ION BEAM ANALYSIS OF COPPER AND COPPER ALLOY COINS

Archaeomelry 30. 2 (1988), 187-197. Printed in Great Britain

ION BEAM ANALYSIS OF COPPER AND COPPER ALLOY COINS

F. BEAUCHESNE. J. N. BARRANDON

Centre Ernest Babelon. CRA, CNRS. 45071 Orleans Cedex, France

L. ALVES, F. B. G I L and M. F . GUERRA

Cenrro de Fisica Nuclear da Universidade de Lisboa ( I N I C ) 1699 Lisboa Codex. Portugal

I N T R O D U C T I O N

Different analytical methods are now available for researchers working on copper or copper alloy coins and objects. Among these different techniques are wet chemical (gravimetric) methods, emission spectrometry, X-ray fluorescence (both wavelength and energy disper- sive), spark source mass spectrometry, neutron and photon activation analysis techniques and ion beam techniques.

A large majority of the laboratories working on archaeological objects use only one or two methods which sometimes are not suitable for the alloy under study. There are only a few works where results obtained on the same samples by different analytical techniques are presented. Chase (1974) compares the results for three objects (two archaeological and one standard) obtained by about thirty laboratories but with methods needing samples of between 5 and 500 mg of the objects. In a more recent work Carter et a f . (1983) present the results obtained for one of the copper alloys, orichalcum, with both non-destructive (X-ray fluorescence (XRF), photon activation analysis) and destructive analytical methods (wet chemistry, neutron activation analysis, atomic absorption analysis). These comparisons are of importance both for the analytical laboratories, as they can point out unsuspected sources of errors, and for the archaeologists as they show on concrete examples whether or not a method is adapted to the archaeological problem.

The results are presented here of a comparison between three non-destructive ion beam techniques: particle-induced X-ray emission (PIXE) using a van de Graaff accelerator, proton activation analysis (PAA) and fast neutron activation analysis (FNAA) using a cyclotron.

In order to verify the accuracy and the precision of these three methods our experiment was first carried out on brass and bronze industrial standards and afterwards on five coins described in table 1, with the following composition: one of copper, two of orichalcum and two of lead-bronze.

E X P E R I M E N T A L P R O C E D U R E

Particle-induced X-ray emission

In the art and archaeological fields PIXE is a well-known method and has been the object of two important publications (Bird et al. 1983, IBA3 workshop 1986). The following points should be noted.

187

188 F. Beauchesne, J . N . Barrandon. L. Alves, F. B. Gil and M . F. Guerra

I I

Table 1 Identification of the five roman coins

Coin No. Date A.D. Emperor Denomination Reference

I ngn 1 Titus As RIC I1 p. 142, No. 191 2 86 Domitien Sestercius RIC I1 p. 194, No. 3 13

Nerva Dupondius RIC I1 p. 232, No. 132 3 98 4 240 Gordian 111 Sestercius RIC IV p. 49, No. 307

244249 Philippe I Sestercius RIC IV p. 90, No. 175 5

For Z > 1 1 PIXE is a multielement analytical method and for heavy elements in light matrices its sensitivity goes down to the ppm level; considering the possibility of having a beam in open air, this method allows point analysis in both small and large objects; it allows a rapid non-destructive analysis.

However PIXE has the disadvantage of being a surface method of analysis, for example the half-thickness for zirconium X-rays in a copper matrix is 3.3 pm.

For our work we used the experimental facilities of the 2 MV van de Graaff accelerator of the LNETI (Sacavem, Portugal), with an analysing proton beam with an energy of 1.5 MeV and an intensity in the range 6-20 nA (Ferreira and Gil 1981). The data collection system uses an Si(Li) detector with a resolution of 175 eV placed at an angle of 45" with the beam axis.

RC"

I

0 50 100 150 200 250 300 Range (Jim)

I I I I R

Figure 1 Comparison between total activity and the activity of a surface slice for Some reactions

Ion beam analysis of copper and copper alloy coins 189

Spectral processing is carried out in a Digital PC-350 with AXIL program and concentration calculations, based on a theoretical formula developed in CFNUL, are obtained with an iterative program also created in CFNUL and running in a PDP 11/23. The irradiation time varies from 20 min to 2 h, depending on the sample purity.

Proton activation analysis This multielement analytical method was developed in the 1970s (Barrandon et al. 1972) and used for ancient metals in 1973 (Barrandon et al. 1973). In that study the variable energy cyclotron of the CNRS (OrlCans) was used. The experimental set-up and the calibration method are the same as those used for the gold coins (Morrison et al. 1985). In the case of copper or copper alloy coins and in order to minimize the production of 65Zn by (p, n) reactions on copper, the irradiations are limited to 5min with a beam current intensity of 1 pA. In that case the detection limits are in the order of 1-lOppm (Barrandon 1986).

In spite of having all the advantages of the nuclear methods, the thickness analysed by PAA is limited. Protons going through matter rapidly lose their energy (the range for 1 1 MeV protons in copper is 288 pm) which leads to a non-uniform activation.

For some reactions, the comparison between total activity and the activity of a surface slice is presented in figure 1, and shows on the one hand that the activated thickness is always lower than the proton range and on the other hand that for proton activation the analysed half-thickness is between 50 and 100pm for a copper matrix; i.e. this method gives preference to the surface layer.

Fast neutron activation analysis It has been shown in a recent study (Beauchesne and Barrandon 1986) that the use of fast neutrons produced by a cyclotron allows a complete non-destructive analysis of copper and copper alloy (brass, bronze and lead-bronze) archaeological objects.

With fast neutrons we can determine, without any interference, the eight major elements of copper alloys (nickel, zinc, arsenic, silver, tin, antimony, gold and lead). Neutrons are obtained from the nuclear reaction produced by 17.5MeV deuterons impinging on a beryllium target.

Table 2 Limits of detection f o r a l o g coin

Element Limit (ppm)

Fe Ni Zn As

Sn Sb Au Pb

Ag

40 2

10 0.5

10 0.5 0.1 0.4

80

190 F. Beauchesne, J . N . Barrandon, L . Alves. F. B. Gil and M . F. Guerra

Table 3 Concentration (%) obtained by PIXE, FAA and FNAA ( C V , certijied values) for the industrial standards

MCI/l(Jd Fe Ni cu Zn As Sn Sh Fb

U E 10 c v PIXE PAA FNAA U E 15 c v PIXE PAA FNAA U Z 51 cv PIXE PA A FNAA uz 53 cv PIXE PAA FNAA

0.3 I 0.37 0.30 0.26

0.97 0.98 0.95 0.83

82.7 0.30 83.1 0.3 I

0.3 0.30 d

14.7 14.8 13.8 12.7

0.36 0.11 0.35 0.32

0.32 0.34 0.30 0.24

-

0.016 0.017

0.15 0.17 0.17 0.18

0.24 0.25 0.26 0.27

87.1 0.17 86.4 0.15

0.17 0.18

d

'I

0.09

0.10 0.095

- 10.75 11.4 11.2 11.5

0.61 0.40 0.58 0.57

0.51 0.62 0.50 0.45

0.10 0.12 0.10 0.1 I

0. I55 0.165 0.145 0. I4

84.34 14.4 83.5 14.2

14.3 13.8 a

0.085 0.1 1 0.075 0.070

I .52 I .60 I .40 1.38

0. I9 0. I4 0.18 0.17

< 0.005 0.0009

0.255 0.30 0.28 0.30

0.025 0.023

0.022 s 0 . 0 2

82.45 16.82 82.6 16.8

16.8 16.8

il

il

0.01

0.01 0.01

- 0.205 0.225 0.22 0.205

0.025 0.06 0.03 0.035

- < 0.005

0.0005

"The concentration of copper is obtained by subtraction.

Table 2 shows the limits of detection for a 10 g coin irradiated for 15 min with a deuteron current of 30 PA.

For FNAA with neutrons obtained with 17.5 MeV deuterons, a half-thickness of 36mm was measured for a copper target. The experimental set-up described in Beauchesne (1986), was used.

D I S C U S S I O N O F T H E A N A L Y T I C A L R E S U L T S

The results obtained for the industrial standards and those for the five coins analysed are presented below.

The standards Table 3 summarizes the results obtained for eight elements by PIXE, PAA and FNAA analysis, and the certified values given by the manufacturer for brass and bronze spectro- graphic standards from Le Centre Technique des Industries de Fonderie. The certified values are obtained with wet chemistry and atomic absorption. The precision given by the manufacturer is about 1 YO for iron, nickel, copper, zinc, antimony and lead, 2% for arsenic and 5% for tin, which we think to be >, 5% relative error for some elements and concentrations. If we look for the major elements (tin, zinc) we note (figure 2 ) that there is good agreement for the zinc concentration values. The precision for the results obtained by PAA and FNAA is of the order of 5% relative error and that for PIXE is slightly better. Tin concentration results (figure 2) are slightly more scattered. This can be explained by the fact

Ion beam analysis of copper and copper alloy coins

f% Zn

191

12.51

%O Sn

12.5

uz 53

UZ 51

UE10

UE15

L cv I PAA FNAA

Figure 2 Comparison of the results Jor zinc and f in in the standards.

that the certified values are given with a relative error of 5% and also by the relatively poor precision of the nuclear methods (relative error 5 % ) for these high concentrations.

If the trace elements whose concentrations vary from 1% to l00ppm are now con- sidered, it can be seen (figure 3) that the results obtained with the nuclear methods are in good agreement with the certified values, while PIXE gives more dispersed and sometimes incorrect results.

Looking carefully at the results for each element, we note the following points. The relative differences between the certified values found for iron concentration and the results obtained by these different methods are always lower than 20%. Nickel concentration is always determined by PIXE and FNAA and the detection limit with PAA is 200 ppm; the relative differences between the results obtained and the certified values are lower than 15%, even if the concentration is of the order of 200 ppm. Arsenic concentration is hardly ever determined by PIXE (detection limit is of the order of 0. I YO) but the results obtained with the activation methods are in good agreement with the certified values (relative differences

In the case of antimony concentration, PIXE gives incorrect results (e.g. UE 10 and UE 15) or is unable to detect it. The limit of detection for antimony, existing in small quantities, depends on the complexity of the spectrum, which is correlated with the complexity of the alloy. With I .5 MeV protons the L-shell of heavy elements is excited and has an energy close

< 10%).


- ,/

If? cv / Ni m

/ A O - Sb

Zn 6 - F e p b A 0 .

/

0.1 1

Figure 3 Comparison of the results,for minor and trace elements in the standarh

to the K-shell of light elements. For our standards this is typically about 0.1 %. FNAA allows the determination of antimony concentration at the ppm level (5 ppm for UZ 53); lead concentration is determined by all three methods, but it should be noted that in the case of low concentration PIXE gives incorrect results (UZ 53).

This comparison of results obtained with industrial samples shows clearly that PIXE is a very sensitive multielement analytical method (in the ppm range) if used for thin or thick samples of light matrices. This characteristic is no longer true in the case of thick samples of medium atomic number elements such as copper and copper alloys. This is a confir- mation of the empirical data published by Fleming and Swann (1986) who showed that the detection limits for copper are in the range of lOOppm for the elements between chromium and bromine.

The coins Table 4 summarizes the results of the analysis obtained for the five Roman coins. We will compare the results for each metal or alloy.


Table 4 Concentration (%) obtained by PIXE. PAA and F N A A for the five Roman coins

Method Fe N i Z n As Ag Sn Sh Au Pb

Coin I PIXEd 0.17 PAA 0.14 FNAA 0.14 Coin 2 PIXE" 0.06 PAA 0.70 FNAA 0.55 Coin 3 PIXE" 0.85 PAA 0.36 FNAA 0.21 Coin 4 PIXEI 0.45 PAA 0.12 FNAA 0.09 Coin 5 PIXEa 0.16 PAA 0.11 FNAA 0.14

0.008 $ 0.01

0.008

0.0 16

0.015

0.02 $ 0.05

0.026

0.041

0.03

0.073

0.035

g 0.02

$0.1

GO.1

0.13 0.1 I 0.1 I

11.0 10.8 10.5

8.8 12.0 12.3

0.22 0.14 0.37

0.12 0.35 0.35

- 0.07 0.044 6 0 . 3 0.045 0.03

- 0.04 0.038 6 0 . 3 0.032 0.04

- -

0.052 $0.3 0.05 1 0.04

0.95 - 0.08 g0 .3 0.08 0.07

0.15 - 0.12 g 0 . 3 0.12 0.07

0.08 0.056 0.055

0.63 0.68 0.60

1.20 0.26 0.25

21.5 9.45 4.5

5.25 5.6 3. I

0.10 0.08 0.1 15

0.16 0.10 0.1 I

0.28 0.16 0.24

0.85 0.10 0.10

0.26 0.1 I 0.11

-

g 0.05 $ 0.0002

-

G 0 . l 0.07

-

g 0.05 0.0005

- g 0.05

0.003

- < 0.05

0.027

0.19 0.09 0.065

0.39 0.30 0.3 1

1.17 0.20 0.17

9.55 9.65 12.6

22.0 15.4 16.2

"PIXE results are the average of the two determinations carried out on different regions.

Copper For the copper coin (coin I ) it appears that the most sensitive of the non-destructive methods is FNAA, whose detection limits are in the ppm range ( 2 ppm for Au), while PAA, because of the low irradiation time ( < 5 min), cannot determine the concentrations of nickel, gold and silver; there is a good agreement between both nuclear methods. PIXE gives quite different results for some elements such as silver, tin and lead. These differences can easily be explained by the fact that these experimental conditions (20 nA, 2 h irradiation), are very near the detection limits of the method for thick samples. These results are in good agreement with those obtained with the standards. Brass The two analysed coins (coins 2 and 3 in table 4) show that, despite the zinc concentration being about lo%, FNAA allows the determination of 5 ppm of gold (coin 3) which is not possible by either of the other two methods. In some cases the results obtained by PIXE can be quite different from those obtained by the other two methods, for example, in the case of the iron concentration for coins 2 and 3 and for zinc, tin and lead concentrations for coin 3.

In order to explain these important differences, we sectioned coin 3 to analyse its bulk by PIXE and to obtain the concentration profile of the major elements (copper, zinc) using an electron microprobe.

The comparison of the results obtained by PIXE for both the surface and the bulk of the coin show (table 5) that the origin of the concentration discrepancies is not the analytical method but the sample itself: the surface layer (patina) sometimes shows a different composition from the bulk of the coin. The composition profiles obtained for copper and zinc at the same depth allow the thickness of the corroded zone (about 100pm) to be measured (figure 4).


,.-++.-.- v-v v v-- Ln ,;. 7 v \w-vyv v v

t

Elemen is PIXE ( face) PIXE (slice)” PAA FNAA

Fe 0.85 0.2 I 0.36 0.2 I Ni 0.02 0.03 0.05 0.026 Zn 8.80 13.80 12.00 12.30 As - - 0.052 0.05 1

- - 0.3 0.04 Ag Sn 1.20 0.26 0.26 0.25 Sb 0.28 0.22 0.16 0.24 Au - - 0.05 0.0005 Pb 1.17 0.11 0.20 0.17

“The analysis of the slice is in fact a bulk analysis.


Figure 5 Depth profiles for copper, tin and lead in lead-bronze coin 4 , profile 1.

The results (figures 5 and 7) confirm the following points. Lead is present as precipitates free from tin and copper at this level of concentration; the concentrations of tin and copper are uniform between the lead precipitates.

These results also show that there is a corroded superficial layer, the patina, which has a variable thickness of 40-100 pm for well-preserved coins; the composition of this layer can vary hugely from one point to another on the surface: lead and tin enrichment in profile 1 for coins 4 and 5 (figures 5 and 7), and lack of these elements in profile 2 for coin 4 (figure 6); there is a large heterogeneity of composition inside this layer, as shown by profile 2 for coin 4, where a zone with hardly any tin and lead is followed by a zone enriched with these two elements (figure 6) .

All these results explain why the values given by the most sensitive surface method (PIXE) are incorrect, as well as in some cases (tin, lead), those obtained by PAA. For PAA the charged particles going through 300 pm of matter rapidly lose their energy, which leads to non-uniform activation. This non-uniformity makes the radioactivity level higher on the surface and lower in the deeper layers.

196 F. Beauchesne, J . N . Barrandon. L. Alves. F. B. Gil and M . F. Guerra

I

Figure 6 Depth profiles,for copper. tin and lead in lead-bronze coin 4 , projle 2.

I ,p-* -l

,

- - 0 - cu r I

.-.- ;-"a, IJm

ioo Figure 7 Depth profiks,for copper, tin and lead in lead-bronze coin 5. profile 1

Ion beam analysis of copper and copper alloy coins

C O N C L U S I O N S

I97

This study confirms that the analysis of archaeological metallic objects needs a good understanding of metal physics as well as of the analytical methods employed.

In the case of copper and copper alloy objects, our results undoubtedly show that the surface analytical methods (PIXE, XRF, low energy ion beam techniques) can only be used after a surface treatment of the objects. As proposed by Carter (1964) ‘one thousandth of an inch of metal should be worn off, but it seems that this is not always adequate. A safer rule would be to wear off a layer of 100pm before carrying out any analysis by surface methods.

This study shows that at the present time, FNAA is a method which allows a fully non-destructive analysis of copper alloy coins. Another possible method is photon activation analysis. However, with this method iron is not determinable (Carter 1983), while with FNAA nine elements can be determined at the ppm level.

R E F E R E N C E S

Barrandon, J. N., Debrun, J. L. and Kohn, A,, 1972, Modern Trends in Activation Analysis, 2-6 Oct., Saclay,

Barrandon, J. N., Debrun, J. L. and Hours, M., 1973.24-29 May, Rome Academia Nationale dei Lincei 11,77-85

Barrandon, J.N., 1986, IBA’ Workshop, Nucl. Instrum. Methods M, 133-141. Beauchesne, F. and Barrandon, J. N., 1986, Rev. d’Arch6om. 10, 75-85. Bird, J. R., Duerden, P. and Wilson, D. J., 1983, Ion beam techniques in archaeology and the arts, Nucl. Sci. Appl.

Carter, G. F., 1964, Preparation of ancient coins for accurate X-ray fluorescence analysis, Archaeometry 7 ,

Carter, G. F., Caley, E. R., Carlson, J. H., Carriveau, G . W., Hughes, M. J., Rengan, K. and Segebade, C., 1983, Comparison of analyses of eight Roman orichalcum coin fragments by seven methods, Archaeometry 25 (2).

Chase, W. T., 1974, Comparative analysis of archaeological bronzes, in Archaeological chemistry, (ed. C. W.

Ferreira, G. P. and Gil, F. B., 1981, Elemental analysis of gold coins by particle induced X-ray emission (PIXE),

Fleming, S. J. and Swann, Ch. P., 1986, PIXE spectrometry as an archaeological tool, Nucl. Instrum. Methods

IBA3 Workshop, 1986, Nucl. Instrum Methods B4, 1-168. Morrison C. , Brenot, C., Calla, J. P., Barrandon, J. R., Pokier, J. and Halkux, R., 1985, Cuhiers Ernest Bubelon,

J . Rudioanal. Chem., 1974.

(1986).

1, 357-516.

106-1 13.

201 -2 1 3.

Beck), pp. 144-185, Washington: American Chemical Society.

Archaeomerry 23 (2). 189-197.

A242, 626-63 I .

No. 2. pp. 1-282, Editions du CNRS.

ION BEAM ANALYSIS OF COPPER AND COPPER ALLOY COINS

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