Development of neutron and X-ray imaging detectors based on MHSP

A Micro-Hole & Strip Plate (MHSP) é uma microestrutura desenvolvida recentemente em instituições universitárias portuguesas. Neste trabalho, o seu desempenho como detector para imagiologia é explorado, tendo como objectivo a imagiologia não só de neutrões, mas também de raios X. A aplicação do método da divisão resistiva de carga é aplicada a uma MHSP especialmente desenhada para sensibilidade em posição. Várias abordagens em termos da electrónica de aquisição de sinal são testadas, sem perder de vista a relação desempenho-preço. Resoluções espaciais abaixo de 1 mm foram obtidas com a MHSP a operar em xénon e tetrafluorometano, com um sistema de detecção a um preço modesto quando comparado com as alternativas, e apropriado para inúmeras aplicações em imagiologia de neutrões e de raios X.

Estudo de Monte Carlo da electroluminescência em detectores gasosos

Uma descrição detalhada do processo de electroluminescência é um prérequisito na optimização de detectores gasosos para sistemas de imagiologia, astrofísica, física de altas energias e experiências de eventos raros. Neste trabalho, é apresentada e caracterizada uma nova e versátil plataforma de simulação da emissão de luz durante a deriva de electrões em gases nobres, desenvolvida usando os programas Magboltz e Garfield. Propriedades intrínsecas da electroluminescência em gases nobres são calculadas e apresentadas em função do campo eléctrico aplicado, nomeadamente eficiências, rendimento e flutuações estatísticas associadas. São obtidos resultados em grande concordância com dados experimentais e simulações Monte Carlo anteriores. A plataforma é usada para determinar as condições óptimas de funcionamento de detectores como o NEXT (Neutrino Experiment with a Xenon TPC) e outros baseados nas micro-estruturas GEM (Gas Electron Multiplier) e MHSP (Micro- Hole & Strip Plate).

An application of cell-cluster theory to a rare gas crystal /|n[by] K. Westera. - 260 St. Catharines [Ont.] : Dept. of Physics, Brock University,

The Lennard-Jones Devonshire 1 (LJD) single particle theory for liquids is extended and applied to the anharmonic solid in a high temperature limit. The exact free energy for the crystal is expressed as a convergent series of terms involving larger and larger sets of contiguous particles called cell-clusters. The motions of all the particles within cell-clusters are correlated to each other and lead to non-trivial integrals of orders 3, 6, 9, ... 3N. For the first time the six dimensional integral has been calculated to high accuracy using a Lennard-Jones (6-12) pair interaction between nearest neighbours only for the f.c.c. lattice. The thermodynamic properties predicted by this model agree well with experimental results for solid Xenon.

Photoacoustic investigation of the optical and thermal properties of selected amorphous chalcogenide semiconductors

This thesis work has mainly concentrated on the investigation of the ,optical and thermal properties of binary semiconducting chalcogenide glasses belonging to the AivB¥5x and AZBXEX families. The technique used for these studies is a relatively new one namely, the photoacoustic (PA) technique. This technique is based on the detection of acoustic signal produced in an enclosed volume when the sample is irradiated by an intensity modulated radiation. The signal produced depends upon the optical properties of the sample, and the thermal properties of the sample, backing material and the surrounding gas. For the present studies an efficient signal beam gas-microphone PA spectrometer, consisting of a high power Xenon lamp, monochromator, light beam chopper, PA cell with microphone and lock-in amplifier, has been set up. Two PA cells have been fabricated: one for room temperature measurements and another for measurements at high temperatures. With the high temperature PA cell measurements can be taken upto 250°C. Provisions are incorporated. in both the cells to change the volume and to use different backing materials for the sample. The cells have been calibrated by measuring the frequency response of the cells using carbon black as the sample

Zeitaufgelöste Photoelektronenspektroskopie an Kaliumatomen in intensiven Femtosekundenlaserfeldern

Gegenstand dieser Dissertation sind Ultrakurzzeitexperimente im Bereich der kohärenten Kontrolle im schwachen wie auch im starken Feld. Untersucht wird am Modellsystem Kaliumatom die resonanzunterstützte Anregungs- und Ionisationsdynamik mit Hilfe der Photoelektronenspektroskopie. Die Kaliumatome werden mittels eines Atomstrahles bzw. mit Hilfe von Dispensern ins Vakuum entlassen. Angeregt und ionisiert werden sie mit Hilfe von Femtosekundenlaserpulsen (mit einer Pulslänge größer/gleich 30 fs). Die Flugzeiten der durch Ionisation freigesetzten Photoelektronen werden mit Hilfe einer magnetischen Flasche detektiert und in ihre kinetischen Energien umkalibriert. Drei Experimente werden in dieser Arbeit beschrieben: Im schwachen Feld sind Interferenzen von freien Elektronenwellenpaketen das Thema: Mit Hilfe eines Analogons zu Thomas Youngs Doppelspaltexperiment wird demonstriert, dass Kohärenzeigenschaften von Femtosekundenlaserpulsen auf Elektronenwellenpakete, welche sich im Kontinuum befinden, übertragbar sind. Als Referenzsystem für das Verständnis der Starkfeldkontrolle interessiert das Autler-Townes-Doublet im Photoelektronenspektrum des Kaliumatoms: Es wird gezeigt, dass sowohl durch das starke Laserfeld, als auch auf Grund der optische Phase zweier Laserpulse, die quantenmechanische Phase eines Atomzustandes gleichermaßen manipuliert werden kann. Das aufgezeigte Experiment liefert die Basis für ein in unseren Laboratorien entwickeltes völlig neuartiges Starkfeldkontrollszenario, welches „Selective Population Of Dressed States“ (SPODS) getauft wurde. Im dritten Experiment wird der Starkfeldbereich auf eine Laserintensität über 3*10^13 W/cm^2 ausgedehnt! Beginnend mit I = 1*10^11 W/cm^2 bis zur eben angegebenen größten Intensität wird mittels linear wie auch zirkular polarisiertem Laserlicht das Kaliumatom bei einer Vielzahl von Messungen ionisiert werden. Mit Hilfe der gemessenen Photoelektronenspektren wird in Abhängigkeit der Laserintensität die Aufspaltung des Autler-Townes-Doublets vermessen. Zudem werden unter Einbeziehung des schwachen und des starken Feldes Steigungsanalysen der Schwell- sowie der Above-Threshold-Ionisation-Photoelektronenausbeute aufgezeigt. Im Gegensatz zu Starkfeldexperimenten an nicht resonanten Systemen (z. B. Xenon), ist der Fall der resonanten Anregung bislang weitgehend ununtersucht. Für die resonante Anregung zeigt diese Arbeit somit erstmals einen Referenzdatensatz.

Studying reactive processes with classical dynamics: rebinding dynamics in MbNO

A new surface-crossing algorithm suitable for describing bond-breaking and bond-forming processes in molecular dynamics simulations is presented. The method is formulated for two intersecting potential energy manifolds which dissociate to different adiabatic states. During simulations, crossings are detected by monitoring an energy criterion. If fulfilled, the two manifolds are mixed over a finite number of time steps, after which the system is propagated on the second adiabat and the crossing is carried out with probability one. The algorithm is extensively tested (almost 0.5 mu s of total simulation time) for the rebinding of NO to myoglobin. The unbound surface ((FeNO)-N-...) is represented using a standard force field, whereas the bound surface (Fe-NO) is described by an ab initio potential energy surface. The rebinding is found to be nonexponential in time, in agreement with experimental studies, and can be described using two time constants. Depending on the asymptotic energy separation between the manifolds, the short rebinding timescale is between 1 and 9 ps, whereas the longer timescale is about an order of magnitude larger. NO molecules which do not rebind within 1 ns are typically found in the Xenon-4 pocket, indicating the high affinity of NO to this region in the protein.

Ligand dynamics in myoglobin: calculation of infrared spectra for photodissociated NO

We present molecular dynamics simulations of the photodissociated state of MbNO performed at 300 K using a fluctuating charge model for the nitric oxide (NO) ligand. After dissociation, NO is observed to remain mainly in the centre of the distal haem pocket, although some movement towards the primary docking site and the xenon-4 pocket can be seen. We calculate the NO infrared spectrum for the photodissociated ligand within the haem pocket and find a narrow peak in the range 1915-1922 cm(-1). The resulting blue shift of 1 to 8 cm(-1) compared to gas-phase NO is much smaller than the red shifts calculated and observed for carbon monoxide (CO) in Mb. A small splitting, due to NO in the xenon-4 pocket, is also observed. At lower temperatures, the spectra and conformational space explored by the ligand remain largely unchanged, but the electrostatic interactions with residue His64 become increasingly significant in determining the details of the ligand orientation within the distal haem pocket. The investigation of the effect of the L29F mutation reveals significant differences between the behaviour of NO and that of CO, and suggests a coupling between the ligand and the protein dynamics due to the different ligand dipole moments.

A theoretical study on the XeF(2) molecule

A relativistic four-component study was performed for the XeF(2) molecule by using the Dirac-Coulomb (DC) Hamiltonian and the relativistic adapted Gaussian basis sets (RAGBSs). The comparison of bond lengths obtained showed that relativistic effects on this property are small (increase of only 0.01 angstrom) while the contribution of electron correlation, obtained at CCSD(T) or CCSD-T levels, is more important (increase of 0.05 angstrom). Electron correlation is also dominant over relativistic effects for dissociation energies. Moreover, the correlation-relativity interaction is shown to be negligible for these properties. The electron affinity, the first ionization potential and the double ionization potential are obtained by means of the Fock-space coupled cluster (FSCC) method, resulting in DC-CCSD-T values of 0.3 eV, 12.5 eV and 32.3 eV, respectively. Vibrational frequencies and some anharmonicity constants were also calculated under the four-component formalism by means of standard perturbation equations. All these molecular properties are, in general, ill satisfactory agreement with available experimental results. Finally, a partition in terms of charge-charge flux-dipole flux (CCFDF) contributions derived by means of the quantum theory of atoms in molecules (QTAIM) in non-relativistic QCISD(FC)/3-21G* calculations was carried out for XeF(2) and KrF(2). This analysis showed that the most remarkable difference between both molecules lies on the charge flux contribution to the asymmetric stretching mode, which is negligible in KrF(2) but important in XeF(2). (c) 2008 Elsevier B.V. All rights reserved.

Investigating the Nature of Noble Gas-Copper Bonds by the Quantum Theory of Atoms in Molecules

We investigated noble gas copper bonds in linear complexes represented by the NgCuX general formula in which Ng and X stand for a noble gas (neon, argon, krypton, or xenon) and a halogen (fluorine, chlorine or bromine), respectively, by coupled cluster methods and modified cc-pVQZ basis sets. The quantum theory of atoms in molecules (QTAIM) shows a linear relation between the dissociation energy or noble gas-copper bonds and the amount of electronic charge transferred mainly from the noble gas to copper during complexation. Large changes in the QTAIM quadrupole moments of copper and noble gases resulting from this bonding and a comparison between NgCuX and NgNaCl systems indicate that these noble gas-copper bonds should be better interpreted as predominantly covalent. Finally, QTAIM atomic dipoles of noble gases in NgNaCl systems agree satisfactorily with atomic dipoles given by a simple model for these NgNa van der Waals bonds.

Gas-filled, flat plate solar collectors

This work treats the thermal and mechanical performances of gas-filled, flat plate solar collectors in order to achieve a better performance than that of air filled collectors. The gases examined are argon, krypton and xenon which all have lower thermal conductivity than air. The absorber is formed as a tray connected to the glass. The pressure of the gas inside is near to the ambient and since the gas volume will vary as the temperature changes, there are potential risks for fatigue in the material. One heat transfer model and one mechanical model were built. The mechanical model gave stresses and information on the movements. The factors of safety were calculated from the stresses, and the movements were used as input for the heat transfer model where the thermal performance was calculated. It is shown that gas-filled, flat plate solar collectors can be designed to achieve good thermal performance at a competitive cost. The best yield is achieved with a xenon gas filling together with a normal thick absorber, where normal thick means a 0.25 mm copper absorber. However, a great deal of energy is needed to produce the xenon gas, and if this aspect is taken into account, the krypton filling is better. Good thermal performance can also be achieved using less material; a collector with a 0.1 mm thick copper absorber and the third best gas, which is argon, still gives a better operating performance than a common, commercially produced, air filled collector with a 0.25 mm absorber. When manufacturing gas-filled flat plate solar collectors, one way of decreasing the total material costs significantly, is by changing absorber material from copper to aluminium. Best yield per monetary outlay is given by a thin (0.3 mm) alu-minium absorber with an argon filling. A high factor of safety is achieved with thin absorbers, large absorber areas, rectangular constructions with long tubes and short distances between glass and absorber. The latter will also give a thin layer of gas which gives good thermal performance. The only doubtii ful construction is an argon filled collector with a normal thick (> 0.50 mm) aluminium absorber. In general, an assessment of the stresses for the proposed construction together with appropriate tests are recommended before manufacturing, since it is hard to predict the factor of safety; if one part is reinforced, some other parts can experience more stress and the factor of safety actually drops.

Secondary emission of nanocrystalline zinc oxide

The Raman and photoluminescence (PL) spectra of nanocrystalline zinc oxide produced by mechanochemical synthesis were measured using a pulsed nitrogen laser (337.1 nm) and xenon lamp (360 nm) as excitation sources in PL measurements and a cw Nd:YAG laser in Raman measurements. PL was observed in the range 400–800 nm. The Raman spectrum of nanocrystalline (90 nm) ZnO was compared to that of coarsegrained ZnO. The Raman bands of nanocrystalline zinc oxide were found to be shifted to lower frequencies and broadened. Laser radiation was shown to cause local heating of zinc oxide up to 1000 K, resulting in photoinduced formation of zinc nanoclusters. Mixtures of zinc oxide and sodium chloride powders are heated to substantially lower temperatures. Under nitrogen laser excitation, the green PL band (535 nm), characteristic of bulk ZnO, is shifted to longer wavelengths by 85 nm. The results are interpreted in terms of light confinement in zinc oxide microclusters consisting of large number of nanocrystallites. The photoinduced processes in question may be a viable approach to producing metal-insulator structures in globular photonic crystals, opals, filled with zinc oxide.

Electron scattering and effects of sources of light on photoconductivity of SnO2 coatings prepared by sol-gel

Electro-optical properties of sol-gel derived 2 mol% antimony or niobium doped tin dioxide films have been measured. The electron density has been calculated considering all the relevant scattering mechanisms and experimental conductivity data measured in the range -197 to 25 degrees C. The results support the hypothesis that both ionised impurity scattering and grain boundary scattering have comparable effects in the resistivity of coatings, for free electron density congruent to 5 x 10(18) cm(-3). We have measured variation of photoconductivity excitation with wavelength using xenon and deuterium lamp as light sources. Results show that the main band in the photoconductivity spectrum is dependent on the spectral light source emission, the excitation peak reaching 5 eV (deuterium lamp). This band is due to the recombination process involving oxygen species and photogenerated electron-hole pairs. (C) 1999 Elsevier B.V. B.V. All rights reserved.

Oxygen related defects excitation and photoconductivity dependence of SnO2 sol-gel films with several light sources

Since oxygen vacancies act as donors in SnO2, the electrical properties are related to deviation from stoichiometric composition. Depending on stoichiometry SnO2 can be highly insulating or may exhibit fairly high n-type conductivity. Since bandgap transitions are in the ultraviolet range, its photoconductivity is strongly dependent on the excitation source. We have measured variation of photoconductivity excitation with wavelength for tin dioxide grown by dip-coating sol-gel technique using several light sources: tungsten lamp, xenon, mercury and deuterium, and present selected results. The main band is obtained in the range 3-4eV according to light source spectrum in the ultraviolet range. The presence of oxygen in the cryostat also affects the spectrum since electron-hole pairs react with adsorbed oxygen specimens. © 1999 OPA (Overseas Publishers Association) N.V. Published by license under the Gordon and Breach Science Publishers imprint.

Effect of light-curing method and indirect veneering materials on the Knoop hardness of a resin cement

This study evaluated the Knoop hardness of a dual-cured resin cement (Rely-X ARC) activated solely by chemical reaction (control group) or by chemical / physical mode, light-cured through a 1.5 mm thick ceramic (HeraCeram) or composite (Artglass) disc. Light curing was carried out using conventional halogen light (XL2500) for 40 s (QTH); light emitting diodes (Ultrablue Is) for 40 s (LED); and Xenon plasma arc (Apollo 95E) for 3 s (PAC). Bovine incisors had their buccal face flattened and hybridized. On this surface a rubber mold (5 mm in diameter and 1 mm in height) was bulk filled with the resin cement. A polyester strip was seated for direct light curing or through the discs of veneering materials. After dry storage in the dark (24 h 37°C), the samples (n = 5) were sectioned for hardness (KHN) measurements, taken in a microhardness tester (50 gF load 15 s). The data were statistically analyzed by ANOVA and Tukey's test (α = 0.05). The cement presented higher Knoop hardness values with Artglass for QTH and LED, compared to HeraCeram. The control group and the PAC/Artglass group showed lower hardness values compared to the groups light-cured with QTH and LED. PAC/HeraCeram resulted in the worst combination for cement hardness values. © 2009 Sociedade Brasileira de Pesquisa Odontológica.

In vitro lingual bracket evaluation of indirect bonding with plasma arc, LED and halogen light

Authors - Magno AFF, Martins RP, Vaz LG, Martins LP Objectives - Evaluate the shear bond strength (SBS) and the adhesive remnant index (ARI) of indirect bonded lingual brackets using xenon plasma arc light, light-emitting diode (LED) and conventional quartz-tungsten-halogen light. Material and Methods - Lingual brackets were bonded indirectly to 60 premolars divided to three groups according to the curing light used: Group 1, plasma arc for 6 s; Group 2, LED for 10 s; and Group 3, halogen light for 40 s. After bonding, the specimens were subjected to a shear force until debonding. The debonding pattern was assessed and classified according to the ARI scores. The mean shear bond strengths were accessed by anova followed by the Student-Newman-Keuls test for multiple comparisons. ARI scores were assessed using the chi-square test. Results - The three groups showed significant differences (p < 0.001), with the averages of group 1 < group 2 < group 3. Groups showed no differences regarding ARI scores. Conclusion - Bonding lingual brackets indirectly with plasma arc, during 60% of the time used for the LED, produced lower SBS than obtained with the latter. Using LED during 25% of the time of the halogen light produced lower SBS than obtained with the latter. These differences did not influence the debonding pattern and are clinically acceptable according to the literature. © 2010 John Wiley & Sons A/S.