Spectral bounds for the Hellmann potential

The method of potential envelopes is used to analyse the bound state spectrum of the Schroedinger Hamiltonian H=-\Delta+V(r), where the Hellmann potential is given by V(r) = -A/r + Be^{-Cr}/r, A and C are positive, and B can be positive or negative. We established simple formulas yielding upper and lower bounds for all the energy eigenvalues.Comment: 9 pages, 2 figures, typos correcte

Thermal analysis of dissimilar weld joints of high-strength and ultra-high-strength steels

Advances in steel production processes in the last two decades have enabled the production of novel materials with improved strength, weldability and usability. Many industries are implementing these novel materials into production, primarily to benefit from the higher strength-to-weight ratio. Improved material properties are especially important for industries engaged in advanced structural engineering and applications such as construction plants, piping systems of nuclear power plants, and products in the automotive and aeronautical sectors. Another trend in modern manufacturing is increased use of dissimilar metals welding, and welding of dissimilar steel grades is becoming common in the economically critical energy sector. Dissimilar welding of high-strength steels is especially advantageous for regions with extreme weather conditions, such as sub-Saharan Africa and Arctic regions. When dissimilar joints for high strength steel (HSS) are welded, gas metal arc welding (GMAW) is extensively used because of its adaptability and controllability (control of input and output) and well-established production quality. The most important issue in dissimilar welding of HSS is control of the thermal cycle, as these steels have rather narrow process parameter windows and are prone to softening in the heat-affected zone (HAZ). This thesis addresses the issue of improving the weld quality of dissimilar material welds through improved understanding of the relationship between the welding parameters and resulting microstructure determining the mechanical properties of the joint. The materials used in this thesis belong to the classes of high strength steels, ultra-highstrength steels (UHSS) with a static strength of 690 to 960 MPa, steels manufactured by a thermo-mechanically controlled process (TMCP) and quenched and tempered (QT) steels. A specific objective is to define favourable heat input values that can improve the quality of dissimilar joints of S690QT-TMCP and S700MC-S960QC. The main difficulty in welding of dissimilar HSS is control of HAZ softening on both sides of the joint. Many factors have an influence on HAZ characteristics, e.g., welding parameters, filler wire composition, and groove geometry. Therefore, selecting parameters that produce the desired properties in both materials being joined is very important. An appropriate choice of parameters results in improved microstructural constitution and mechanical properties. The research has been carried out using three methods: literature review, numerical modelling and experimental validation. The literature review formed the first part of the study and examined thermal effects on the microstructural constituents and mechanical properties of dissimilar HSS welds. A numerical model of the thermal cycle was developed in second part of the study to understand the effect of cooling rate on changes in microstructure. The third part of this thesis comprised experimental validation of the influence of cooling rate on the properties of the weld joints. The literature analysis demonstrated the feasibility of developing a numerical model predicting the thermal cycle of dissimilar welds made with GMAW by improving understanding of thermal transfer in the HAZ. In the experimental part, the thermal cycle data obtained during numerical modelling were applied to dissimilar welds of HSS and UHSS. Based on analysis of numerical and experimental data, optimum welding conditions were proposed, and their accuracy further validated with standard mechanical testing: Vickers hardness testing, tensile testing, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) mapping. Microscopic analysis of the specimens was used to determine the process parameters having the most significant influence on the microstructure and mechanical properties of the welded joint. Analytical and experimental approaches to control heat transfer in the weld joints of dissimilar HSS have been developed and experimentally validated in this thesis. The findings enable weld quality to be improved and the microstructural constituents and mechanical properties of the joints to be optimised by precise control of heat input. The proposed approach allows the number of tests needed for welding parameters definition to be reduced by providing an improved understanding of the effect of heat transfer on microstructure characteristics

HARD PHOTON PRODUCTION IN NUCLEUS-NUCLEUS COLLISIONS AT 30 MeV/u AND 44 MeV/u

Doubly differential cross-sections for Bremsstrahlung production have been measured in the reactions 40Ar + 197Au at 30 MeV/u and 86Kr +12C, AgNat and 197Au at 44 MeV/u. A qualitative analysis of the characteristics of the γ-ray emission suggests strongly that the initial proton-neutron collisions are the main source of nuclear Bremsstrahlung

Generalized Ladder Operators for the Dirac-Coulomb Problem via SUSY QM

The supersymmetry in quantum mechanics and shape invariance condition are applied as an algebraic method to solve the Dirac-Coulomb problem. The ground state and the excited states are investigated using new generalized ladder operators.Comment: 9 pages, Revtex. Preprint CBPF NF-032/0

Limitation in detecting African cassava mosaic geminivirus in the lignified tissues of cassava stems

Field-infected cassava stems whose leaves were identified with differential symptoms of African cassava mosaic geminivirus (ACMV) disease were assessed for index of severity of symptoms (ISS) todetermine their infection status by scoring young resultant plants in the laboratory. Extracts of young stem tissues and leaves of emergent shoots from topped stems were also assayed for occurrence ofACMV by the enzyme-linked immunosorbent assay (ELISA). Geminivirus disease incidence and severity of symptoms were most abundant on leaves of plants from cuttings of field-symptomatic (I) stems of the moderately-resistant TMS 4(2)1425 (63.2%) and the susceptible TMS 60506 (51.1%) genotypes while those from apparently symptom-free (H) stems of the resistant genotype TMS 30001 recorded significantly the least disease (0%). Similarly, the index of severity of symptoms on all plants (ISSAP) and diseased plants only (ISSDP) were significantly highest on leaves of TMS 4(2)1425 (2.42 and 2.83), and were significantly the least (1) on leaves of TMS 30001, respectively. ACMV was not detected inextracts of all sections of lignified cassava stems by ELISA as all absorbance values were below threshold (0.0890). However, the virus was detected at greater concentrations in leaves of emergentaxillary shoots regenerating on topped plants of all node types of TMS 60506 and TMS 4(2)1425 as well as the base (node 1) of TMS 30001 stems but not on those from middle and uppermost nodes (10 and20, respectively) of the latter genotype. The highest absorbance values were recorded on shoots on node 1 of TMS 60506 (0.1720 ± 0.096), TMS 4(2)1425 (0.1640 ± 0.115) and TMS 30001 (0.1580 ± 0.080) inthat order, while the least values were on nodes 10 (0.0298 ± 0.020) and 20 (0.0289 ± 0.019) of TMS 30001

Suppression of soft nuclear bremsstrahlung in proton-nucleus collisions

Photon energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy nuclei to investigate the influence of the multiple-scattering process on the photon production. Relative to the predictions of models based on a quasi-free production mechanism a strong suppression of bremsstrahlung is observed in the low-energy region of the photon spectrum. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium.Comment: 12 pages, 3 figures, submitted to Phys. Rev. Let

Exploration de la notion géométrique d'aire des polygones par un groupe de transformations du plan eudidien

Québec Université Laval, Bibliothèque 201