Littérature scientifique sur le sujet « Fibre scintillanti »

This article analyzes articulations among urban enclaves, finance capital, and glass architecture by exploring MGM’s corporate investments in the Las Vegas CityCenter development and the Chinese enclave of Macau. CityCenter is an unsuccessful $9 billion master-planned urban community financed by MGM and Dubai World. Macau is a former Portuguese colony and Special Administrative Region of the People’s Republic of China which has, since its return to the PRC in 1999, replaced Las Vegas as the world’s most lucrative site of casino gaming revenue. Taken together, CityCenter and Macau are illustrative of the political economy and cultural logics of financialization. Foreign investment from Las Vegas entrepreneurs has vitrified Macau, transforming it into a phantasmagoria of glass resorts. Macau in turn plays a crucial functional role in capitalism’s recomposition in East Asia, similar to the autochthonous role of the Italian city-states of Venice and Genoa in the historical origins of capitalism. In order to ‘read’ the cities of Las Vegas and Macau, I explore intertextual legibilities among fictitious capital that relies on glass fiber-optic technology to enable grand architectural projects; expressionist fictional representations of glass architecture and its utopian transformative potential; and glass buildings that themselves dissimulate in a manner not unlike fiction.

I materiali con proprietà di scintillazione trovano ampio impiego in applicazioni che riguardano la rivelazione di radiazioni ionizzanti, quali il monitoraggio e la diagnostica per immagini, la dosimetria in campo medico, la sicurezza nazionale ed industriale, e la fisica delle alte energie. Recentemente, la scoperta di nuovi scintillatori veloci ed efficienti ha rappresentato un campo di ricerca in attivo e continuo sviluppo. Fra i numerosi sistemi indagati, le fibre ottiche scintillanti hanno suscitato grande interesse grazie alla loro estrema versatilità che permette la progettazione di rivelatori con un design innovativo. In questa tesi si propone lo studio di fibre scintillanti di silice che mostrano efficienti proprietà luminescenti quando drogate con ioni di terre rare, come Cerio e Praseodimio. L’argomento è trattato sia da un punto di vista fondamentale che applicativo, al fine di migliorare ed ottimizzare la resa del materiale perché possa essere impiegato in rivelatori di nuova generazione. A questo scopo, si è deciso di studiare gli effetti dell’esposizione ad elevate dosi di radiazioni ionizzanti sulla trasparenza del materiale. L’ottimizzazione della concentrazione del drogante luminescente nella matrice di silice, nonché dei processi di sintesi sol-gel e di filatura della fibra ha permesso di ottenere un’efficiente propagazione della luce all’interno della fibra stessa. É stata altresì dimostrata la fattibilità di una rivelazione simultanea di luce Cherenkov e di scintillazione, esponendo a fasci di elettroni altamente energetici un piccolo prototipo di calorimetro realizzato con fibre scintillanti drogate con Cerio. Le fibre di silice possono quindi essere considerate promettenti candidate in quell’ambito della calorimetria che prevede l’utilizzo della doppia lettura del segnale Cherenkov e di scintillazione per compensare le fluttuazioni energetiche caratteristiche dell’interazione con adroni pesanti. Inoltre, una completa conoscenza dei fattori che limitano l’efficienza di scintillazione è di primaria importanza per una futura ingegnerizzazione del materiale: l’esistenza di difetti di punto, che competono con i centri luminescenti nella cattura dei portatori di carica generati a seguito dell’interazione con la radiazione ionizzante, risulta essere la principale causa della presenza di una componente temporale lenta nella cinetica di ricombinazione. É stato quindi condotto uno studio approfondito del ruolo dei difetti nelle fibre di silice, al fine di comprenderne la natura e gli effetti sulle proprietà di scintillazione. I risultati ottenuti in questa tesi hanno confermato il potenziale di questa tecnologia per applicazioni in rivelatori per fisica delle alte energie e gettato le basi per un ulteriore futuro sviluppo del materiale. Questo lavoro è stato svolto presso i laboratori del Dipartimento di Scienze dei Materiali dell’Università di Milano-Bicocca, in collaborazione con il Centro Europeo per la Ricerca Nucleare (CERN, Ginevra, Svizzera) e con il Lawrence Berkeley National Laboratory (Berkeley, USA). Parte dei dati è stata ottenuta in collaborazione con Saint Gobain Research (Aubervilliers, Parigi, Francia) e con l’Istituto di Fisica dell’Accademia delle Scienze della Repubblica Ceca (Praga, Repubblica Ceca).
Scintillating materials find a wide variety of applications in ionizing radiation detection systems, monitoring and imaging, real time dosimetry in the medical field, homeland and industrial security, and high energy physics. In the recent years, the development of new, fast, and performing scintillators has been an active field of research. Scintillating fiber technology freshly raised a lot of interest because its extreme flexibility can provide a powerful tool for innovative detector designs. This thesis focuses on the study of scintillating fibers made of silica glass which show efficient luminescent properties when activated with rare-earth ions, like Cerium and Praseodymium. Both fundamental and practical aspects are discussed, in view of the improvement and optimization of the material performances for application perspectives in the future generation of high energy physics detectors. With this objective, the effects of high dose levels of ionizing radiation on the transparency of the material are studied. The fine-tuning of the activator content incorporated in the silica matrix and of the sol-gel synthesis and fiber drawing processes allow to obtain a good light guiding and a well-controlled optical quality. The feasibility of a simultaneous readout of Cherenkov and scintillation light is demonstrated in high energy calorimetry conditions, probing Ce-doped silica fibers embedded in a small detector prototype exposed to beams of electrons. Silica fibers can be considered as promising candidates in the framework of the dual readout calorimetry approach, which aims at compensating the energy fluctuations, inherent to the detection of hadronic particles. A deep understanding of the factors limiting the scintillation performances is of primary importance for future material engineering: they are found to be mainly related to the presence of point defects, which compete with the luminescent centers in capturing the free carriers created upon irradiation and introduce a delay in the recombination kinetics. A fundamental study of the role of defects in silica fibers, detrimental for the scintillation efficiency, is proposed and discussed. The potential of silica fibers for applications in high energy physics detectors is outlined and further optimization of the material technology is foreseen. This work was performed at the Department of Materials Science at the University of Milano - Bicocca, in collaboration with the European Organization for Nuclear Research (CERN, Switzerland) and with the Lawrence Berkeley National Laboratory (US). Some measurements were carried out in collaboration with Saint Gobain Research (France) and the Institute of Physics of the Czech Academy of Sciences (Czech Republic).

Le developpement de la physique des particules fait apparaitre un interet croissant pour des detecteurs disposant d'une haute resolution spatiale et temporelle. Les microfibres optiques plastiques scintillantes permettent de repondre a ces exigences. Les travaux de recherche presentes dans ce memoire traitent de la realisation et la caracterisation de fibres optiques scintillantes de faible diametre. Une premiere partie est consacree au rappel des caracteristiques principales des fibres optiques en polymere. Les phenomenes de luminescence et les mecanismes qui la gouvernent, avec un interet particulier pour la scintillation sont ensuite developpes. Les choix des materiaux de coeur et des dopants sont justifies. Deux techniques de fabrication de microfibres sont proposees. La premiere consiste a remplir des capillaires en verre de 100 microns de diametre avec un polymethylphenylsiloxane commercial. Les avantages des siloxanes sur le polystyrene comme materiau de coeur sont ensuite detailles. Differents essais sont effectues afin d'ameliorer la transmission de la lumiere dans les microfibres obtenues. Le deuxieme axe de recherche concerne la realisation de faisceaux de microfibres de 50 microns de diametre par etirage secondaire d'un assemblage de 331 fibres meres, en polystyrene, de 1 mm de diametre. Une solution au probleme de retrait des fibres meres lors de l'etirage secondaire est donnee. Les faisceaux de microfibres obtenus sont caracterises par leur longueur d'attenuation. Une interpretation de la degradation de la transparence entre les fibres meres et les microfibres est proposee

Cette thèse a pour sujet le développement d’un détecteur à fibre scintillante plastique pour la dosimétrie des faisceaux de photons de basses énergies. L’objectif principal du projet consiste à concevoir et caractériser cet instrument en vue de mesurer la dose de radiation reçue au cours des examens d’imagerie diagnostique et interventionnelle. La première section est consacrée à la conception de six différents systèmes et à l’évaluation de leur performance lorsqu’ils sont exposés à des rayonnements de hautes et basses énergies. Tous les systèmes évalués présentaient un écart type relatif (RSD) de moins de 5 % lorsqu’ils étaient exposés à des débits de dose de plus de 3 mGy/s. Cette approche systématique a permis de déterminer que le tube photomultiplicateur répondait le mieux aux conditions d’exposition propres à la radiologie. Ce dernier présentait une RSD de moins de 1 % lorsque le débit de dose était inférieur à 0.10 mGy/s. L’étude des résultats permis également de suggérer quelques recommandations dans le choix d’un système en fonction de l’application recherchée. La seconde partie concerne l’application de ce détecteur à la radiologie interventionnelle en procédant à des mesures de dose à la surface d’un fantôme anthropomorphique. Ainsi, plusieurs situations cliniques ont été reproduites afin d’observer la précision et la fiabilité du détecteur. Ce dernier conserva une RSD inférieure à 2 % lorsque le débit de dose était supérieur à 3 mGy/min et d’environ 10 % au débit le plus faible (0.25 mGy/min). Les mesures sur fantôme montrèrent une différence de moins de 4 % entre les mesures du détecteur et celles d’une chambre d’ionisation lors du déplacement de la table ou du bras de l’appareil de fluoroscopie. Par ailleurs, cette différence est demeurée sous les 2 % lors des mesures de débit de dose en profondeur. Le dernier sujet de cette thèse porta sur les fondements physiques de la scintillation dans les scintillateurs plastiques. Les différents facteurs influençant l’émission lumineuse ont été analysés afin d’identifier leur contribution respective. Ainsi, la réponse du détecteur augmente de près d’un facteur 4 entre un faisceau de 20 kVp et 250 kVp. De ce signal, la contribution de la fluorescence produite dans la fibre claire était inférieure à 0.5 % lorsque les fibres étaient exposées sur 10 cm par des faisceaux de 20 à 250 kVp. Le phénomène d’extinction de la fluorescence par ionisation a également été étudié. Ainsi, l’atténuation du signal variait en fonction de l’énergie du faisceau et atteignit environ 20 % pour un faisceau de 20 kVp. En conclusion, cette étude suggère que les détecteurs à fibres scintillantes peuvent mesurer avec précision la dose de radiation reçue en imagerie diagnostique et interventionnelle, mais une calibration rigoureuse s’avère essentielle.
This thesis deals with the development of a plastic scintillation detector for low energy photon eams. The main goal of the project consists in the design and characterization of this tool in the aim of measuring the radiation dose involved during diagnostic and interventional radiology examinations. The first section is devoted to the design of six different systems and to their performance evaluation when they are exposed to high and low energy radiation. For all photodetectors, the relative standard deviation (RSD) was less than 5% for dose rates higher than 3 mGy/s. This systematic approach identified the photomultiplier tube as the most appropriate photodetector for radiology specific beam qualities. Indeed, its RSD was less than 1% when the dose rate was below 0.10 mGy/s. The result analysis allowed the suggestion of some guidelines for the selection of an appropriate detector for a specific application. The second part was about this detector application to interventional radiology procedures by performing dose measurements at an anthropomorphic phantom surface. Several clinically relevant setups were reproduced to observe the detector’s accuracy and reliability. The RSD remained under 2% when the dose rate was more than 3 mGy/min and about 10% at the lowest dose rate (0.25 mGy/min). Phantom measurements showed a dose rate difference between the detector and the ion chamber of less than 4% when moving the table’s height or rotating the fluoroscope gantry. Moreover, this difference was below 2% for depth dose rate measurements. The last subject of this thesis was about the fundamental physics of scintillation within the plastic scintillators. The factors affecting the light emission were analyzed in order to identify their respective contribution. The detector’s response increased by a factor of about 4 when the tube potential varied between 20 kVp and 250 kVp. The clear optical response was below 0.5%of the scintillator’s light when the fibers were exposed on 10 cm-long by these beam qualities. he ionization quenching phenomenon was also investigated. The signal attenuation varied with the beam energy and reached about 20% for a 20 kVp beam quality. In conclusion, this study suggests that the plastic scintillation detectors can accurately measure the radiation dose involved in diagnostic and interventional radiology, but a rigorous calibration is essential.

Preparation et caracterisation de polymeres entrant dans la fabrication de fibres optiques; ameliorations du procede de fabrication de la fibre pour pallier les fluctuations de diametre. Presentation des phenomenes de luminescence et des theories existantes dans le domaine de la scintillation; justifications du choix de polystyrene ou de polymethylstyrene pour les fibres scintillantes; probleme des fibres scintillantes de faible diametre; presentation d'une methode de preparation de materiau de coeur et mesures de transparence