Project presentation

Project's title: DACIAN GOLD AND SILVER PROJECT PN-II-ID-PCE-2011-3-0078

Project's summary:

Scientific context and motivation.

The application of the atomic and nuclear techniques in the study of archaeological objects gives the historian or the archaeologist "materials" information that can help the understanding of the way of life during the ancient times. This knowledge is necessary to test the authenticity and provenance of artifacts and to prepare and carry out the necessary restorations. All these objectives are common to the very large community of people working in the field of archaeometry, i.e. the "application of science to art and archaeology". In the case of investigating metallic artifacts, the domain is called "archaeometallurgy". For all these research activities it is important to emphasize that the multidisciplinary community of action is essential, putting together physicists, chemists, archaeologists, numismatists, historians, geologists and conservators from different laboratories, institutions and museums.

We intend to perform, using some of the most advanced spectrometric techniques, compositional analyses - elemental, layers structure (e.g. gilding, silvering, tinning), micro-inclusions (PGE elements, Sn, Sb, Te, Pb etc in gold samples) - on ancient (Geto-Dacian, Geto-Thracian, Greek - classical, Hellenistic, Roman) gold and silver objects (coins included) from Romanian Museums. The obtained information will provide essential clues for provenance-authentication studies of the objects:

  • gold origin (alluvial, primary, geological deposits), refining and alloying
  • silver origin (mines, metallurgical obtaining), alloying
  • gilding and silvering procedures
  • modern and contemporary forgeries
  • Political, commercial, military aspects - goldsmiths and minting workshops, trade routes, payments, looting and significance (spiritual or material) of gold and silver objects can be also cleared by such compositional analyses.

    Method and approach.

    Trace elements are more significant for provenance of archaeological metallic artifacts than the main components (20, 21, 22, 23, 24). For gold, the most promising elements are Platinum Group Elements (PGE), Sn, Te, Sb, Hg, Pb. For ancient gold artifacts (jewelry and coins) found on Romania's territory, the very possible use of Carpathian gold must be considered (25). Such studies involve the compositional study of both artifacts and Carpathian native gold samples - placer (gold sand or nuggets found in rivers bed) and primary (gold obtained from mining), to determine the fingerprints of gold from this geological area. Literature published information about other antiquity-relevant gold sources - Anatolia, Macedonia, today Bulgaria, must be also obtained. Similar analyses will be performed on silver artifacts (coins, adornments, toreutics) to determine silver provenance (Au, Bi, Sb trace elements), alloying technologies (Cu, Pb), plating and silvering (amalgamation, thin foil on bronze core, etc) forgeries (26, 27). The analysis of archaeological objects will be performed mainly by XRF (X-Ray Fluorescence) spectrometers, a total non-destructive procedure, but with a reduced sensitivity for trace elements in the ppm (parts per million) region. For a better sensitivity we intend to use a very new advanced technique - LA-ICP-MS - Laser Ablation Inductively Coupled Mass Spectrometry (28, 29), based on a new Perkin-Elmer equipment (Laser Nd 213 nm) which will start to work in our Department in 2012. The investigation of small fragments of geological native Carpathian gold is performed using micro Proton Induced X-ray Emission (micro-PIXE) technique and micro Synchrotron Radiation X-Ray Fluorescence (micro SR-XRF). Taking into account the reduced size of the native gold samples, especially for the ones coming from placers - several hundreds µm in diameter, the micro-analysis was strongly required. In special cases, when a practically non-invasive sampling procedure (100 to 300 microns diameter samples) will be accepted, micro-PIXE and micro-SR-XRF will be also used for archaeological objects.

    Brief description of the analytical methods

    For the X-MET 3000TX XRF Portable Spectrometer, the exciting X-ray beam is generated by a 40 kV tube with Rh-anode. The detection system is a PIN silicon diode detector with Peltier cooling. The resolution of the detector is 270 eV for the Ka line of Mn (5.89 keV). The measurement spot size is about 30 mm2.The X-MET XRF analyzer has a Hewlett-Packard (HP) iPAQ personal data assistant (PDA) for software management and data storage.

    The stationary SPECTRO MIDEX spectrometer has a 50 kV Mo-anode tube and a Peltier cooled Si drift chamber detector, with 170 eV resolution for the Ka line of Mn (5.89 keV). The typical diameter of the measurement spot is 0.7 mm, but it can be optimized for different tasks to 0.2 mm, 0.6 mm, 1 mm or 2 mm with four integrated - software controlled - collimators. A double video system with different magnifications is used to determine the exact measurement position in the large sample chamber.

    X-Ray Fluorescence (XRF) can be also performed at "Horia Hulubei" National Institute of Nuclear Physics and Engineering, Bucharest, using a "classical" spectrometer based on a Am-241 (30 mCi) radioactive source and a HPGe detector. Pure gold (99,99%) sample from the National Bank of Romania is used to extract the 26.4 keV ? peak (from 241Am source) contribution. The sensitivity in the Tin region (Sn, Sb, Te) of this spectrometer is much lower than for the other spectrometers if long spectra (1-2 hours) are acquired.

    At the AN2000 accelerator of INFN-LNL Legnaro (Italy), the experiment is carried out with a 2 MeV proton microbeam (9 mm2 beam area), the beam current between 400 pA and 1 nA. The characteristic X-rays are measured with a Canberra HPGe detector (with 140 eV FWHM at 5.9 keV). A Mylar filter (52 mm thickness, 11 % hole) in front of the X-ray detector is used to reduce the intensity of the peaks in the low spectral region (below 4 keV). Map and point spectra are acquired. The map is 2x2 mm2, the point has the dimension of the beam, 3x3 µm2. The quantitative analysis of the X-ray spectra is performed using GUPIXWIN software. At the AGLAE Louvre accelerator, Paris (France) micro-PIXE facilty, the samples are bombarded with a 3 MeV proton micro-beam (roughly 50 ?m diameter) extracted into air. Because of the reduced size of the samples, point measurements (no scanning) are made. The X-ray spectra are acquired for a fixed dose, corresponding to an irradiation with a 10 nA beam current for about 15 minutes. For the characteristic X-rays acquisition, two Si(Li) detectors are used: low-energy (1 - 10 keV) - for the determination of matrix elements and high-energy (5 - 40 keV) - for trace-elements determination. The elemental concentrations are obtained by processing the spectra with GUPIXWIN code.

    At the micro-PIXE Scanning Nuclear Microprobe Facility of ATOMKI Debrecen (Hungary) map and point spectra are obtained using a H+ beam of 3 MeV energy focused down to 3 x 3 ?m? ("point") with the intensity of 300-400 pA for the irradiation of the samples, accumulated charge 0.5-1.5 ?C. The scanned area varies between few square microns to 1 x 1 mm2, depending on the sample size and structure. The experimental setup consists of a super ultra thin windowed (SUTW) and a Be windowed Gresham type Si(Li) X-ray detectors, each having an area of 30 mm2 and 136 eV resolution (at Mn Ka line). The detectors are placed at 135ŗ geometry to the incident beam. This setup allows the detection of low and medium energy characteristic X-rays (0.2 - 9 keV) and medium and high energy X-rays (> 4 keV) simultaneously. Elemental concentrations for Z = 6 are derived from the X-ray spectra obtained by the two detectors. PIXE spectra are evaluated by the GUPIXWIN and PIXEKLM-TPI program codes.

    At BESSY Synchrotron Radiation Facility, Berlin Germany), micro-SRXRF facility, point spectra are acquired at 35 keV excitation energy using a spatially resolved synchrotron radiation XRF set-up dedicated to analysis. The beam is focused to a size of 300x150 µm2. The metal (especially gold) samples are mounted in air in a special frame for passé - partouts on a motorized xyz stage at an angle of 45o to the X-ray beam. Fluorescence signals are collected for 300 s each by an HPGe detector, with no filtering. A video system and a long distance microscope allow monitoring and selection of samples analyzed points. Relative elemental concentration is determined using a procedure based on different metallic standards. Data analysis is performed by means of the PyMCA software.

    At the ANKA/ISR Synchrotron Karlsruhe (Germany), FLUO-beam-line, 2-dimensional scans with the beam focused to 6×7-10 µm2 are performed. The scan area could be visualized with an optical microscope and recorded with a camera. The detector used is a HPGe crystal.

    The maximum excitation energy of the X-rays in experiments is 32.5 keV.

    The metal (gold) samples are mounted in air on a dedicated frame, put on a motorized xyz stage and positioned at an angle of 45o to the primary X-ray beam. The identification of the peaks and the off-line data analysis is performed with PyMCA and AXIL. Relative concentrations of minor and trace elements are determined using a procedure based on various standards and fundamental parameter calculations. Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) is widely used as a powerful micro analytical technique for solid sample analysis in a variety of scientific fields including geological, biological, archaeometrical, environmental, nuclear and metallurgical sciences. Within an 'ablation cell' an amount of matter is extracted from the sample by focusing a laser beam on its surface. The energy provided by the laser is absorbed by the sample resulting in vaporization and generation of aerosol which is subsequently transported by a 'carrier gas' through tygon tubes to the ICP-MS instrument. Here the sample is firstly ionized by the ICP-torch and then analyzed by the Mass Spectrometry instrumentation.

    Impact, relevance, applications.

    The realization of the present project will offer unique opportunities for archeological researchers, offering the chance of performing compositional analysis of artifacts from Romanian cultural heritage patrimony which were never investigated up to now and thus will valorize the equipments and the experience in the field of archaeometry of the atomic and nuclear physics researchers. The project will create opportunities for education and training - physics and archaeology students, and also conservators, curators and restorers, valorizing at a high level the Romanian national cultural heritage resources. The project will allow a better understanding of the key-periods in Romanians' history and, due to its results publication in prestigious journals, will lead to an increasing international visibility of Romanian archaeometrical research.

    The project will consolidate the Romanian archaeo-metallurgy network, involving the participation of Horia Hulubei National Institute of Nuclear Physics and Engineering, National History Museum of Romania, Institute of Archaeology "Vasile Parvan", Bucharest University, National History Museum of Transylvania, Bucharest University, other Romanian Archaeological and History Museums.

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