Index of content:
Volume 26, Issue 8, 01 August 1955
26(1955); http://dx.doi.org/10.1063/1.1722139View Description Hide Description
Methods of treating the Boltzmann equation for the case of a sinusoidally distributed source of fast neutrons in a material containing hydrogen are discussed and compared. The effects of the simplifying assumptions commonly employed in reactor calculations are analyzed. A technique which is particularly suitable for accurate numerical integration of the Boltzmann equation in the above case is described. Numerical results are being obtained by this method, and will be presented and compared with results obtained from other approximate methods in a subsequent paper.
26(1955); http://dx.doi.org/10.1063/1.1722140View Description Hide Description
The tracer diffusion of iron in 18−8 stainless steel has been measured over a wide temperature range by the surface activity decrease method, using radioactive Fe55. The results show that the lattice diffusion coefficients can be described by the equation D=0.58 exp(−67 100/RT) cm2 sec−1. The effect of variations in grain size has been determined, and it has been shown that when the grain size is small, grain boundarydiffusion contributes appreciably to the over‐all diffusion process, this effect decreasing with increasing temperature.
26(1955); http://dx.doi.org/10.1063/1.1722141View Description Hide Description
These papers, designated I and II, discuss measurements of excess Ba (and/or Sr) in practical oxide coated cathodes. More than 300 cathodes of various designs were prepared, aged for periods up to 50 000 hours, tested thermionically, and then analyzed chemically.
Despite its great importance, the accurate measurement of excess Ba in practical cathodes is a problem whose complexity has not been adequately realized. Although the methods of the present experiments avoided many difficulties, limitations still exist. The difficulties and limitations, as well as the capabilities, can best be discussed in terms of typical measurements; we emphasize this aspect in I.
26(1955); http://dx.doi.org/10.1063/1.1722142View Description Hide Description
Using methods given in paper I, emission and Ba content are given on (a) 125 coatedcathodes, (b) 15 coated filaments; H2 evolution by 19 uncoated cathodes was also measured. For analysis, each was isolated from the remainder of its tube. Coatings varied in composition from pure SrO to pure BaO and in density from 1 to 5 mg per cm2. Ba was also measured in cathodecoatings removed from their support metal. Essentially all cathodes contained excess Ba, but no cathode structure gave any correlation between Ba content and emission.
These experiments contribute the following toward understanding practical oxide cathodes: If excess Ba (or Sr) is important, the significant amounts are probably less than one atom of Ba in 106 molecules of (BaSr)O. Because essentially all cathodes, including some with extremely low emission, contained at least this amount, we believe that performance is seldom limited by excess Ba. The concentration of Ba (or Sr) was not affected by using pure BaO or SrO coatings or Pt supports, although each of these steps led to very low emission; the highest emissions were obtained by operating at relatively low temperature during life, but this retained no more Ba in the coat. These major effects require interpretation.
26(1955); http://dx.doi.org/10.1063/1.1722143View Description Hide Description
A quantitative discussion of the forward characteristic is given for point contact rectifiers. It shows that the theoretical value of d lnI/dV in the exponential region is nearly q/kT, and that the experimental slopes which are a fraction of this value cannot be the result of the spreading voltage. Recombination of injected carriers has little effect except at large currents beyond the exponential region. Relations are derived for the limit of very large currents, which show that γ approaches a value smaller than (1+b)−1, the hole concentration at the contact approaches a maximum value proportional to the equilibrium concentration of holes at the surface, and the spreading voltage becomes a linear function of the current.
26(1955); http://dx.doi.org/10.1063/1.1722144View Description Hide Description
The point‐source, spherical shock wave moving into a constant density γ‐law gas is considered in the limit of infinite shock strength from the point of view of the Richtmyer‐von Neumann viscosity technique. A similarity solution of this problem is shown to exist and is obtained for various boundary conditions with γ=1.4. The solutions are obtained analytically in that part of the flow field not involving viscosity, and numerically in the other parts of the flow field. It is found that whereas all discontinuities of the physical parameters are removed by the viscosity there remain discontinuities in the slopes of these parameters at the shock front. It is indicated, moreover, that the complete flow field depends upon the form and magnitude of the viscosity.
Geometrical Acoustics. II. Diffraction, Reflection, and Refraction of a Weak Spherical or Cylindrical Shock at a Plane Interface26(1955); http://dx.doi.org/10.1063/1.1722145View Description Hide Description
The method of a previous paper is applied to the reflection and refraction of a weak spherical or cylindrical shock wave at a plane interface and by a plane slab. The method is also extended to apply to the diffractedwave which appears in these problems whenever total reflection occurs. From the results the leading terms in the asymptotic expansions for high frequency of the fields produced by periodic point or line sources over a plane interface are also obtained. These terms, which apply to electromagnetic as well as acoustic fields, are in exact agreement with results previously obtained by much more complicated methods.
26(1955); http://dx.doi.org/10.1063/1.1722146View Description Hide Description
The position and width of the x‐ray beam transmitted through calcite in Laue diffraction has been measured. For the extinction case, the transmitted beam has the same position and width as the incident beam. For the anomalous transmission case, the transmitted beam widens showing that the radiation propagates through the crystal along the diffracting planes. These results are in general accord with the theoretical calculations of von Laue.
26(1955); http://dx.doi.org/10.1063/1.1722147View Description Hide Description
The absence of weak detonations in available experience is felt to pose a problem since the well‐known rapid condensation processes encountered in the flow of expanding gases have all the characteristics and present, in fact, examples of weak detonations. As such, they would suggest that the initiation mechanism producing weak detonations will have to be different from that commonly adopted for strong detonations. The physical aspects of rapid condensation processes are discussed and compared with the conditions obtained by Friedrichs for the existence of weak detonations.
26(1955); http://dx.doi.org/10.1063/1.1722148View Description Hide Description
Techniques for the preparation by vacuum evaporation of single thin layers and laminated structures of ferromagnetic alloys (principally in the iron‐nickel system) are described. The magnetic properties are related to those parameters of the process which may be chosen to yield materials having desired characteristics. A method for producing films with extreme hysteresis loop rectangularity is described. Questions of suitable substrate materials, evaporable dielectrics for use in insulating multilayer deposits, and control of alloy composition, are considered in detail.
26(1955); http://dx.doi.org/10.1063/1.1722149View Description Hide Description
The readjustment of the mean exit pressure to its steady‐flow equilibrium level, following the arrival of a shock wave at an open end of a duct, has been investigated. For a cylindrical duct in which the gas is initially at rest, a solution based on acoustic theory was obtained in the form of a Fourier integral. The nature of this solution was established as a series of damped harmonic oscillations superposed on an exponential pressure decay. The actual pressure decay curve was obtained by numerical integrations. The same problem was also attacked experimentally using a shock tube of 3.23 inches i.d. From pressure records taken some distance from the open end, the ``effective'' exit pressure was derived.
It was found that the exit pressure deviates appreciably from its steady‐flow equilibrium value during a time in which a sound wave could travel about three duct diameters. Satisfactory agreement between theory and experiment was observed for shock pressure ratios up to about 1.9 (in air) although, near this limit, significant deviations owing to wall friction were observed toward the end of the pressure decay region. For still stronger shocks, the influence of the decay time on the phenomena inside the duct becomes unimportant while frictioneffects dominate. The possible significance of the lag in the establishment of steady‐flow boundary conditions in practical applications is discussed, in particular, for cases where the acceleration of the gas by the reflected expansion wave may be of importance.
26(1955); http://dx.doi.org/10.1063/1.1722150View Description Hide Description
The magnetic domain structure of polycrystalline grain‐oriented 3¼% silicon steel has been investigated by the magnetic‐powder‐pattern technique and the orientation of individual grains determined by the etchpit optical‐goniometer technique. A semiquantitative relationship has been found between the domain patterns and crystalline orientation. The observed powderpatterns can be grouped into five types, depending upon the angle, φ, between the rolling plane and the <100> direction lying closest to the rolling plane. When the rolling direction is known and φ is <10°, semiquantitative grain‐by‐grain orientation information can be obtained solely from the domain patterns.
The behavior of two of the types of patterns in an applied magnetic field is shown, and on the basis of this behavior, a three‐dimensional domain configuration is proposed for one of the less complicated surfacepatterns.
26(1955); http://dx.doi.org/10.1063/1.1722122View Description Hide Description
Water superheated to the limit in an open tube develops an internal negative pressurePn (tending to tear the molecules apart) equal to the saturation vapor at that temperature, less one atmosphere. Kendrick, Gilbert, and Wisner heated water in an open thin‐walled capillary U‐tube to 270°C for 5 sec before it exploded (Pn =53 atmos). Using their method, the writer heated three tubes to 264°, 266°, and 267°C for 5 sec or more before explosion occurred (Pn =48 to 51 atmos). The cohesive strength of water is thus sufficient to withstand an internal negative pressure of over 51 atmos at 267°C. At this temperature an additional negative pressure (applied externally through centrifugal force) would rupture the water column. It has been predicted from van der Waals' equation that the external negative pressure which the system could withstand would vanish at 273°C, in fair agreement with experiment.
26(1955); http://dx.doi.org/10.1063/1.1722123View Description Hide Description
26(1955); http://dx.doi.org/10.1063/1.1722124View Description Hide Description
The role of space charge as a factor influencing the potentials and potential gradients in a mass spectrometerion source of the electron bombardment type is calculated. Planar equipotential surfaces are assumed, and the analysis then becomes that of a plane parallel positive ion diode. The ``cathode'' of the ion diode may be either emission‐limited or space‐charged‐limited. The analysis considers the charge of the electrons in the ionizing sheet and the charge of the positive ions in the diode.
At a critical gas pressure the influences of the positive and negative charges on the potential of the ionizing region are equal and opposite, for small ionizing current. This concept leads to a pressure normalization in terms of the critical pressure. For a given source geometry and electron bombarding energy one can construct universal curves which give the potentials and the potential gradients as a function of the normalized gas pressure and the ratio of the ionizing electron current density to the repeller voltage. Experimental data are in agreement with the predictions of the theory.
26(1955); http://dx.doi.org/10.1063/1.1722125View Description Hide Description
The results of a measurement of the slowing down distribution in water of neutrons from a point fission source to indium resonance energy, 1.458 ev, are given. The second, fourth, sixth, and eighth moments of the measured distribution are calculated and have the values, , , , and .
Slowing Down Distribution to Indium Resonance of U235 Fission Neutrons from a Point Fission Source in Two Aluminum Light Water Mixtures26(1955); http://dx.doi.org/10.1063/1.1722126View Description Hide Description
26(1955); http://dx.doi.org/10.1063/1.1722127View Description Hide Description
Metalsemiconductor contacts of a number of different metals were made on n‐ and p‐type germanium using jet etching and plating techniques. Current voltage curves taken on 12 of these metals on 5 ohm‐cm n‐type germanium showed rectification which follows the diode equation . No correlation was found between the reverse saturation current densities of these diodes and such properties of the metals as work function, electromotive force, etc. For those metal contacts possessing the lowest saturation current densities, calculations indicated the current crossing the contact was to a large percent hole current and that the magnitude of the hole current was controlled primarily by the geometry of the diode. All metals plated on 5 ohm‐cm p‐type germanium produced ohmic contacts of resistivity comparable to the spreading resistance expected for the diode geometry used.
For indium diodes, a study of rectificationversusresistivity indicated that the barrier produced on both n‐ and p‐type germanium with plated contacts is one to electron flow rather than hole flow. When the assumption of only hole current crossing the barrier was made, it was shown that the I–V curves calculated from the diode theory, for different resistivities of germanium, were in qualitative agreement with the measured curves. Curves of zero voltage conductance versus temperature for different resistivities of germanium were also found to be in good agreement with those calculated on the assumption of all hole current.
26(1955); http://dx.doi.org/10.1063/1.1722128View Description Hide Description
Self‐sustained ion and electron plasma oscillations have been observed simultaneously in an ion neutralized electron beam traveling in a transverse magnetic field. The observed frequencies of oscillation agree with the Langmuir‐Tonks theory for plasma oscillations. Electron plasma oscillations occur when the electron plasma frequency equals the gyro frequency of a circular orbit in the magnetic field.
26(1955); http://dx.doi.org/10.1063/1.1722129View Description Hide Description
The ac power flow in an electron beam moving in weak electric fields is given a precise definition. Since it is a quantity quadratic in the applied field strength its evaluation requires the quadratic perturbation theory of the beam to be worked out. Quadratic expressions in the first‐order variables of the beam and circuit may also be written down which have the dimensions of power and are shown to satisfy a conservation theorem. The relations between the exact quantities of the nonlinear theory and the constructs from linear theory are discussed.