INVESTIGATION OF CHEMICAL COMPOSITION OF MARBLE SAMPLES BY PROMPT-GAMMA ACTIVATION ANALYSIS

Prompt-gamma activation analysis (PGAA), as a non-destructive analytical method can provide a lot of valuable information for different fields of materials sciences, especially for archaeometrical research too.

The physical base of the method is that the atomic nuclei become excited following the capture of thermal or "cold" neutrons. The nuclei get rid of their excitation energy by emitting prompt- photons (See Fig. 1.). All the elements emit prompt gamma rays after neutron capture, and the energy of this radiation is characteristic for the element (or rather for the isotope). The intensities of the gamma rays are proportional to the concentration of the given element (or isotope). Thus the prompt-gamma radiation is convenient for multi-elemental analysis.

The sensitivities for different elements basically depend on the neutron capture cross-section of a given nucleus, i.e. the probability of the neutron capture into the nucleus. Sensitivities can be improved by increasing intensity of the neutron beam and the decreasing "temperature" (i.e. energy) of the neutrons. The detection limits can be improved by longer measurements or by irradiation of larger samples too. From practical points of view, the measurements last from a few minutes to 8-10 hours. The usual mass of the samples range from 0.1 g to 10 g.

The result of the analysis is independent of the physical and chemical form of the sample, and – in the first order – the matrix-effects can be neglected.

We would like to mention, that PGAA – as a volumetric analytical method – can be important complementary for other surface analytical techniques, such as PIXE and XRF.

Similarities and differences in various samples – based on the chemical composition – can be investigated with different mathematical calculations. In most cases we use Principal Component Analysis (PCA).

Great advantage of the PGAA method, that – because of the relatively low neutron flux ( 10⁶–10⁸ neutron/cm^2 s) – can be considered absolutely non-destructive. After measurements, following the disintegration of short lived nuclides the sample (object of art) can be given back to the owner in its original state. The measurements don’t require any special sample preparation; almost every kind of sample can be placed into the neutron beam. Because of these features, PGAA is unique method in archaeometry research.

Prompt-gamma radiation followed by neutron capture – i.e. (n, ) nuclear reaction - was used for chemical analysis as early as in the 1960’s. PGAA has become a routine analytical method parallel to the development of the high-resolution semiconductor (HPGe) detectors and the high intensity guided neutron beams. From the 1990s the most up-to-date PGAA-techniques is represented by neutron sources moderated by liquid hydrogen and special neutron guides. Such laboratories are NIST, USA (1993); JAERI, JAPAN (1993); University Texas, USA (1996); PSI, SWITZERLAND (1997).

The measurement system at the Budapest Research Reactor, which was put into operation in 1995, also continues its work with cold neutron source since the spring of 2001. The current intensity of our beam is  7 10⁷ neutron/cm^2 s; the temperature of the neutrons is  20 K. This cold neutron source results in the increase of sensitivity by a factor of thirty. The view of the Budapest PGAA system can be seen on Fig. 2.

Though the PGAA method has already been applied for investigation of archaeological objects at different laboratories in the 1980s [1] and the 1990s [2, 3], applications as a routine are not known. At the Institute of Isotope and Surface Chemistry we apply PGAA successfully in different archaeological research:

• Investigation of bronze objects (fibulas, coins, buckles) from Roman and Avar age

• Investigation of Neolithic polished stone tools and raw material

In 2001 we performed the first PGA analysis of marble by cold neutron beam of the Budapest Research Reactor.

The test measurement was made on sample "K9" provided by Judit Zöldföldi. The approximate mass of the sample was 5 g, its size was  2cm  2cm  0.3cm

As a result of a 17 hours long measurement we were able to identify Mg and Si as traces, in addition to major component (CaCO₃). According to the ICP-MS measurements, concentrations of all the identified trace elements (Li, Rb, Sr, Y, Zr, Nb, Mo, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Tl, Pb, Bi, Th and U) vary between 0.001 and 5 ppm, in most cases well bellow 1 ppm. Out of the trace elements, which can be measured by ICP-MS, only Sm, Eu and Gd were around the detection limits of our PGAA system. Other trace elements were below our detection limits. The approximate detection limits for our PGAA-system can be seen in the Table.

Finally we have to establish, that our recent PGAA-system is not suitable for investigation of marble samples, even with the "cold neutron source".

[1] M. D. Glascock, T. G. Spalding, J. C. Biers and M. F. Cornman: Analysis of Copper-based Metallic Artifacts by Prompt Gamma-ray Neutron Activation Analysis, Archaeometry 26, 1 (1984), 96-103

[2] K. Sueki, Y. Oura, W. Sato, H. Nakahara, T. Tomizawa: Analysis of archaeological samples by the internal monostandard method of PGAA, Journal of Radioanalytical and Nuclear Chemistry, 234, Nos. 1-2 (1998) 27-31.

[3] Y. Oura, A. Saito K. Sueki, H. Nakahara, T. Tomizawa, T. Nishikawa, C. Yonezawa, H. Matsue, H. Sawahata: Prompt gamma-ray analysis of bronze, Journal of Radioanalytical and Nuclear Chemistry, 239, No. 3 (1999) 581-585.

[4] Zs. Kasztovszky, Zs. Révay, T. Belgya, B. Fazekas, J. Östör, G. L. Molnár, A. Vaday, A. Figler: Prompt gamma activation analysis of Roman brooches, Proceedings of the 31^st International Symposium on Archaeometry, (in print).

[5] Zs. Kasztovszky, Zs. Révay, T. Belgya, G. L. Molnár: Non-destructive analysis of metals by prompt-gamma activation analysis at the Budapest Research Reactor, Journal of Radioanalytical and Nuclear Chemistry, 244, No.2 (2000) 379-382.

[6] Zs. Kasztovszky, A. Vaday: Prompt-gamma activation analysis of late Roman brooches, Proceedings of Workshop on Ancient Metallurgy between Oriental Alps and Pannonian Plain, 2000, Trieste, (ed. Alessandra Giumlia-Mair), pp. 171-187.

[7] Zs. Kasztovszky, A. Vaday, Zs. Révay, T. Belgya, G.L. Molnár: Prompt gamma activation analysis of bronze archaeological objects, Proceedings of the 32^nd International Symposium on Archaeometry, (submitted).

[8] T. Vida, Zs. Kasztovszky: Der Messingbeschlag aus Gic, Westungarn, Die Awaren am Rand der byzantinischen Welt, Monographien Aus Frühgeschichte Und Mittelalterarchaologie, 2000, Innsbruck, Austria (Hrg.. Falko Daim), pp. 305-325.

[9] Zs. Kasztovszky, Gy. Szakmány, Zs. Révay, T. Belgya, G.L. Molnár: Investigation of neolithic greenschist polished stone tools from Carpathian-Pannonian Basin, Hungary by prompt gamma activation analysis, Proceedingsof the 32^nd International Symposium on Archaeometry, (submitted).

[10] Zs. Kasztovszky, Gy. Szakmány, in Lithotheca II. Comparative Raw Material Collection of the Hungarian National Museum 1990-1997., Magyar Nemzeti Múzeum, Budapest, 2000, (eds. Katalin.T. Bíró, Viola T. Dobosi, Zs. Schléder)