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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
41

Application of the direct timing method in the ZEUS Central Tracking Detector

Nash, Jonathan January 1990 (has links)
No description available.
42

Adiabatic processes in cavit QED and the coherent control of trapped ions

Constantinos, Lazarou January 2009 (has links)
No description available.
43

On the development of a far-infrared bolometric detector using a 2DEG as the absorbing medium

Bacchus, Ian Dean January 2008 (has links)
This thesis describes the development of a far-infrared bolometric detector using a two-dimensional electron gas (2DEG) as the detecting medium. The 2DEG is formed at a AlGaAs/GaAs heterojunction made of layers of undoped GaAs and AlGaAs and highly doped (5 x lO126 3) AlGaAs. A 2DEG layer grown in this way in a molecular beam epitaxy (MBE) system is generally within 100 nm of the surface of the wafer and is subsequently patterned by etching away the surrounding wafer material and leaving a mesa containing the buried 2DEG. Ohmic contact to the 2DEG is achieved either through a diffusion of charge carriers into the contact region. Using a 2DEG as the absorbing medium in bolometers will yield detectors that are fast, sensitive, and frequency selective. The low electron densities in 2DEGs (1011 cm<super>-2</super>) allow large-area devices with extremely low thermal conductance between the electrons and the semiconductor lattice (e.g. Appleyard, et al. 1). The fast time constant (on the order of 1 ps) of the electron relaxation time in the 2DEG would allow for very high bandwidth spectroscopy. This thesis presents an overview of the use of 2DEG bolometers and a detailed study of their properties relevant for use as THz HEBs or CEBs. Chapter 1 briefly outlines the importance of Terahertz astronomy. Chapter 2 presents an introduction to bolometer theory. Chapter 3 provides a description of the electrical, thermal, and magnetic properties of 2DEGs. Chapter 4 outlines the equations governing the operation of 2DEG HEBs and CEBs and contains computer-simulated data. Chapter 5 describes our device fabrication, testing methods, and gives the results of our measurements. Finally, this thesis concludes with a discussion of the results of the tests and possible interpretations in terms of different physical models for electron-photon interactions in the 2DEG.
44

One-pot methodology for the synthesis of polysubstituted pyridines and terpyridines

Chapaneri, Krishna January 2008 (has links)
Polysubstituted pyridines are prepared by a one-pot three-component cyclocondensation process, developed by modification and improvement of the traditional Bohlmann-Rahtz reaction. The synthesis combines a 1,3-dicarbonyl compound, ammonia, and an alkynone without the use of an additional acid catalyst. This three-component heteroannulation reaction proceeds by tandem Michael addition-heterocyclization with total control of regiochemistry and the resulting library of pyridines is isolated in good yield. Modified Bohlmann-Rahtz procedures were applied to the synthesis of a range of terpyridines, by a two- and three-component condensation of 2,6-propynoylpyridine derivatives and a range of enamines, or 1,3-dicarbonyl compounds and ammonia, proceeding in moderate to good yield using a range of conditions. The synthesis of fluorescent cyanopyridines with desirable photophysical properties from p- aminocrotononitrile and a variety of heterocyclic alkynones was established by one-pot Bohlmann-Rahtz reaction in excellent yields. These cyanopyridines can be generated in good yield, rapidly, using microwave irradiation. Primary thioamides are prepared in excellent yield from the corresponding nitriles by treatment with ammonium sulfide in methanol, at room temperature for electron deficient aromatic nitriles or under microwave irradiation at 80 C or 130 C in 15-30 minutes for other aromatic and aliphatic nitriles without the need for chromatographic purification.
45

Experimental techniques for the study of the CMB anisotropy and polarization, and the Sunyaev-Zel'dovich effect

Rossinot, Philippe January 2004 (has links)
Observational cosmology is one of the most active fields in modern astrophysics. Following the discovery of the microwave background by Penzias and Wilson (Penzias & Wilson 1965) and the success of COBE-DMR, research groups throughout the world have pursued the study of the primary and secondary anisotropics in the cosmic microwave background (CMB). Instrumentation designed specifically for these studies have resulted in ever more sensitive detections of the CMB fingerprints. This thesis describes work on three bolometric instruments operating at cryogenic temperatures that have been designed to study the CMB. We present a design study with initial assembly and testing on the first of these, a new type of instrument---the millimetre-wave bolometric interferometer prototype (MBI-P). This instrument combines two well-known techniques: bolometric detection and interferometry, and could be used to observe the CMB on small angular scales with high sensitivity. Our investigation includes the design of the optics, cryogenics and mechanics of the prototype. We present a prototype that has a clear potential to demonstrate the technology involved. We then describe our contribution to Polatron, a bolometric polarimeter designed to study the polarization of the CMB. We assess its performance by analysing the noise coupling between the mechanical cryocooler and the bolometers. We demonstrate that Polatron is not able to fulfil the technical requirements in terms of sensitivity, necessary to achieve its science goal. Finally, we discuss the thermal behaviour of the cold-amplifier stage of Bolocam---a millimetre-wave bolometric camera, now operating at the Caltech sub-millimetre observatory (CSO), one of whose main scientific objective is the study of the CMB secondary signatures. Having modified the cryogenic design, we measure and analyse its performance which has significantly lowered the cryogen consumption.
46

Novel atmospheric monitoring for the H.E.S.S. site and its industrial applications

Hadjichristidis, Christos January 2009 (has links)
This thesis concerns the atmospheric monitoring instrumentation for the H.E.S.S. (High Energy Stereoscopic System) gamma-ray telescope site and the adaptation of such instruments for commercial use. The effect of the atmosphere on the H.E.S.S. telescopes' response has been demonstrated and the technicalities associated with the atmospheric monitoring instruments have been studied in depth. The responses of a LIDAR (Light Detection And Ranging) and a transmissometer have been checked by customised MODTRAN (MODerate resolution atmospheric TRANsmission) routines. This process revealed a malfunction of the LIDAR, whose raw data was independently treated to yield meaningful results. More importantly, the `Durham-designed' transmissometer, manufactured to operate during the night in parallel with the H.E.S.S. telescopes, was successfully adapted for day-light operation. As a result Durham prototype gained strong interest from Aeronautical & General Instruments Limited (AGI) in Dorset, who are particularly interested in the airport applications, and see the Durham instrument as a potential replacement for the transmissometer which they manufacture currently and is coming to the end of its useful design life. Durham University and AGI drew up a license agreement to pursue further development of the instrument. The resulting Durham aviation transmissometer meets the accuracy requirements for the Runway Visual Range (RVR) assessment imposed by both the World Meteorological Organisation (WMO) and the International Civil Aviation Organisation (ICAO). Moreover, the Durham instrument is easy to align, uses very little power, and is lightweight and portable, enabling its use not only in civil airports, at altitudes exceeding all prior-art aviation transmissometers, but also in tactical military applications, such as remote landing strips.
47

Realisation of a cold mixture of rubidium and caesium

Harris, Magaret L. January 2008 (has links)
This thesis describes a new apparatus designed to study cold, ultracold, and quantum degenerate mixtures of rubidium and caesium atoms. The Rb- Cs mixture is prepared using a double magneto-optical trap (MOT) system in which a two-species pyramid MOT acts as a source of cold atoms for a 'science' MOT. The first results of experiments on the magneto-optically trapped mixture are presented, including measurements of trap loss rates due to single-species and interspecies inelastic collisions. A technique for reducing interspecies loss by spatially separating the MOTs during loading is described. This technique allows 50-50 mixtures of Rb and Cs atoms to be loaded into a magnetic trap at close to their respective maximum single- species atom numbers. Alternatively, one species can be loaded with arbitrarily small amounts of the other. The displaced MOT technique is thus аn excellent starting point for investigations of interspecies Feshbach resonances and sympathetic cooling of Rb-Cs mixtures in magnetic and optical traps. In addition, a model of polarisation spectroscopy based on numerical integration of population rate equations is described. Theoretical polarisation spectra generated by the model are shown to agree with experimental spectra for the F = I + 1/2 → F' transitions in Rb and Cs. An investigation of the sub-Doppler dichroic atomic vapour laser locking (DAVLL) technique demonstrates how locking signals can be optimised for the Rb D2 transitions. The role of polarisation purity in generating the spectra is discussed, and impurities are modeled using a Jones matrix approach. Comparisons with polarisation spectroscopy and DAVLL are used to enhance understanding of atom-light interactions in spectroscopic systems, and indicate methods for optimising locking signals for use in cold atom experiments.
48

An embedded adaptive optics real time controller

Saunter, Christopher D. January 2007 (has links)
The design and realisation of a low cost, high speed control system for adaptive optics (AO) is presented. This control system is built around a field programmable gate array (FPGA). FPGA devices represent a fundamentally different approach to implementing control systems than conventional central processing units. The performance of the FPGA control system is demonstrated in a specifically constructed laboratory AO experiment where closed loop AO correction is shown. An alternative application of the control system is demonstrated in the field of optical tweezing, where it is used to study the motion dynamics of particles trapped within laser foci.
49

Development of electron based dissociation techniques in mass spectrometry for the structural characterisation of small organic ions and modified proteins

Prakash, Aruna Sunithi January 2012 (has links)
The work detailed herein describes the developments made using electrons to initiate bond dissociation via electronic excitation. Electron Capture Dissociation (ECD) uses low energy electrons to analyse multiply charged cations, providing a complementary series of product ions to vibrational excitation techniques Collision-Induced Dissociation (CID) and Infrared Multiphoton Dissociation (IRMPD). ECD has been adapted to analyse small singly charged ions by increasing the electron energy, known as Electron-Induced Dissociation (EID). The effect of electron energy has been studied, indicating optimal results occurring at 18 - 20 eV. EID has been carried out on a range of small organic molecules, resulting in a high degree of fragmentation. EID results suggest that bond dissociation can occur via multiple dissociation mechanisms, forming a combination of odd-electron and even-electron species, resulting in a unique set of product ions. Developments have been made to combine Liquid Chromatography (LC) with EID in order to analyse complex mixtures of small organic molecules with a wide dynamic concentration range. In-depth analysis has been carried out by LC-EID and LC-CID on the pharmaceutical reaction mixture of cediranib, allowing structural inferences to be made for the ten unknown low abundance components observed in the sample, proving each compound to be analogous to cediranib. Investigations of protein modifications have focussed on two proteins; the Multiple transferrable resistance Regulator (MtrR) protein and the Matrix (M) protein. ECD and CID successfully identified and located an insertion on the flexible N-terminus of the M protein that could not be resolved by X-ray crystallography. Mass spectrometry analysis has been used to identify the chemical alterations of both proteins resulting from reactions with small molecules, and ECD has been used to confirm the location of the reaction site for one modified peptide.
50

Experiments on ultracold quantum gases of 85Rb and 87Rb

Haendel, Sylvi January 2011 (has links)
This thesis describes a new apparatus designed to study ultracold gases of rubidium. The apparatus comprises a six-beam MOT chamber and a dierential pumping stage leading into a 'science chamber'. This science chamber is constructed from a rectangular glass cell. Atomic gases of rubidium are collected in a MOT and then transferred into a magnetic quadrupole trap. This quadrupole trap is mounted on a motorised translation stage. This setup transports the atoms into the science chamber, where they are transferred into a static quadrupole trap which is built around the glass cell. During the transport the atoms are deected over a glass prism, which shields the science chamber from stray rubidium from the MOT chamber. The magnetic transport is studied in detail and the deection over the glass prism is fully described simulating the displacement of the quadrupole trap. Using the magnetic quadrupole trap in the science chamber to store one rubidium isotope, we are able to load the other rubidium isotope in the MOT chamber and transfer it also into the science chamber. There, the two magnetic traps are merged and variable ratios of isotopic mixtures can be created. The merging of the two quadrupole traps could be employed in future experiments to cool 85Rb sympathetically with 87Rb. In the science chamber forced radio-frequency evaporation is performed and the loading of a far-detuned dipole trap is studied. Initially the dipole trap is realised as a hybrid trap, a single beam dipole trap in combination with the quadrupole trap. Further studies include the loading of a crossed beam dipole trap. We demonstrate that the apparatus is capable of producing 87Rb condensates. Preliminary studies of 85Rb in the dipole trap are included which hopefully in future will lead to a quantum degenerate gas of 85Rb.

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