Alfredo Castellano

Applications of non-cryogenic portable EDXRF systems in archaeometry

Abstract In this paper the most relevant developments in the realisation of portable Energy-Dispersive X-ray Fluorescence (EDXRF) equipments are discussed. In particular, the latest advances in non-cryogenic (Peltier cooled) X-ray detectors and miniaturised X-ray generators are shown. The energy resolution of the new detection systems is adequate to resolve the characteristic X-ray emission lines of contiguous elements. This small size and low power make the system ideal for portable instrumentation and have stimulated the development of small- and low-power X-ray generators which can be used for the excitation of fluorescence radiation in a broad energy range (5–40 keV). Finally, the use of EDXRF related to archaeometric research (pigments in ancient paintings and major elements in the metal alloys) is emphasised. Recent results obtained with new HgI 2 and silicon PIN detector systems combined with miniaturised highly stable air-cooled X-ray generators are described.

Portable equipment for energy dispersive X-ray fluorescence analysis of Giotto’s frescoes in the Chapel of the Scrovegni

Abstract Photon induced energy dispersive X-ray fluorescence (EDXRF) analysis is a valuable technique for the study of works of art, because it is nondestructive, multielemental, simple and relatively inexpensive. For this reason EDXRF is a very popular analytical technique in archaeometry. Portability of EDXRF equipments is extremely useful and almost mandatory in many cases, such as analysis of frescoes, of large paintings, bronzes, brasses and gold alloys, and so on, especially when located in museums. EDXRF analysis generally involves an area of a few mm2, and a thickness between μm and fractions of mm and, therefore, the analysis is superficial and dependent on the surface conditions. The frescoes by Giotto in the “Chapel of the Scrovegni” in Padua were systematically analysed in the period July 2001–March 2002 in more than 300 points, before, during and after restoration, in order to detect the possible presence of superficial sulphur and to test various sulphur cleaning procedures. Further all pigments were systematically analysed in order to determine their composition. Golden haloes were also analysed and different pigment layers were detected under the gold leaf; from the EDXRF analysis the attribution of chemical elements to the proper layer was possible.

A portable apparatus for energy-dispersive X-ray fluorescence analysis of sulfur and chlorine in frescoes and stone monuments

Abstract A simple and portable equipment was constructed, based on the energy dispersive X-ray fluorescence. It is composed of a small size X-ray tube, a thermoelectrically cooled Si-PIN detector, and a multi channel analyzer. The tube has a Pd anode, and works at 3–30 kV, 0.1 mA. At 3–5 kV, it emits Pd-L lines and bremsstrahlung radiation, and therefore is suited to the analysis of low atomic number elements, such as P, S, Cl. Working at 30 kV the tube emits Pd-K lines and bremsstrahlung radiation, and is able to excite medium atomic number elements up to tin (K-lines) and high atomic number elements (L-lines). The Si-PIN detector has a 25 μm Bc-window and energy resolution of about 190 eV at 5.9 keV. It has a thickness of about 300 μm and thus an efficiency rapidly decreases at energies larger than about 20 keV. The equipment is completed by a portable multi channel analyzer. The described system was first tested in Laboratory and then employed to analyze mainly sulfur and chlorine in the follwoing frescoes and monuments: • frescoes of Piero della Francesca (church of S. Francesco, Arezzo) • frescoes of Domenichino (cathedral of Fano) • Roman frescoes (church of S. Clemente, Rome) • lapideous altars (church of S. Croce, Lecce).

A simple scanner for Compton tomography

A first generation CT-scanner was designed and constructed to carry out Compton images. This CT-scanner is composed of a 80 kV, 5 mA X-ray tube and a NaI(Tl) X-ray detector; the tube is strongly collimated, generating a X-ray beam of 2 mm diameter, whilst the detetor is not collimated to collect Compton photons from the whole irradiated cylinder. The performances of the equipment were tested contemporaneous transmission and Compton images.

From Giotto to De Chirico to Verrocchio: analyses of paintings and historical bronze alloys availing of portable EDXRF equipment

Energy-dispersive X-ray fluorescence (EDXRF) has been used successfully for some time now in non-destructive diagnostic examinations of art works, especially for paintings and bronze works. Furthermore, such examinations may be carried out with portable equipment, allowing studies in situ. EDXRF-analysis of paintings generally provides the following information: –possible presence of elements on the surface (sulphur, chlorine) due to pollution; –identification of the elements, and therefore giving a good indication of the pigments, employed by the artist; and –identification of previous restoration areas, by detecting the presence of “modern” elements, such as titanium, zinc, cadmium, etc. Recently, the famous Chapel of the Scrovegni painted by Giotto in Padua in 1303–1305 was analysed in detail to obtain all the information described above. Another example of application of EDXRF-analysis to paintings was a recent examination of 11 paintings believed to be by De Chirico the outcome of which was compared ...

Laser ablation threshold of cultural heritage metals

In this work we determined experimentally the threshold fluence of the most common metals found in cultural heritage, e.g. copper, silver and their alloys. We carried out the ablation process in air at atmospheric pressure with 8 ns pulsed Nd:YAG and 23 ns pulsed KrF lasers, at 532 and 248 nm, respectively. We irradiated every target by a fixed number of laser shots (repetition rate of 1 Hz) at several laser fluence values. Then, the resulting craters were characterized by a stylus surface profiler in order to obtain the dependence of ablation rate on laser fluence F. Here, we defined the ablation raate as ablated matter thickness for single laser pulse, x. Therefore, we identified the ablation threshold fluence, Fth, as the fluence value below which no ablation process would occur.

New tomographic methods using X-ray tubes

Abstract Different kinds of images of the section of a sample can be deduced from the interaction of X-rays with matter: 1. (1)|When two X-ray beams are employed, the energies of which closely bracket a photoelectric discontinuity of an element in the sample, then the difference of two images carried out with the pair of X-rays is only sensitive to the element itself. This kind of tomography, called “differential tomography”, gives the distribution of the element in the section. 2. (2)|Fluorescent X-rays emitted by the voxel identified by the intersection of the collimated incident beam with the collimated secondary beam can be mapped versus the x – y position of the voxel: an image can be obtained (called XRF-tomography) giving the distribution of all fluorescent elements. 3. (3)|When the diameter of the beam in the transmission tomography is reduced to the order of magnitude of microns, then a “microtomography” is obtained, in which the geometrical resolution is of the same order of magnitude.

Experimental results of UV laser cleaning on a silver Carlino coin

Silver artefacts and particularly ancient silver artefacts present the serious problem of turning black due to ambient contamination. The black colour is the consequence of a layer forming on the surface made of acanthite and jalpaite; both compounds are sulphur composts. Recently, a UV laser cleaning technique has demonstrated to be very promising in the processing of cultural artefacts. To operate on artefacts without contaminating the bulk, we first determined the laser fluence threshold of the bulk; second, we controlled the surface compounds, and finally we operated the laser irradiation. In this work, we utilised certified silver and silver/copper samples in order to perform preliminary studies. The threshold fluences we found were 780 and 510 mJ/cm2 for the pure Ag and for the Ag/Cu alloy, respectively. The jalpaite concentration on the Ag/Cu sample increased after a laser dose of 25 J/cm2, whereas for a higher laser dose its value lowered, pointing out that complex processes take part during the laser application. We successfully applied the technique on a Carlino coin made of silver coined in 1689 under King Carlo II. The analyses were performed by two different techniques that are able to find the percentage of elements and the chemical compounds: the energy dispersive X-ray fluorescence and the X-ray diffractometer spectrometer. We found that the sulphur concentration decreased upon the increase of the laser dose. Operating on the coin up to a dose of 280 J/cm2, the sulphur concentration deceased by up to 20%, while the coin looks clearer just after 50 J/cm2.

Selective laser cleaning of chlorine on ancient coins

Results about the efficiency of the laser cleaning on the reduction of corrosion products from the surface of ancient coins are reported. In this work an ancient copper coin datable from 1500 to 1600 A.D. and a UV excimer laser were utilized. The goal of this work consists to study the potentiality of UV laser treatment in the reduction of the chlorine concentration on the coin surface which is the main responsible of the corrosion processes of the ancient coins. We used Energy Dispersive X-Ray Fluorescence (EDXRF) and X-Ray Diffraction (XRD) techniques to estimate the chemical composition of the coin surface, before and after UV excimer laser cleaning. In particular, we measured the chlorine, copper and calcium concentrations. We found that a radiation dose of about 19 J/cm2 was able to reduce the chlorine concentration from 2.3 % w/w to 0.6 % w/w without damaging the metal bulk.

Thermoelectrically cooled semiconductor detectors for portable energy-dispersive x-ray fluorescence equipment

Thermoelectrically cooled semiconductor detectors, such as Si- PIN, Si-strip and HgI2, coupled to miniaturized low-power x-ray tubes, are well suited for constructing portable systems for energy-dispersive x-ray fluorescence analysis (EDXRF) of samples of archaeological interest. The Si-PIN detector is characterized by a thickness of about 300 micrometer, an area of about 2 by 3 mm2, an energy resolution of about 250 - 300 eV at 5.9 keV and an entrance window of 75 micrometers. The Si-strip detector has approximate the same area and thickness, but an energy resolution of 145 eV at 5.9 keV. The efficiency of these detectors is around 100% from 4 to 10 keV, and then decreases versus energy, reaching 10% at 30 keV. Coupled to a miniaturized 10 kV, 0.1 mA, Ca-anode or to a miniaturized 30 kV, 0.1 mA, W-anode x-ray tubes, completely portable systems can be constructed, which are able to analyze K-lines of elements up to about silver, and L-lines of heavy elements. The HgI2 detector has an efficiency of about 100% in the whole range of x rays, and an energy resolution of about 200 eV at 5.9 keV. Coupled to a small 50 kV, 1 mA, W- anode x-ray tube, a portable system can be constructed, for analysis of practically all elements. These equipments were applied to analysis in the field of archaeometry and in all applications for which portable systems are needed or at least useful (for example x-ray transmission measurements, x-ray microtomography and so on). More specifically, concerning EDXRF analysis, ancient gold samples were analyzed in Rome, in Mexico City and in Milan, nuragic bronzes in Sassari, ceramics of various origin in Merida, La Habana and Sassari, and sulfur (due to pollution) in an old Roman fresco in S. Stefano Rotondo (Rome). Concerning transmission measurements, ancient copper coins and wood samples were analyzed, and microtomographic measurements are in progress to improve the quality of the image.