[B._Beckhoff,_et_al.]_Handbook_of_Practical_X-Ray_(b-ok.org).pdf

A particular requirement for the investigation of

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tinguish original from later additions or to assess the impact of restoration. A particular requirement for the investigation of works of art is the use of techniques that are non-destructive or only need minimal sampling [562]. After analysis, the unchanged sample should preferably be available for fur- ther studies. In accordance to Lahanier et al. [563], the ideal procedure for analysing art, historical, or archaeological objects should be non-destructive (respecting the physical integrity of the object), but also fast (to analyse large numbers of similar objects or to investigate a single object at various loca- tions), universal (to analyse many materials and objects of various shapes and dimensions), versatile (allowing to obtain average compositional information but also permitting local analysis of small areas), sensitive (so that object grouping and other types of provenance analysis can be done not only by means of major elements but also by means of trace element fingerprints), and multielemental (to obtain simultaneously information on many elements in a single measurement). As X-ray techniques meet most of these requirements, analyses of objects of artistic or archaeological value with XRF [564], TXRF [565, 566], SRXRF [567], and micro-XRF [562] are now quite frequent. The importance of X-ray techniques for the study of art and archaeological objects was for example emphasized by a special millennium edition of the journal X-Ray Spectrometry on Cultural Heritage (2000 Vol. 29 No. 1). Indeed, XRF represents one of the most suitable methods to obtain qualitative and semiquantitative information on a great diversity of materials. In conventional XRF equipment a vacuum chamber is usually part of the spectrometer to determine elements between Z = 11 and about 19 (Na to K). Using vacuum limits the possible object size and may cause damage to fragile pieces. As Schreiner [568] first demonstrated, a purging of the detection path with helium effectively reduces the absorption of low-energy lines even in an open arrangement. Unfortunately, many spectrometers have the draw- back of being stationary, which may limit their application to art objects. The transport of fragile objects into a laboratory may involve unacceptable risks, or in certain cases (e.g., wall paintings) is even impossible. First, portable XRF spectrometers of the last decades, which were primarily developed for geochemical and metallurgical purposes, are usually equipped with a radionu- clide source and, because of low excitation intensities, are restricted to large spot sizes of the incident beam. In the last years, a new generation of com- pact and portable XRF set-ups have been designed consisting of air-cooled low-power X-ray tubes, detectors without the need of liquid nitrogen cooling, such as silicon drift chamber detectors and miniaturized electronics [569, 570].
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Methodological Developments and Applications 689 A high lateral resolution is often a crucial factor in the analysis of art and archaeological objects. The actual requirement of the spot size strongly depends on the type of object and material: as a rule, about 1 cm for met- als, stones and ceramics, about 0.5 mm for paintings and 0.1–0.05 mm for
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