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Strumenti e metodi per l'analisi XRF di materiali di interesse culturale
No full textIl progetto riguarda l’uso della fluorescenza a raggi X per l’analisi non-distruttiva degli elementi con Z>15-20 nei materiali di interesse culturale (leghe di argento e di rame, paste vitree, pigmenti inorganici, ceramiche, ossidiane). L’obiettivo principale è la realizzazione di uno spettrometro portatile equipaggiato con un sistema di focalizzazione del fascio primario che permetta di combinare un’elevata risoluzione spaziale (dell’ordine di poche centinaia di micron) con la capacità di eccitare efficientemente la fluorescenza degli elementi nell’intervallo 15<Z<83 (con spettri piccati fino a ~ 35 keV), fornendo quindi una radiazione primaria di energia relativamente elevata.
Si è anche messa a punto una procedura di analisi quantitativa che utilizza PyMCA, un software in costante sviluppo basato sul metodo dei Parametri Fondamentali, sia su campioni omogenei che multistrato. Per questi ultimi si sono fatte prove su campioni di argento dorato e di acciaio rivestito, ed è stato anche possibile misurare in modo non-distruttivo gli spessori delle dorature e dei rivestimenti con incertezze dell’ordine del 10%.
Parallelamente è stata condotta una campagna di analisi all’interno del Museo del Duomo a Guardiagrele sulla croce di argento dorato e smaltato di Nicola da Guardiagrele, con l’impiego congiunto di uno spettrometro XRF portatile e un micro-XRF trasportabile dotato di camera porta-campioni, allo scopo di prendere atto degli eventuali problemi legati all’uso delle lenti policapillari su oggetti reali e di effettuare i necessari confronti con spettrometri portatili di tipo tradizionale. In particolare, si è osservata da un lato l’elevata risoluzione spaziale del micro-XRF (spot ~ 100 μm, contro 1.7 mm), dall’altra i migliori limiti di rilevabilità del XRF tradizionale (0.001 wt% per l’argento misurato sulla K-alpha, contro 0.03 wt%) e la sua applicabilità a oggetti di grandi dimensioni.
La parte principale della ricerca ha riguardato la realizzazione e caratterizzazione della sorgente, un tubo a raggi X a trasmissione (anodo di tungsteno, tensione fino a 50kV, corrente fino a 200 μA) con un sistema di focalizzazione, una lente policapillare che produce uno spot di diametro 100-150 μm (a seconda dell’energia). La progettazione meccanica dello spettrometro ha riguardato con particolare attenzione la sicurezza radiologica, la portatilità e la flessibilità: i disegni costruttivi sono pronti, lo spettrometro portatile verrà realizzato a breve. In parallelo si sono valutati i limiti di rivelabilità del sistema ottenuto assemblando i componenti in un setup di laboratorio, per tutti gli elementi di interesse pratico (si riescono ad eccitare le righe di fluorescenza fino ai 35 keV). La conclusione è che limiti di rivelabilità sono uguali o migliori di quelli del sistema micro-XRF utilizzato per le misure nel museo di Guardiagrele, dotato di una sorgente ben più intensa. Sorprendentemente, tali limiti sono nettamente migliori per gli elementi leggeri, la qual cosa apre grandi potenzialità per l’analisi delle paste vitree. Considerando che nel progettare uno strumento portatile, tutto quello che si guadagna in portatilità generalmente lo si perde in prestazioni analitiche e cioè in riproducibilità e limiti di rivelabilità, ottenere un sistema che peserà 5-6 kg con prestazioni simili o migliori di quelle di un sistema sostanzialmente da banco rappresenta un risultato di notevole rilievoThe project is about the X-Ray Fluorescence non-destructive analysis of elements with Z>15-20 in materials of cultural interest (silver and copper alloys, glass matrices, inorganic pigments, potter, obsidians). The main goal is the realization of a portable spectrometer equipped with a primary beam focusing systems alloying the combination of high spatial resolution (few hundreds of microns) and capability of exciting elements in the range 15<Z<83 (fluorescence peaks up to ~ 35 keV), by providing a relatively high energy primary radiation.
In addition, a procedure for the quantitative analysis by PyMCA, a software based on the Fundamental Parameter method, has been optimized for both homogeneous and multilayer matrices. For the latter case some tests on gilded silver and coated steel samples have been carried out, and also the coating thickness was non-destructively estimated with uncertainties of about 10%.
In the meantime an investigation campaign at Museo del Duomo in Guardiagrele on a gilded and enamelled silver cross by Nicola da Guardiagrele has been carried out, by the combined use of a portable XRF spectrometer and a transportable micro-XRF device equipped with a sample chamber. The aim of this work is the evaluation of the performance limits and advantages of the polycapillary optics use on real objects and the comparison between the two instrumentations. In particular, the micro-XRF provides higher spatial resolution (spot ~ 100 μm, against 1.7 mm) but the conventional XRF system has better detection limits (0.001 wt% for Ag K-alpha, against 0.03 wt%) and can be used also on big size objects.
The main part of the work has been the realization and characterization of the source, a transmission anode X-ray tube (W anode, voltage 50 kV, current 200 μA) coupled with a focusing polycapillary lens producing a spot diameter of 100-150 μm (according to the energy). The mechanical design of the spectrometer has been focused on the radio-safety, portability and flexibility aspects. The limits of detection of the system, assembled in an laboratory experimental setup, have been evaluated for the elements of interest. The results have shown that the detection limits are similar or even better than those of the micro-XRF system used in the case study, although this one employs a more intense source. In particular, the LOD are much better for the light elements, making the new device suitable of the analysis of glass matrices
X-ray fluorescence investigation of gilded and enamelled silver: The case study of four medieval processional crosses from central Italy
No full textThe presence of multilayered structures is common in such cultural artefacts as paintings, corroded metals, objects that underwent a whatever form of surface qualification. One of the most usual and complete ways to investigate such structures is observing a cross section, which requires sampling. There are however situations where at least part of the stratigraphic information can be derived non-destructively: the literature shows that X-ray fluorescence (XRF) has frequently been used, in recent years, for this purpose, with special regard to paintings and gilded metals. Aim of this paper is to further explore the suitability of XRF-based techniques to characterise multilayered structures. This is achieved by introducing improvements, with respect to previous works, in both equipment and data processing. The method, that has been developed for gilded and enamelled silver artefacts, relies on optimum excitation conditions provided for silver and on the relationship existing between the ratio AgKα/AgKβ of its fluorescence lines and the gilding thickness itself. The coating (gilding or enamel) thickness is derived by verifying the condition CKα,Ag = CKβ,Ag, where CKα,Ag and CKβ,Ag are the mass fractions of silver calculated on the lines AgKα and AgKβ, respectively. The calculations are carried out by PyMCA, a Fundamental Parameters code that implements the analysis of multilayered samples. As a case study we investigated in situ the four processional crosses of Borbona, Sant'Elpidio, Rosciolo and Forcella, made of a wood core with attached gilt and embossed silver sheets and enamelled silver plates. The analyses allowed to distinguish ancient restorations from original parts, to characterise the enamels and find their composition consistent with the dates of manufacturing and, as regards the cross of Rosciolo, to hypothesize the contribution of different "hands" in its manufacturing. © 2013 Elsevier B.V
"Size effect" in the fatigue behavior of Friction Stir Welded plates
No full textComparative fatigue tests were carried out on Friction Stir Welded specimens of a 2195-T8 aluminum-lithium alloy that differed significantly in width. The width of the larger specimens was over thirteen times greater than that of the small specimens. Fatigue results showed a clear "size effect", i.e. fatigue life of large specimens was about 40% of the corresponding value of small specimens. The Equivalent Initial Flaw Size methodology was adopted to correlate the two sets of results. Fatigue crack initiation life was disregarded with respect to crack propagation life, and fatigue life was evaluated only as propagation of a small pre-existing defect. Following this methodology, test results of small specimens were used to evaluate the initial equivalent flaw contained in each specimen. It was assumed that this data followed a normal distribution. The equivalent initial flaw in larger specimens was evaluated by simple geometrical considerations. A very good assessment of mean fatigue life and scatter in the fatigue results of large specimens was obtained by simulating the propagation of these defects. Calculations were carried out by taking also welding residual stresses into account, but the results demonstrated that this effect was not significant
Some contraindications of hole expansion in riveted joints
No full textComparable fatigue tests were carried out on aluminium alloy 7475-T7351 double butt joints with untreated and with cold expanded holes. Surprisingly, the behaviour of the specimens having cold expanded holes was worse than that of specimens with untreated holes. This result was attributed to the surface upset which was present in cold expanded holes. Several hypotheses were formulated and experimentally verified to overcome this problem, such as deeper hole deburring, rivet diameter, joint design and grip material, steel instead of aluminium alloy.
Additional tests demonstrated that the problem was not present in sealed joint as the surface upset was hidden in the sealant thickness. Sealants have detrimental effects in the fatigue resistance of riveted joints, as they increase the load transferred by rivet bearing. The hole expansion is beneficial in this condition, while in un-sealed joints its effect must be accurately evaluated.
Other authors too highlighted possible problems due to surface upset, up to suggest to eliminate it. Only in a very few cases, a reduction of fatigue life as a consequence of hole expansion was observed
Análisis mediante XRF de una cruz procesional medieval de plata originaria de Italia centra
No full text
The medieval silver cross of Rosciolo: an XRF approach to the non-destructive investigation of gilt and enamelled metals
No full textGoing Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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