Molecular Fullerides

The closed shell structures of certain all-carbon fragments originally observed in mass spectroscopy experiments leads to the enhanced stability of these species, known as fullerenes, which have excited sufficient interest amongst chemists and physicists over the last decade to warrant the award of the 1996 Nobel Prize for Chemistry to their discoverers. ¶ Studies of the stability, symmetry, and consequent remarkable properties of fullerenes began in earnest in 1991 with the development of a technique enabling the production and purification of macroscopic quantities of material. The best known and most widely studied fullerene is the truncated icosahedral C60 molecule, which forms the basis of the present work

Investigating the stability of a laser-based plasma x-ray source

The use of laser based plasma x-ray sources (LPXS) has quickly expanded during the past decades due to rapid development of ultrafast laser systems. These sources are used in many research applications such as emission, absorption and particularly time-resolved spectroscopy. From the LPXS x-ray pulses are generated when an intense laser pulse is focused onto a liquid or solid interface in gas or vacuum. In this thesis we investigate the source stability which we define as the x-ray flux and spectrum reproducibility from each generated x-ray pulse. Understanding the stability is of great importance for its research applications. A basic theory of the LPXS is introduced describing relevant parameters for the source stability. Two parameters of relevance for the LPXS stability were investigated: The laser pointing and pulse energy fluctuations. These were experimentally determined using a beam profiler, capturing the beam profile of each laser pulse at 1 kHz. The relevance of these parameter fluctuations to the source stability is discussed based on these measurements. An existing LPXS setup was reconstructed in preparation for shot to shot stability measurements. A synchronized chopper system was built to decrease the laser pulse frequency for single pulsed mode and a program was developed for analyzing the x-ray photons captured by an x-ray CCD camera. Problems with the laser system prevented successful gathering of data within the time of the project. Future measurements based on these preparations will reveal the stability of the LPXS.Det mänskliga ögat observerar världen i formen av synligt ljus med olika färger. Allt ljus kan beskrivas som elektromagnetiska vågor med en viss våglängd, längre våglängder ger rött och kortare ger violett ljus. Utanför det för ögat synliga ljuset finner man röntgenstrålning med våglängder mycket kortare än både violett och ultraviolett ljus. Ända sedan dess upptäckt för över 100 år sedan har man funnit mängder av användningsområden för denna typen av ljus. Röntgenstrålning passerar oförhindrat genom många fasta material och används därför inom sjukvården för att se inuti kroppen. Den korta våglängden gör det även möjligt att studera mycket små saker som annars är osynliga med vanligt ljus. Röntgenstrålning är därför ett utmärkt verktyg inom forskning för att undersöka egenskaper hos molekyler och atomer. Det finns många olika metoder idag för att producera röntgenstrålning. I detta arbete har en mycket speciell typ av röntgenkälla undersökts, nämligen en s.k.laser-plasma-röntgenkälla. Denna röntgenkälla genererar korta intensiva röntgenpulser likt en stark kamera blixt. Röntgenkällan fungerar genom att fokusera en stark laserpuls med en enorm effekt på 1.5 biljoner watt och en kortvarighet på 40 femtosekunder (femtonde decimalen av en sekund). Laserpulsen fokuseras på en vattenstråle inte mycket bredare än ett hårstrå. Då den intensiva laserpulsen träffar vattenytan värms ytan upp så pass kraftigt att molekylerna och atomerna slits isär, vilket bildar ett plasma likt solen, bestående utav fria joner och elektroner. Under den kortvariga, men mycket våldsamma interaktionen accelereras och kolliderar elektroner, vilket resulterar i en skarp blixt av röntgenstrålning. Denna skarpa röntgenblixt kan riktas mot ett material som skall undersökas. När röntgenblixten interagerar med materialet kan ljuset som sänds ut från materialet detekteras av en mycket känslig detektor, likt den i en digitalkamera, där bilden kan återskapas. Den informationen som sensorn fångar in kan då analyseras för att ta reda på egenskaper hos materialet. Ju snabbare och intensivare röntgenblixten är destu bättre blir bildkvaliteten. Den extremt kortvariga röntgenblixten gör det möjligt att observera egenskaper hos molekyler och atomer med en mycket god upplösning. Likt en digitalkamera är det mycket viktigt att bildkvaliten är stabil och inte ändras mellan varje bild. I detta arbete undersöks stabiliteten hos denna röntgenkällan genom att bestämma laserpulsens träffsäkerhet och styrka på den hårtunna vattenstrålen. Information om dess stabilitet kan användas för att i slutändan förbättra stabiliteten hos röngenkällan och på så vis förbättra bildkvaliteten med denna speciella kamera för framtida forskning

Inherent dose-reduction potential of classical ghost imaging

Classical ghost imaging is a computational imaging technique that employs patterned illumination. It is very similar in concept to the single-pixel camera in that an image may be reconstructed from a set of measurements even though all imaging photons or particles that pass through that sample are never recorded with a position resolving detector. The method was first conceived and applied for visible-wavelength photons and was subsequently translated to other probes such as x rays, atomic beams, electrons, and neutrons. In the context of classical ghost imaging using penetrating probes that enable transmission measurement, we here consider several questions relating to the achievable signal-to-noise ratio (SNR). This is compared with the SNR for conventional imaging under scenarios of constant radiation dose and constant experiment time, considering both photon shot noise and per-measurement electronic readout noise. We show that inherent improved SNR capabilities of classical ghost imaging are limited to a subset of these scenarios and are actually due to increased dose (Fellgett advantage). An explanation is also presented for recent results published in the literature that are not consistent with these findings

X-ray attenuation models to account for beam hardening in computed tomography

We introduce a beam-hardening correction method for lab-based X-ray computed tomography (CT) by modifying existing iterative tomographic reconstruction algorithms. Our method simplifies the standard Alvarez-Macovski X-ray attenuation model [Phys. Med. Biol. 21, 733 (1976)] and is compatible with conventional (i.e., singlespectrum) CT scans. The sole modification involves a polychromatic projection operation, which is equivalent to applying a weighting that more closely matches the attenuation of polychromatic X-rays. Practicality is a priority, so we only require information about the X-ray spectrum and some constants relating to material properties. No other changes to the experimental setup or the iterative algorithms are necessary. Using reconstructions of simulations and several large experimental datasets, we show that this method is able to remove or reduce cupping, streaking, and other artefacts from X-ray beam hardening and improve the self-consistency of projected attenuation in CT. When the assumptions made in the simplifications are valid, the reconstructed tomogram can even be quantitative.Australian Research Council (LP150101040); partner company FEI (now Thermo-Fisher) through Linkage (Project LP150101040)

Beating Darwin-Bragg losses in lab-based ultrafast X-ray experiments

The use of low temperature thermal detectors for avoiding Darwin-Bragg losses in lab-based ultrafast experiments has begun. An outline of the background of this new development is offered, showing the relevant history and initiative taken by this wor

Buckminsterfullerene: C60

A study of the recently discovered molecular allotrope of carbon, C60 (buckminsterfullerene) is presented. The preparation of this potentially useful new material is described, along with its characterisation by X-ray powder diffraction, scanning electron microscopy, mass spectrometry, infrared spectroscopy and ultraviolet/visible/near-infrared spectroscopy. To a lesser extent, its congener C70 is also described. ¶ The newly discovered spectra of the C601-, C602-, C603-, and C604- anions of C60 are characterised and used in a novel preparation of the superconducting Rb3C60 salt in liquid ammonia. This method is the first to allow determination of the oxidation state of C60 during synthesis, and also provided the first near-infrared spectroscopic observation of the C605- anion. The nature of the synthesis is such as to allow stoichiometric synthesis of other alkali metal compounds also. The products of the liquid ammonia synthesis were analysed by X-ray diffraction and microwave susceptibility (to determine superconductivity temperatures)

The use and characterization of a backilluminated charge-coupled device in investigations of pulsed x-ray and radiation sources.

Examinations of bremsstrahlung and energetic electron beams from a novel laser plasma source motivate and assist characterization of a backthinned, backilluminated direct detection x-ray charge-coupled device (CCD), a topology that is uncommon in hard x-ray work. Behavior toward pseudomonochromatic ((55)Fe) and multichromatic ((241)Am) sources is briefly reviewed under optimized noise conditions. Results collectively establish the previously unknown functional depth structure. Several modes of usage are illustrated in approximately 4-20 keV x-ray laser plasma source investigations, where the significance of the characterization is briefly discussed. The spectral redistribution associated with this CCD topology is unfavorable, yet appropriate analysis ensures that sufficient spectral information remains for quantitative determination of broadband x-ray flux and spectra in essentially single laser shot measurements. The energy dependence of nascent electron cloud radii in silicon is determined using broadband x-rays from the laser plasma source, turning the narrow depletion depth to advantage. Finally, the characterization is used to quantify recent x-ray spectral explorations of the water jet laser plasma source operating under aspirator vacuum. These results will have key value for establishment of laboratory based ultrafast extended x-ray absorption fine structure experiments using microbolometric detectors

Laser generated 300 keV electron beams from water

Abstract in Undetermined300 keV electron beams with energy peaked in the range 280–390 keV were generated by focusing a high contrast ratio but temporally double pulsed 800 nm ultrafast laser onto a flowing water jet under both helium atmosphere at ambient pressure and water aspirator vacuum conditions, using laser intensities in the range 1015–1018 Wcm−2. Their characteristics have been investigated as functions of inter-pulse delay, incidence geometry and laser pulse chirp. Shot-to-shot variation of the beams' equatorial and azimuthal distributions was also recorded in real time. Measurements of the emitted charge and energy have been performed. Secondary X-ray emission arising from impingement of the electron beams on the target chamber walls and other parts of the apparatus have been identified. Preliminary results after transition to a high repetition rate laser system have shown similar behavior. Approaches for improvements and applications are suggested