Volume 26, Issue 6, 01 June 1955
Index of content:
26(1955); http://dx.doi.org/10.1063/1.1722063View Description Hide Description
This paper is concerned with theoretical estimates of wide‐angle radiation in pencil beam antennas. In reflector‐type antennas this energy consists of direct radiation from the feed and scattered energy from the reflector. Approximate methods for the evaluation of these contributions are discussed, together with their simplifying assumptions. To get some numerical indications, calculations were made for paraboloidal reflectors of different f/D ratios and a class of primary patterns which provide an approximate representation of a great many common feeds. The results are presented in graphical form to provide useful design information. They show the dependence of the wide‐angle radiation level on feed and dish parameters.
26(1955); http://dx.doi.org/10.1063/1.1722064View Description Hide Description
The effect of low‐temperature (liquid nitrogen) reactor irradiation on the white‐to‐grey tin transformation has been investigated. It was found that, compared to an unirradiated pure sample, the transformation, measured by dilatometry, is drastically accelerated by prior irradiation. The irradiation apparently eliminates the normally very long induction period. The kinetic behavior of reactor irradiated samples and of samples seeded with grey tin were found to be qualitatively similar. These results indicate that the defects introduced into white tin by reactor irradiation serve as nuclei, or at least embryos of nucleation, for the subsequent phase transformation. Whether point defects, their aggregates, or the strains surrounding the displaced atoms are responsible for the artificial ``seeding'' cannot yet be decided. Reactor irradiation appears to be a promising new tool for studying nucleation and growth processes since quite uniform artificial nucleation can be accomplished this way.
26(1955); http://dx.doi.org/10.1063/1.1722065View Description Hide Description
Number and energy gamma‐ray albedos have been calculated for a material of Z eff=13, corresponding to both concrete and aluminum. The Monte Carlo method was used, the calculations being performed on an IBM‐701. The single‐ and multiple‐scattered components were evaluated separately; the multiple‐scattered component was considerably the larger of the two in most cases, being as much as three times as large as the single‐scattered component. For moderately small incident angles, the emergent photons are distributed approximately proportional to the cosine of the normal angle of emergence. The spectra peak around 150–250 kev and in some cases have a second peak at higher energy; such secondary peaks move upward in energy and increase in importance as the angle of incidence increases. The average energy of the emergent photons varies from 0.33 to 1.4 m o c 2. For the multiple‐scattered component the average energy varies from 0.30 to 0.93 m o c 2. Buildup factors have been calculated for 2 m o c 2 gammas normally incident on aluminum of 2 and 4 mean free paths thickness.
26(1955); http://dx.doi.org/10.1063/1.1722067View Description Hide Description
The reverse current and carrier lifetime have been measured in a series of germanium diodes as a function of temperature between room temperature and liquid nitrogen temperature. The lifetime reaches a plateau at low temperatures, and its behavior can be explained in terms of the Hall‐Shockley‐Read recombination theory. Log iR vs 1/T exhibits a break to a shallower slope at lower temperatures, which can be explained in terms of charge generation by recombination centers in the space‐charge region.
Theoretical and Experimental Study of Electromagnetic Scattering by Two Identical Conducting Cylinders26(1955); http://dx.doi.org/10.1063/1.1722068View Description Hide Description
The scattering of a cylindrical TEM wave by two parallel, identical conducting circular cylinders is developed as a special case of a theoretical analysis which treats the scattering by an arbitrary array of cylinders. Only the case with the incident E‐vector parallel to the axes of the cylinders is considered, and attention is focused on the mutual effects present when the cylinder diameter and spacing are comparable to a wavelength.
The approximations made in the theory are tested experimentally using 3 cm microwaves in a parallel plate region. Significant departures from the results of the independent scattering hypothesis as predicted by the theory have been confirmed experimentally.
26(1955); http://dx.doi.org/10.1063/1.1722069View Description Hide Description
A high‐speed automatic electron‐trajectory tracer was employed to study the relative loss in deflection sensitivity and linearity caused by continuous resistive helical and conventional band‐type cathode‐ray tube postdeflection accelerators. A total of 144 trajectories was traced for six different electrode configurations immersed in a wedge‐type electrolytic tank that simulated a particular tube envelope. Beam‐deflection angles of 5.5°, 13.5°, 21.5°, and 30° were employed with maximum potential differences of 0, 2.3, 4.8, 7.3, 9.8, and 12.3‐kv postdeflection acceleration and a 2.7‐kv initial electron energy. The relative error of the resultant trajectories plotted is estimated at one percent. Of the configurations tested, the performance of the continuous resistive helix was far superior in deflection sensitivity and slightly better in deflection linearity. The results also indicate that a linear, stepped voltage distribution [(V 0/N)·n], although poorer than the continuous‐helix distribution, may give better performance than that resulting from a stepped, geometrical voltage distribution (V 0, V 0/2, V 0/4, etc.).
26(1955); http://dx.doi.org/10.1063/1.1722070View Description Hide Description
The nonmagnetic radio‐frequency principle of mass selection appears attractive because it promises to result in a mass spectrometer of simple construction and rugged nature. Accurate evaluation of the use of this principle is difficult because of the lack of published performance data. In an effort to obtain performance data a 5‐stage Bennett‐type tube with stage separation of 5–9‐4–7 cycles has been constructed and operational variables studied theoretically and experimentally.
The experimental results are encouraging. They indicate the tube is capable of sufficient stability, resolving power, and sensitivity to be used for many analytical and research problems. Electronic circuits required to achieve a reasonable stability are relatively simple.
26(1955); http://dx.doi.org/10.1063/1.1722071View Description Hide Description
Siliconn‐p‐n junction transistors have been made from rate‐grown single crystals. Using gallium and antimony as doping agents, single crystals of n‐type silicon have been grown containing up to five p‐regions from 0.0005 to 0.002 inches wide which are suitable for the production of transistors. The ohmic contact to the p‐type base region was made by alloying an aluminum wire using techniques similar to those employed in the fabrication of aluminum‐silicon diodes.
The electrical evaluation of n‐p‐ntransistors produced from these single crystals is described. Alphas in excess of 0.9 at −1 ma emitter current have been obtained, and collector saturation currents of 10−6ampere/cm2 are common. With base‐layer widths of about 0.0005 inch, the alpha cutoff occurs at approximately 5 megacycles. The units (approximately 0.04×0.04×0.5 inches in size) have been operated at power levels in excess of 1.5 watts in air with no special provision for heat dissipation.
Magnetic Materials for Digital Computer Components. II. Magnetic Characteristics of Ultra‐Thin Molybdenum‐Permalloy Cores26(1955); http://dx.doi.org/10.1063/1.1722072View Description Hide Description
Measurements of the flux‐reversal time τ as a function of the applied magnetic fieldH have been made for ⅛‐mil, ¼‐mil, ½‐mil, and 1‐mil 4–79 molybdenum‐Permalloy tape cores. There measurements have been taken at seven temperatures, ranging from −196°C to 270°C. From these measurements, the switching coefficient Sw =(H−H 0)τ is found, where H 0 is the threshold field value for irreversible domain‐wall motion. Determination of the switching coefficient as the tape thickness is varied permits a separation of the spin‐relaxation and eddy‐current contributions to switching delay. These contributions are studied individually as functions of temperature. Upon increasing the temperature over the range considered, the eddy‐current contribution and the threshold field value H 0 are approximately halved, and the spin‐relaxation contribution is reduced by 20 percent. All these factors lead to a faster flux reversal at higher temperature for a given applied field. This behavior is in agreement with theoretical predictions.
26(1955); http://dx.doi.org/10.1063/1.1722073View Description Hide Description
The one‐dimensional contact region common to shock‐tube flows is idealized by the simple model of initial step change in temperature and density at constant pressure. For weak transitions, neglect of pressure effects, and linearization of the conservation equations gives spatially monotonic temperature and density profiles with the contact region thickening as the square root of time. Experimental results are noted from shock‐tube studies of two air/air contact regions. Insofar as comparison of theory and experiment is possible, the theory for weak transitions appears reasonable.
26(1955); http://dx.doi.org/10.1063/1.1722074View Description Hide Description
An apparatus for measuringstress relaxation of plastics at constant strain in the time range of 0.01 to 2.5 sec after load application is described. Experimental results obtained indicate that rigid plastics at room temperature may be classified as either brittle or ductile.Brittle plastics sustain only low strains without fracture at the testing speed of the apparatus and undergo little relaxation of stress in this time range; ductile plastics sustain higher strains and undergo considerable relaxation of stress. The factors affecting relaxation behavior are briefly discussed. Increasing the strain, temperature, or plasticizer content generally increases the relaxation rate.
Many of the results yield a linear plot of stress vs logarithmic time, as has been reported in the literature for various materials tested at longer times. By application of one form of the Eyring absolute rate theory, an average free energy of activation for the relaxation process can be calculated. The value thus calculated for polymethyl methacrylate is in the range reported in the literature for other polymers.
An empirical measure of ``toughness'' can be based on these results. For many plastics this measurement is in accord with service performance.
26(1955); http://dx.doi.org/10.1063/1.1722075View Description Hide Description
Experimental results of the near zone diffraction field of circular and elliptical apertures and of the infinite slit are presented. The investigations were made on apertures which were illuminated by a plane polarized electromagnetic wave and whose characteristic dimensions were varied over a considerable range with respect to the wavelength of the incident radiation.
Most of the measurements are compared with predictions from recently published approximate theories and in a few cases with exact solutions. In general the agreement between theory and experiment is found to be satisfactory. Some of the discrepancies are discussed.
26(1955); http://dx.doi.org/10.1063/1.1722076View Description Hide Description
The low‐frequency noise spectra of germanium and silicon rectifiers have been determined. Two methods were employed: a numerical analysis according to the Wiener‐Khintchine relationship, and a frequency translation photo playback method. No evidence of a deviation from a 1/fa (a>1) noise power law was found down to frequencies as low as 6×10−5 cps.
Calculation of the Space Charge, Electric Field, and Free Carrier Concentration at the Surface of a Semiconductor26(1955); http://dx.doi.org/10.1063/1.1722077View Description Hide Description
In the study of the surface properties of semiconductors, it is valuable to know the relation between carrier density at the surface and the shift in electrostatic potential from the bulk to the surface. By a solution of Poisson's equation utilizing Boltzmann statistics, the space charge,electric field, and change in free carrier concentration have been calculated for a semiconductor surface. The results are expressed as a function of the deviations of the Fermi energy from its intrinsic value in the bulk and at the surface. The calculated curves may be used for any nondegenerate semiconductor at any temperature, provided that the donor and acceptor levels are completely ionized. Numerical values are given for germanium and silicon at room temperature.
26(1955); http://dx.doi.org/10.1063/1.1722078View Description Hide Description
Currents preceding breakdown have been measured between closely spaced tungstenelectrodes in high vacuum. It is found that field emissioncurrents sufficient to evaporateanode metal flow before breakdown. These currents follow the Fowler‐Nordheim equation when field magnification due to surface irregularities on the cathode is taken into account. The field magnification is a function of distance at electrode separations less than 4×10−4 cm.
Explanation of the observed breakdown at low voltage and small spacing requires an unusually high yield of electrons at the cathode per ion formed in the gap. Furthermore there is no measurable direct enhancement of the current by ionization even at higher voltages. The high electron yield must therefore exist over the entire observed range of breakdown voltages. This high yield is satisfactorily accounted for by the increase in field emission due to the positive ion space charge, which in turn increases the positive ion current density until there is breakdown. It is shown that breakdown occurs when the field emissioncurrent is increased by only 65 percent. This condition is reached with the ion current density much smaller than the electron current density.
26(1955); http://dx.doi.org/10.1063/1.1722079View Description Hide Description
X‐ray diffraction profiles from filed solidsilver specimens were measured with a Geiger counter spectrometer using crystal‐reflected Cu Kα radiation. It is shown that for values of n>7, the assumption of a Gauss strain distribution fits the observed Fourier coefficients far better than a Cauchy distribution. The results obtained from solid specimens are consistent with those previously measured for silver filings. The influence of background errors is discussed.
26(1955); http://dx.doi.org/10.1063/1.1722080View Description Hide Description
The energy relations associated with the plastic deformation by cold working of a 75 percent gold—25 percent silver alloy at two temperatures were investigated. The samples were produced by drilling under controlled conditions, which is a suitable method of deformation. The energy stored in the chips was measured by tin solution calorimetry and the total energy expended in the deformation was determined from dynamometer measurements.
The results for cutting with a drill having the most suitable geometry were as follows. The energy stored at room temperature was 0.50 cal/g and the corresponding value for 78°K was 1.48 cal/g. The values of the total energy of deformation were 53.5 cal/g at room temperature and 109 cal/g at 78°K. The true stress‐true strain curve at room temperature was normal for a nonferrous metal and the curve for 78°K showed the behavior expected at low temperature from a face‐centered cubic metal.
26(1955); http://dx.doi.org/10.1063/1.1722081View Description Hide Description
The work functions of the low emitting crystallographic planes of tungsten, (011) and (112), are not well known. In previous experiments scattered secondaries and some other disturbing effects covered the small emission particularly of the (011) plane. A method combining the field emission microscope with a probe collector has been applied. The current density in the (011) plane turned out to be 4 to 5 orders of magnitude smaller than in strongly emitting planes. Applying the Fowler‐Nordheim theory, φ011 was found between 5.70 and 5.99 ev and φ112 between 4.65 and 4.88 ev depending on the temperature at which the tungsten crystal has been annealed previously. This is not a temperature effect of the work function, but the result of freezing in thermal imperfections of the planes. The work function of the ideal (011) plane appears to be as high as 5.99 ev. The existence of such a high value is further suggested by the ionization of aluminum on a hot polycrystallinetungsten wire, indicating that a part of the surface has a work function as high as the 5.96 ev ionization energy of aluminum.
26(1955); http://dx.doi.org/10.1063/1.1722082View Description Hide Description
This paper presents an intensive study of the viscous and elastic properties of the poly methacrylates. Variation of these properties is studied as a function of temperature in the range 60<T<160°C, as a function of molecular weight in the range 5×103<M<5×106, and as a function of plasticizer concentration over the range 0–100 percent. Capillary viscometers were used in the low‐viscosity systems and tensile creep and recovery measurements under very small loads were used to study the less fluid systems. Principal effort was confined to polymethyl methacrylate but enough work was done using the butyl and decyl esters to show that the whole series of these polymers fits into a relatively simple picture. Consequently, given a certain minimum amount of data, it is now possible to predict the viscoelastic behavior of these polymers in the range of variables given above.
The theory of viscoelasticity previously given has been extended so as to take account of the effects of chain entanglements and distributions in molecular weight. Moderately good agreement is found between theory and experiment.
26(1955); http://dx.doi.org/10.1063/1.1722083View Description Hide Description
An experimental study was made of the behavior of two rubbers, Butyl (GR‐I‐18) and Paracril‐35, in the low extension region (0 to 15 percent), at temperatures ranging from −40°C to +60°C. The results have been compared with the Guth‐James theory of rubber elasticity. In general, good agreement with theory was observed. Two effects predicted by the theory have been confirmed, namely, the nonlinear dependence of stress upon temperature at constant extended length, and the shift of the thermoelastic inversion extension to higher values with increasing temperature. Small deviations, which increased with increasing extension and with decreasing temperature, were observed.