This thesis includes work on both microchannel plate X-ray detectors and X-ray polarimetry, which although essentially distinct, have a common link through X-ray photocathodes. The source(s) of the background noise count rate in microchannel plates are investigated. Various noise mechanisms assessed include outgassing, cosmic rays, field emission, internal radioactivity, ion feedback and thermal emission. Experimental measurements are compared with calculations from a Monte Carlo model based on the assumption that radioactive decay (by beta emission) of elements within the microchannel plate glass is the major source of dark noise. The performance of Caesium Bromide as an X-ray photocathode for enhancing the quantum detection efficiency of microchannel plates is reported and compared with that of Caesium Iodide. A new type of microchannel plate configuration, a Sandwich Plate, consisting of three standard microchannel plates bonded in permanent contact is examined for use as an X-ray photon counting detector. This investigation includes a study of the correlation between gain reduction with increased count rate and the size of the illuminated area. An evaluation is made of Galileo Long Life (L2) Microchannel plates operated in pulse counting mode with special emphasis on the stability of the gain as a function of abstracted charge. Further evidence for radioactivity as the major source of background noise is obtained; L2 plates contain different radioactive isotopes compared to the 'standard' (Mullard) microchannel plate glass. The design and performance of a new type of polarimeter, a Photoemission Polarimeter, for use in soft X-ray astronomy is presented. The polarimeter utilises the linear polarisation sensitivity in the photoemission from a CsI photocathode. Possible sources of instrumental modulation are evaluated by comparing experimental measurements with calculations from a Monte Carlo model. The sensitivity of the photoemission polarimeter is compared with X-ray polarimeters presently used in X-ray astronomy.

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This thesis addresses one of the major limiting factors in the performance of photomultipliers, that is that the photocathodes employed often only absorb a small fraction, typically less than 25%, of the power in the incident light. Current photocathodes are almost exclusively planar and the starting point of the thesis is the mathematical modelling of both, semitransparent and reflective planar photocathodes. The analysis shows that the absorptance of semitransparent photocathodes increases for light incident beyond the critical angle needed for Attenuated Total Internal Reflection (ATIR). Reflective type planar photocathodes could certainly have their absorptance enhanced by use of silver rather than nickel substrates, as increases in absorptance of 2-3 times are possible for red light. The proposed method for remedying the inherent loss in sensitivity of photomultipliers caused by the non-total absorption of light in the photocathode was to employ a ridged substrate in the photocathode. The ridged substrate, glass or metal for semitransparent and reflective type photocathodes respectively, allows the light multiple interactions with the photoemissive layer. In the case of semitransparent photocathodes ATIR would mean no power is transmitted for those interactions that take place beyond the critical angle of incidence. The mathematical modelling and subsequent analysis of ridged photocathodes show enhanced absorptance (20-30 fold improvements are certainly achievable), especially for light in the red end of the operating spectral range. Further gains in quantum efficiency can follow by the reduction of the optimum photocathode thickness, resulting from the structure, while maintaining high absorptance. Some subwavelength structures are also modelled and analysed to ascertain whether this route could be used to improve the absorptance of photocathodes, the results are inconclusive but generally indicate anti-reflective, rather than absorbing properties. Finally the extremely sparse nature of published permitivity data has been rectified by our own measurements for the permitivities of certain photocathodes over a wide wavelength range.

Ce travail de thèse porte sur l'étude et le développement de sources électroniques à base de nanotubes de carbone modulées par voie optique appelées photocatodes. L'objectif de ces dernières est de les utiliser dans les tubes à ondes progressives, en remplacement des sources thermoïoniques actuelles, qui émettent un faisceau d'électrons continu. Ce nouveau type de dispositif permettrait de développer une nouvelle génération d'amplificateurs hyperfréquence large bande, plus compacts, plus légers et ayant un fort rendement pour les satellites de communication. Ces sources modulées reposent sur l'association de nanotubes de carbone avec des photodiodes P-i-Ns. Les photodiodes agissent comme des sources de courant tandis que les nanotubes jouent le rôle d'émetteurs d'électrons. Une modulation optique des photodiodes induit ainsi une émission modulée du faisceau d'électrons. Des études théoriques, couplées à des simulations, ont abouti à la compréhension détaillée du fonctionnement des photocathodes et à la connaissance de leurs performances. Par ailleurs, des résultats traitent de la fréquence de coupure qui limite le dispositif mais apportent néanmoins des perspectives d'améliorations. La fabrication des photocathodes a été menée à terme grâce à la mise au point de trois nouveaux procédés technologiques : une passivation des photodiodes InP-InGaAs-InP par une bi-couche de Slice/Nitrure de Silicium empêchant toute gravure ionique du substrat pendant la croissance des nanotubes. une technique de croissance de nanotubes de carbone à basse température (550 °C-600 °C) limitant la diffusion des dopants dans les matériaux semiconducteurs de type III-V. un recuit LASER des nanotubes de carbone améliorant leur qualité cristalline et diminuant leur résistivité. Enfin, les caractérisations du courant en fonction de la tension et les mesures fréquentielles des échantillons ont confirmé les résultats annoncés par la théorie. Une modulation du faisceau électronique contrôlée par voie optique a pu être mesurée jusqu'à 1.1 GHz, même si la fréquence de coupure actuelle se limite à 400 MHz. La réalisation des photocatodes a ainsi pu être démontrée. De surcroît, les résultats prometteurs aux vues des perspectives d'évolutions, permettent d'envisager une intégration proche des photocathodes dans un tube à ondes progressives.

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Für viele Anwendungen in der Grundlagenforschung, Medizin und industriellen Entwicklung sind Beschleuniger der entscheidende Antrieb. Vor allem Elektronenbeschleuniger sind als Synchrotronquellen unter den brillantesten Quellen für Strahlung im Infrarot- bis Röntgenbereich und damit unerlässlich für eine Vielzahl von Anwendungen und analytischen Methoden. Photoinjektoren stellen als Elektronenquellen für Beschleuniger eine wichtige Komponente für die Entwicklung von Lichtquellen wie Freie-Elektronen-Laser, sowie für neue Beschleunigerkonzepte wie Linearbeschleuniger mit Energierückgewinnung dar. Die Photokathode und der Anregungslaser definieren dabei mit der Quantenausbeute (QE) und der intrinsischen Emittanz zentrale Kenngrößen des Photoinjektors. Diese Arbeit beschreibt die Entwicklung von Alkali-Antimonid Photokathoden für die Anwendung in einem Photoinjektor mit supraleitendem Hochfrequenz-Resonator. Alkali Antimonide zeigen generell eine hohe QE und Cäsium Kalium Antimonid (Cs-K-Sb) im speziellen verspricht eine geringe intrinsische Emittanz aufgrund der Ionisierungsenergie, die nur knapp unter der Photonenenergie der grünen Anregungslaser liegt. Mit der Inbetriebnahme eines Präparations- und Analysesystems konnte die Abscheidung dünner Schichten von Cs-K-Sb sowie die Messung der QE und chemischen Zusammensetzung erzielt werden. Dabei wurde mit der Ko-evaporation der Alkalimetalle eine neue Wachstumsmethode etabliert und hinsichtlich der Prozessstabilität und Qualität der erzeugten Proben mit der sequenziellen Methode verglichen. Schließlich beschreiben die Inbetriebnahme eines Prototyps des Photoinjektors und erste erfolgreiche Kathodentransfers im Vakuum einen wichtigen Schritt hin zum Betrieb eines Beschleunigers mit einer Cs-K-Sb Photokathode im supraleitenden Hochfrequenz-Injektor. Diese Kombination erlaubt die Erzeugung eines Elektronenstrahls mit niedriger Emittanz und hohem mittleren Strom. / For many disciplines in basic and applied research, medicine and industrial development accelerators are an important driving force. Especially electron accelerators as synchrotron sources are among the brightest sources of radiation from the infrared to the X-ray regime and thus fundamental to a broad range of analytical techniques. Photoinjectors as electron sources for accelerator applications are a key component for the development of light sources such as free electron lasers as well as new accelerator concepts like energy-recovery linacs. The photocathode and drive laser define the quantum efficiency (QE) and intrinsic emittance of the photoemission process and thus central figures of merit of the photoinjector. This work focuses on the development of alkali antimonide photocathodes for the application in a superconducting radio frequency photoinjector. Alkali antimonides generally exhibit a high QE and cesium potassium antimonide (Cs-K-Sb) specifically is expected to release electrons with a low intrinsic emittance as the photoemission threshold is close to the photon energy of common, green, drive laser wavelengths. A preparation and analysis system has been commissioned for the deposition of Cs-K-Sb thin film photocathodes and their analysis regarding QE and chemical composition. A new deposition technique, the alkali metal co-deposition, was established and compared to the sequential deposition in terms of process reliability and quality of the produced samples. The work concludes with a report of the commissioning of a prototype of the photoinjector and successful cathode transfers in ultra-high vacuum, which represents an important technological advancement towards the operation of an accelerator with the combination of Cs-K-Sb photocathodes and an SRF injector. This combination makes the generation of an electron beam with low emittance and high average current possible.