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Roentgen-Ray Tubes W. D. Coolidge and E. E. Charlton Research Laboratory General Electric Co. Schenectady 5, N. Y. Excerpt Since its birth, the roentgen-ray tube has undergone many radical changes. The general method of producing roentgen rays is, however, still the same, namely by accelerating electrons to a high velocity and then suddenly stopping them by collision with a solid body, the so-called target. Depending upon the method used in generating the electrons, roentgen tubes may be classified into two general groups, gas tubes and high-vacuum tubes. In the first group, the electrons are freed from a cold cathode by positive ion bombardment, thus necessitating a certain gas pressure. In the second group, the vacuum is made as good as possible and the electrons are freed from the cathode either by heat, by bombardment from other electrons, or by the use of a potential gradient high enough to remove them electrostatically. As First Used by Röntgen: The first roentgen tube (Fig. 1) was of a form previously employed by Crookes in his experiments on electrical discharges through rarefied gas. The electrons liberated by positive ion bombardment from the flat aluminum cathode were emitted in a direction perpendicular to its surface and, under the impressed voltage gradient, traveled in straight lines to the glass wall of the tube, where they generated roentgen rays. Early Modifications: This first roentgen-ray source was soon greatly improved by Campbell-Swinton through the introduction of a platinum foil target and by Professor H. Jackson through the substitution of a concave cathode for the original flat one. A later step of great importance was the addition of a device for regulating the vacuum. The early tubes were small and easily ruined, as so little energy was necessary to melt the thin electrodes and to overheat portions of the glass envelope. Later Improvements: The power of the tubes was greatly increased by making them larger and with more massive cathodes and targets (Fig. 2). The development of the target in particular received much attention, resulting in a change from the use of a simple piece of sheet platinum to a platinum-faced disk of nickel brazed to a massive block of copper. This greatly increased the rate of heat flow away from the focal spot and, at the same time, increased the heat storage capacity. A further substantial increase in tube power was later obtained by the development of a tungsten-faced copper target, consisting of a disk of wrought tungsten onto which a large mass of oxygen-free copper had been cast in a vacuum. The principal properties desired in the target facing are high atomic number for maximum roentgen-ray efficiency, a high melting point, high thermal conductivity to allow maximum energies for a given size of focal spot, and a low vapor pressure to reduce the amount of target metal vaporized. Of all the chemical elements, tungsten combines these properties to the highest degree. Copyrighted 1945 by The Radiological Society of North America, Inc.
Radiology – Radiological Society of North America, Inc.
Published: Nov 1, 1945
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