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Volume 19, Issue 7, 01 July 1948
19(1948); http://dx.doi.org/10.1063/1.1698178View Description Hide Description
In this work roentgen rays and cathode rays of several million‐volts energy have been applied to an investigation of their biological, photo‐chemical, and germicidal effects, particularly as they are related to the processing of foods and biological materials. A constant‐potential electrostatic generator, together with an acceleration tube, was used to produce continuous streams of electrons with homo‐geneous and controllable energy. [R. J. Van de Graaff, K. T. Compton, and L. C. Van Atta, Phys. Rev. 43, 149 (1943).] These high energy electrons were utilized both for the production of penetrating roentgen rays and for the direct irradiation of materials. The mechanism of the biological action of both roentgen rays and cathode rays is discussed, as well as the energy considerations in their application to various absorbers. The companion paper [C. G. Dunn, W. L. Campbell, H. Fram, and A. Hutchins, J. App. Phys. 19, 605 (1948)] reports on measurements of the lethal action of these radiations on a wide variety of micro‐organisms and also on the effect of the radiations on enzymes,vitamins, and certain whole food products. Both investigations have been conducted cooperatively by the Departments of Electrical Engineering and Food Technology at the Massachusetts Institute of Technology.
The systematic and quantitative study of the effects of roentgen and cathode rays on various elementary forms of living matter is regarded as an essential prelude to their possible widespread application to the preservation of foods and the sterilization of various biological materials. It was realized that the mechanisms of biological action of high energy roentgen rays and cathode rays are similar and depend closely upon the ionization energy absorbed per gram of material. The studies were begun with roentgen rays since these were immediately available and permitted quantitative measurement of the energy absorbed in the biological materials. Later, the water‐cooled gold x‐ray target was replaced with an aluminum cathode‐ray window, and corresponding studies were begun on the direct application of high energy electrons to these materials. During the period of these biological studies the x‐ray source was also used in a daily clinical program of deep cancer therapy under the medical direction of Dr. Richard Dresser and also in the investigation of the physical properties of high energy radiations. [R. Dresser, Radiology, in publication.]
Biological and Photo‐Chemical Effects of High Energy, Electrostatically Produced Roentgen Rays and Cathode Rays19(1948); http://dx.doi.org/10.1063/1.1698179View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698180View Description Hide Description
This paper presents a mathematical discussion of retarded control systems. The analysis presented is applicable to general control systems and is based on the solution of a linear differential equation of infinite order. The mathematical investigation is performed by operational methods, and the question of stability is studied by the use of the complex variable.
Erratum: Response of Linear Resonant Systems to Excitation of a Frequency Varying Linearly with Time19(1948); http://dx.doi.org/10.1063/1.1698181View Description Hide Description
19(1948); http://dx.doi.org/10.1063/1.1698182View Description Hide Description
Studies of the rising sun design for magnetron oscillators have been extended to large numbers of cavity resonators. Successful operation of pulsed magnetrons having as many as 38 cavity resonators has been achieved in the 1.25‐cm wave‐length region. The open cavity resonators heretofore used in rising sun magnetrons were found unsuited for oscillators containing a large number of cavities. The extension to higher numbers was made possible by the use of closed‐end resonators. This modification in cavity geometry produces a favorable redistribution of the spectrum of the resonant modes of the rising sun structure, thereby making possible the operation of the magnetron in the desired π‐mode without disturbance from other modes. The increase in the number of cavity resonators for magnetrons should serve to facilitate the extension of present magnetron techniques to the millimeter range and to increase the radiofrequency power obtainable from magnetrons at 1 cm and at longer wave‐lengths.
19(1948); http://dx.doi.org/10.1063/1.1698183View Description Hide Description
The semipositive character of the dissipation function of any physical 2n‐terminal network imposes some restrictions on the realizability of prescribed transmission losses between the various pairs of terminals when they are connected to given impedances. These restrictions are most easily expressed in terms of elements of the efficiency matrix of the network, which is defined in the first part of this paper. Discussions of the application of matrix calculus to the solution of reactive and resistive, transformer‐type, and general 2n‐terminal networks are presented.
A very useful tool in solving telephone transmission problems is the conventional theory of 4‐terminal transducers, often called 4‐pole networks. Although this theory is sufficient for the study of most circuits, some problems involve networks with a larger number of terminals, which are often referred to as 2n‐terminal, or 2n‐pole networks. This paper is a contribution to their theory.
The particular point that will be considered in this paper is the restriction imposed by physical realizability on the arbitrary choice of transmission losses between various pairs of terminals connected to a given set of terminating (generator or receiver) impedances.
Transmission of Monoenergetic Slow Neutrons through Solid Solutions and Mechanical Mixtures of TiC and WC19(1948); http://dx.doi.org/10.1063/1.1698184View Description Hide Description
TiC and WC form continuous substitutional solid solutions at least up to 25 percent TiC+75 percent WC by weight when heated to 4000°F, TiC being the solvent and WC the solute. Total neutron scattering cross sections of a series of finely powdered solid solutions and corresponding mechanical mixtures of the two carbides were determined as a function of the mole percent of WC from the measured transmission of monoenergetic neutrons with energies 0.08138, 0.03479, 0.02056, and 0.01526 ev or wave‐lengths λ=1.004A, λ=1.536A, λ=1.998A and λ=2.319A, respectively. The cross sections of both the solutions and the mixtures increase with increasing mole percent of WC. For mixtures they are the same as the additive cross sections, each constituent scattering independently of the other. They are analogous to x‐ray scattering for the mixtures in which each constituent gives its own diffraction pattern and the patterns of the two constituents are superimposed. The neutron cross sections of solid solutions are lower than those of the mixtures at low percentages of WC but increase rapidly with the concentration of the solute molecules. They appear to be sensitive both to the structure of the solvent and the concentration of the substituting atoms.
19(1948); http://dx.doi.org/10.1063/1.1698185View Description Hide Description
The work that was done in this laboratory on the development of a resonant cavity type linear accelerator is briefly described. The experimental results demonstrate that electrons entering at low energies will accelerate to a higher energy in passing through a group of cavities of equal β(λ0/2) spacing and along which appears a standing wave field distribution. Starting at 57 kv, an energy of 0.345 Mev was attained with two cavities. An 8‐cavity accelerator of cavity length corresponding to β=0.75 had a maximum output of 0.57 Mev. Experimentally determined energies were in fair agreement with theoretical values at equal axial gradients. Excitation power was found to be proportional to [(Mev)2/Length](1/Q)×coupling coefficient. The power required for an axial gradient of 1 Mev/ft., even for the most favorable design, was high and would require the full output of one HK7 type magnetron for each foot of accelerator length. Energy spread in the accelerated beam was large for the two cavity tubes but small for the longer 8‐cavity tube. It is believed that the apparent low energy spread observed in the 8‐cavity case resulted from a limited range in entering angle brought about by the reduced axial gradient. There was little evidence of radial defocusing. A change in resonance wave‐length amounting to +0.000225 cm per degree centigrade was observed. There was indication that the magnetron would follow a resonance change of approximately 5°C.
A Method for Measuring the Complex Dielectric Constant of Gases at Microwave Frequencies by Using a Resonant Cavity19(1948); http://dx.doi.org/10.1063/1.1698186View Description Hide Description
A sensitive method has been developed for the measurement of the complex dielectric constant of low pressure gases at X‐band microwave frequencies. The gas, whose complex dielectric properties are to be determined, is contained in a resonant cavity which is a part of the microwave circuit. With the introduction of the gas, the real part of the dielectric constant changes the resonant frequency of the cavity, while the imaginary part changes the amplitude and breadth of the cavity response curve. By a rapid variation of the frequency across the cavity resonance, the real and imaginary parts can be conveniently and accurately determined from measurements on an oscilloscope. Tests on such a system have been performed with satisfactory results. Sample data are presented for methylchloride and deuterated ammonia. This method can be used for the measurement of the resonant dispersion and absorption of microwaves by gas molecules. Also, the same method can be used for the measurement of loaded and unloaded Q‐values of a cavity.
19(1948); http://dx.doi.org/10.1063/1.1698187View Description Hide Description
The faces of carbon or graphite brushes, polished by rubbing against a rotating copper disk and subsequently examined with the electron microscope by means of surface replicas, are found to consist of many tilted ``fingers,'' which are the projecting portions of the large and irregular graphite plates composing the brush. These graphite projections show a directional overlap, which is determined by the direction of motion of the disk, and when viewed stereoscopically they are found to be inclined at various angles up to nearly 90° with reference to the plane of the rotating base. They account for a previously observed ``directional memory'' of graphited surfaces and they show that such surfaces in general cannot be regarded as consisting simply of graphite plates oriented with the hexagonal planes flat and parallel to the plane of motion.
The brush track on the rotating copper base is found to contain small islands of carbon deposited from the brush. These islands consist of large numbers of graphite wear‐fragments, which are individually of very small size in comparison with the projections of the brush face. These fragments are clearly derived from the projections by a tearing apart of their edges and are packed tightly and with random arrangement into the tool marks of the copper smoothing over the irregularities of the latter. It is not determined whether this ``carbonfilm'' of the brush track is continuous.
The wear‐dust of unlubricated graphite is found to consist of particles which are very small in average size compared with those from which brushes are manufactured. The size of these wear‐fragments extends below the resolving power of the electron microscope, and this is consistent with recent data on similar particles of a size determined independently by adsorption measurements.
19(1948); http://dx.doi.org/10.1063/1.1698188View Description Hide Description
A theory is developed describing the electromagnetic radiation from dielectric rods used as terminations to rectangular wave guides. The radiationpatterns are computed from the fields due to equivalent magnetic and electric currents on the surfaces of the rods. These equivalent currents are determined by use of Schelkunoff's equivalence principle applied to approximate fields within the dielectric. Specific calculations are carried out for rods whose average dimensions are 0.9″×0.25″ and whose lengths vary from three to ten wave‐lengths. Experimental patterns are obtained for eight polystyrene rods, some of which are variable in length. Agreement with theory is found for rods from three to six wave‐lengths long; differences between experiment and theory are discussed.
19(1948); http://dx.doi.org/10.1063/1.1698189View Description Hide Description
The peak pressure of spherical shock waves from small explosive charges has been determined as a function of distance by measuring the velocity of propagation and applying the velocity‐pressure relation derived from the Rankine‐Hugoniot equations. The charges were composed of TNT or pentolite (TNT/PETN 50/50 by weight), weighed from 1.45 to 8 lb., and were spherical or cylindrical in shape, except for one series of small rectangular blocks. Measurements on the non‐spherical charges were made in the plane through the center of the charge, perpendicular to the axis.
The pressure‐distance relations for the four principle charge types are given by the following fitted equations, in which Π represents excess peak pressure in atmospheres, and the distance, scaled according to charge weight, is given by the non‐dimensional variable Z=R/(ρτ)⅓ where R is the distance from a charge of volume τ and specific gravity ρ:
½‐lb. rectangular blocks, TNT,;8‐lb. cylinders, Pentolite,;4‐lb. cylinders, TNT,;3.75‐lb. spheres, Pentolite,.These equations are valid for values of Z between approximately 18 and 110. The indicated probable error of the fitted curves is of the order of one percent for intermediate distances, increasing to from two to seven percent at the extremes of the distance range covered.
The curve for spherical charges is in agreement with the Kirkwood theory, and the results for cylinders having various length/diameter ratios indicate large dependence of pressure on charge shape for values of Z up to 50 at least.
19(1948); http://dx.doi.org/10.1063/1.1698190View Description Hide Description
Contrast difficulties in the observation of fine detail in thin specimens may be overcome by employing dark‐field observation. This introduces the added complication, however, that the imaging electrons are now inhomogeneous, having experienced a most probable energy loss of the order of 20 volts. The present calculations of the effect of chromatic aberration and diffraction show that with an objective aperture smaller than the ``optimum aperture,'' and centered optics, such inhomogeneities do not appreciably affect the resolution, even in the extreme case that the electrons are uniformly distributed in energy.
19(1948); http://dx.doi.org/10.1063/1.1698191View Description Hide Description
The Young's modulus and strain coefficient of electrical resistivity were determined for some Bi–Pb, Bi–Sn, Bi–Se, and Bi–Ni alloys. It was found that the modulus of elasticity decreases rapidly in the regions of solid solution contrary to the predictions of Guillet. The strain coefficient of resistivity was found to increase rapidly within the limits of solid solubility while the stress coefficient of electrical resistivity remained constant.