Side Walls Of Cavity Research Articles

Determination of the dose distribution of therapeutic electrons at interfaces

The variation of dose close to tissue/inhomogeneity or tissue/air interfaces was investigated in slab and anthropomorphic phantoms for electron beams between 4 and 20 MeV. Effective source‐surface distance (ESSD), beam obliquity, backscatter from internal interfaces, and dose to the sidewalls of air cavities were investigated using an Attix ion chamber, different thicknesses of thermoluminescent dosimeters, and EGS4 and MCNP Monte Carlo simulations. Surface dose was found to vary due to the SSD and beam obliquity. Relative surface dose decreased as the SSD increased due to the effect of low energy scattering from the cone and ESSD determined at the surface differed from the ESSD measured at The difference was found to be most significant for the combination of high energy beams and small field sizes. Surface dose increased with angle for obliquely incident beams from approximately 30° up to a maximum, which was found at approximately 80°, and then decreased rapidly. This dose maximum angle increased with increasing electron beam energy for both surface and near surface depths. Dose variations at internal tissue/inhomogeneity interfaces are due to disequilibrium of electron scatter. The backscatter of electrons (EBF) from inhomogeneities was found to be dependent on the effective atomic number and relative electron density of the scatterer. Where dose upstream of the scatterer was reduced due to a lack of backscattered electrons. EBF was dependent on the mean energy of the beam incident on the interface, but was independent of the shape of the energy spectrum. The energy of backscattered electrons at the interface was generally low, but increased with increasing atomic number of the inhomogeneity and was related to the upstream extent of backscattered electrons. Dose variation along the side wall of a cavity was dependent on the angular distribution of the beam entering the cavity and dimensions of the cavity.

Experimental investigation of high Si/Al selectivity during anisotropic etching in tetra-methyl ammonium hydroxide

This work contributes to the understanding of anisotropic etching of silicon, for microsystems technology, by studying Si/Al selectivity during anisotropic etching of silicon in tetra-methyl ammonium hydroxide (TMAH). By under-etch experiments using a wagon-wheel mask pattern, Si/Al selectivity is studied in relation to trends in etch anisotropy, etched surface morphology, and variations of under-etch behavior with mask-edge angle. TMAH at 5 wt % is used, with or without the additives: dissolved silicon and ammonium persulfate. High Si/Al selectivity is accompanied by obvious changes in the roughness, flatness, and etch rate of the {100} cavity bottoms, by large changes in anisotropy as seen in under-etch rate curves, by lower ratio of {101} to {100} etch rates, and by more regular 〈101〉-oriented steps on non-{111} cavity sidewalls. The conditions are consistent with a low rate of attack of 〈101〉-directed periodic bond chains in the Si lattice.

Full-wave analysis for antennas with mixture structure of planar elements and waveguides

Interest in planar antennas for small satellite communication earth terminals is growing rapidly. A planar element-waveguide mixture structure antenna is analyzed using a spectral domain approach and eigenfunction technique. The difficulty in satisfying the boundary conditions on the side walls of cavities and waveguides in the spectral domain is overcome. The confidence in our method is strengthened further by the convergence of the mode series and the fine agreement between the theoretical and experimental results.

Modellization of losses in TE/sub 011/-mode waveguide bandpass filters

A mode-matching technique for the analysis of TE/sub 011/ mode waveguide cylindrical bandpass filters including losses is presented. The modes of a lossy radial waveguide are derived and the generalized scattering matrix of the lossy cavity coupled by two rectangular apertures is computed enforcing an impedance boundary condition on the cavity sidewall. Cavity sidewall losses as well as top and bottom wall losses are therefore taken accurately into account. Numerical and experimental results are given for a four cavity filter in Ka band. >

Quasi-TEM study of microshield lines with practical cavity sidewall profiles

This paper presents the quasi-TEM characteristics of microshield lines with practical cavity sidewall profiles. A conformal mapping method is used for the derivation of the electrical parameters of the structures. In this study, numerical results for the characteristic impedances of air-suspended microshield lines with both positive and negative sidewall slopes are presented. Simple and explicit CAD-oriented expressions are proposed for the design and analysis of rectangular-shaped microshield line. Comparisons are made between the results obtained by these formulas and by a standard numerical technique. Furthermore, the sensitivities of the electrical parameters of a rectangular-shaped microshield line to an imperfect sidewall etching process, leading to nonvertical sidewall profiles, are also examined.

Cavity-restrained multistacked dielectricomnidirectional antenna for microwave andmillimetre-wave wireless communications

A novel microwave omnidirectional antenna is proposed for wireless communications. This antenna is constructed with cavity-restrained multistacked dielectric discs. Vertical polarised omnidirectional radiation patterns are obtained from radiative ring slots in the side wall of dielectric-metal cavities operating in the TM01δ mode. High omnidirectional gain is realised with stacked cavities with multiradiative slots. Ring slots between the adjacent cavities are used to ensure the excitation of the desired radiating mode in phase, which actually eliminates the feed network. A special technique is adopted for excitation of the antenna from the coaxial line, with which very good matching is achieved.

Heat transfer at the base of a cylindrical cavity oriented perpendicular to a freestream flow

The average and local heat transfer at the base surface of a cylindrical cavity whose opening is perpendicular to an oncoming freestream have been determined experimentally. During the course of the experiments, the cavity depth was varied from zero to 65% of the diameter, while the freestream Reynolds number ranged between about 5000 and 50,000. The average heat transfer coefficient decreased sharply with increasing cavity depth, the reduction being slightly greater than 50% at a depth-diameter ratio of 0.10 and greater than 90% when the depth-diameter ratio was 0.65. For the finite-depth cavities, the local heat transfer coefficient attained its maximum value at the center of the base surface and decreased with increasing radial distance from the center until a minimum was reached, after which there was a moderate increase adjacent to the outer edge of the base. The minimum occurred at the location where the radial outflow along the base separated from the surface, and a toroidal-shaped recirculation zone occupied the corner region at the intersection of the cavity base and side wall. In contrast, for the zero-depth cavity (i.e. a free disk oriented perpendicular to the oncoming flow), the local coefficient attained its minimum at the center of the surface and increased with increasing radial distance from the center.

Low Reynolds number flow over cavities

Low Reynolds number flow over rectangular cavities is analyzed. The problem is posed to simulate towing tank experiments of Taneda [J. Phys. Soc. Jpn. 46, 1935 (1979)]. A very good agreement exists between numerical and experimental results. The dividing streamline separates from the cavity side wall below the upper corner. The separation point moves toward this corner when the aspect ratio decreases. Flow structure inside the cavity changes considerably with the aspect ratio. Only corner vortices exist in a cavity of aspect ratio W/h=4.0. A decrease of the aspect ratio leads to the enlargement and eventual merger of these vortices. The merger begins with the formation of a stagnation point separating two vortex centers inside the cavity. These vortex centers become progressively weaker and merge to form a single central vortex in a cavity of aspect ratio W/h=2.0. Further decrease of the aspect ratio results in the enlargement of the new corner vortices and their eventual merger. This process begins in a cavity of aspect ratio between 0.6 and 0.575 and is finished in a cavity of aspect ratio 0.5, where two central vortices have been identified. The further decrease of the aspect ratio leads to the sequential repetition of this process and creation of additional central vortices.

Acoustical wall panel

An acoustical wall panel has a core of a plurality of layers of glass fiber pressed together and having a density ranging from approximately two pounds per cubic foot at the front surface to approximately six pounds per cubic foot at the rear surface. A plurality of cavities are formed on the front surface of the core. The front of the panel is covered with fabric, and the rear is covered with a septum to provide a barrier to the transmission of sound pressure waves. The cross-sectional area, depth and spacing of the cavities are selected to improve absorption of sound in the intelligence range of speech while having less absorption in the lower frequency range to permit a desirable background or ambient noise to exist. The spacing and shape of the cavities also provides a smooth, continuously changing curvature in the cavity side walls to increase the surface area and enhance the capacity of the panel to absorb sound at flanking angles of incidence.

The development of enamel structure in rat incisors as compared to the teeth of monkey and man.

The rat incisor is an excellent model system in which to study amelgenesis. However, the information obtained has not been extrapolated to the human because of alleged structural differences between the teeth. The obvious differences include continuous eruption in rat incisors and an enamel rod pattern in rats which seemingly differs from the keyhole pattern of human enamel. A comprehensive analysis was made of those features of enamel structure considered fundamental to the understanding of its formation. This was done by applying the knowledge of amelogenesis obtained in rat incisors to the teeth of monkey and man. The following points of basic similarity were established between these species: (1) Interrod enamel is secreted first. It forms the side walls of cavities which are initially occupied by Tomes' processes. (2) The formation of interrod cavities is followed by deposition of enamel rods within these spaces. (3) The rods conform to the shape of the cavities and are secreted from one surface of Tomes' process. (4) At the initial site of rod deposition its enamel is continuous with the interrod enamel wall. (5) Growth of the rod compresses the process to one side of the cavity resulting in an arcade-shaped "space" between the rod and the remaining interrod walls. This study demonstrates that it is no longer necessary to postulate a keyhole structure for primate enamel, and it has established that a fundamental similarity exists in the basic structure and in the mode of formation of enamel in all three species.

Investigation of jet curtains for chemical laser application

A method is presented for passively containing reacting gas within the cavity of a chemical laser using a combination of cavity wall displacement and jet curtains. Suggesting several advantages over the pressure approach, the concept was evaluated in an application to a DF laser using the hydraulic analogy. Analyses were made to determine the flow characteristics of the confined and unconfined laser gas, the jet curtain requirements, and basic information for the design of the test hardware (water table). Experiments were conducted on the water table with the jet nozzle size, jet exit Froude number, and wall displacement as the variables. The results of these experiments qualitatively demonstrated the feasibility of the containment concept and demonstrated that for a given nozzle size, the ideal wall displacement is that which requires the minimum jet Froude number for containment. N a chemical laser, the power is extracted from the cavity in a transverse direction to the gas flow. This means that the optical system requires apertures in the cavity walls and that these be located downstream of the entrance to the cavity and sized to approximately span the iasing zone. The heat release within the Iasing zone will generate a static pressure rise which, when combined with the aspirating effect of the flow across the apertures, will result in a transverse pressure gradient and a lateral expansion of the laser gas. Some of this gas will enter the tunnels leading to the mirror cavities and may reach and contaminate the mirror surfaces unless measures are taken to prevent it. Reference 1 describes a method of preventing the laser gas from entering the tunnel by injecting a noncontaminating gas into the mirror cavity and thereby increasing the flow static pressure there to a point where it matches the laser gas static pressure at the aperture. This is referred to as matched pressure'* injection. This paper discusses a recently completed study of a more passive method of gas containment in which an aerodynamic barrier is used much in the same manner as described in Ref. 2. In this concept, the gas is permitted to expand freely in the lateral direction, and its capture is achieved by a lateral displacement of the cavity sidewalls. In addition, a constant Mach number inert gas jet located at each aperture and flowing across it in the direction of the laser gas serves as a barrier or curtain to prevent the latter's entrance into the mirror cavities. When wall displacement is sufficient to capture the laser gas plus a major portion of the flow from the jets, while diverting the remainder into the mirror cavity, then this containment concept exhibits the following desirable features: 1) Capturing the expanded gas rather than forcibly con- taining the unexpanded gas eliminates the introduction of shock disturbances in the Iasing zone. 2) The placement of the coflowing jet adjacent to the laser gas when coupled with an adequate wall displacement will prevent the formation of a large zone of recirculating laser gas at the tunnel entrance as a result of the latter's partial en- trance into the tunnel. Such a recirculation can bring harmful deactivants back to the start of the Iasing zone. 3) The coflowing nature of the jet minimizes its distur- bance impact upon the laser gas compared with jets used to replenish the mirror cavities for pressure injection.

Trapped radiation in injection lasers

The effect of trapped radiation on the stimulated emission threshold and power in injection lasers is investigated theoretically and experimentally. The density of trapped radiation is determined by the relation between the gain and the loss coefficients for this radiation in the active region. The last value depends upon treatment of the cavity sidewalls and laser diode geometry. The absorption of spontaneous emission in the active diode region provides for an additional optical excitation of the crystal and can result in a decrease of threshold current. The GaAs laser diodes with Fabry-Perot cavities and four-ended resonators were studied. The lasing threshold for the axial modes is increased with the diode width and that for the nonaxial radiating modes is decreased with it. The luminescence spectra from the cavity ends and sidewalls are investigated as a function of diode length and current density. It is shown that the gained luminescence can be an essential source of energy losses in injection lasers and leads to limitations of dimensions of laser diodes with planar p-n-junctions. Various methods for suppression of harmful radiation and for increase of the stimulated emission power in injection lasers are discussed.

Vacuum Resonant-Cavity Dielectrometer for in Situ Measurevents of Complex Permittivity in a Simulated Space Environment

The instrument consists of an invar cylindrical cavity of variable length, suitably excited in the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">014</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">015</sub> modes by a fixed X-band frequency of 9.28 GHz. The inside surface of the cavity is silver plated. Disc-shaped specimens are placed on top of a micrometer-driven noncontacting plunger that forms one of the end-plates. The other end-plate is a vacuum-tight flange with a window to permit particulate or ultraviolet radiation to enter the cavity and bathe the specimen. Signal coupling hoses are placed along the cavity side wall on the same plane, at a distance 3 λ/4 from the top end-plate. Input ports are two tectangular slots machined lengthwise along the major axis of the cavity and coupled 180 degrees out of phase at the points of maximum magnetic field. The output port is an orifice located halfway between the input slots. A side-arm vacuum flange below the cavity connects to a sputter-ion vacuum pump. Another similar flange provides an insertion port for the specimens. The shaft of the plunger is isolated from the chamber by a stainless steel bellows. To facilitate specimen insertion, the plunger-micrometer assembly is lowered by a rack and pinion. Precise repositioning is achieved by two conical indexing plugs. Relative dielectric constant of a specimen is obtained from the shift in cavity resonant length when the specimen is inserted. Loss tangent is obtained from the shift in half-power resonance bandwidth. Dielectric measurements under vacuum have been made with the specimen irradiated with ultraviolet light.