No description available.

The initial aim of the work presented in this thesis was to increase the soft x-ray quantum detection efficiency of a tandem-pair microchannel plate detector by the use of a CsI deposition photocathode. This aim was achieved. The coating technique and initial measurements are presented herein. After showing the use of such photocathodes, we investigated their stability and reproducibility. The effects of storage in poor vacuum, high vacuum and desiccated air are presented as is the stability of CsI photocathodes under prolonged X-ray bombardment. One consequence of the use of CsI is that a degree of energy resolution can be conferred upon a microchannel plate detector. We present further research in this field, including measurements performed on detectors with eight micron diameter channels. A feature of microchannel plate operation that is undesirable is the phenomenon of gain degradation. We performed a series of lifetests on a number of microchannel plate detectors.

No description available.

The molecular structures of 18 Group VI organometallic compounds were determined by single crystal X-ray diffraction. Most of these complexes either contain the 'Cp'M(NO)' fragment (where Cp'=Cp(ɳ⁵-C₅H₅ ) or Cp⁻(ɳ⁵-C₅Me₅ ) ) or were derived from such compounds. The purpose of this work was to determine the structures of representative types of Group VI nitrosyl complexes and related compounds and to compare their features. Also, an effort was made to study any structural changes that accompanied reactions involving ancillary ligands. Fourteen of the structures that were determined contain a metal-nitrosyl linkage that is nearly linear in all cases. One of these compounds is the first example of a Group VI nitrosyl complex containing two different alkyl groups. The structures of two of its reaction products show that one of the alkyl groups can form a second metal to carbon bond via an orthometallation process. Five other nitrosyl alkyl complexes that were studied contain benzyl groups. In these structures, the benzyl ligands bond to the central metal atoms in ɳ²-type linkages, through both the methylene carbon and the ipso carbon atoms. In the compounds studied, this unusual mode of attachment is not influenced by the nature of other ligands on the metal or by methyl substitution on the phenyl ring. Other neutral nitrosyl complexes studied were the products of reactions involving Group VI diene compounds with acetone. Two structures are the result of similar insertion of an acetone molecule into one of the metal-diene bonds with the formation of a new metal to oxygen bond. The orientation of the diene ligands, however, is different in these structures due to steric interactions between these groups and cyclopentadienyl ligands in the complex. A third compound, produced upon prolonged reaction of the parent diene nitrosyl complex with acetone, has a trimeric structure containing bridging acetone groups and no diene ligands. Three cationic nitrosyl structures showed some small differences from the other nitrosyl complexes studied. The metal to nitrosyl linkages are nearly linear but the bond lengths are slightly different from those found in other compounds. Metal to nitrogen distances are slightly longer and nitrogen to oxygen bond lengths are slightly shorter in these cationic complexes. This observation suggests that in these cations, the metal to nitrosyl π backbonding is weaker than in other complexes. Three of the structures presented were not derivatives of nitrosyl containing complexes. One of the compounds was isolated upon oxidation of a dioxoalkyltungsten complex. Its structure is a rare example of an organometallic oxo peroxo complex. Reaction of this product with tetracyanoethylene gave a charge-transfer complex; structural studies of this compound identified a 2:1 organometallic:TCNE complex with interactions involving the peroxo groups of the organometallic molecules and the central ethylene carbon atoms of the TCNE molecule. Treatment of a dioxoalkyltungsten complex with a substituted isocyanate did not give the anticipated diamido compound. Instead, the starting complex reacted with three isocyanate molecules; the structure of the product is essentially that of a solvated diamido complex, with one amido group and a substituted urea ligand. In the nitrosyl complexes studied, a range of ancillary ligands (alkyl groups such as trimethylsilylmethyl, triphenylethyl, benzyl; diene ligands; chloro or acetonitrile groups) does not alter the linear nature of the metal-nitrosyl fragment. In all cases, the nitrosyl group is attached to the metal centre through only the nitrogen atom and the M-N-O bond angle does not deviate significantly from 180°. / Science, Faculty of / Chemistry, Department of / Graduate

Dimethyltellurium diiodide is known in two forms; the crystal structure of the β-form has been determined by X-ray diffraction; Mo-Kα scintillation counter data were used for this analysis in which the heavy atoms were located from the Patterson function, the carbon atoms by a difference synthesis and refinement was by least-squaresmethods. β-dimethyltellurium diiodide is ionic, [Me₃Te]⁺ [MeTeI₄]⁻, and is built up from trigonal pyramidal Me₃Te⁺ cations with Te - C = 2.07 Å, C - Te - C = 95°, and square pyramidal MeTeI₄⁻ anions with Te - C = 2.15 Å, Te - I = 2.84 - 2.98 Å. The ions are bridged by four weak Te... I interactions (distances 3.84, 3.88, 3.97, 4.00 Å), which complete a distorted octahedral environment around each tellurium atom. The crystal and molecular structures of 2-biphenylylferrocene and 4-biphenylylferrocene have been investigated in order to compare the configuration of the rings of the biphenyl and ferrocene groups in these two molecules. The structure of 2-biphenylylferrocenje, has been determined with visual Gu-Kα data. The iron atom position was found by Patterson methods, the carbon positions from successive Fourier summations. The positional and thermal parameters were refined by least-squares. The cyclopentadienyl rings are eclipsed, the first six-membered ring of the biphenyl group is rotated 43° out of the cyclopentadienyl plane and the outer six-membered ring is rotated 58° out of the plane of the first six-membered ring. These rotations relieve the strain which would exist in a planar model for the C₅H₄ . C₆H₄ . C₆H₅ group. The mean bond distances are Fe - G = 2.05 Å, C - C (cyclopentadienyl) = 1.44 Å. Using Fe-Kα scintillation counter data, the structure of 4-biphenylylferrocene has been determined by Patterson and Fourier methods and refined.by least-squares. The two crystallographically independent molecules in the unit cell have slightly different conformations. In one the cyclopentadienyl rings are oriented about midway between the eclipsed and staggered conformations and the first six-membered ring is rotated 6° out of the plane of the cyclopentadienyl ring to which it is bonded, with the second six-membered ring rotated a further 9°. In the second molecule the cyclopentadienyl rings are only about 5° form the fully eclipsed position and the six-membered ring rotations are 0° and 10°. The mean bond distances are Fe-C = 2.07 Å, C-C (cyclopentadienyl) = 1.48 Å, C-C (biphenylyl) = 1.43 Å, C-C (between rings) = 1.48 Å. The intermolecular separations correspond to, van der Waals' interactions. / Science, Faculty of / Chemistry, Department of / Graduate

The structures of five compounds of group III elements have been determined by single crystal X-ray diffraction, three boron compounds, one aluminum compound, and one gallium compound: 1» B,B-diphenylboroxazolidine (2-atninoethyl diphenyl-borinate) , C14H16BNO. 2. B ,B-bis (p-f luorophenyl) boroxazolidine , C14H14BF2NO. 3. 4 ,4-dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-bor-anatacyclopentane, C15H18gBNO2. 4. N-methyldiethanolaminogallane dimer, C10H24Ga2N2O4 5. (pentahaptocyclopentadienyl) hydridomolybdenum-μ-di-me th ylaluminum-μ-[ methylaluminum-di- iju.-pentaha_p_to-(monohapto)cyclopentadienyl)dimethylaluminum] (pentaha]3-tocyclopentadienyl)hydridomolybdenum, C25H35Al3Mo2. Crystals of B,B-diphenylboroxazolidine are monoclinic, a = 13.840(1), b = 8.9169(5), c = 10.170(1) A, = 98.85 (1)° , Z = 4, space group P21/n. The structure was determined by direct methods, and refined by electron-density and full-matrix least-squares procedures to R 0.041 for 1458 reflexions. The five-membered boroxazolidine ring is in the half-chair conformation. Bond angles in the ring range from 99.7 for OBN to 110.1° for BOC. Bond lengths are as follows: mean B-C, 1.616(2), B-H, 1 .653 (3), B-0, 1.484(3), C-N, 1 .485 (3), C-O, 1.413(3), mean C-C (aromatic) , 1 .392(11 ), and C-C, 1.505(4) A. The structure consists of discrete molecules linked by 0...H-N hydrogen bonds (2,874(2) A) to form continuous spirals about the 21 axes. Crystals of B,B-bis (p-fluorophenyl)boroxazolidine are orthorhomfcic, a = 13.442 (4), b = 10.214(3), c = 9. 823 (2) ft, Z = 4, space group P212121. The structure was solved by direct methods, and refined by electron-density and full-matrix least-squares procedures to R 0.047 for 1234 reflexions. The five-membered boroxazolidine ring is in a distorted half-chair conformation , Bond angles in the ring range from 99.9(2) for OBH to 108.2(2)° for BOC. Bond lengths are: mean B-C, 1.621:, (3), B-N, 1.652 (4), B-O, 1.471(4), C-N, 1.491(4), C-0, 1.4 18(4), mean C-F, 1.371(1), mean C-C (aromatic) , 1 .390 (13), and C (sp3) -C (sp3) , 1.494 (6) A. The structure consists of discrete molecules each linked to six others by an extensive network of O...H-N (0...N = 2.941(3) A), F...H-N (F...N = 3.171(4) A), and F...H-C (F...C = 3.318(5) A) hydrogen bonds. Crystals of 4,4-dimethyl-2,2-diphenyl-1,3-dioxa-4-azonia-2-boranatacyclopentane are orthorhombic, a = 17.043 (3), b= 6.289 (1), c= 13.024 (2) A, Z= 4, space group Pna2i. The structure was determined by direct methods, and was refined by full-matrix least-squares procedures to R 0,071 for 1100 reflexions. Bond angles in the five-membered ring , which has a distorted half-chair conformation, range from 101.5(4) for OBO to 107.1(4)° for NOB. Bond lengths are: mean B-C, 1.632 (8), B-0, 1.506(7) and 1.556 (8), N-0, 1.409(5), C-O, 1. 378 (9), C-N, 1.467-1.509(7-10), mean C-C(aromatic), 1.395(25)A. The structure consists of discrete molecules separated by normal van der Waals distances. Crystals of the N-methyldiethanolaminogallane dimer are orthorhomfcic, a = 19.112(4), b = 9.947(2), c = 7.709(2) A, Z = 4, space group P212121 . The structure was determined by Patterson and Fourier synthesis and was refined by full-matrix least-squares procedures to a final R of 0.056 for 1477 reflexions. The structure provides the first known crystallographic example of pentacoordinate gallium, the dimerization of HeH (CH2CH2O)2GaH occurring via the formation of a four-membered Ga2O2 ring. The coordination about the gallium is distorted trigonal bipyramidal with an angle of 151.2(4)° between the axial substituents. The mean bond distances are: Ga-N, 2.192(5), and Ga-O, 2.018(2) for axial ligands; Ga-O, 1.847(2), 1.960 (8), and Ga-H, 1.41(4) for equatorial ligands; O-C, 1.419(14), C-N, 1.470(7), C-C, 1.520 (12), and C-H, 1.00 (13) A. The molecule has C2 symmetry to within experimental error. There are possible C-H...0 hydrogen bonds (C...O, 3.13 (1)-3.44 (1) A) in the structure. Crystals of the hydridomolybdenum complex, C25H35Al3MO2, are orthorhombic, a = 19.398 (4) , b = 14.438 (9) , c = 9.0 35 (2) I, Z =4, space group P212121, The structure was determined by Patterson and Fourier syntheses, and refined by full-matrix least-sguares procedures to R 0.066 and Rw 0.063 for 1213 reflexions. The molecular structure exhibits several unusual features: C5H4 groups which are pentahapto to the molybdenum atoms and are involved via the unique carbon atom in multicentre bonding to two aluminum atoms, one of which occurs as an Al(Me)2 unit and the other an AlMe unit which also bridges the two molybdenum atoms. The third aluminum atom is probably involved in a Mo-H-Al (Me)2-H-Mo linkage. Mean bond distances are: Mo-Al, 2.659 and 2.974, Al-C(terminal) , 2.00, Al-C (bridge), 2.05 and 2.33, Mo-C (cyclopentadienyl) , 2. 285, and C-C (cyclopentadienyl) , 1.389 A. / Science, Faculty of / Chemistry, Department of / Graduate

No description available.

In order to elucidate the effects of nanostructuring on electron behavior in vanadium oxides, a suite of x-ray spectroscopy techniques was employed to comprehensively characterize the electronic structures of V2O5 and VO2 nanoparticles and compare them to their bulk counterparts. V2O5 and VO2 nanoparticle powders were characterized via PXRD, TEM, and HR-TEM to confirm size, purity, and crystallinity. Additionally, DSC and temperature-varied PXRD measurements on both VO2 samples confirmed the structural aspect of the monoclinic to rutile metal-insulator phase transition, and UV-Vis measurements allowed for Kubelka-Munk analysis on the V2O5 samples. XAS measurements enable the comparison of unoccupied conduction band states, while XES and RIXS measurements reveal occupied valence band states and the individual vanadium and oxygen PDOS below the Fermi level. XPS measurements of both core and valence band states both confirmed the valence band structure revealed by XES and also provide information on core-state energy levels. In the case of V2O5, the valence band O 2p states are upshifted in the nanoparticle sample, while the lowest V 3d conduction band states are unshifting but provide more available unoccupied states for excitation. These changes produce a shrunken bandgap in the V2O5 nanoparticles that is in line with much previous computational work, but unexpected from previous experimental results and defies the Moss-Burstein effect usually observed in V2O5. The resulting changes in band structure are attributed to a higher concentration of oxygen vacancy defects in the nanoparticle sample. Additionally, electron correlation effects in V2O5 nanoparticles are found to be enhanced relative to the bulk, likely due to added electron presence in the V 3d split-off band. In the case of VO2, dramatic changes in both the valence band and conduction band states are observed both below and above the structural phase transition temperature. These changes (lowered unoccupied conduction band states coupled with broadened and upshifted occupied valence band states) also lead to nanoparticle bandgap reduction and enhanced metallicity. The enhanced metallic nature of the VO2 nanoparticles is again attributed to the increased presence of surface oxygen vacancy defects, as well as a V2O3-like surface reconstruction. Additionally, electron correlation effects are found to be reduced in the VO2 nanoparticle samples relative to the bulk, unlike in the case of V2O5.

No description available.

No / The aim of this research was to optimize x-ray image quality to dose ratios in the cardiac catheterization laboratory. This study examined independently the effects of peak x-ray tube voltage (kVp), copper (Cu), and gadolinium (Gd) x-ray beam filtration on the image quality to radiation dose balance for adult patient sizes. Methods: Image sequences of polymethyl methacrylate (PMMA) phantoms representing two adult patient sizes were captured using a modern flat panel detector based x-ray imaging system. Tin and copper test details were used to simulate iodine-based contrast medium and stents/guide wires respectively, which are used in clinical procedures. Noise measurement for a flat field image and test detail contrast were used to calculate the contrast to noise ratio (CNR). Entrance surface dose (ESD) and effective dose measurements were obtained to calculate the figure of merit (FOM), CNR2/dose. This FOM determined the dose efficiency of x-ray spectra investigated. Images were captured with 0.0, 0.1, 0.25, 0.4, and 0.9 mm Cu filtration and with a range of gadolinium oxysulphide (Gd2O2S) filtration. Results: Optimum x-ray spectra were the same for the tin and copper test details. Lower peak tube voltages were generally favored. For the 20 cm phantom, using 2 Lanex Fast Back Gd2O2S screens as x-ray filtration at 65 kVp provided the highest FOM considering ESD and effective dose. Considering ESD, this FOM was only marginally larger than that from using 0.4 mm Cu at 65 kVp. For the 30 cm phantom, using 0.25 mm copper filtration at 80 kVp was most optimal; considering effective dose the FOM was highest with no filtration at 65 kVp. Conclusions: These settings, adjusted for x-ray tube loading limits and clinically acceptable image quality, should provide a useful option for optimizing patient dose to image quality in cardiac x-ray imaging. The same optimal x-ray beam spectra were found for both the tin and copper details, suggesting that iodine contrast based imaging and visualization of interventional devices could potentially be optimized for dose using similar x-ray beam spectra.