Lipponen, M.

Rosenberg, R. J.

Zilliacus, R.

English

Journal.fi

THE ANALYSIS OF TRACE ELEMENT CONCENTRATION IN ANCIENT OBJECTS BY NEUTRON ACTIV ATION ANALYSIS M. LIPPONEN, R.J. ROSENBERG and R. ZILLIACUS Technical Research Centre of Finland , Reactor Laboratory, SF-02150 Espoo Introduction Neutron activation analysis is commonly used to determine trace element concentra-tions in ancient objects in order to evaluate their origin. At the Reactor Laboratory trace elements have been analyzed from some copper and brass objects and also from samples of old ceramics. Because of its sensitivity neutron activation analysis is a suitable technique for the analysis of small amounts of materials. The sample size has to be small when rare and hence valuable objects are analyzed. Sample preparation Copper and brass objects In the analysis of meta! objects the sample preparation was carried out in the following way: The samples were taken by drilling a 4 mm deep hole with a steel drill 3 mm in diameter. The first millimeter was removed before taking the sample so as to avoid pos-sible contamination caused by the altered surface layer. The samples of some milli-grams were dissolved in quarz ampoules with ultrapure HNO3 and diluted to 200 J,ti in order to avoid seif absorption during irradiation. The brass samples did not dissolve in HNO3 and therefore HCI was added drop by drop until the sample was completely dissolved. A reagent blank was also prepared. The standard was prepared of liquid solutions of the appropriate elements. Ceramic samp/es The ceramic samples were homogenized in an agate mortar. About 80 mg of the sam-ples and the same volume of standards were weighed into small polyethylene capsules (0.5 ml in volume). Two of the standards were rock standards made by the Geological Survey of Finland, while the other two were artificial silicon-based Standards made in the Reactor Laboratory. Analysis Irradiation of the samples and standards took place in a research reactor Triga Mark II in Otaniemi, Espoo, where the neutron flux varied from 1012-1013 n/ cm2 s depend-ing on the irradiation position. 411 The samples were measured with a -y-spectrometer which comprises a sample changer, a Ge(Li)-detector and a multichannel analyzer (Vänskä et al., 1981). The spectrometer was equipped with an on-line microcomputer which computes the elemental concentrations of the samples immediately after the measurement has ended. Copper and brass objects The standards and samples were irradiated for 30 h in the central tube of the reactor. After the irradiation the samples were transferred into polyethylene capsules using a Pasteur pipette. This was done in order to lower the background activity and to im-prove the geometry of the measurement. The samples were measured 5 days and 13 days after irradiation for one hour. The blank contained small peaks of 51Cr, 60Co, 187W, 198Au and 122Sb these corresponding to negligible concentrations as compared to the results reported. Co is an exception because of a high 60Co background in the Laboratory. A little cobalt is also found by a (n, a) reaction from copper. The effect of these errors was corrected in the results which are shown in Table 1 . Ceramic samples Three separate irradiations were performed to analyze 30 elements from the ceramic samples. Table / . Concentration in µ,g / g. Sample Brass ring Copper ring Sword Knife Plate Brass ring Copper ring Sword Knife Plate As 280 137 1090 Ag 464 35 4.6 90 551 Cr 59 252 Table 2. Trace element concentration of short-lived nuclides . Sample V No. µ,g / g 101 2 93 3 129 4 135 5 100 6 145 7 134 8 91 9 109 10 102 412 Sb w Au 1360 <20 15 110 0.018 66 0.24 1290 <3 1.2 1896 33 9.0 Co Ni Zn 55 658 59700 5.5 5.5 24 270 8.8 < 10 95 1600 161 6830 < 150 Al Mn Ofo Ofo 10.6 0.37 9.4 0.21 8.4 0.16 10.4 0.16 8.8 0.16 10.3 0.12 10.6 0.11 8.4 0.29 9.8 0.19 9.7 0.52 Table 3. Average detection limits for whole rock and till samples. Element Detection limit Element Detection limit (ppm) (ppm) Na 250 Sn 100 Sc 0.5 Sb 0.1 Cr 40 Cs 0 .6 Fe 2500 Ba 80 Co 2.5 La l.~ Ni 40 Sm 0.05 Zn 100 Lu 0.05 As Hf 2 Se 4 Ta 0 .5 Br 0.6 w 2 Rb 15 Au 0.003 Mo 1.5 Th 0.4 Ag 3 u 0 .3 The short-lived nuclides V, Al and Mn were measured after a 20 s irradiation in a thermal flux of 1.2- 1012 n cm-2 s- 1• The cooling and measurement times were two minutes. The results of these elements are in Table 2. Sodium and potassium were analyzed using an irradiation time of 5 hours. The samples were measured after two days for 20 minutes. These nuclides could also be analyzed after epithermal irradiation but the sensitivity is better in the above procedure. The rest of the elements were analyzed using epithermal irradiation which improves the sensitivities above all of As, Br, Mo, W and Au . This method reduces the total activity of the sample which is mainly due to the 24Na-activity. The detection limits of some till samples can be seen in Table 3. The detection limits were calculated applying the definition of Currie (1968). In the Reactor Laboratory epithermal irradiation has been routinely used to analyze exploration samples for mining companies (Rosenberg et al., 1982). The ceramic sam-ples were analyzed in the same way . The samples and the standards were irradiated for 25 hours. After 5 days the samples were measured for 20 minutes . Some of the results have been collected in Table 4. The other elements gave only determination limits. Activation analysis is a moderately accurate technique the relative error being of the order of± 10 0/o . Discussion The copper ring was found at the Suovaara stone age site in Polvijärvi, Finland. lt has been described by Taavitsainen (1982). The copper ring seems to have a very high degree of purity. The trace element composition is characteristic of pure natural cop-per. The ring contains As and therefore the region of the Urals, USSR, is thought to be the most probable area of origin and not the area of Eastern Karelia, where the object was found. The sword is probably from Northern Germany. The origin of the other objects has not yet been determined. The ceramic samples were found in Harjavalta, Finland. lt is quite obvious that the determination of the origin of these objects will require additional work . Comparing for instance the elemental concentrations of the samples with the clay used in different areas can be a start in solving this problem. 413 Tab/e 4. Trace element concentration o f ce ramic samples. Sample Na K Br As Sc Ba Cs Rb 0/o 0/o 1 0.81 3. 16 6.5 2.9 24 21 70 6.2 148 2 0.65 3.36 7. 3 2.3 23 1300 9.5 191 3 0.67 2.64 7.0 4.3 20 860 7 .5 147 4 0.87 3. 11 3.7 4.2 25 1110 9.2 179 5 0.81 2.56 10.0 1.6 18 1230 8.7 166 6 0.75 2.94 4.8 3.7 20 1580 9.7 186 7 0. 79 3. 17 4.9 4.1 22 940 9.9 192 8 0. 70 2.38 5.8 3.5 16 690 7.7 148 9 0.77 3.04 5.2 3.7 19 1180 9.1 197 10 0 .66 2.65 8.4 3.1 20 2750 9. 7 184 Cr Ni Fe Co La Sm u Th 1 138 62 74800 30 56 7 .3 6.8 24 2 129 97 70600 36 58 8.0 6.4 22 3 449 158 67000 32 59 6.9 3.5 18 4 153 57 77400 28 62 7 .3 5.0 24 5 91 34 51300 21 51 6.4 4.9 23 6 123 66 60600 21 49 6.2 4.6 20 7 105 61 68000 26 63 7. 9 4.8 24 8 799 174 6 1400 34 56 7. 1 4. 1 16 9 126 91 59900 34 59 7.9 5. 1 20 10 116 52 61200 29 62 8.7 6.4 21 Concentrat ions in /J-g / g. REFERENCES C urrie, L. A ., 1968. Limits for quantitative detection a nd quantitative determinat ion . Anal. Chem. 40: 3, 586-593. Rosenberg, R . J. , Kaisti la, M. and Zi lliacus, R., 1982. Instrumenta l epithermal neutron activation ana lys is of solid geochemical samples. J. Radioanal. C hem . 71, 419- 428. Taavitsainen , J-P ., 1982. A copper Fing from Suovaara in Polvijärvi, Northern Karelia. Fennoscandia anti-qua I, 41-49. Vänskä , L., Rosenberg , R. J . and Pitkänen, V., 1981. An automatic gamma spectrometer for acti vati on ana lysis. Nuclear Instruments and Methods 2 13, 343 - 347. 414 

The analysis of trace element concentration in ancient objects by neutron activation analysis

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The analysis of trace element concentration in ancient objects by neutron activation analysis

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