摘要:

Details of the image formation in the low temperature field ion microscope are discussed. The hopping height of the rebounding gas atom, which depends on the atom's polarizability, the tip temperature, tip radius, and field, is significant for the resolution. Photographs of tungsten and rhenium surfaces with the atomic lattice resolved and in different states of disorder are presented. A color printing technique, which permits finding quickly a few displaced atoms among the many thousand that are visible, is described.

ISSN:0021-8979    

DOI:10.1063/1.1722557

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

The physical principles underlying the design and performance of electron guns suitable for the generation of high‐density electron beams (beam current density of the order of 200 amp/cm2) are discussed. Initiation of the flow of a cylindrical electron beam focused by a uniform magnetic field is described, with emphasis on the design of the electron gun. The principles underlying the Pierce design procedure for space‐charge limited electron guns is treated, including the limitations and the modifications necessary when used for very high‐density beams. Means are suggested for partial compensation of the electric field distortion due to the anode aperture of the gun. The effects on the beam and resulting limitations in gun design due to thermal velocities of emission are described.The experimental techniques for study of gun and beam performance are described, together with some results of these studies. These techniques include detailed measurement of beam characteristics, such as current density and transverse velocity distributions, both with and without a magnetic field. These measurements illustrate a number of deviations of actual electron flow from that usually considered in the simple theories.

ISSN:0021-8979    

DOI:10.1063/1.1722575

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

The formation of positronium in gases can be investigated by observing the energy spectrum of the annihilation radiation. The increase in positronium formation due to a static electric field has been measured in various gases. For the rare gases the increase has a sigmoid dependence on the ratio of electric field to pressure. The fraction of the positrons that form positronium increases by a maximum factor of 1.5, 1.4, and 2.1 for He, Ne, and A, respectively. A similar increase was observed for H2, D2, and N2. No effect of the electric field was found in the polyatomic gases CO2, CH4, C2H6, and CCl2F2, but a small anomalous decrease in positronium formation was seen in SF6.The increase in positronium formation occurs in the presence of an electric field because low energy positrons, which die by direct annihilation with atomic electrons without positronium formation in the absence of an electric field, can gain energy from the electric field to cross the threshold energy for positronium formation in the gas. A theory based on the Boltzmann equation describes the behavior of the positrons under the combined influence of the electric field and molecular collisions, and provides the basis for obtaining from the observed data values of the elastic scattering cross section of positrons by the rare gas atoms. The cross sections are 0.023&pgr;a02, 0.12&pgr;a02, and 1.5&pgr;a02±25% for He, Ne, and A, respectively. Quantitative interpretation for the polyatomic gases is more difficult.An optical search for the 2430 A Lyman line from positronium is described.

ISSN:0021-8979    

DOI:10.1063/1.1722568

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

By the use of radioactive tracer techniques it is shown that the strontium evolution from an oxide coated cathode provides a measure of the rate of reduction of the coating. For the cases of carbon, aluminum, manganese, and tungsten the results indicate that the rate of reduction of the coating is controlled by diffusion of the reducing agents from the base nickel. The rate at which common reducing agents (excluding carbon) reach the nickel‐coating interface is proportional tot−½for lifetimes up to 104hours. For oxide coated cathodes operated without current drain the thermionic emission as measured by pulse means is observed to correlate with the rate of reduction of the coating. The development of thermionic activity can be understood in terms of a model in which the concentration of donors in the oxide particles is controlled by the surface concentration of the alkaline earth metals. This surface concentration is controlled by the rate of reduction of the coating, with the further postulate that there is a maximum effective surface concentration.

ISSN:0021-8979    

DOI:10.1063/1.1722569

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

The permeation process consists of the steps: adsorption, solution, diffusion, and desorption. These are discussed for glasses, polymers, and metals. There are two generalizations: (1) All gases go through all polymers. (2) No rare gas permeates any metal. Criteria are given for measurements of the permeation process.In glasses the permeation rate depends on atomic size and glass composition. Some special processes which may simulate permeation in vacuum devices are discussed: oxide dissociation, electrolysis in ceramic materials, nuclear reactions, and ionic impact.

ISSN:0021-8979    

DOI:10.1063/1.1722570

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

The effect of dislocations in Ge upon the diffusion of Cu has been studied using Ge crystals having different etch‐pit counts as well as on Ge specimens bent on a (112) axis. The percentage of the total Cu introduced in a given time which attains the acceptor state was measured by comparing conductivity to radioactivity determinations employing Cu64. In addition, the detailed diffusion processes have been studied as a function of time and etch‐pit density by means of autoradiographs taken both parallel and perpendicular to the direction of diffusion and also by delineation ofp‐nboundaries.The two most important findings are: (1) the slow attainment of acceptor equilibrium especially in Ge having low concentrations of dislocations and (2) the identification of dislocations as the initial loci of acceptor copper in Ge.The results call for a revision of our ideas on the mechanism of diffusion of Cu. The view is advanced that Cu atoms enter Ge interstitially at a high rate (>10−4cm2/sec), but that their flow and ultimate concentration is limited by the effective rate of generation and equilibrium concentration of vacancies chiefly at dislocations.

ISSN:0021-8979    

DOI:10.1063/1.1722571

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

A simple, rigorous geometrical representation for the Schro¨dinger equation is developed to describe the behavior of an ensemble of two quantum‐level, noninteracting systems which are under the influence of a perturbation. In this case the Schro¨dinger equation may be written, after a suitable transformation, in the form of the real three‐dimensional vector equationdr/dt=&ohgr;×r, where the components of the vectorruniquely determine &psgr; of a given system and the components of&ohgr;represent the perturbation. When magnetic interaction with a spin ½ system is under consideration, ``r'' space reduces to physical space. By analogy the techniques developed for analyzing the magnetic resonance precession model can be adapted for use in any two‐level problems. The quantum‐mechanical behavior of the state of a system under various different conditions is easily visualized by simply observing howrvaries under the action of different types of&ohgr;. Such a picture can be used to advantage in analyzing various MASER‐type devices such as amplifiers and oscillators. In the two illustrative examples given (the beam‐type MASER and radiation damping) the application of the picture in determining the effect of the perturbing field on the molecules is shown and its interpretation for use in the complex Maxwell's equations to determine the reaction of the molecules back on the field is given.

ISSN:0021-8979    

DOI:10.1063/1.1722572

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

99.994% pure aluminum single crystals were deformed in tension at 4.2°K, 78°K, and 295°K. The deformation at 4.2°K produced fine slip, observed by electron microscopy, and gave rise to a linear stress strain curve. At 78°K at low strain one again observed fine slip and a linear stress strain curve; at large strains (resolved shear strain above about 0.25) a weak slip band structure developed progressively and the slope of the stress strain curve decreased. The stress strain curves were orientation dependent at all temperatures.

ISSN:0021-8979    

DOI:10.1063/1.1722573

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

The effect of particle‐size variations on the detonation velocity of the two‐component solid explosive, Composition B, has been studied at various diameters and shown to be important. An attempt has been made to apply the diameter‐effect theory of Eyring and that of Jones to explain the experimental data, but without complete success.Certain particle‐size effects in Composition B have been shown to persist to infinite diameter, and a plausibility argument in terms of the Kirkwood‐Wood theory has been offered.The presence of large crystals in low‐density RDX has been shown to lead to a ``super velocity'' which still prevails at infinite diameter. Experiments using large spheres of Composition B have been performed which also demonstrate a ``super velocity'' which may be related to the effect found in RDX.

ISSN:0021-8979    

DOI:10.1063/1.1722574

出版商:AIP    

年代:1957

数据来源: AIP

摘要:

Description is given of a negatively biased bare wire probe with suitable amplification and pulse‐height discrimination to be used in detecting flame fronts in turbulent motion. Typical measurements are given of a Bunsen flame's most probable position, its rms displacement from this position, and its degree of continuity during unstable burning. The treatment of data and the precision and limitations of the method are discussed.

ISSN:0021-8979    

DOI:10.1063/1.1722576

出版商:AIP    

年代:1957

数据来源: AIP