Analysis of Gold jewellery Artifacts CHARACTERIZATION OF ANCIENT GOLD SOLDERS BY PIXE G. Demortier L.A.R.N., Facultés Universitaires de Namur, B-5000 Namur, Belgium For many years it bas been uridely assumed that in amfacts from antiquity in whíchparts were joined by soldering or brazing, such solders or brazes were cadmium free. Recent analyses usingparticle inducedX- orgamma-ray emission techniques have castdoubt upon this hypothesis. When analyzing jewellery items the ideal method of analysis time (10 -9 to 10 -20 s) and they may be called prompt reactions by should be quantitative, accurate, reliable, topographical, but above comparison with delayed emission of particles and/or radiations all non-destructive and suitable to give the chemical composition obtained in activation analysis: the emission of photons (X and/or of different parts of a piece of jewellery without any sampling, even 'y-ray) and charged particles induced by low energy charged particle at microscopic level. bombardment stops when the incident beam is stopped. Particle Induced X-ray Emission (PIXE) and Particle Induced Rutherford elastic scattering (mainly in the reverse direction) has Gamma-ray Emission (PIGE, sometimes called PIGME), are two been successfully used in material analysis for quantitative techniques used specially with the objects under investigation at determination of heavy elements in matrices of low atomic weight. atmospheric pressure, which exhibit most of these qualities. When At a given reverse angle, the energy of the scattered particles used simultancously, these two techniques are capable of solving depends only on the ratio of the mass of this incident projectile to most of the problems of interference often present in prompt that of the nucleus collided with: the greater is the mass of the target physical methods of analysis. nucleus the greater is also the energy of the scattered particle. This The physical principles of elemental analysis of solids using kind of interaction does not change the nature of the collided nuclei. prompt signals induced during the irradiation of the objects with The second type of reaction (emission of charged particles other projectiles heavier than electrons are described followed by it than the incident one) may give analytical information when the discussion of the application of these principles for topographical incident particles have an energy which is sufficient to cross the analyses of gold jewellery items. Special attention is focussed on the Coulomb barrier of the target nucleus. The analytical interest of this determination of the chemical composition of narrow regions type of reaction is thus restricted to the analysis of elements of low situated at solders evident or expected from the construction of the atomic weight, and mainly for lithium, boron, carbon and nitrogen item. for which the limitofdetection lies in the 10 tg/gregion. This kind A new approach to ancient gold brazing with regard to the of interaction gives rise to a transmutation of the collided nucleus. presence of cadmium is discussed. It takes into account recent Both these abovementioned reactions are then of little interest observations obtained from experiments performed to attempt to for the purposes of the analysis of elements in heavy matrices such rediscover a possible ancient technology for the fabrication of as those contained in gold artifacts. brazing alloys involving only rudimentary means, that is the use of The Coulomb repulsion between the incident particle and a amixture of gold (or electrum) and natural cadmium mineral, the positive atomic nucleus of the material under analysis does not allow colour of which is close to that of gold, in a charcoal furnace. the proton to penetrate in heavy nuclei to eject particles. Fortunately, when the emission of such massive particles is Prompt Analytical Techniques forbidden, the quantum mechanical behaviour of nuclear Let us briefly consider from an analytical point of view the interactions allows -y-ray emission, even when the Coulomb barrier physical principles of prompt nuclear and atomic interactions does not allow the incident partide to collide with the nucleus. The induced by the bombardment of an object with particles the energy parameters governing this kind of reaction have been extensively of which lies in the MeV range. studied during the beginning of the second half of this century and When protons of energy in the MeV range interact with the identification of nearly all the chemical elements by energy atomic components of a sample, four types of reactions useful for dispersive photon detectors is nowpossible (1). The elements used analytical purposes may be produced: in gold jewellery include gold, topper, silver, zint, cadmium and (1) Elastic scattering of the incident particles iron (encountered either as a component or as an impurity) and, in (2) Nuclearreactions giving rise to emission of charged particles the case of modern sophisticated materials, nickel, germanium, (3) Coulomb excitation or resonant absorption immediately rhodium, palladium and platinum. When the incident proton followed by 'y-ray emission energy is increased from < 3 to up to 10 MeV, transmutation of (4) Ionization of electrons from their innershells followed by hard elements may then be induced and residual activity of irradiated X-ray emission. samples expected (2). This phenomenon is used for analytical All the signals from these reactions are produced in a very short purposes but the delayed activity of the sample is often undesirable. GoldBull., 1984, 17,(1) 27 The limits of detection by prompty-ray analysis are better than ionizations give rise to 5 isolated X-ray peaks: LQ, La, Li, L(^, Ly 1 per cent for all of the above mentioned elements and for a time which are more intense than K lines and thus more valuable for of bombardment of less than 1h and when no major interference accurate analysis. The ratios of the intensities of these lines are well is suspected. established and do not vary with the chemical environment of the This sensitivity is often sufficient to determine the elements. The author has written in depth on the treatment of PlXE concentrations of the most abundant elements of the alloy but the data elsewhere (3,4). In Table I are gathered the most useful shape of the y-ray spectrum with an important `background' from the long tail Compton scattering does not allow sufficient accuracy to be reached due to statistical uncertainty on the determination Table 1 of the areas of each characteristic peak. Figure 1 gives an illustration CharacteristicX:rays Usetul br analysis of Metals in Gold of such a spectrum recorded when a modern soldering alloy (75 Jeweltery gold! 3 silver/ 8 topper/ 2 zint/ 12 cadmium per cent is irradiated in a beam of 3 MeV protons). These reactions leading to the emission X ray I Interferences ofy-rays may be used for the characterization of the most abundant components of all materials but they are especially useful for the Element line energy, main secondary analysis of lithium, boron, beryllium, fluorine, sodium, keV element energy, magnesium and aluminium in all kinds of matrices: the limit of keV detection for all these light elements is in the range of 10 .tg/g in Fe Ka 6,40 Mn 8:49 Dy,Sm materials whose matrices are composed of elements of high atomic K(3 7.05 Co 6.93 mass. The use of the fourth prompt phenomenon (X ray production) Ni Ka 7.47 Dy; Eu solves the problem of lack of accuracy for the analysis of heavy and Kp 8.30 W, Tm medium mass elements. When each proton loses its energy in the Cu Ka 8.05 Er, Ta, Tb thick sample, it removes electrons from their specific shells. After Kp 8.80 Os, Lu, Hg a hole is created in a K or L atomic shell, electronic rearrangements occur with emission of K X-rays (and also Auger electrons Zn Ka 8.64. Au 8.50 Re, Yb 9.65 Au 9.70 Ta,W particularly for elements of low atomic mass). For the medium mals Kp elements, it is possible to observe two X-ray lines (Ka and K(i) in Ge Ka 9.85 Hg 9.98 W a good Si(Li) detector; for heavier elements, four lines (Ka, Ka z , KR. 11.10 Pt 11.10 Po, Pb K13, and K(3 2 ) may be detected. For heavy elements L shell Pd Ka 21.10 To Kol 23.80 In 24.00 K02. 24.30 In 24.20 Ag Ka 22.15 Ru 22.07 24.94 Sn 25.05 KP, Kat 25.45. Sn 25.27 Cd. Ka 23.15 Rb 23.16 26.10 Sb. 26,10 K111 Kat 26,64 Sb 26.40 L' 8:27 Cu 8.05 Lei 9A0 in 9.98 Ga Lil • 8977' Ge 0.85 Lp 11.10 Ge 11.10 Se ly 12.95 Se, Br Au Lt 8.50 zn 864 La 970 29.85 .Ge 3.85 Lq 10.30 TI 10.27 G .MI t4 FIPY ENERGY keV L3 11.50 Se, As, Pb, Eig.1 Gamina-rayspetttvmobtainedduringthehradiátionwith3MeV proton of a modern gold braae.contaiáing gold, eiher, eopper, eadtniuny Hg, Mg and zins LY 13.38 Rb 28 GoidBuil , 1984, 17, (1) (b) ,rN WFZ F lu. X-9AV ENERGY, keY Fig. 2(a) -tay spectrum obtained during thé irradiation oFa pure gold sai»pe (b) Enlargetnent of tli spectrum shown in Fig. 2(a) ut the vertical scale. Note the fine characteristic X-ray Tinesanti the`broaden ng oftheX-ray lines. at: the bottoni of In and lui (e) X-ray spectrum obtained during, the irradianon of a cadmium- containing so4dër. The spectrum i recorded usinga fod of zint between the atuple anti the de eetor, givlug rise to zint lines.by fluorescente.