Volume 29, Issue 9, 01 September 1958
Index of content:
29(1958); http://dx.doi.org/10.1063/1.1723425View Description Hide Description
Central‐source neutron multiplication measurements have been made on small spheres of U233, U235, U238, and Pu239. Measurements were made on four sizes of spheres, varying approximately from 1 in. to 2.5 in. in diameter. In the case of U233, only one sphere, 1.245 in. in diameter, was available. The three neutron sources, mock‐fission, Po–Li, and Po–Be, were 0.4‐in. diameter spheres. They have widely different neutron spectra. Multiplications were measured with a flat‐response long counter, and with U235, U238, and Np237 spiral fission chambers, under conditions such that the error introduced by room‐scattered neutrons was negligible. Results of the multiplication measurements are given. The measurements have been analyzed to obtain the leakage spectra from the spheres in terms of three velocity groups. The Np237 and U238 fission chambers serve as threshold detectors defining the lower energy of the second and third velocity groups, respectively.
29(1958); http://dx.doi.org/10.1063/1.1723426View Description Hide Description
Apparatus was developed by which beams of cadmium and of sulfur were simultaneously directed to a common point on a temperature controlled substrate in a demountable vacuum apparatus.Cadmium sulfide crystals were obtained. The various types of crystals: spiroidals, globules, ribbons, and needles are described together with the conditions tending to produce each type. Crystal size can be increased by repeated exposure to the beams.
29(1958); http://dx.doi.org/10.1063/1.1723427View Description Hide Description
It has been found that single crystals of the ferrimagneticgarnets are transparent to visible light. Thin sections (e.g., 0.005 cm thick) transmit sufficient light for microscopic examination. The transmitted light undergoes a nonreciprocal rotation similar to the Faraday effect. By virtue of this rotation many features of the domain structure are readily visible. A magnetic birefringence has also been observed which can be used to differentiate domains whose magnetization lies perpendicular to the line of sight. It is possible to study domain structure at any temperature below the Curie point. The effects of strains set up by the polishing procedure are discussed. Finally, mention is made of possible combined microwave‐optical experiments, as well as experiments aimed at understanding the switching process.
29(1958); http://dx.doi.org/10.1063/1.1723428View Description Hide Description
The oxygen content of liquidsilicon increases proportionally to the oxygen content of the ambient helium‐oxygen mixture for oxygen partial pressures pO2°<8 mm Hg. This observation is explained by the proportionality between the oxygen partial pressure pO2° in the ambient gas and the SiO partial pressure on the surface of liquidsilicon. For pO2°>8 mm Hg the silicon melt is covered by a film of silica and asaturation value of about 2×1018 oxygen atoms per cm3 is observed in the crystallized sample.
29(1958); http://dx.doi.org/10.1063/1.1723429View Description Hide Description
The dependence of the rate of the oxidation of silicon in oxygen‐helium mixtures on gas composition at 1410°C is discussed theoretically. At low oxygen contents of the gas, no layer of solid silica is expected to occur and the rate of attack due to formation of volatile SiO is supposed to be proportional to the oxygen partial pressure in the bulk gas. Above a critical oxygen partial pressure, solid silica may be formed and the rate of attack under steady‐state conditions may drop by several powers of ten.
29(1958); http://dx.doi.org/10.1063/1.1723430View Description Hide Description
Data have been obtained on the pyroelectric and piezoelectric behavior of samples of a polycrystallinebariumtitanateceramic in an effort to determine the possible existence of a primary pyroelectric effect. It is shown that the total pyroelectric effect is in a direction opposite to the piezoelectric contribution resulting from volumetric changes and that, as a consequence, there must exist a primary pyroelectric effect. The time constant of the samples for generation of electrical signals from heat pulses was less than 12 μsec. Practical applications of bariumtitanatesensors may be found in the measurement of heat radiation beams having large cross sections.
29(1958); http://dx.doi.org/10.1063/1.1723431View Description Hide Description
The rate of diffusion of indium in single crystals for two orientations has been measured using the radio‐active isotope In114. The diffusion coefficients in the directions parallel to the a axis and c axis are given by the Arrhenius relations Da =34.1 exp(−25 800/RT) cm2 sec−1 and Dc =12.2 exp(−25 600/RT) cm2 sec−1, respectively. The anisotropy is discussed in terms of the vacancy mechanism of diffusion.
29(1958); http://dx.doi.org/10.1063/1.1723432View Description Hide Description
The thermoelastic dissipation due to an edge dislocation moving at an arbitrary speed is calculated analytically and numerical results presented. It is found that, at usual stress levels, this mechanism has only a minor effect in limiting dislocation speed.
29(1958); http://dx.doi.org/10.1063/1.1723433View Description Hide Description
This paper presents the general equation for self‐diffusion in deforming media. Methods of analyses for determining the self‐diffusivity as a function of time for prescribed strain histories are given for special cases of linear diffusion. No unrealistic assumptions relative to the constancy of the diffusivity with time are made.
29(1958); http://dx.doi.org/10.1063/1.1723434View Description Hide Description
A reanalysis of data on the thermal conductivity of certain metal oxides has shown that the deviation from a T −1 law may be fitted to a law exponential in T −1. Thus the deviations may be ascribed to the flow of excitions. The contribution to the thermal conductivity from radiative transfer is shown to be too small to fit the observed deviations.
29(1958); http://dx.doi.org/10.1063/1.1723435View Description Hide Description
A theory describing polarization reversal in BaTiO3 c‐domain crystals based on observed time and field dependence of polarization P=f(E,t) is presented, and the activation field α as well as mobility μ are related to the constants appearing in the basic differential equation. The theory gives time and field dependence of the switching current at constant temperature. Although the theory does not imply restriction on the applied field E(t), results are given only for pulsed and sinusoidal fields. The effect of skin properties on the bulk crystal has been considered. Dependence of coercive field on applied field and frequency are readily obtained. Switching resistance is calculated.
29(1958); http://dx.doi.org/10.1063/1.1723436View Description Hide Description
The variable parameter (or parametric) principle of amplification is characterized by a typical arrangement in which a variable energy storage element, such as an inductor or a capacitor, is suitably coupled to two resonant circuits. If the value of the energy storage element is made to vary in the proper way, energy is fed from the source which drives the element (that is, the pump) to the fields of both the resonant circuits. This paper describes the behavior relative to gain, band width, and noise of this type of amplifier.
Specifically, it is shown that to increase gain, the Q of one of the resonant circuits, the one commonly called the idling circuit, must be increased or the variation in the variable reactance must be increased. The band width is inversely proportional to this Q and to the voltage gain. Hence, for high gain, the amplifier is normally a narrow band device. One of the most important sources of noise is the thermal noise originating in the idling circuit. However, in principle this source can be reduced indefinitely by making the idling frequency approach the pumping frequency or by artificially cooling the idling circuit. In this fashion very low noise figures should be possible.
The parametric principle can also be applied to producing frequency conversion with large conversion gain. The appendix presents the expressions for gain, band width, and noise figure for this application. The behavior of the converter relative to gain, band width, and noise is quite similar to that of the amplifier.
29(1958); http://dx.doi.org/10.1063/1.1723437View Description Hide Description
Spheres of copper,lead,tin,iron,aluminum,zinc,silver, and lead‐tin alloys were accelerated to velocities of 0.75 to 2.25 km/sec and impacted normally upon targets of the same material as the pellets. Conditions were maintained so that pellets lost no mass before striking the target. The target mass was large compared to the mass of the pellet, so the targets could be considered semi‐infinite.
The volume of the crater produced was found to be directly proportional to the kinetic energy of the pellet in the energy range investigated.
The penetration varied linearly with the velocity or momentum of the pellet. The area of the crater as measured in the plane of the original surface of the target was found to be directly proportional to the momentum of the pellet at the time of the impact.
In the case of the lead‐tin alloy series, a correlation was observed between the crater parameters, the phase diagram of the alloys, and various functions of the pellet mass and velocity for the following series of alloys: 100% lead; 90% lead, 10% tin;...; 10% lead, 90% tin; 100% tin.
29(1958); http://dx.doi.org/10.1063/1.1723438View Description Hide Description
Permeability, coercive force,remanence, and hysteresis and core losses of a 3% singly oriented silicon‐iron alloy are measured in the temperature range of 30° to 700°C. Their temperature dependence is interpreted in terms of either the intrinsic properties of a ferromagnetic or domain theory. The magnetization curve is analyzed on the basis of the crystallographic characteristics of the texture in the material. Domain distribution is discussed at each stage of magnetization. Application of the analysis leads to the establishment of the relationship between the temperature coefficient of permeability and the magnetization processes.
29(1958); http://dx.doi.org/10.1063/1.1723439View Description Hide Description
Analytical equations have been derived for the transmission factor applicable to the case of a rather weakly absorbing sample of circular cross section and larger than the x‐ray beam. Calculations are presented for liquid nitrogen and it is shown that under certain practicable conditions the transmission factor is constant for a rather wide range of the scattering angle.
29(1958); http://dx.doi.org/10.1063/1.1723440View Description Hide Description
When two circuits such as filters or transmission lines are coupled together by means of distributed reactances which can vary sinusoidally in time and space, energy can be converted between waves of different frequencies in a variety of ways. If the waves on the two transmission lines are characterized by frequencies ω1 and ω2 and phase constants β1 and β2, while the coupling reactances vary as ω=ω1−ω2 and β=β1−β2+Δβ, power P 1 at ω1 is converted into power P 2 at ω2 and vice versa in a manner reminiscent of waves on coupled passive circuits, except that a relationship (P 2/ω2)=(P 1/ω1) is obeyed. If the group velocities on both transmission lines are in the same direction, the direction of power transfer reverses periodically with distance. If they are not in the same direction, the power transfer increases monotonically with distance for small Δβ but reverses periodically for Δβ larger than a certain limit.
When the coupling reactance varies as given by ω=ω1+ω2 and β=β1+β2, parametric amplification is possible in the form of exponentially growing waves at frequencies ω1 and ω2 if the group velocities are in the same direction, and in a form reminiscent of the backward wave amplifier when the group velocities are not in the same direction. In both cases the excess energy is supplied by the variable coupling reactance and can be indefinitely large.
Possible applications of new principles of these coupled circuits to broad‐band frequency converters, frequency‐channel selectors, wide‐band amplifiers, tunable narrow‐band amplifiers, and oscillators are described. Noise performances of the circuits are also discussed.
29(1958); http://dx.doi.org/10.1063/1.1723441View Description Hide Description
The general solution is obtained for the index of refraction of a variable‐index, spherical lens which will form perfect geometrical images of the points of two given concentric spheres on each other. One conjugate sphere is assumed to be outside the lens or at its surface, while the other may be either inside, outside, or at the surface. If one of the spheres is of infinite radius, the lens will focus a parallel beam perfectly at a point on the other sphere.
It is shown that the index of refraction may be specified arbitrarily, subject to two simple conditions, in an outer shell of any desired thickness less than the radius of the lens. The index of the central portion is then expressed in terms of a function which is tabulated, and of an integral involving the index of the outer shell. Some properties of the general solution are discussed, and various special solutions are derived.
29(1958); http://dx.doi.org/10.1063/1.1723442View Description Hide Description
The collection of a minority gas species at the cathode in a glow discharge, termed cataphoresis, appears not to have been completely successfully explained. Observation indicates that in all cases, the minority gas segregated at the cathode, must be capable of being very effectively ionized by the vehicular gas. The recent discovery of the effective creation of ion complexes such as NeHe+ and NeA+ in pressure dependent reactions above some mm of Hg pressure by Oskam, the independent direct observation of these ions and the production of ions such as HeH+, NeH+, and AH+ with 1% H2 in the inert gas by Weimer, using the effusion mass spectrometer of Pahl, render a basis for the extension of the Druyvesteyn theory developed for metallic ions in inert gas to the general process, aid in the explanation of the Hg‐Xe anomaly observed by Kenty, and indicate certain needed corrections which are acting to make the process more efficient.
- LETTERS TO THE EDITOR
29(1958); http://dx.doi.org/10.1063/1.1723443View Description Hide Description
29(1958); http://dx.doi.org/10.1063/1.1723444View Description Hide Description