摘要:

Mass‐scattering cross sections of biological specimens as determined by several authors are compared with the values calculated for carbon from the scattering theories of Lenz, Burge and Smith, and of Sarkar. It is found that the electron scattering theory of Sarkar for carbon gives a better representation of scattering by biological specimens.

ISSN:0021-8979    

DOI:10.1063/1.1709768

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The nucleation of droplets of polyethylene, polyethylene oxide, polyoxymethylene, nylon 6, poly (3,3‐bis‐chloromethyloxacyclobutane), isotactic polypropylene, and isotactic polystyrene, suspended in silicone oil, has been investigated. Under conditions of slow cooling (0.1°C/min), two different phenomena were observed; occasional droplet solidification at low degrees of subcooling, the number of droplets solidified and the temperature at which nucleation occurred being a function of the thermal melt history; and a sudden catastrophic, history‐independent, nucleation phenomenon at large subcooling. The latter event may be identified as a critical phenomenon occurring at a subcooling &Dgr;Tcwhich is only slightly dependent upon the rate of cooling.For polyethylene and isotactic polypropylene, the &Dgr;Tc's correspond closely with values obtained by other authors using different suspending media and have been identified with homogeneous nucleation. Homogeneous nucleation may also be explored by the method of kinetic‐rate measurements, with the advantage that rate constants can be evaluated directly rather than by theoretical derivation. Interfacial energies calculated from nucleation theory using the first (slow‐cooling) method, have been compared in this work with those obtained from the second (isothermal) method. Such comparison shows that the isothermal method gives values of the same order as, but somewhat lower than, the slow‐cooling method.For the other polymers, most of which are glass‐forming, it is not entirely clear that the catastrophic event corresponds with homogeneous nucleation. Whereas critical supercooling and solidification rates resemble those for homogeneous nucleation, there is evidence that the interfacial energies recovered are too low and the circumstances surrounding the catastrophic event suggest that the suspending medium may be catalyzing nucleation.

ISSN:0021-8979    

DOI:10.1063/1.1709769

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The damping of plasma oscillations in velocity‐modulated electron beams with large‐velocity spreads has been analyzed. It is shown that the observed spatial decay of rf current along the beam (a loss of the fundamental interference pattern) can be characterized by the exponential decay of one of the space‐charge waves. The exponential‐decay rates for this wave are extracted from the data of over 50 experiments which exhibited a spatial damping. The parameters of the measured ``near‐Maxwellian'' distribution of velocities (cutoff with a slow ``tail'') associated with each decay lead to the characterization of an equivalent temperature for the beam condition of each experiment. This is used to calculate the damping‐determining parameters (ratio of plasma wavenumber to Debye wavenumber,k/kD, or equivalent). The experimentally observed damping rates for a variety of beam conditions are found to vary withk/kDin a consistent fashion and are also comparable with rates extracted from electron‐beam theories. This is interpreted as a demonstration of a damping mechanism without collisions, and leads to the ascription of the decay as Landau damping in electron beams. Further experiments, in which the decay is observed as a function of the driving amplitude of the electron‐beam plasma oscillations, are studied as possible evidence of Landau damping in electron beams.

ISSN:0021-8979    

DOI:10.1063/1.1709770

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

A solution is presented to the practical problem of adapting known theoretical multicoil magnetic field generators for compensation of slightly inhomogeneous fields. Three concentric orthogonal sets of coils are used as uniform field generators. The two sets used to compensate the strong field components are each an array of four parallel, equal‐size square coils, separated by ⅓ of a side length, i.e., uniformly spaced on the surface of a cube. Each of these sets generates a magnetic field uniform to 2×10−4within the central sphere of diameter equal to 0.2 of the side length of a coil. The outer pair of coils (22 turns each) and the inner pair (13 turns each) are series connected to a single current supply. By shunting small fractions of the current from one‐half of two of the three coil sets, and by appropriate location of two pairs of small coils near the main degausser coils (also series connected), field gradients that result in variations of the order of 2×10−3of the uncompensated field over the central sphere can be compensated. Construction and performance specifications are given.

ISSN:0021-8979    

DOI:10.1063/1.1709771

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Impact shear tests of the Kolsky thin‐wafer type were used to determine the effect of temperature and strain rate on the critical resolved shear stress for slip in aluminum single crystals at strain rates of 104sec−1and in the temperature range 20° to 500°K. The aluminum deformed in a viscous manner in that the flow stress was proportional to the plastic strain rate. The behavior was found to be temperature‐dependent. The results are discussed in terms of dislocation damping models where the friction force acting on a dislocation results from, at cryogenic temperatures, electronic viscosity, and at higher temperatures, phonon viscosity. The theories predicted general agreement as to the magnitude of the observed damping but some discrepancy was found to exist between the observed and theoretical temperature dependence of the damping.

ISSN:0021-8979    

DOI:10.1063/1.1709772

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

It is known that in ferromagnetic films, the Bitter powder pattern of a Ne´el‐type domain wall is much more visible than that of a Bloch wall. The Bitter powder particles are held in place by the magnitude of the stray fields from the domain walls. Calculations on the basis of the simplest wall models show that the magnitude of the component of the Ne´el wall stray field parallel to the film surface is identical with the magnitude of the Bloch wall stray field perpendicular to the film surface. This is not found to be true if the calculations are made using a theoretical Bloch wall model proposed by Brown and LaBonte and an empirical Ne´el wall model as measured with Lorentz microscopy techniques by Fuchs. These calculations show that the ratio of the magnitude of the Ne´el wall stray field component parallel to the film surface to the magnitude of the Bloch wall stray field component perpendicular to the film surface as a function of the distance above the film surface increases almost linearly to a value of 29 at 10 000 Å.

ISSN:0021-8979    

DOI:10.1063/1.1709773

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Replication electron microscopy has confirmed that the microstructure of precious opal consists of spherical particles of amorphous silica, SiO2, packed in a cubic close‐packed array. Twinning relationships have been observed obeying the spinel twin law. Regions related by twinning correspond to the bands or striations observed under the light microscope. A specimen of common opal displayed a unique topography upon etching.

ISSN:0021-8979    

DOI:10.1063/1.1709775

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

Pack‐rolling has been utilized to roll foils of Mg, Al, Sc, Ti, V, Mn, Fe, Y, Zr, Mo, Ag, In, Nd, Gd, and Dy for use asK‐ andLIII‐edge filters in x‐ray experiments in the 1–15 keV range. Sc, V, Y, Zr, Ag were readily rolled to 0.5–1.0 &mgr; thickness; Mg, Ti, Mn, Gd, and Dy to 1.8–4.5 &mgr;; and Mo and In to about 6.0 &mgr;. Details of the rolling procedures for producing 10–20 cm2foils for monochromatizing x‐ray beams are given. The thickness uniformity of the pack‐rolled foils was found to vary from ±10% at >4 &mgr; to ±30% at <1 &mgr;, as measured by241Am alpha transmission. The successful fabrication of thin, pigmented, plastic films for Ge, SrF2, and Sc2O3K‐edge filters is briefly described.

ISSN:0021-8979    

DOI:10.1063/1.1709776

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

An easily fabricated bulk form of Josephson junction has been used as a harmonic generator. Harmonically generated power emitted by these nonresonant junctions has been directly detected at about 12 Gc/sec, corresponding to the second or third harmonic of the input frequency. The harmonic output is sensitive to the bias point in theV‐Icharacteristic and to the input power level as expected from Josephson's equations. The coupling to waveguide of the radiation emitted by the nonresonant junctions used in these experiments is superior to that achieved in earlier work using self‐resonant junctions as microwave generators.

ISSN:0021-8979    

DOI:10.1063/1.1709777

出版商:AIP    

年代:1967

数据来源: AIP

摘要:

The surface rearrangement of nickel and platinum due to (1) an electric field (build‐up) and (2) surface tension (annealing) as the net driving force, has been studied by field‐electron emission microscopy over the temperature range 510°–750°K for Ni and 550°–850°K for Pt. An Arrhenius‐type relationship between temperature and time was found in both cases, so that an activation energy could be associated with each process. It is assumed that this is an average activation energy for surface diffusion.The results of the field build‐up experiments gave an activation energy ofQf=19.1±1.9 kcal/mole for Ni over the negative field range (28–38)×106V/cm, andQf=26.3±2.6 kcal/mole for Pt over the negative field range (27–39)×106V/cm. It was possible to measure the effect of the electric field using a method devised earlier by Bettler and Charbonnier. This gave calculated zero‐field activation energies of 20.6±2.1 kcal/mole for Ni and 29.7±3.0 kcal/mole for Pt. The results of the annealing experiments (no electric field) gave an activation energy ofQ0=21.4±2.1 kcal/mole for Ni and 29.5±3.0 kcal/mole for Pt. (The foregoing indicated uncertainties are estimated over‐all limits of error.) It is concluded from the agreement between the results of the two different types of measurement that either type of measurement (for fcc metals) may be used to determine the same quantity within a 10% uncertainty.

ISSN:0021-8979    

DOI:10.1063/1.1709778

出版商:AIP    

年代:1967

数据来源: AIP