Index of content:
Volume 28, Issue 1, 01 January 1957
- SPECIAL ISSUE ON ELECTRON PHYSICS
28(1957); http://dx.doi.org/10.1063/1.1722557View Description Hide Description
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 rheniumsurfaces 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.
28(1957); http://dx.doi.org/10.1063/1.1722575View Description Hide Description
The physical principles underlying the design and performance of electron guns suitable for the generation of high‐density electron beams(beamcurrent 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 beamcharacteristics, 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.
28(1957); http://dx.doi.org/10.1063/1.1722568View Description Hide Description
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 0.023πa 0 2, 0.12πa 0 2, and 1.5πa 0 2±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.
28(1957); http://dx.doi.org/10.1063/1.1722569View Description Hide Description
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 to t −½ for lifetimes up to 104 hours. 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.
28(1957); http://dx.doi.org/10.1063/1.1722570View Description Hide Description
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.
- REGULAR ARTICLES
28(1957); http://dx.doi.org/10.1063/1.1722571View Description Hide Description
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 of p‐n boundaries.
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 (>10−4 cm2/sec), but that their flow and ultimate concentration is limited by the effective rate of generation and equilibrium concentration of vacancies chiefly at dislocations.
28(1957); http://dx.doi.org/10.1063/1.1722572View Description Hide Description
A simple, rigorous geometrical representation for the Schrödinger 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 Schrödinger equation may be written, after a suitable transformation, in the form of the real three‐dimensional vector equation d r/dt=ω×r, where the components of the vector r uniquely determine ψ of a given system and the components of ω 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 how r varies under the action of different types of ω. 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.
28(1957); http://dx.doi.org/10.1063/1.1722573View Description Hide Description
99.994% pure aluminumsingle 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.
28(1957); http://dx.doi.org/10.1063/1.1722574View Description Hide Description
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.
28(1957); http://dx.doi.org/10.1063/1.1722576View Description Hide Description
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.
28(1957); http://dx.doi.org/10.1063/1.1722577View Description Hide Description
The basic interactions of nonuniform shock waves in one‐dimensional unsteady and two‐dimensional steady flow, are analyzed by mapping into the hodograph plane, neglecting effects of third and higher orders in the shock strength. The resulting equations are linear and can be solved by straightforward numerical calculation.
28(1957); http://dx.doi.org/10.1063/1.1722578View Description Hide Description
Equations have been derived for velocity, temperature, and pressure distribution in a two‐dimensional compressible viscousvortex with a steady state component of radial flow.Radial velocity has been assumed small compared with the tangential component, and heat transfer has been neglected. The equations have been based upon the Navier‐Stokes equation in its most general form, the First Law of Thermodynamics, and the Gas Laws. A reference radius has been defined where viscous stress is zero; from this, nondimensional forms have been set up and generalized charts prepared for ready visualization and numerical applications. Because of the classical concept of viscosity as used in the Navier‐Stokes equation, results are strictly applicable to laminar flow only; a discussion is presented regarding possible application to turbulent flow.
28(1957); http://dx.doi.org/10.1063/1.1722579View Description Hide Description
The effects of sample size on the apparent properties of ferrites observed by microwavecavityperturbation methods are treated. Retardation effects associated with the propagation of electromagnetic waves in the specimen are considered. Results are given for the idealized cases of rods of infinite length and disks of infinite extent. Methods of adaptation to finite rods and disks are suggested. An approximate solution is given for the sphere which is in qualitative agreement with some aspects of the experimental observations. A formula relating the shift in resonance field to the sphere size is presented: .
28(1957); http://dx.doi.org/10.1063/1.1722580View Description Hide Description
A semiempirical method for the determination of mass absorption coefficients for the very soft x‐ray region of 5 to 50 A is described. A universal function for the absorption by K‐shell electrons and another for absorption by L and extra‐L shell electrons are given in table form to permit the calculation of mass absorption coefficients for elements of atomic number up to 36. Calculated and observed values from available absorption data are compared with those predicted by quantum theory. Tables are given for the mass absorption coefficients for nine useful wavelengths in the ultra‐soft region, 8.34, 13.3, 17.6, 21.7, 23.7, 27.4, 31.6, 36.3, and 44 A.
Fabrication of Multiple Junctions in Semiconductors by Surface Melt and Diffusion in the Solid State28(1957); http://dx.doi.org/10.1063/1.1722558View Description Hide Description
Techniques are described for the preparation of multiple junction structures in semiconductors by solid state diffusion of impurities from a region of a doubly doped crystal into an adjacent region of different impurity concentrations. The change in impurity concentrations from one region to the other is achieved initially by surface melting and regrowth, and is later modified by diffusion. Conditions which must be imposed on the doping levels, segregation coefficients, and diffusion constants of the doping materials employed will be discussed. p‐n‐p, n‐p‐n, and n‐p‐n‐p structures have been fabricated in germanium.
28(1957); http://dx.doi.org/10.1063/1.1722559View Description Hide Description
The electrical resistances of thin films of 24 metals, deposited on glass substrates (near room temperature) by evaporation, sputtering, or electroplating, have been measured before and after artificial aging by heating in vacua. Reductions in resistance of 25 to 50% as a result of aging were commonly noted. Evaporated films were reduced from approximately 2ρb to 1.3ρb (resistivity of the bulk metal) and sputteredfilms from 4–10ρb to 1.5–1.8ρb . A preferred aging temperature, specific to each metal but influenced somewhat by film thickness, was noted. This temperature agreed closely with the temperature of recrystallization of the metal. Thin metal films, as usually deposited, appear to be in a state of strain not associated with the bulk metal as it crystallizes from the melt. The application of heat energy to the film promotes the removal of strains, occluded and adsorbed gas, and the growth of the crystallites of the film. These concurrent actions reduce the electrical resistance of films of 1000 A thickness to a value usually in the range 25–75% above that of the bulk metal.
28(1957); http://dx.doi.org/10.1063/1.1722560View Description Hide Description
Secondary waves of electroluminescence exhibit specific trends with varying frequency and temperature. As has been assumed before, secondary waves are caused by effects of charge polarization inside the phosphor crystals; however, there has been no previous theoretical approach to this problem which leads to a description of the behavior of secondary waves with respect to frequency and temperature. An attempt has therefore been made to treat this phenomenon mathematically, using a simple model of charge polarization. During electroluminescent excitation conduction electrons are drawn to the anodic crystal parts where they become trapped. When the alternating field externally applied passes zero these electrons, under the influence of a polarization field, may return to luminescence centers and thus give rise to light emission. The return process depends on the trap depth, the temperature, the applied external field, and its frequency. This model can indeed describe the observed behavior, so that there seems to be little doubt as to the actual origin of secondary waves. As an interesting corollary of these considerations, the occurrence of secondary waves may be a means for determining electron trap depths and for detecting shallow traps where ``glow curves'' fail.
28(1957); http://dx.doi.org/10.1063/1.1722561View Description Hide Description
A semigraphical scheme is presented for the synthesis of a linear, finite, discrete network, in which the desired network response is prescribed in the time domain. The method is based on the approximation of the real part of a desired system function by a sequence of impulses along a contour parallel to the imaginary axis of the complex‐frequency plane. A simple scheme of error evaluation is proposed. Examples illustrating the synthesis procedure are also considered.
28(1957); http://dx.doi.org/10.1063/1.1722562View Description Hide Description
A criterion is presented for the applicability of cavity perturbation theory for measuring permeabilities and dielectric constants of materials. In the literature, the perturbation criterion is usually stated as: the rf fields in the cavity with and without the sample are approximately equal. This is the same as stating that changes in the stored energy in the cavity, upon introduction of the sample, are small. This criterion is shown to be too strong, and usually violated in experiments. A weaker but sufficient criterion, for all commonly encountered Q values, is that changes in the over‐all geometrical configuration of the rf fields must be small upon introduction of the sample. Experimentally, this means that for small sample measurements only the percentage change in the real part of the frequency must be small. Data are presented to demonstrate the above ideas.
28(1957); http://dx.doi.org/10.1063/1.1722563View Description Hide Description
The investigation of excess currents on silicon and germanium diodes has shown that inversion layers give an approximate logarithmic increase of excess current with applied voltage. Approximately ohmic excess currents are observed not only in the presence of water vapor but also in the presence of vapors of certain organic liquids. These excess currents can be shown to be due to a surface conductance which has a different bias voltage dependence from that exhibited by inversion layers. Evidence is presented which indicates that the holes induced in the liquid film (outer surface states) move in an applied field much like holes in a molten semiconductor, provided the liquid film is coherent. This added conductance is responsible for the ohmic behavior in the presence of condensable vapors.