Volume 26, Issue 3, 01 March 1955

Review of the Measurements of the Resonance Absorption of Neutrons by Uranium in Bulk
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Resonance Absorption of Neutrons by Spheres
View Description Hide DescriptionBy concentrating uranium into lumps, one can decrease its resonance absorption as compared with the resonance absorption of the same amount of uranium spread out uniformly in a moderator. It is shown here that this decrease is caused principally, not by the diminished over‐all density of the neutrons at the lump, but by their changed energy distribution. This shows deep minima at the resonance lines, i.e., where the uranium absorbs most strongly. The considerations here presented show that the decrease in absorption is largest for the low energy resonances of U^{238} which are responsible for the bulk of the absorption in the case of uniform distribution. As a result, the resonance absorption of uranium lumps extends over a very large energy region–up to about 5000 ev. It can be decomposed, approximately, into two parts: the mass absorption, proportional to the number of U^{238} atoms present; and a surface absorption, proportional to the surface of the lump. The total decrease of the resonance absorption produced by lumping (the ``advantage factor'' of lumping) can be estimated to reach, under favorable conditions, a value as high as 20.

Effect of Geometry on Resonance Absorption of Neutrons by Uranium
View Description Hide DescriptionThe total resonance absorption of neutrons by spheres of uranium and uranium oxide imbedded in graphite and bombarded with essentially a dE/E spectrum is found to be given by 1/α=c[1+(κ/ρR)] where α is the advantage factor, that is, the ratio of absorption of uranium when distributed uniformly throughout graphite to the absorption when it is lumped in a sphere. R is the radius of the sphere and ρ is its density. c and ρ were measured in various parts of a large graphite block exposed to cyclotron neutrons from the reaction Li (p,n). Values of c and ρ, respectively, are 0.051 and 5.1 g/cm^{2} for U_{3}O_{8}. In a newer notation, A = 10.9 barns and μ = 1.95 g/cm^{2}.

Effect of Temperature on Total Resonance Absorption of Neutrons by Spheres of Uranium Oxide
View Description Hide DescriptionIn the expression 1/α=c[1+(κ/ρR)] for the advantage (α) of decreased resonance absorption of uranium in a dE/E neutron spectrum brought about by lumping UO_{2} into spheres, the temperature dependence of c is measured and κ is measured also. ρ is the density of UO_{2} in g/cm^{3}; R the radius of the sphere in cm. In the range 20° to 1000°C, Δc/ΔT averages 1.8×10^{−6} per degree centigrade and is positive as expected. This temperature dependence decreases by about a factor of three from the lowest to the highest measured temperatures. κ is 5.06 g/cm^{3} leading to a value of μ in the expression [Phys. Today 5, 32 (May, 1952)]

Temperature Dependent Creep in Zinc Crystals
View Description Hide DescriptionTensile creep measurements beginning in a tensile strain region of about 10^{−6} have been made upon a set of seven single crystals of zinc of 99.99^{+} percent purity. Measurements were made both by means of electrical resistance strain gauges and an optical lever system. Some of the measurements made in the latter apparatus lasted 100 hours or more, and in both systems,measurements were made at several temperatures.
The creep results obtained in this work can be best described by the single empirical relation in which η is the shear strain, t is the time after completion of the application of stress to the crystal, and b and m are constants for a given creep test under given experimental conditions. It is found that b increases with the stress but does not vary greatly with temperature. The values of m are practically independent of stress but vary with temperature, and appear to fit best the equation, in the temperature interval investigated. For these crystals T_{0} = 1580°K, m _{0}=120, and the temperature at which m=1 is 58°C. Values of m>1 are interesting in dislocation theory.

Creep of Zinc Crystals
View Description Hide DescriptionThe plastic extension of zinc crystals in the region just beyond the elastic limit is found to obey the empirical law, η=bt^{m} in which η is the resolved shear strain, t is the time, m is a constant with a value close to 0.5 for initially unstrained crystals, and b is dependent on the resolved shear stress. Application of a theory due to Koehler shows that b should be related to stress (σ) in such a way that a plot of ln(mbσ^{2}/Δσ) against 1/σ should be a straight line of slope −cG. In this, Δσ is the stress increment from one creep curve to the next, G is the shear modulus, and c is the concentration of anchor points which determines the lengths of segments of dislocation lines which generate according to the Frank‐Read mechanism. The experiments are in fair agreement with this theory and values of c obtained are roughly equal to the concentrations of metallic impurities. The small amount of dissolved gases appears to play no significant part in the creep process.

Analysis of Scintillation Spectrometer Observations of the Penetration of Cs^{137} Gamma Radiation through Water
View Description Hide DescriptionSpectral modifications of Cs^{137}gamma radiations produced by penetration and diffusion through water have been observed. A single crystal of NaI(Tl) was used to observe the pulse‐height distributions produced by radiation from both plane parallel and point isotropic sources. Inversion of the pulse‐height distribution was carried out by use of an analog computer to obtain the spectral distribution of radiation.

Bistatic Radar Cross Sections of Surfaces of Revolution
View Description Hide DescriptionThe results obtained by applying the current‐distribution method to the problem of approximating the scattering cross section, when the transmitter and receiver are separated, are presented for several simple geometric configurations. The method is applied for the case in which the transmitter is located on the axis of revolution and the ratio l/λ is large, where l is a ``characteristic dimension'' of the body and λ is the wavelength. These results indicate that in most of the cases considered the cross section increases as the angle between the receiver and the transmitter increases.

Electromagnetic and Acoustic Scattering by a Semi‐Infinite Body of Revolution
View Description Hide DescriptionThe first two terms of Kline's asymptotic expansion are obtained for the scattering of a plane wave incident along the axis of a perfectly reflecting semi‐infinite body of revolution. When this method is applied to the paraboloid the exactelectromagneticsolution is obtained in closed form. The accuracy of the method of physical optics is studied by using the asymptotic expansion.

Electromagnetic and Acoustical Scattering from a Semi‐Infinite Cone
View Description Hide DescriptionThe value of the nose‐on back scattering cross section of a semi‐infinite cone is determined by the exact methods of electromagnetic and acoustical theory, and by physical optics. It is shown that, to the degree of approximation used, the electromagnetic value and the physical‐optics value are equal. The acoustical value is found to be less than the electromagnetic value by a factor which depends only on the cone angle; both are proportional to the square of the wavelength. It is shown that the electromagnetic and physical‐optics answers agree with experimental data to within a factor of two. The electromagnetictheory results obtained hold for the cases in which the half‐cone angle is close either to 0 or to π/2.

Techniques for Nuclear Magnetic Resonance Measurements on Granular Hygroscopic Materials
View Description Hide DescriptionAn investigation was made of the effects of nonuniformity in the magnetic fields and in the test specimens employed in the measurement of nuclear magnetic resonance absorption. Particular attention was given to the determination of relative integrated line intensity from derivative maxima independently of the uniformity of concentration of the subject nuclei through the available sample volume. The principal objective was to devise techniques applicable to granular hygroscopic materials. A technique applicable for the low‐moisture range was tested for materials containing 7 to 16 percent water and found to be satisfactory.

Gap Problem in Antenna Theory
View Description Hide DescriptionThe so‐called gap problem in antenna theory is considered critically. It is shown that whereas there exist problems related to transmission‐line end‐effect and coupling between antenna and line, there is no gap, and hence no gap problem, when a physically realizable complete transmitting system is considered rather than an antenna with a fictitious mathematically convenient driving mechanism.

Acceleration of Slender Bodies of Revolution through Sonic Velocity
View Description Hide DescriptionThe linearized theory of slender bodies in arbitrary motion at zero angle of attack has been worked out. The results have been applied to a smooth body accelerating uniformly through sonic velocity. The results theory can be used to estimate the nonlinear or transonic effects.
For an accelerating body, the parameter (bl/c ^{2})^{½} is important where 2b = acceleration, 2l = length of body, c = sound speed at infinity. For sufficiently high (bl/c ^{2})^{½}, transonic effects can be neglected. Using linearized theory to estimate the ratio of nonlinear terms in the differential equation gives

Use of Space Charge in Electron Optics
View Description Hide DescriptionThe electron optical properties of a cylindrical space charge cloud are derived. The possibility of achieving a system free from either spherical or chromatic aberration by combining a space charge lens with a space charge free converging lens is examined. It is concluded that the achromatization of a thin electric or magnetic lens is possible only if the resultant action of the combination is divergent. The correction of the spherical aberration of a high quality lens, such as an electron microscope objective, is shown to be impossible on account of electron interaction effects.

Radiation Characteristics of a Conical Helix of Low Pitch Angle
View Description Hide DescriptionThe present work is an extension of the author's prior work on the radiation characteristics of and current distribution in a conical helix, the helix used in the present case having a pitch angle much lower than of that studied previously. In this case the condition of the phase velocity adjusting itself automatically to maintain maximum directivity condition (which enables the radiation to be confined in the axial mode over a large band of frequencies) does not hold. Nevertheless the major fraction of energy could be radiated in the axial direction when the geometry of the helix and the distance of the apex from the ground were properly chosen, the conical helix together with the ground being regarded as tapered transmission lines (with the ground as one and the helix wire—with the distance of its elements from the ground line increasing continually,—as the other line).
Measurements of the field intensity and the current distribution were made in the frequency range of 100 Mc/sec to 500 Mc/sec with a conical helix antenna arrangement as follows. The helix was of six turns, the radius increasing uniformly from 5 cm at the apex to 65 cm at the base within an axial distance of 60 cm. The apex of the helix was placed at a distance of 17.5 cm from the ``ground'' provided by a 6‐ft square copper netting of close mesh. It was found that within the range of frequency measured the major fraction of the energy was radiated in the axial direction. Theoretical expressions for E _{φ} and E _{θ} were derived by assuming a linear current distribution, and the calculated radiation patterns were found to agree with the experimentally determined ones.

Studies of the Diffraction of Electromagnetic Waves by Circular Apertures and Complementary Obstacles: The Near‐Zone Field
View Description Hide DescriptionThe diffraction of electromagnetic waves by circular apertures in extended conducting screens and by complementary obstacles was investigated experimentally to determine the structure of the field in the near zone. Techniques for measuring both the electric and magnetic field distributions are presented and the general experimental problems are discussed. Data are given for the tangential components of the electric and magnetic vectors in the principal planes in the immediate neighborhood of the diffracting structures, and axial distributions for the corresponding components. The results extend the range of the data available previously. The predicted uniformity of the tangential magnetic field in the aperture plane, for the case of a wave incident normally, is verified. An experimental test was made also of the electromagnetic Babinet principle.

Replica Studies of Bulk Clays
View Description Hide DescriptionThe electron microscope examination of the surfaces of bulk clays by replica techniques is difficult because of the porosity of the clays and the necessity of using hydrofluoric acid to dissolve the clay from the replicas. The surfaces of some bulk endellite clays were examined by means of collodion, silica, and preshadowed replicas. Collodion and two‐step silica replicas were unreliable and difficult to use because of penetration of the collodion or plastic into the pores of the clay. Collodion strippings were of some value in removing layers of clay particles from the surface for direct examination in the microscope. Best results were obtained with the preshadowing technique which employed a shadowing layer of platinum backed by a layer of evaporated carbon or by Parlodion. Micrographs of replicas prepared by the latter technique show the surface of a highly porous endellite clay to consist of tubes in various orientations.
 LETTERS TO THE EDITOR


Velocity Spectrography of Electron Dynamics in a Traveling Field
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A Novel Microwave Attenuator Using Germanium
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Circularly Polarized Cavities for Measurement of Tensor Permeabilities
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