Volume 34, Issue 11, 01 November 1963
Index of content:
34(1963); http://dx.doi.org/10.1063/1.1729154View Description Hide Description
Large single crystals of calciumniobate Ca(NbO3)2, grown by the Čzochralski technique, are transparent and can be doped with rare earth or transition metalions. Laser action has been observed in calciumniobatedoped with trivalent neodymium, holmium, praseodymium, erbium, and thulium.
34(1963); http://dx.doi.org/10.1063/1.1729155View Description Hide Description
Exo‐electron glow curves for an oxide surface on aluminum have two principal peaks. The analysis supports the theory of thermostimulated exo‐electrons originating in traps below the conduction band. The aluminumoxide surface had two types of traps, both approximately one electron volt below the conduction band, and attempt‐to‐escape frequencies of 4×1010 sec−1 and 4×109sec−1.
34(1963); http://dx.doi.org/10.1063/1.1729156View Description Hide Description
A quantitative analysis of pulse sizes recorded in an air Geiger counter at 200 Torr was made, relating counting rates of various pulse types at different sensitivities and potentials along the counting plateau. Using this information and a determination of the onset of the self‐counting potential, data concerning counts, pulse size, distance of pulse propagation down the tube, and pulse velocities were obtained for pressures of 100, 200, 300, and 400 Torr. Triggering on the steep rise of the electrical pulse stimulated by α particles at one end of the counter, the time of passage of the luminous pulse from one photomultiplier located at the α particle gun, and a second viewing of the counter at a given distance along the axis could be recorded on an oscillograph screen. Large fluctuations of number of ions per pulse, distance of propagation down the tube, and velocities were noted. All these quantities increased with potential as the onset of steady corona was approached. There was a close correlation between pulse size, distance of propagation, and velocity. These are readily accounted for by the number of ions and photoionizing photons created in the avalanches produced by the initial triggering event.
34(1963); http://dx.doi.org/10.1063/1.1729157View Description Hide Description
It has been known for many years that cylindrical plasma columns subjected to a transverse electric field exhibit dipole resonanceeffects. Very little attention has been given to the modification of this phenomenon by a magnetic field. This paper gives a theory for the splitting of the resonance that occurs under these conditions and describes experiments carried out using mercury‐vapor discharges at a pressure of about 1 μ Hg to verify the theoretical prediction. Good agreement was obtained for magnetic fields giving an electron cyclotron frequency up to about half the geometric mean‐resonant frequency, and it is suggested that the effect provides a useful diagnostic technique for the measurement of mean electron density and magnetic field in discharge columns.
Application of the Garrett Method to Calculation of Coil Geometries for Generating Homogeneous Magnetic Fields in Superconducting Solenoids34(1963); http://dx.doi.org/10.1063/1.1729158View Description Hide Description
The design and construction of a superconducting solenoid generating a magnetic field of high homogeneity is discussed. The design is based on the Garrett mathematical description of a magnetic field produced from a set of axially symmetric coils. The experimental results for a 1.75 in. i.d. solenoid, operating at either 10 or 24 kg and possessing end‐correcting coils in an arrangement determined by theory, indicate a central field homogeneity of 1/105 over about 1 in. at 10 kg and 0.75 in. at 24 kg. At approximately 9.4 kg it was possible to clearly resolve the three chemically shifted proton NMR peaks of ethyl alcohol. This result indicated a homogeneity of 1/106 over the sample at a single point in the accessible field volume.
34(1963); http://dx.doi.org/10.1063/1.1729159View Description Hide Description
A method is given for calculating the potential distribution of a system consisting of two coaxial cylinders, each of unit radius, separated by a distance of 2a along the axis. The potentials of the left‐ and right‐hand cylinders are taken as −1 and +1, respectively. The problem can be solved if the potential φ(1,z) on r = 1 or its inverse Fourier sine transformA(λ) is obtained. The dual integral equations for A(λ) are derived from the boundary condition on the cylinders and the continuity condition of the potential and of its normal derivative on the gap (r = 1, |z|≤a) between the cylinders. The solution A(λ) is expressed by the following series: .The coefficients an in this series satisfy an infinite set of linear equations. The values of an for several values of the half‐gap length a are calculated by successive approximations and are shown in a table.
34(1963); http://dx.doi.org/10.1063/1.1729160View Description Hide Description
The mixing of the first four axial‐mode frequencies of a ruby laser in a bulk, room temperature, CdSe crystal to obtain microwave beat frequencies in the 1–5 Gc/sec range is described. A photoconductive mixing analysis based on a monomolecular recombination process is found to give excellent agreement with the experimental data. The beat frequency power was found to vary as the square of the dc bias voltage applied across the crystal, the square of the incident light power, and inversely as the square of the beat frequency. A minimum lifetime of 10−9 sec, a cross section of 1.3×10−16 cm2, a modulation index of 0.2, and a maximum beat frequency power of 0.2 mW were obtained in the experiments.
34(1963); http://dx.doi.org/10.1063/1.1729161View Description Hide Description
Emission from most practical electron sources is nonuniform over the emitting area. Ordinary metallic surfaces are polycrystalline and their emission comes from many different small crystal facets whose work functions can vary over a wide range of values. As an example, the range of work functions of tungsten goes from a low of 4.3 eV for the (116) face to a high of 5.9 eV for the (110) face. Substantially improved uniformity of work function and current density would result if the entire emitting surface contained only a single‐crystal plane. The use of a single crystal as a means of obtaining such a surface is costly and wasteful since its unique bulk properties are superfluous and do not in any way contribute towards the improved performance. Even more important, using a single crystal inherently limits one to planar geometries.
Many polycrystallinematerials of arbitrary shape, among them the refractory metals, when prepared by chemical vapor deposition can be made to exhibit a high degree of uniaxial crystal orientation with their preferred axis centered about the direction of growth. Given proper treatment after deposition, the surface of such materials predominantly consists of a single‐crystal plane.
In this paper, X‐ray, optical, and thermionic‐emission measurements are described which were made on vapor‐deposited tungsten prepared by the hydrogen reduction of tungsten hexafluoride. By comparing volt‐ampere characteristics of high‐vacuum diodes, one of which contains a conventional tungsten filament and the other a chemically vapor deposited one, of identical dimensions, it is found that there is an expected improvement in emission uniformity due to the uniaxial grain orientation in the latter.
By a similar comparison of cesium‐neutralized energy‐converter diodes, it is shown that the improved surface uniformity results in significantly improved power quadrant characteristics.
34(1963); http://dx.doi.org/10.1063/1.1729162View Description Hide Description
Discharge currents which flow in carefully distilled hexane on the removal of the applied potential were investigated for predischarge stresses ranging from 10–750 kV/cm. The duration and shape of the discharge characteristics were found to depend on the electrodematerial, on the surface state of the cathode, and on the predischarge stress. It is believed that the discharge currents are the result of a redistribution of charges in the gap in the absence of an external field. Using a model based on the theory of simple diffusion within a liquid bulk, an equation is developed which shows good agreement with the experimental results.
34(1963); http://dx.doi.org/10.1063/1.1729163View Description Hide Description
An expression for the output power of a laser in terms of the measured single‐pass gain is discussed for two cases: homogeneously and inhomogeneously broadened spectral lines. The results are applied to the 6328‐Å laser line in the helium‐neon mixture, but only an order‐of‐magnitude calculation is possible because of the lack of data. Finally, several of the assumptions made during the derivation are discussed.
34(1963); http://dx.doi.org/10.1063/1.1729164View Description Hide Description
The spatial distribution of the energy emitted by GaAs lasers having Fabry‐Perot resonators was investigated. In general, the diodes emit several fan‐shaped beams of comparable magnitude at angles up to 10° with respect to the normal of the mirrors in the junction plane. These beams have widths from ¾° to several degrees in the plane of the junction and about 10° in the plane perpendicular to the junction. The radiation patterns are usually asymmetric about the normal to the polished faces of the diode and this asymmetry is mirrored on the opposite side of the diode. We conclude that because our Fabry‐Perot mirrors are not perfectly parallel, the most prevalent modes of our diodes illuminate the radiating surface with phase and intensity distributions which vary strongly along the junction. This variation is frequently nearly periodic.
34(1963); http://dx.doi.org/10.1063/1.1729165View Description Hide Description
Between about 0.1 and 20 mm Hg, ion concentration is related to the current collected by a negative cylindrical probe by the equation (in practical units) ,where λ e is the electron mean free path, μ is the ion mobility,Vp and rp are the probe voltage and radius, and i is the probe current per unit length. The plasma is not perturbed because the probe current is supplied by ionization occurring at the sheath boundary.
Using this equation, the following values of the dissociative‐recombination coefficient α in cm3/sec were obtained from pulse discharge afterglow measurements: argon, 3.6×10−7 for high excitation and 1.4×10−6 for moderate excitation; oxygen, 2.0×10−6; mercury, 3.5×10−6. For nitrogen α was found to be proportional to pressure and equal to 1.7×10−7 at 1 mm Hg. For cesium the above equation does not apply, and the value α = 1.7×10−7 was obtained from experimental results using conventional probe theory. The frequency of three‐body conversion of atomic to molecular ions was found to be 151 sec−1 for argon and 37 sec−1 for helium.
34(1963); http://dx.doi.org/10.1063/1.1729166View Description Hide Description
Modification of a conventional light‐scattering photometer for measurements in the near ultraviolet is described. As a test of the instrument, unperturbed dimensions are determined for a series of polystyrene fractions covering a thirtyfold range of molecular weight. Values of the Flory hydrodynamic parameter are found to be nearly independent of molecular weight, averaging 10−21 Φ = 2.65±0.24, but there is a fairly definite indication that the polystyrene chain is non‐Gaussian for molecular weights below 3×105. Use of a 313‐mμ wavelength extends the measurement of radii of gyration of coiling molecules down to 100 Å, which is about the upper limit which has been achieved by low‐angle x‐ray diffraction.
34(1963); http://dx.doi.org/10.1063/1.1729167View Description Hide Description
In a rotating coordinate system the Coriolis force and centrifugal force can account for the curved trajectories followed by moving particles. It is shown that particle trajectories diverging from a source fixed in the rotating system may come to a focus at another point fixed in the system. Both first‐ and second‐order focusing are obtained and an example of each is given. Expressions are derived for velocity dispersion and for the transverse aberration at the focus.
34(1963); http://dx.doi.org/10.1063/1.1729168View Description Hide Description
This work describes a simple apparatus used to measure power spectra of Barkhausen noise in an analysis frequency band between 25 and 104 cps. Because previous work by the authors showed that when the specimen is in the form of a closed magnetic circuit and unlaminated the intensity of the noise has but small dependence on the core dimensions, unlaminated ring‐shaped samples were used in this work. Spectra obtained with a magnetizing field frequency of 0.1 cps are given for iron, 6% siliconiron, grain‐oriented siliconiron, ultraperm, mumental, nickel, and Permalloy C. It is observed that the intensity of the noise depends to a large extent on the nature of the ferromagnetic material. No interpretation of this fact is given, as it appears very complicated to understand.
34(1963); http://dx.doi.org/10.1063/1.1729169View Description Hide Description
The inductive behavior of a closed system of superconductors is investigated with the purpose of attaining high magnetic fields. High‐field superconductors do not exhibit the Meissner effect, and the inductive behavior of a system formed of these materials can be predicted fairly accurately from the conservation law of flux linkage which follows from the zero resistivity. Simple flux shielding or trapping is quantitatively analyzed for a tubular geometry. The flux concentration scheme is discussed for several practical configurations. In this scheme, a low field over a large volume (externally applied field) is compressed into a smaller volume to achieve the desired intensification in field strength. The energy of the external magnetic field can be utilized more effectively by flux pumping, but this requires intricate switching sequences.
34(1963); http://dx.doi.org/10.1063/1.1729171View Description Hide Description
A region of dielectric dispersion has been found in ferroelectric lead titanate zirconate ceramics at frequencies about 1Gc/sec. The losses associated with the dispersion are quite high, the dissipation factor rising to about 0.4 at 2Gc/sec. The magnitude of the dispersion is halved in the measured frequency range (500 Mc/sec to 2 Gc/sec) by electrical orientation (poling) of the ceramic. Two mechanisms which may account for the effect, inertial clamping of the piezoelectric response of ferroelectric domains and clamping of domain‐wall motion, are discussed.
34(1963); http://dx.doi.org/10.1063/1.1729172View Description Hide Description
The asymptotic value of the isothermal thermomagnetic figure of merit is obtained for a two‐band multivalleyed semiconductor or semimetal where the concentration of holes and electrons is equal. The bismuthband structure is used as a specific model. It is shown that at strong magnetic fields the isothermal Nernst coefficient Nji i is independent of the magnetic fieldBk . The isothermal electrical resistivity is proportional to (Nji i )Bk 2 and the isothermal thermal conductivity is equal to the sum of the lattice thermal conductivity κ ii 0 and the strong field ambipolar contribution (κ ii i )amb, where (κ ii i )amb is proportional to Nji i . The isothermal Ettingshausen coefficient Eji i is also obtained and it is shown that κ ii i Eij i = −Nji iT. The largest figure of merit is obtained when the electron and hole mobilities are large and approximately equal, when the lattice thermal conductivity is small, and when the Fermi energy is approximately equal to +kT.
34(1963); http://dx.doi.org/10.1063/1.1729173View Description Hide Description
The nucleation and movement of 180° domain walls in BaTiO3 crystals with salt‐water electrodes has been observed directly by partial switching and repeated differential etching at applied fields from 2 to 450 kV/cm. Most of the polarization was found to be reversed by sidewise growth of new domains at these fields, as it is at lower fields, but the sidewise wall velocities are higher, as high as 105 cm/sec. The nuclei are originally circular cylinders, becoming squared up by sidewise growth whose lowest velocity is along the 100 crystal axis. The sidewise wall velocity and the nucleation rate each vary as E 1.4, thereby causing the known E −1.4 variation of ts . The nucleation rate dependence is not understood, but the wall‐velocity dependence agrees strikingly with a previously uncalculated prediction of the Miller‐Weinreich theory. This indicates that, at both high and low applied fields, the apparent sidewise motion of 180° domain walls is due to the nucleation and forward growth of knife‐shaped steps on the previously existing wall.