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A new method for preparing thin samples of pottery shards for pixe analysis
Nuclear Instruments and Methods 202 (1982) 487-491 North-Holland Publishing Company
A NEW METHOD FOR PREPARING F O R PIXIE A N A L Y S I S A.J. H O U D A Y E R
*, P. B E A U D O I N
487
THIN SAMPLES
OF POTrERY
SHARDS
a n d L. L E S S A R D
Laboratoire de Physique Nuelbaire, Universitb de Montrbal, MontrbaL Qubbec, Canada, H3C 3.17
Received 24 November 1981 and in revised form 22 March 1982
A novel method, based on the centrifugal precipitation technique, has been used for preparing thin (1 mg/cm 2) uniform and homogeneous samples of pottery shards for PIXE analysis. The reproducibility of the results has been tested on standard Perlman-Asaro samples and ancient Greek pottery shards. The abundances of more than 12 elements can be reliably measured.
1. Introduction The PIXE (proton induced X-ray emission) method of elemental analysis has been successfully applied to a variety of samples. Although not equally sensitive to all elements, it offers a quick and accurate method of analysis for a wide variety of elements. In our laboratory, an experimental set-up has been developed [ 1] and utilized for many years, in particular for the analysis of samples of medical interest [2]. For these organic materials, problems of self-absorption of X-rays, uniformity of thickness and composition can be readily minimized. Recently, we have been interested in the analysis of pottery shards for archaeological purposes. In this particular case, there exists obvious problems of material uniformity; one would like to use samples thin enough to minimize self absorption, of uniform composition and capable of sustaining a beam of sufficient intensity to keep counting times at a reasonable level. The purpose of this paper is to present a method of preparing pottery shard samples which we have found suitable for PIXE analysis. We compare our results with those of a neutron activation analysis of the same shards.
2. Description of the method Our problem has much in common with the preparation of targets of refractory materials for nuclear reaction studies. Such targets have to be uniform, must be of
* CEGEP Ahuntsic, 9155 rue St-Hubert, Montr6al, Qu6bec, Canada, H2M 1Y8. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
a suitable thickness (typically, for many purposes m g / c m 2) and capable of sustaining rather intense beams. In many cases, it is either not practical, or too costly, to evaporate such materials, especially when one deals with enriched isotopes. A solution to this problem has been to make a suspension of the material in oxide form, and then deposit it by either simple gravity precipitation, suction through a filter, or electrophoresis [3-5]. Unfortunately, these methods produce targets with non uniform thickness which are unable to withstand intense beam bombardment. An alternative method has been developed by Sugai [6] and later refined by Richaud [7] to overcome these difficulties. This method, called the centrifugal precipitation technique (CPT) has been used to produce targets of powdered isotopic material in the range of 0.1-50 m g / c m 2 with remarkable thickness uniformity and capable of accepting reasonable beam current intensities. It consists essentially in depositing the fine grained isotopic powder of metal oxide or carbonate from a suspension in a suitable liquid, such as liquid paraffin or a plastic solution, by centrifugation onto an appropriate backing. To achieve target thicknesses of less than 1 m g / c m 2, the suspended powder must be of uniform and very fine grain size. We have developed a method of preparing pottery shard samples based on CPT. Contrary to target materials which exist usually in powder form, pottery samples have to be treated to obtain a powder of proper grain size. This treatment must be such that the final product is typical of the average pottery composition. At the same time, one must ensure that no contamination is produced in the process. Furthermore, the target backing must not introduce spurious X-ray peaks. Finally, it is useful, in order to obtain an absolute measurement of the elemental composition, to add an element of known abundance, for marking purposes.
488
A.J. Houdayer et al. / Preparing thin samples
2.1. Collection of the powder from the shards After having superficially cleaned the pottery shards with research grade acetone and isopropyl alcohol, holes were drilled in the shard using a high speek drill with an ultra-clean tungsten carbide drill bit (diameter: 0.317 cm). The initial powder collected from the surface was discarded and only that obtained from below the surface (3-5 mm hole depth) was kept. In order to ensure homogeneity, powder was collected and thoroughly mixed from a minimum of four holes drilled at different locations on each shard. The grain of the powder thus obtained was then further reduced in size by carefully grinding it with an agate mortar and pestle. This procedure of grinding also improves the homogeneity of the powder. In this fashion, it was possible to obtain a homogeneous powder of grain diameter smaller than 3/tin. It has also been noted by Sugai [6] that the process of grinding the powder in an agate mortar introduces only minute traces of silicon. 2.2. The C P T method The centrifugal tube that we used is essentially a copy of Richaud's design. It consists of six parts (see fig. 1). A teflon tube (teflon is chosen because of its very low reactivity with the products used) defines exactly the dimensions of the target. In our case, the diameter is 1 cm, giving a target area of 0.785 cm 2. The tube, of length 8.80 cm, contains, after closing the assembly, a suspension volume of 6.75 cm 3. This tube fits snugly into an aluminum tube 8.65 cm long, with an internal diameter of 1.82 cm. At the botton of this system, a thin backing film (X-ray mylar, 2.5 ktm thick)* is glued to a 0.10 cm thick aluminum target frame, and is pressed by a neoprene ring (0.30 cm thick) and a 0.50 cm thick aluminum cover. This cover is fixed by four screws and compresses the neoprene disk, the target holder and mylar film to the centrifugal tube assembly. The purpose of the neoprene (or polyethylene) disk is to press uniformly and seal perfectly the mylar backing at the base of the teflon tube. It was found that pressing with the neoprene disk on the aluminum ring support (as in fig. 1) produced virtually no wrinkle on the mylar backing. For our purposes, the optimum target thickness is approximately 1 m g / c m 2 in order to minimize selfabsorption of X-rays and beam energy loss. Since our target has an area of 0.785 cm z, about 1 mg of powder is needed. In order to separate and wet each grain of the powder, the powder is first poured in a clean glass tube, then, using an Eppendorf 1000 /~1 pipette, 2 ml of
* From Chemplex Industries.
,
ALUMINUM
LSOO
COVER
2- NEOPRE NE DISK 3 ALUMINUM 4
MYLAR
5-ALUMINUM 6- T E F L O N
TARGET FRAME
FILM TUBE TUBE
IO00
3,000
I
Fig. I. Centrifugal tube assembly. All units are in cm.
research grade ethyl acetate is added. This suspension is well mixed by inserting the glass tube in an ultrasonic vibrator for about 10 min. To achieve a very homogeneous suspension, a 5 ml solution of collodion in ethanol (with a concentration of 10-40% depending on the powder density; for pottery, a solution of 30% is chosen) is then added and the glass tube is again placed in the ultrasonic vibrator for 20 min. As a result of this procedure an acceptable suspension is produced with no observed precipitation at the bottom of the glass tube. The suspension (-- 7 ml) is then rapidly poured into the centrifugal tube mount and then put in a centrifuge which is rotated at 3300 rpm for about 90 min. A period of 60 min is sufficient to obtain a very transparent solution, which means that essentially all the powder is then deposited on the mylar film. It is found however that the target is much more resistant to vibration if the centrifugation time is extended to 90 min. After this period, the transparent solution (ethyl acetate, collodion, ethanol and minute traces of powder) is poured out and the target holder carefully dismounted from the centrifugal tube. The targets are then heated for about 30 min at 60°C in order to evaporate the remaining collodion solution from the target material. 2.3. Marking procedure For an absolute determination of the elemental composition of the target (or sample); it is imperative to know the weight to extract the thickness of the deposited material. This is done by carefully weighing the aluminum and mylar target holder before and after deposition with a microgramatic balance (2 ~tg accuracy). Because of the uncertainty introduced in the weighing procedures by a small amount of collodion solution which is not completely removed by the heating
A.J. Houdayer et al. / Preparing thin samples operation described in the previous paragraph, it was decided to add a k n o w n a m o u n t of an element which would not produce X-ray peaks in the region of interest. This was achieved by v a c u u m evaporation of metallic silver. To ensure a u n i f o r m making of a large group of samples a rotating wheel was constructed, capable of supporting up to 26 target holders, carefully centered above the filament, at a distance of 80 cm. D u r i n g a typical evaporation, the wheel would rotate ~ 50 times. It was verified that, to within 4%, the silver coating was the same on all 26 targets. Two different m e t h o d s were employed for determining the absolute c o n c e n t r a t i o n from this information. T h e coating of silver was first calibrated using X-ray fluorescence technique. This then served as a secondary s t a n d a r d for determining the remaining abundances. Special care h a d to be exercised to take into account the a b s o r p t i o n of X-rays in the pottery matrix. Since samples of Perlman a n d Asaro s t a n d a r d pottery were always included a m o n g the 26 targets, it was also possible to use this s t a n d a r d for relative concentration determination, the silver peak playing the role of a cross calibration. In the latter case, since all the targets h a d (within 10%) the same thickness, it was not necessary to include any self a b s o r p t i o n corrections.
489
2.4. Determination of relative abundances To o b t a i n relative abundances, we employed a computer p r o g r a m ( T O U R I X ) which has been described in detail in ref. 2. It incorporates subroutines for unfolding Si(Li) X-ray spectra for up to 30 elements, taking into account relative X-ray intensities, relative yields for various elements at the p r o t o n energy used (3 MeV), a n d detector efficiency. Since this program has been described extensively in ref. 2, it will not be discussed further here. The possibility of analyzing a n d collecting spectra simultaneously more than compensates for the extra labor involved in preparing samples, by comparison with other methods such as n e u t r o n activation.
3. Results
and
discussion
We tested our m e t h o d for preparating pottery shard samples by collecting spectra of the Perlman and Asaro s t a n d a r d pottery on m a n y different sample targets, and b y doing a n i n d e p e n d e n t analysis of the composition of ancient G r e e k from Asine (called Asi). We b o m b a r d e d our samples with a 3 MeV p r o t o n b e a m delivered by the Universit6 de M o n t r r a l t a n d e m accelerator, using an
A Fe
I06
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Ca
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i0 ~
I00
I
I
I
I
I
200
300
400
500
600
CHANNEL
I 700
800
NUMBER
Fig. 2. (A) Spectrum of Perlman-Asaro standard pottery. The resolution of the Si(Li) detector is 195 eV at the energy of the Fe K,~ line (Q ~ 1200 # C). (B) Spectrum of the mylar-collodion backing with silver marking. Au and Cu are impurities associated with silver used for marking. The mylar (obtained from Chemplex Industries) has a thickness of 2.5 /~m (Q ~ 800 ~tC).
A.J. Houdayer et al. / Preparing thin samples
490
Table 1 Comparison of spectra from two different Asaro standard pottery samples, (See text for details.) Element
Composition a)
PIXE ratio b)
Indep. det. of composition c.d)
K Ca Ti Cr Mn Fe Co Ni Cu Zn Ga As Br Rb Sr Y
13 500400 < 200 7820340 1154 40.95 10170120 14.0615 2792o 595 598 44.446 30.822 2.39 706 14522 2.84
1.06
13 900850
1.06 1.40 2.40 1.08 0.63 1.20 1.31
10260630 8613 10816 9800250 155 29029 52 7
1.23 0.94 1.29 1.11 -
377 327 741o 13415 _
a) Composition of standard pottery from Perlman and Asaro [8,9]. b) Ratio of PIXE analysis of two different standard pottery samples. c) Independent determination of the composition in ppm or indicated in %. d) Statistical error only; systematic error of 5% to silver deposition not included.
experimental set up described in detail in ref. 2. Typically, with beam intensities of 10-15 nA, on samples 1.1 m g / c m 2 thick, the detector count rate was ~ 3 0 0
cps. We carefully chose our target samples to be within 10% of the average thickness to minimize variations of the corrections due to self-absorption. Comparison was always made with X-ray spectra of the mylar-collodi o n - s i l v e r samples to determine possible contaminations introduced by the backing or the deposition and marking procedures. We chose to use a thin gold scatterer a meter upstream from the target to average the beam over possible remaining target inhomogeneities. Fig. 2 presents a typical Perlman and Asaro standard pottery spectrum together with a background spectrum obtained with the b a c k i n g - c o l l o d i o n - s i l v e r samples described above. In table 1, we show a comparison between spectra taken from two different Asaro standard pottery sampies. In spite of the very small amount of material used ( ~ 1 rag), the composition agreement is excellent. Using our silver marking procedure, we extracted an independent composition evaluation of the standard pottery, taking into account the sample thickness, the selfabsorption in the target matrix and the absorption in the silver coating. Again we obtained a very good agreement with the published values. F r o m this analysis, we conclude that, using our procedure, we can obtain reliable estimates of at least 12 elements. Because of the predominance of the iron lines in such samples, estimates of nearby elements like Mn, Co and Cr are less reliable if the abundances of these elements are less than say 100 ppm. It should however be emphasized that even in this case, the results obtained are not seriously in error. A similar analysis was done with Greek potteries of sufficiently different composition to study the reliability of the method. The results are shown in table 2. The
Table 2 Comparison of spectra of pottery samples of different composition. (See text for details.) Element
K Ca Ti V Cr Mn Fe Co Ni As Rb Columns A: Columns B: Columns C: a) Statistical
Asi 11
Asi 18
A
B
C a)
A
B
C ")
2.397% 6.2 2% 0.514% 1404 3441o 117016 5.2014% 34.85 325103 9.4 n 1379
1.01 0.93 0.99 1.29 0.96 1.17 1.09
2.9524% 6.55% 0.564% 1615 38527 118480 4.9024% 1015o 23012 164 m
2.28m% l 1.33% 0.574% 12l 4 211 m 825L8 4.4916% 29.2 t8
1.11 0.95 1.14 0.85 1.04 1.35 1.01 0.96
3.6629% 13.41 ~% 0.474% 6914 19414 67071 4.5022% 4625
4.38 131m
0.89
19011
1.36 0.92
Concentration given by neutron activation technique (unpublished results) in ppm or indicated in %. Ratio of PIXE analyses of two different targets. Independent determination of the composition in ppm or indicated in %. error only; systematic error of 5% to silver deposition not included.
A.J. Houdayer et al. / Preparing thin samples
491
elemental composition has been investigated by neutron activation technique. For the elements for which a reliable comparison can be made, the agreement is good. In this particular case, the higher abundance of Cr and Mn allows a good evaluation.
ing compounds: beryllium oxide (2 mg/cm2), boron oxide (1 mg/cm2), tungsten oxide (1.5 mg/cm2), zirconium oxide (1 mg/cm2). In these cases, the mylar backing was replaced with ultra pure tantalum (0.005 m m thick). Other heavier elements could also be measured but at higher incident proton energies.
4. Conclusions
The authors wish to thank Dr. M. Attas for kindly supplying Greek pottery samples from Asine and for his neutron activation analysis. We also thank Prof. S. Monaro for the use of his PIXE analysis system. Finally the financial support of the National Research Council of Canada is gratefully acknowledged.
We have developed a method for preparing uniform and reproducible samples of potteries amenable to PIXE elemental analysis. Comparison with potteries of known composition shows that one can obtain with our method reliable estimates of the elements K, Ca, Ti, Fe, Ni, Cu, Zn, Ga, As, Br, Rb and Sr. The very high abundance of the element Fe in our samples (1-5%) does not allow an accurate determination of the nearby elements like Cr, Mn and Co whenever the abundances of those elements is less than - - 1 0 0 ppm. It is not possible to obtain element abundances in the 10 ppm region under the experimental conditions that we used (10-15 nA, 1 h bombardments, 1 m g / c m 2 samples). We believe however that the range of elements that can be reliable measured is sufficient for most ceramic provenience studies in archaeology. Furthermore, within the same experimental constraints, it is possible to make systematic studies of a rather large number of samples without unduly taxing precious accelerator time. This method can readily be adapted to other types of materials provided they can be put in powdered form. In particular, for nuclear reaction studies, we successfully prepared targets of good uniformity of the follow-
References
[1] R. Lecomte, P. Paradis, S. Monaro, M. Barrette, G. Lamoureux and H.A. M~nard, Nucl. Instr. and Meth. 150 (1978) 289. [2] S. Monaro and R. Lecomte, Int. J. Nucl. Med. and Biol. 8 (1981) 1. [3] W. Parker, M. de Croi~s and K. Sevier, Jr., Nucl. Instr. and Meth. 7 (1960) 22. [4] R.K. Jolly and H.B. White, Jr., Nucl. Instr. and Meth. 97 (1971) 103. [5] V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 431. [6] I. Sugai, Nucl. Instr. and Meth. 145 (1977) 409. [7] J.P. Richaud, Nucl. Instr. and Meth. 167 (1979) 97. [8] I. Perlman and F. Asaro, Archaeometry 11 (1969) 21. [9] I. Perlman and F. Asaro, Science and archaeology, ed., R.H. Brill (MIT Press, Cambridge, Mass., 1971).
A NEW METHOD FOR PREPARING F O R PIXIE A N A L Y S I S A.J. H O U D A Y E R
*, P. B E A U D O I N
487
THIN SAMPLES
OF POTrERY
SHARDS
a n d L. L E S S A R D
Laboratoire de Physique Nuelbaire, Universitb de Montrbal, MontrbaL Qubbec, Canada, H3C 3.17
Received 24 November 1981 and in revised form 22 March 1982
A novel method, based on the centrifugal precipitation technique, has been used for preparing thin (1 mg/cm 2) uniform and homogeneous samples of pottery shards for PIXE analysis. The reproducibility of the results has been tested on standard Perlman-Asaro samples and ancient Greek pottery shards. The abundances of more than 12 elements can be reliably measured.
1. Introduction The PIXE (proton induced X-ray emission) method of elemental analysis has been successfully applied to a variety of samples. Although not equally sensitive to all elements, it offers a quick and accurate method of analysis for a wide variety of elements. In our laboratory, an experimental set-up has been developed [ 1] and utilized for many years, in particular for the analysis of samples of medical interest [2]. For these organic materials, problems of self-absorption of X-rays, uniformity of thickness and composition can be readily minimized. Recently, we have been interested in the analysis of pottery shards for archaeological purposes. In this particular case, there exists obvious problems of material uniformity; one would like to use samples thin enough to minimize self absorption, of uniform composition and capable of sustaining a beam of sufficient intensity to keep counting times at a reasonable level. The purpose of this paper is to present a method of preparing pottery shard samples which we have found suitable for PIXE analysis. We compare our results with those of a neutron activation analysis of the same shards.
2. Description of the method Our problem has much in common with the preparation of targets of refractory materials for nuclear reaction studies. Such targets have to be uniform, must be of
* CEGEP Ahuntsic, 9155 rue St-Hubert, Montr6al, Qu6bec, Canada, H2M 1Y8. 0167-5087/82/0000-0000/$02.75
© 1982 North-Holland
a suitable thickness (typically, for many purposes m g / c m 2) and capable of sustaining rather intense beams. In many cases, it is either not practical, or too costly, to evaporate such materials, especially when one deals with enriched isotopes. A solution to this problem has been to make a suspension of the material in oxide form, and then deposit it by either simple gravity precipitation, suction through a filter, or electrophoresis [3-5]. Unfortunately, these methods produce targets with non uniform thickness which are unable to withstand intense beam bombardment. An alternative method has been developed by Sugai [6] and later refined by Richaud [7] to overcome these difficulties. This method, called the centrifugal precipitation technique (CPT) has been used to produce targets of powdered isotopic material in the range of 0.1-50 m g / c m 2 with remarkable thickness uniformity and capable of accepting reasonable beam current intensities. It consists essentially in depositing the fine grained isotopic powder of metal oxide or carbonate from a suspension in a suitable liquid, such as liquid paraffin or a plastic solution, by centrifugation onto an appropriate backing. To achieve target thicknesses of less than 1 m g / c m 2, the suspended powder must be of uniform and very fine grain size. We have developed a method of preparing pottery shard samples based on CPT. Contrary to target materials which exist usually in powder form, pottery samples have to be treated to obtain a powder of proper grain size. This treatment must be such that the final product is typical of the average pottery composition. At the same time, one must ensure that no contamination is produced in the process. Furthermore, the target backing must not introduce spurious X-ray peaks. Finally, it is useful, in order to obtain an absolute measurement of the elemental composition, to add an element of known abundance, for marking purposes.
488
A.J. Houdayer et al. / Preparing thin samples
2.1. Collection of the powder from the shards After having superficially cleaned the pottery shards with research grade acetone and isopropyl alcohol, holes were drilled in the shard using a high speek drill with an ultra-clean tungsten carbide drill bit (diameter: 0.317 cm). The initial powder collected from the surface was discarded and only that obtained from below the surface (3-5 mm hole depth) was kept. In order to ensure homogeneity, powder was collected and thoroughly mixed from a minimum of four holes drilled at different locations on each shard. The grain of the powder thus obtained was then further reduced in size by carefully grinding it with an agate mortar and pestle. This procedure of grinding also improves the homogeneity of the powder. In this fashion, it was possible to obtain a homogeneous powder of grain diameter smaller than 3/tin. It has also been noted by Sugai [6] that the process of grinding the powder in an agate mortar introduces only minute traces of silicon. 2.2. The C P T method The centrifugal tube that we used is essentially a copy of Richaud's design. It consists of six parts (see fig. 1). A teflon tube (teflon is chosen because of its very low reactivity with the products used) defines exactly the dimensions of the target. In our case, the diameter is 1 cm, giving a target area of 0.785 cm 2. The tube, of length 8.80 cm, contains, after closing the assembly, a suspension volume of 6.75 cm 3. This tube fits snugly into an aluminum tube 8.65 cm long, with an internal diameter of 1.82 cm. At the botton of this system, a thin backing film (X-ray mylar, 2.5 ktm thick)* is glued to a 0.10 cm thick aluminum target frame, and is pressed by a neoprene ring (0.30 cm thick) and a 0.50 cm thick aluminum cover. This cover is fixed by four screws and compresses the neoprene disk, the target holder and mylar film to the centrifugal tube assembly. The purpose of the neoprene (or polyethylene) disk is to press uniformly and seal perfectly the mylar backing at the base of the teflon tube. It was found that pressing with the neoprene disk on the aluminum ring support (as in fig. 1) produced virtually no wrinkle on the mylar backing. For our purposes, the optimum target thickness is approximately 1 m g / c m 2 in order to minimize selfabsorption of X-rays and beam energy loss. Since our target has an area of 0.785 cm z, about 1 mg of powder is needed. In order to separate and wet each grain of the powder, the powder is first poured in a clean glass tube, then, using an Eppendorf 1000 /~1 pipette, 2 ml of
* From Chemplex Industries.
,
ALUMINUM
LSOO
COVER
2- NEOPRE NE DISK 3 ALUMINUM 4
MYLAR
5-ALUMINUM 6- T E F L O N
TARGET FRAME
FILM TUBE TUBE
IO00
3,000
I
Fig. I. Centrifugal tube assembly. All units are in cm.
research grade ethyl acetate is added. This suspension is well mixed by inserting the glass tube in an ultrasonic vibrator for about 10 min. To achieve a very homogeneous suspension, a 5 ml solution of collodion in ethanol (with a concentration of 10-40% depending on the powder density; for pottery, a solution of 30% is chosen) is then added and the glass tube is again placed in the ultrasonic vibrator for 20 min. As a result of this procedure an acceptable suspension is produced with no observed precipitation at the bottom of the glass tube. The suspension (-- 7 ml) is then rapidly poured into the centrifugal tube mount and then put in a centrifuge which is rotated at 3300 rpm for about 90 min. A period of 60 min is sufficient to obtain a very transparent solution, which means that essentially all the powder is then deposited on the mylar film. It is found however that the target is much more resistant to vibration if the centrifugation time is extended to 90 min. After this period, the transparent solution (ethyl acetate, collodion, ethanol and minute traces of powder) is poured out and the target holder carefully dismounted from the centrifugal tube. The targets are then heated for about 30 min at 60°C in order to evaporate the remaining collodion solution from the target material. 2.3. Marking procedure For an absolute determination of the elemental composition of the target (or sample); it is imperative to know the weight to extract the thickness of the deposited material. This is done by carefully weighing the aluminum and mylar target holder before and after deposition with a microgramatic balance (2 ~tg accuracy). Because of the uncertainty introduced in the weighing procedures by a small amount of collodion solution which is not completely removed by the heating
A.J. Houdayer et al. / Preparing thin samples operation described in the previous paragraph, it was decided to add a k n o w n a m o u n t of an element which would not produce X-ray peaks in the region of interest. This was achieved by v a c u u m evaporation of metallic silver. To ensure a u n i f o r m making of a large group of samples a rotating wheel was constructed, capable of supporting up to 26 target holders, carefully centered above the filament, at a distance of 80 cm. D u r i n g a typical evaporation, the wheel would rotate ~ 50 times. It was verified that, to within 4%, the silver coating was the same on all 26 targets. Two different m e t h o d s were employed for determining the absolute c o n c e n t r a t i o n from this information. T h e coating of silver was first calibrated using X-ray fluorescence technique. This then served as a secondary s t a n d a r d for determining the remaining abundances. Special care h a d to be exercised to take into account the a b s o r p t i o n of X-rays in the pottery matrix. Since samples of Perlman a n d Asaro s t a n d a r d pottery were always included a m o n g the 26 targets, it was also possible to use this s t a n d a r d for relative concentration determination, the silver peak playing the role of a cross calibration. In the latter case, since all the targets h a d (within 10%) the same thickness, it was not necessary to include any self a b s o r p t i o n corrections.
489
2.4. Determination of relative abundances To o b t a i n relative abundances, we employed a computer p r o g r a m ( T O U R I X ) which has been described in detail in ref. 2. It incorporates subroutines for unfolding Si(Li) X-ray spectra for up to 30 elements, taking into account relative X-ray intensities, relative yields for various elements at the p r o t o n energy used (3 MeV), a n d detector efficiency. Since this program has been described extensively in ref. 2, it will not be discussed further here. The possibility of analyzing a n d collecting spectra simultaneously more than compensates for the extra labor involved in preparing samples, by comparison with other methods such as n e u t r o n activation.
3. Results
and
discussion
We tested our m e t h o d for preparating pottery shard samples by collecting spectra of the Perlman and Asaro s t a n d a r d pottery on m a n y different sample targets, and b y doing a n i n d e p e n d e n t analysis of the composition of ancient G r e e k from Asine (called Asi). We b o m b a r d e d our samples with a 3 MeV p r o t o n b e a m delivered by the Universit6 de M o n t r r a l t a n d e m accelerator, using an
A Fe
I06
c,~ !rico ~'1
10 s
A+ i
,rl
104 _.1 3 bd Io Z "1-
I ¢r" lad (!.
I
I
I
I
Ca
,0~ I-'-
z,g
~]
K..: Ti
Fe
~l
.":;~:.:J"::'i:.iLl
~Zn
A. .++
O (,.)
:."' "
::..
?.
. i:L •~..i~ .i.
~":.:,+~":: ',:!:.~::c.=~.ii::;?..:!?i: ..:~ ~,:;.,::/~::;):.;. . I. @:I:.:,.~.:/: . :~
i0 ~
I00
I
I
I
I
I
200
300
400
500
600
CHANNEL
I 700
800
NUMBER
Fig. 2. (A) Spectrum of Perlman-Asaro standard pottery. The resolution of the Si(Li) detector is 195 eV at the energy of the Fe K,~ line (Q ~ 1200 # C). (B) Spectrum of the mylar-collodion backing with silver marking. Au and Cu are impurities associated with silver used for marking. The mylar (obtained from Chemplex Industries) has a thickness of 2.5 /~m (Q ~ 800 ~tC).
A.J. Houdayer et al. / Preparing thin samples
490
Table 1 Comparison of spectra from two different Asaro standard pottery samples, (See text for details.) Element
Composition a)
PIXE ratio b)
Indep. det. of composition c.d)
K Ca Ti Cr Mn Fe Co Ni Cu Zn Ga As Br Rb Sr Y
13 500400 < 200 7820340 1154 40.95 10170120 14.0615 2792o 595 598 44.446 30.822 2.39 706 14522 2.84
1.06
13 900850
1.06 1.40 2.40 1.08 0.63 1.20 1.31
10260630 8613 10816 9800250 155 29029 52 7
1.23 0.94 1.29 1.11 -
377 327 741o 13415 _
a) Composition of standard pottery from Perlman and Asaro [8,9]. b) Ratio of PIXE analysis of two different standard pottery samples. c) Independent determination of the composition in ppm or indicated in %. d) Statistical error only; systematic error of 5% to silver deposition not included.
experimental set up described in detail in ref. 2. Typically, with beam intensities of 10-15 nA, on samples 1.1 m g / c m 2 thick, the detector count rate was ~ 3 0 0
cps. We carefully chose our target samples to be within 10% of the average thickness to minimize variations of the corrections due to self-absorption. Comparison was always made with X-ray spectra of the mylar-collodi o n - s i l v e r samples to determine possible contaminations introduced by the backing or the deposition and marking procedures. We chose to use a thin gold scatterer a meter upstream from the target to average the beam over possible remaining target inhomogeneities. Fig. 2 presents a typical Perlman and Asaro standard pottery spectrum together with a background spectrum obtained with the b a c k i n g - c o l l o d i o n - s i l v e r samples described above. In table 1, we show a comparison between spectra taken from two different Asaro standard pottery sampies. In spite of the very small amount of material used ( ~ 1 rag), the composition agreement is excellent. Using our silver marking procedure, we extracted an independent composition evaluation of the standard pottery, taking into account the sample thickness, the selfabsorption in the target matrix and the absorption in the silver coating. Again we obtained a very good agreement with the published values. F r o m this analysis, we conclude that, using our procedure, we can obtain reliable estimates of at least 12 elements. Because of the predominance of the iron lines in such samples, estimates of nearby elements like Mn, Co and Cr are less reliable if the abundances of these elements are less than say 100 ppm. It should however be emphasized that even in this case, the results obtained are not seriously in error. A similar analysis was done with Greek potteries of sufficiently different composition to study the reliability of the method. The results are shown in table 2. The
Table 2 Comparison of spectra of pottery samples of different composition. (See text for details.) Element
K Ca Ti V Cr Mn Fe Co Ni As Rb Columns A: Columns B: Columns C: a) Statistical
Asi 11
Asi 18
A
B
C a)
A
B
C ")
2.397% 6.2 2% 0.514% 1404 3441o 117016 5.2014% 34.85 325103 9.4 n 1379
1.01 0.93 0.99 1.29 0.96 1.17 1.09
2.9524% 6.55% 0.564% 1615 38527 118480 4.9024% 1015o 23012 164 m
2.28m% l 1.33% 0.574% 12l 4 211 m 825L8 4.4916% 29.2 t8
1.11 0.95 1.14 0.85 1.04 1.35 1.01 0.96
3.6629% 13.41 ~% 0.474% 6914 19414 67071 4.5022% 4625
4.38 131m
0.89
19011
1.36 0.92
Concentration given by neutron activation technique (unpublished results) in ppm or indicated in %. Ratio of PIXE analyses of two different targets. Independent determination of the composition in ppm or indicated in %. error only; systematic error of 5% to silver deposition not included.
A.J. Houdayer et al. / Preparing thin samples
491
elemental composition has been investigated by neutron activation technique. For the elements for which a reliable comparison can be made, the agreement is good. In this particular case, the higher abundance of Cr and Mn allows a good evaluation.
ing compounds: beryllium oxide (2 mg/cm2), boron oxide (1 mg/cm2), tungsten oxide (1.5 mg/cm2), zirconium oxide (1 mg/cm2). In these cases, the mylar backing was replaced with ultra pure tantalum (0.005 m m thick). Other heavier elements could also be measured but at higher incident proton energies.
4. Conclusions
The authors wish to thank Dr. M. Attas for kindly supplying Greek pottery samples from Asine and for his neutron activation analysis. We also thank Prof. S. Monaro for the use of his PIXE analysis system. Finally the financial support of the National Research Council of Canada is gratefully acknowledged.
We have developed a method for preparing uniform and reproducible samples of potteries amenable to PIXE elemental analysis. Comparison with potteries of known composition shows that one can obtain with our method reliable estimates of the elements K, Ca, Ti, Fe, Ni, Cu, Zn, Ga, As, Br, Rb and Sr. The very high abundance of the element Fe in our samples (1-5%) does not allow an accurate determination of the nearby elements like Cr, Mn and Co whenever the abundances of those elements is less than - - 1 0 0 ppm. It is not possible to obtain element abundances in the 10 ppm region under the experimental conditions that we used (10-15 nA, 1 h bombardments, 1 m g / c m 2 samples). We believe however that the range of elements that can be reliable measured is sufficient for most ceramic provenience studies in archaeology. Furthermore, within the same experimental constraints, it is possible to make systematic studies of a rather large number of samples without unduly taxing precious accelerator time. This method can readily be adapted to other types of materials provided they can be put in powdered form. In particular, for nuclear reaction studies, we successfully prepared targets of good uniformity of the follow-
References
[1] R. Lecomte, P. Paradis, S. Monaro, M. Barrette, G. Lamoureux and H.A. M~nard, Nucl. Instr. and Meth. 150 (1978) 289. [2] S. Monaro and R. Lecomte, Int. J. Nucl. Med. and Biol. 8 (1981) 1. [3] W. Parker, M. de Croi~s and K. Sevier, Jr., Nucl. Instr. and Meth. 7 (1960) 22. [4] R.K. Jolly and H.B. White, Jr., Nucl. Instr. and Meth. 97 (1971) 103. [5] V. Verdingh, Nucl. Instr. and Meth. 102 (1972) 431. [6] I. Sugai, Nucl. Instr. and Meth. 145 (1977) 409. [7] J.P. Richaud, Nucl. Instr. and Meth. 167 (1979) 97. [8] I. Perlman and F. Asaro, Archaeometry 11 (1969) 21. [9] I. Perlman and F. Asaro, Science and archaeology, ed., R.H. Brill (MIT Press, Cambridge, Mass., 1971).