At the heart of accelerator-based techniques commonly used in heritage science are the ion beams and synchrotron radiation (SR) produced by electrostatic and synchrotrons respectively. Upon incidence of both these radiation types, a nuclear reaction is stimulated, and the resulting emitted radiation reveals physical and chemical information about the object. Cultural heritage objects are rarely uniform and thus any accelerator-based technique must be able to tackle their heterogenous, mixed or layered nature. Another important subject of consideration is the safe analysis of cultural heritage objects. It is paramount that any radiation applied on cultural heritage objects leaves little, if any, impact. You can read more about the IAEA efforts on safe analysis on the Accelerator Knowledge Portal's (AKP) Safe Analysis of Cultural Heritage Objects and Materials page.
Ion beam analytical techniques are top candidates for the non-invasive analysis of cultural heritage objects
The strengths and limitations of Ion beam analysis techniques can be explained the characteristic behaviour of ions: the number of ions is relatively stable as they travel through the sample until they reach a certain depth meanwhile their energy decreases as a function of depth. The means that IBA have limited bulk sensitivity, but sample depth information can be obtained via energy measurements. In general, ions with energy on the scale of MeV, produced by an electrostatic accelerator, are used to probe the sample. This stimulates a nuclear reaction i.e., the emission of radiation such as x-rays, gamma-rays or charged particles whose energy and amount are measured by radiation detectors. The result is a spectrum which is then processed with the relevant software or code. You can find a selection of such codes on the Accelerator Knowledge Portal Codes page.
Popular ion beam analysis techniques for cultural heritage analysis
Particle Induced X-ray Emission (PIXE)
Rutherford Back Scattering (RBS)
Nuclear Reaction Analysis (NRA)
Particle Induced Gamma Emission (PIGE)
Particle Induced X-ray Emission (PIXE) is an extremely popular technique, in cultural heritage analysis, used for the identification of elements heavier than sodium by measuring the energy of an x-ray emitted as the result of the target atoms transitioning back to a lower energy state after being bombarded by charged particles, usually protons. The technique is highly sensitive with the ability to detect quantities as little as 1µg/g of an element in a sample. Its main applications are the measurements of trace elements and the bulk analysis of materials. One reason PIXE is favoured by heritage science professionals for decades because it uses low current beams which cause virtually no damage to the sample. PIXE has been used to study the coloration of ancient pre-Colombian ceremonial pottery [1], uncover the technological innovations of Cyrene glass workers [2] and could be a vital tool in the combat of illicit trade of paintings [3].
In the Rutherford Back Scattering Spectrometry (RBS) technique, an ion, typically 4 He, is shot at and subsequently repelled or back scattered by the nucleus of the target atom. By measuring the energy of the repelled ion, the mass of the nucleus can be obtained. This technique is especially suitable for determining high-Z elements i.e., elements with a large atomic number. In addition, it has a probing depth of 1-10 µm meaning that RBS is great for studying gildings, for instance as shown in this case study on Neo Assyrian ivories [4], or in general, layers of heavy element on a light substrate. It has also been used for the comparison of elemental composition in bones between a modern man and Egyptian mummy [5] and the study of ancient swords [6]
To learn more about the hands-on procedure of these IBA techniques and more, visit the IBA methods-practical demonstrations page here on the Accelerator Knowledge Portal.
Claire Pacheco, Head of the C2RMF-New AGLAE facility (Paris, France), is positioning a heritage object for analysis at the New AGLAE Ion beam accelerator.
(Credit: C. Hargoues/C2RMF).
The collimated, coherent and highly brilliant nature of X-rays micro-beam has generated much interest in the use of synchrotron-based techniques for application in cultural heritage analysis.
The suite of synchrotron-based techniques used for cultural heritage analysis primarily centre on the use of X-rays micro-beam (an x-ray with a spot size on the order of microns), although other parts of electromagnetic spectrum can be used. Again, the strengths and limitations of synchrotron-based techniques can be explained by the characteristic behaviour of photons. As x-rays travel through a sample, their energy is constant meanwhile the number of photons reduces exponentially. This means that synchrotron- based techniques provide no information about the sample depth but offer high sensitivity.
A typical synchrotron facility consists of a linear accelerator and booster which bring the electrons up to the desired energy, and a storage ring where electrons pass through bending magnets and insertion devices like undulators and wigglers to produce the synchrotron radiation (SR). The SR is directed to various end stations called beamlines, with a specific energy range, where the sample is placed. An X-ray micro-beam (or µ-x-rays) can be obtained, especially that of 3rd and 4th generation facilities, which is high flux, collimated, coherent with precise energy tunability, making it highly desirable for applications in cultural heritage analysis.
Popular synchrotron-based techniques for cultural heritage analysis [7]
µ-X-ray fluorescence (µ-XRF)
µ-X-ray absorption spectroscopy (µ-XAS)
X-ray diffraction (XRD)
SR-based FT-infrared micro-spectroscopy (SR-FT-IR)
SR-based x-ray computed tomography (SR-XCT)
µ-X-ray absorption spectroscopy (µ-XAS) is a useful technique for obtaining information about local electronic structure. X-ray photon energy (1-40keV) is selected, using a crystalline monochromator, such that core electrons for a particular element are excited. The resulting spectrum features various absorption edges that correspond to various characteristic core electron transitions. The lower and higher energy regions of the XAS spectrum are considered as different spectroscopic techniques i.e., X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) respectively. These are incredibly sensitive, may be used on nearly any type of material and can be applied in air, all characteristics fitting of for application in the field of cultural heritage analysis where it has been used to understand the composition of ancient glass in Italy [8] and luster decoration from 13th century Hispano-Moresque era [9].
Internal inspection of an object is possible with synchrotron radiation-based X-ray computed tomography (SR-XCT). X-rays passing through the object which are then attenuated depending on the density and thickness of the material, resulting in a 2D X-ray intensity distribution scan. It provides reconstructed 3D images of the internal structure by combining thousands of these 2D projections, obtained at different angles of rotation, using a reconstruction program. The technique is an excellent candidate for cultural heritage analysis because it is non-destructive, requires no sample preparation and provides high resolution images on the millimetre scale. In a 2009 study, the intricate internal carvings of a 16th century prayer nut with a diameter of just 4cm was revealed using this technique [10]. SR-XCT has also been used to assess the internal condition of stringed instruments of historical relevance [11].
The information gained from using these accelerator-based techniques support the work of archaeologists, conservators, curators and other cultural heritage professionals in determining questions about the origin, composition, provenance, manufacturing technique, previous conservation efforts and more. Together with collaborators in humanities, this information can be used to make conclusions about the object's function within the society in which it existed and even offer clues about the society itself.
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