摘要:

We present a new method which extends the use of thesingleprojection radiographic Abel method, hitherto applicable only to objects of circular and elliptical cross sections, to objects having general, noncircular symmetries. This is done by developing a new integral equation that is similar in applications to Abel’s equation, and includes it as a special case. The use of the new equation is discussed for objects having a smooth and convex cross‐section boundary (e.g., elliptic), a piecewise smooth convex boundary (e.g., bi‐parabolic), and a boundary with regions of zero curvature (e.g., polygons). Specific examples are given for each of these three classes, and analytic inverses are calculated for these cases. Also, numerical inversion of the integral equation is given, showing satisfactory results. We show that in contrast to Abel’s equation in many cases the kernel of the integral equation is non‐singular. Consequently, fairly simple inversion techniques are sufficient. Finally, theazimuthalvariation of the transmitted intensity is employed to provide a convenient and fast nondestructive evaluation test of the deviation of the radiographed object from a prescribed symmetry. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362665

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The cross sections of the1H(3He,1H)3He reaction have been measured at two different angles (20° and 30°) for3He particles incident energies between 1.9 and 3.0 MeV. This reaction offers an alternative to the classical1H(4He,1H)4He elastic recoil reaction and can be more attractive because3He incident particles induce nuclear reactions on light elements such as12C,14N, or16O. The new technique has been applied to silicon implanted with low energy hydrogen as well as a CxHy/Cu/CuxHy/Si specimen. For this multilayer, carbon, hydrogen, and carbon can be analyzed together using three different charge particle detectors. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362618

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A new method is presented for deriving the Fano factor,F, and the mean energy per ion pair,Wi, in counting gases. It is based on the technique of individual counting of single ionization electrons induced in low‐pressure gas samples by soft x‐ray photons. A correlation of the experimental data with a detailed simulation of the electron deposition and counting process permits the extraction of the Fano factor and the mean energy per ion pair values. We present data ofFandWifor C2H6and Ar/C2H6over the energy range of 100–1500 eV. The energy dependence of these parameters reflects the atomic level structure of the gases. We discuss in detail the accuracy of this technique and its advantages and limitations. Ways are proposed for improving the technique and for broadening the energy range. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362619

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Chirality may provide the coupling, and hence the conversion, between two eigen polarization modes: TE and TM modes. In this paper an analysis of mode interaction in chirowaveguides is realized, taking into account longitudinal components of polarization currents. Special consideration is given to so‐called induced polarization effects which are caused by abrupt discontinuities of longitudinal polarization currents on the boundary of chiral–nonchiral media. Because of induced polarization effects, an asymmetry in mode coupling may take place. The method of investigation may be a powerful tool for the analysis of mode interactions due to both material and geometric effects. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362620

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Differential scanning calorimetry, Raman scattering, optical absorption, and upconversion of infrared to green luminescence have been studied for Er3+‐doped lead‐germanate glass 55GeO2⋅20PbO⋅10BaO⋅10ZnO⋅5K2O (GPBZK) and lead‐tellurium‐germanate glass 30GeO2⋅30PbO⋅30TeO2⋅10CaO (GPTC). Judd–Ofelt intensity parameters of Er3+in the two host glasses were determined and used to calculate radiative transition rates and lifetimes. Values of the radiative quantum yield of the4S3/2→4I15/2transition and the infrared (797 nm) to green (547 nm) upconversion efficiency of Er3+were obtained. It has been found that the4S3/2→4I15/2radiative transition rate of Er3+in GPTC glass is about twice that in the GPBZK glass and the upconversion efficiency in the GPTC glass is about four times larger than that in the GPBZK glass. These host‐dependent properties are mainly attributed to the enhanced local field and the reduced multiphonon rate in lead‐tellurium‐germanate glass compared to lead‐germanate glass. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362621

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Using spatially dependent multimode rate equations, we perform the small signal analysis of the push‐pull distributed feedback laser. A closed form expression for the amplitude modulation response has been obtained. It shows explicitly that the resonance frequency of push‐pull distributed feedback lasers is determined by the spacing between the lasing mode and its closest antisymmetric side mode which has design implications. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362622

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Linearly polarized CO2laser beams have been used to investigate the effects of polarization on aluminum alloy weldability. Bead‐on‐plate welding tests have been performed on A5083 (AlMg4.5Mn0.7) alloy plates with quasi‐TEM00‐mode beams whose polarization direction is parallel or perpendicular to the welding direction, referred to as the parallel or the perpendicular polarization beam, respectively. Comparison of the fusion zone shapes between the two cases shows that deeper penetration, and hence more efficient optical energy coupling to the work can be obtained with the parallel polarization beam. This effect can be interpreted as being due to the higher Fresnel absorption at the keyholefrontwall for the parallel polarization beam than for the perpendicular polarization beam. It is also found that the polarization effect is much more evident in the case of He gas shielding than in the case of Ar gas shielding. The weaker polarization effects observed in the case of Ar may be due to more densely formed laser‐induced plume or plasma because of lower ionization potential and much smaller thermal conductivity of Ar when compared with those of He. Under such conditions the energy coupling may be dominated by plasma absorption (the inverse bremsstrahlung absorption). It has so far been considered that in aluminum welding, the energy coupling is dominated by the plasma absorption, because relatively strong plasma is observed in aluminum welding. But our results indicate that the Fresnel absorption process has an important role on the energy coupling mechanism under certain conditions. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362623

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Two bi‐functional chromophores have been developed which possess second‐order nonlinear optical and charge transporting properties. The second‐order nonlinearity arises from conjugated structures terminated by donor and acceptor groups and the charge transporting property results from a triphenylamine moiety. The bi‐functionality of the chromophores is confirmed by the large electro‐optic coefficients and high photoconductivities of composites containing these chromophores. These chromophores have been incorporated into an inert polymer to form photorefractive composite materials when doped with a photocharge generation sensitizer. The electro‐optic coefficients and photoconductivity of these composites have been measured as a function of the applied electric field as well as of the chromophore concentration. The dependence of the four‐wave mixing diffraction efficiency, holographic grating writing rate, and two‐beam coupling gain on the applied electric field and on the chromophore concentration have been evaluated. A quantitative relationship between the chromophore concentration and a number of photorefractive parameters including electro‐optic coefficient, photoconductivity, four‐wave mixing diffraction efficiency, and grating writing rate has been established. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362582

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

An algorithm for the enhancement of digital images is described. The algorithm is based upon the analogy that may exist between the description of a macroscopic system composed of many particles, and a digital image composed of many pixels. The analogy assumes that the intensity in a digital image fluctuates, so that the algorithm takes into account that fluctuations must decrease to a minimum in such a way that an enhanced image may be thought of as an image in an equilibrium state, leading to a quantitative criterion to stop the enhancement process. This may be taken as the starting point of a computer aided vision system, the next step being the image segmentation leading to the identification of the various patterns forming the image and which is described in a forthcoming paper. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362617

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The exceptional wave theory developed in the last two decades is applied to trigonal crystals, widely used in surface acoustic wave devices: quartz, lithium niobate, and lithium tantalate. For quartz all crystallographic orientations where one of bulk acoustic waves satisfies the stress‐free mechanical boundary conditions, have been found and plotted on the stereographic projection of a unit wave normal sphere as ‘‘exceptional wave lines.’’ The possibility of existence and the main features of pure shear, quasishear, and quasilongitudinal exceptional waves are examined in three cut families with Euler angles (0°,&thgr;,0°), (90°,90°,&psgr;), and (0°,&thgr;,90°). These orientations are analyzed both without and with piezoelectric effect by means of analytical and numerical techniques. The obligatory existence of undamped surface skimming bulk waves or leaky waves is proved for selected cuts of trigonal crystals, such as a well‐known 41°‐rotatedYcut of lithium niobate. The behavior of these waves is compared in three crystals. A simple relation between elastic moduli of trigonal crystals with point symmetry of 32 or 3mhas been derived, which allows the existence of a longitudinal type of exceptional waves. Such waves have been discovered in quartz. They are similar to those found earlier in lithium tetraborate and in both crystals they give rise to piezoelectrically coupled high velocity leaky waves with small attenuation. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.362624

出版商:AIP    

年代:1996

数据来源: AIP