2010). INAA is based on the measurement of the gamma rays produced in samples when they are irradiated with neutrons. Thus, some atoms capture neutrons, turn into radioactive isotopes and produce prompt gamma rays, as in Equation 1 (Skoog et al. 2007):

!" ! ! !!!!!!!" !! ! !"!!

!" ! !! (1) Then the radioactive nuclei decay into stable isotopes by producing beta particles and often delayed gamma rays.

During this step, some nuclei do not decay to their parent atom but to another one. It is called transmutation. An example of this reaction is given in Equation 2 (Skoog et al. 2007):

!"!!!" !! ! !"!"

!" ! !!!! ! ! (2)

INAA studies on trade glass beads have resulted in the creation of a large database which may be used as a tool to date beads by comparison of their chemical compositions. However, new questions are being asked by archaeologists. The two most important are: “How were these beads made?” and “Is it possible to make sub-groups in the dated groups in order to follow the penetration of beads from their coastal point of arrival into the interior of North America?” Raman spectroscopy may be used to investigate the structure of glass and thus to answer questions about the manufacture of beads. That method has already proven to be very good at finding the “shape” of the opacifiers, to identify the type of glass and to determine different groups of beads based on their glass type (Colomban 2003, Colomban et al. 2006, Tournié 2009). Since a large number of glass beads have been analysed with INAA, the transmutation of some atoms and the beta and gamma emissions of the radioisotopes produced, may introduce changes in the glass structure. Our study aims to verify the possibility of permanent modification of the glass structure as a result of the use of INAA and thus, the possibility to apply Raman spectroscopy to pre-analyzed beads. Presented here are preliminary results obtained on ten white trade glass beads. Methodology Ten white trade glass beads were selected from ClFi-10, an Amerindian site located at the head of the St-Maurice River, because of their colour (white, the most commonly found colour) and their state of degradation.

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Beads were first observed with a microscope. Then Raman spectroscopy and INAA were carried out. To finish and evaluate the influence of INAA, microscopic observations and Raman spectroscopy analyses were done again. The Ip (polymerization index) and the “Qn model” were used to evaluate the rearrangement of the atoms in the glass matrix. Indeed, as explained in previous studies (e.g. Colomban 2003, Colomban et al. 2006), Ip can be used to evaluate the rate of silica in a glass and thus is related to the heating temperature. On the other hand, the “Qn model” is applied only on the stretching massif and investigates the proportions of each kind of non-bonding oxygen atoms. Because of the presence of modifiers such as sodium or potassium, some oxygen atoms of the silicate tetrahedrons are not bonded to another oxygen atom. They are called “non-bonding oxygen”. If a silica tetrahedron is bonded to another one, it is called Q1; if it is bonded to two other silica tetrahedrons, it is a Q2, and so on. This model will be used to investigate the possible changes in the glass structure after INAA. The Raman instrument is located in the CERMA, laboratory of Chemistry at University Laval, Quebec City, Canada. It is a LABRAM 800HR (Horiba Jobin Yvon, Villeneuve d’Ascq, France) coupled to a microscope Olympus BX 30 with a fixed plate and a long focal x100 objective. The detector is an Open electrode CCD (1024 ! 256 pixels). The laser is green, a 514.5nm Argon (Coherent, INNOVA 70C Series Ion Laser, Santa Clara, CA). Tournié (2009) also used a green laser and a long distance x100 or x200 objective. Only the long distance x100 objective was available at the CERMA. The tests carried out with the 633nm laser, x100 and x200 objectives show less intense spectra. The spectra were recorded with Labspec and analysed with Grams. To calculate the Ip, the background was made with four fixed points (300cm-1, 700cm-1, 850cm-1 and 1300cm-1) and areas under scattering and stretching massifs were calculated between 290cm-1 and 720cm-1, and 720cm-1 and 1220cm-1. For the Qn model, Grams software was used with parameters detailed in Table 1. These parameters are mostly inspired from the ones used and described in Tournié (2009). Table 1. Parameters used with Grams to apply “Qn model”.

Qn Wavenumber limits Width limits Q0 750-850 50-100 Q1 850-950 50-100 Q2 950-1050 50-100 Q3 1050-1100 50-100 Q4 1100-1200 50-100

For INAA, the glass beads were irradiated serially for up to 1 minute at a neutron flux of 5x1013 neutrons.cm-2.sec-1 at the McMaster Nuclear Reactor. Four to six minutes after irradiation, the induced radioactivity was counted for three minutes using a hyper-pure germanium detector-based gamma-ray spectrometer. Large samples (>200 mg) were irradiated for 10 seconds and successively smaller samples were irradiated for longer periods. This process produced analytical concentration data for Co, Sn, Sb, Mg, Cu, Na, V, Al, Mn, Cl and Ca. The samples were recounted for 3-5 minutes the next day, using the same spectrometer, to measure the concentrations of longer-lived radioisotopes of

Au, Na, As, Sb and K. The Na measurements were used to link both counts. Elemental concentrations were calculated using the comparator method. LA-ICP-MS analyses were carried out at the Earth Sciences department, University of Quebec in Chicoutimi, Canada. Few results of this study are presented here. Results The results are presented by instrumentation and with a comparison between results before and after INAA. Microscope observations Under microscope observations, eight glass beads do not show any trace of chemical degradation (Fig. 1), one shows cracking (Fig. 2) and the last one, iridescence and “craters” which are linked to advanced chemical degradation of the glass in surface (Fig. 3).

Fig. 1. Surface of a glass bead with no chemical degradation visible. After INAA, microscope observations were carried out again. No changes were seen on the beads (Fig. 4 and Fig. 5) apart for the chemically degraded bead on which the iridescence seems to increase (Fig. 6). At this stage, INAA does not seem to have an influence on the glass beads apart from a possible increase of iridescence on one bead. Raman spectroscopy analyses The main use of Raman spectroscopy in glass studies is glass and opacifier/colorant characterization. The ten beads analysed give three main spectra. Interpretation is given after background reduction and refers to Colomban et al. (2006) and Colomban and Tournié (2007). The first kind of spectra with glass peaks at 565cm-1, 950cm-1 and 1090cm-1 and an opacifier peak at 635cm-1, is a calcium and sodium-rich glass opacified with cassiterite (SnO2) (Fig. 7a).

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Fig. 2. Surface of a glass bead with cracking.

Fig. 3. Surface of a glass bead with advanced chemical degradation.

Fig. 4. Surface of a glass bead with no chemical degradation visible after INAA.

Fig. 5. Surface of a glass bead with cracking after INAA.

Fig. 6. Surface of a glass bead with advanced chemical degradation after INAA (iridescence increased?). It was recorded on four beads. The second one with glass peaks at 580cm-1, 950cm-1, 990cm-1 and 1090cm-1 and opacifier peaks at 483cm-1 and 636cm-1, is a calcium-rich glass, containing sodium, opacified with calcium antimonate (Fig. 7b). It was recorded on five beads. The last one with glass peaks at 470cm-1, 980cm-1 and 1025cm-1 and an opacifier peak at 535cm-1, is a lead glass, maybe containing potassium, opacified with a mixture of lead, antimony and calcium (Fig. 7c). It was recorded in only one bead, the same one showing cracking at its surface. After INAA, the Raman spectra recorded have the same shapes and peaks at the same places (Fig. 8a, b, c).

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Fig. 7. Raman spectra recorded on beads before INAA (a: first type, b: second type, c: third type).

Fig. 8. Raman spectra recorded on beads after INAA (a: first type, b: second type, c: third type). Following the characterization, Raman spectra processing is made to calculate the Ip and the “Qn model”. Results on spectra recorded before INAA are presented in Tables 2 and 3. Ip results are split into two groups: one with Ip about 0.6 (three beads) and another one with Ip between 0.75 and 1 (seven beads). However, these two groups can be considered as only one in terms of the melting temperature, about 800-1000°C (Colomban et al. 2006). The three types of glass previously made with the peaks are confirmed by the “Qn model”.

Table 2. Ip results on Raman spectra before INAA.

Sample Polymerization

index

Max 500

massif

Max 1000

massif T10378 0,84 579 1073 T10386 0,81 576 1076 T10444 0,64 573 1077 T10520 0,58 576 1072 T14653 0,81 577 1087 T16024 0,81 571 1097 T16106 0,82 568 1097

T19505.10 0,60 460 1024 T21518.1 0,80 556 1098 T22013 0,79 566 1098

Table 3. “Qn model” results on Raman spectra before INAA.

Sample vQ0 vQ1 vQ2 vQ3 vQ4 AQ2/ AQ1

AQ2/ AQ3

T10378 779 954 1031 1089 1150 1,53 1,70

T10386 782 954 1026 1088 1158 0,88 0,67

T10444 778 957 1022 1081 1144 0,59 0,52

T10520 771 954 1023 1083 1162 0,62 0,55

T14653 783 954 1016 1089 1166 0,71 0,28

T16024 787 954 1028 1097 1162 0,79 0,40

T16106 787 954 1026 1097 1160 1,06 0,38

T19505.10 796 954 1020 1076 1162 1,25 5,03

T21518.1 786 954 1042 1098 1145 2,52 1,08

T22013 782 954 1028 1096 1155 1,61 0,62

After INAA, the calculations of Ip and “Qn model” reveal differences between the results (Tables 4 and 5). Table 4. Ip results on Raman spectra after INAA (changes are in bold).

Sample Polymerization

index

Max 500

massif

Max 1000

massif T10378 0,85 578 1071 T10386 0,82 574 1079 T10444 0,81 579 1069 T10520 0,70 573 1068 T14653 0,74 584 1083 T16024 0,80 567 1094 T16106 0,81 564 1094

T19505.10 0,63 467 1024 T21518.1 0,82 558 1098 T22013 0,81 560 1094

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Table 5. “Qn model” results on Raman spectra after INAA (changes are in bold).

Sample vQ0 vQ1 vQ2 vQ3 vQ4 AQ2/AQ1 AQ2/AQ3

T10378 777 958 1024 1083 1149 0,67 0,55

T10386 775 952 1024 1087 1155 1,26 0,90

T10444 775 954 1015 1083 1163 0,92 0,50

T10520 773 954 1028 1087 1161 0,80 0,79

T14653 779 954 1016 1085 1167 0,61 0,29

T16024 784 954 1028 1098 1162 1,33 0,53

T16106 781 954 1028 1095 1157 1,48 0,61

T19505.10 796 954 1021 1096 1157 1,78 35,93

T21518 786 954 1048 1099 1144 2,42 1,33

T22013 782 954 1033 1097 1155 1,83 0,79 The Ip results are mostly homogenous with only two beads increasing their Ip, but staying in the same interval as previously mentioned for the melting temperature. Most of the changes are in the “Qn model” results. The most important one is seen with bead T19505.10, in which the Q3 peak almost completely disappears. Variations of the Q1 and Q2 peaks were recorded on five other beads. These can be the results of rearrangements of atoms in the glass matrix. Discussion Results show that few changes are recorded following INAA. Most of them do not affect the characterization of the glass and opacifiers/colorants as well as the Ip, the main information from Raman spectra. Changes are mostly in the “Qn model” and more precisely in the Q1, Q2 and Q3. In the case of glass degradation, the Q2 peak decreases and a fine peak at 996cm-1 appears (Tournié 2009). However, the glass beads impacted with INAA are Ca-rich glass and thus have got a peak at 990cm-1. In that case, it is very difficult to see another peak at 996cm-1. Both will overlap. Moreover, the decrease of the Q2 peak was observed only on one bead. So, changes can be seen as due to a rearrangement of the atoms in the glass. The Q3 peak almost totally disappears after INAA on the lead glass bead (T19505.10) and its place moved from 1070cm-1 to 1090cm-1. Q1 and Q2 peaks change in the calcium and sodium rich glass. Transmutations of atoms may play a role in these variations, especially three of them (Equations 2, 3, 4):

!"!!!" !! ! !"!"

!" ! !!!! ! ! (2)

!!"

!" !! ! !"!"!" ! !!! ! ! (3)

!"!"

!" ! ! !!!! ! ! !!"

!" ! !!! (4) In the case of T19505.10, LA-ICP-MS analyses reveal about 10wt% of K in the glass. The important decrease of Q3 may be related to the transmutation of K into Ca which forces silica tetrahedrons to bond to other ones or to break bonds to

make more ionic bonds with Ca. The transmutation of Na and Ca in the other beads may be responsible for the variations of Q1 and Q2. As transmutations turn atoms into other ones, the rearrangement should impact only Q1, Q2 and Q3 peaks. Results presented previously tend to confirm this point of view. On the other hand, six of the ten beads seem to be affected by a rearrangement of their glass structure. Transmutation is here seen as a hypothesis and other parameters may be involved. This is because beads with the same chemical composition do not vary in the same way. As LA-ICP-MS analyses were carried out only before INAA, it is not possible to evaluate possible K, Na and Ca rate changes in the affected beads.

Fig. 9. PCA with “Qn model” results before INAA.

Fig. 10. PCA with “Qn model” results after INAA. The “Qn model” is often used to make sub-groups in a type of glass group (e.g. Colomban et al. 2006). Results before

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and after INAA were processed with JMP using principal component analyses (PCA) (Fig. 9 and 10). PCA before INAA made the same groups as given by the characterization of the beads. As only ten beads were analysed and as they are close to each other, it is difficult to make sub-divisions in these groups. On the other hand, PCA after INAA, does not show the same division of the glass. These results are not correct as, for example, a lead glass is close to a calcium-rich one. Conclusions In this article, ten trade glass beads were analysed with microscopy, Raman spectroscopy and INAA to evaluate possible impacts of neutron activation on glass, and to assess the feasibility of Raman analyses after INAA. Preliminary results show no significant impacts in the case of characterization and Ip calculations. Variations are seen in the “Qn model” processing and in the categorisation following its results. The hypothesis is given that these changes are due to transmutations of K, Na and Ca which are modifiers and stabilizers of glass. However, other parameters, such as particles and rays emitted by the decays of radioactive isotopes, may play a part on this rearrangement too. Further analyses on trade glass beads of different colours and shapes should be carried out to evaluate hypotheses presented in this study. Following results presented here, recommendation is given to carry out Raman spectroscopy analyses before INAA when applicable and to use Raman on already irradiated beads only for characterization and not for glass sub-division grouping. Acknowledgements The authors thank the CERMA, Chemistry department, Laval University (Quebec, Canada) for the use of their instruments and their precious advice. We are grateful to Hydro-Québec for their financial support, to Laval University and UQAC for their in kind and in cash support. References Bonneau, A., Moreau, J.-F., Auger, R., Hancock, R.G.V., Emord,

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