As a result, we now have three electrons (cathode rays) travelling to the anode and three positively charged gas ions (canal rays) travelling to the cathode. In the following figure we see that the electron travelling to the anode has ionized two more gas molecules. The next figure shows the electron travelling towards the anode and the positively charged nitrogen ion travelling to the cathode. The next drawing shows that one of these molecules has been ionized by background radiation. Of course, in reality many more electrons and gas ions would be involved than are pictured.įirst we see a partially evacuated tube containing residual nitrogen (air) gas molecules: The fundamentals of this process are illustrated in the next series of drawings. Upon striking the cathode, some of the positively charged gas ions dislodge electrons which join the other electrons accelerating towards the anode. The resulting positively charged gas ions (known as anode, canal or channel rays) accelerate towards the cathode, and the freed electrons (cathode rays) accelerate towards the target. If the potential is sufficiently high, many of the electrons that accelerate towards the anode will pick up sufficient kinetic energy to cause a further ionization of the residual gas. Free ions are always present due to interactions with of the gas cosmic rays and background gamma rays. The application of even a relatively low potential between the anode and cathode will cause any free ions in the gas (electrons and positively charged gas molecules) to migrate to the electrodes. Corrections and suggestions for improvement to the following discussion are welcome. The physics of a gas discharge tube are complicated, and I only understand a little. The primary ways that X-ray tubes differ involve the construction of the target and the design of the regulator. Adding one or two special appendages to the tube (regulators or regenerative devices) helped control the gas pressure inside the tube. Making the target more massive was the primary method used to prevent it from overheating. Perhaps the most important were an overheating of the target when the tube was under heavy use, short and long-term variability in the gas pressure, reversals in the direction of the current through the tube (inverse discharges), and electrical discharges that might puncture the glass wall of the tube. Several problems had to be overcome when designing the tubes. For therapy, less penetrating X-rays were desired. For diagnostic imaging, more penetrating X-rays were preferred. The higher the applied voltage, the higher the energy of the X-rays, and the more penetrating they became. The greater the current (on the order of a milliamp) supplied by the operator to the tube, the greater the intensity of the emitted X-rays. When the electrons struck the target, X-rays were emitted. If the tube had no anticathode, the anode almost always served as the target. If the tube had an anticathode, it was the target. When a sufficient electric potential (high voltage) is applied across the tube's electrodes, a stream of electrons (aka cathode rays) travels through the gas from the cathode to the target. Tubes with both an anode and anticathode were often referred to as bi-anode tubes. It was not unusual for X-ray tubes to have three electrodes: a negatively charged cathode, a positively charged anode, and what was known as an "anticathode." The latter (aka auxiliary anode) was usually given a positive charge, but it sometimes had no electrical charge at all. The configuration of these tubes varied, but there were always at least two electrodes (an anode and a cathode) that might, or might not, be located at opposite ends of tube. Tubes designed for X-ray work usually contained air, although some (e.g., Snook tube) employed helium or hydrogen. Depending on the type of tube, the residual gas might or might not be air. What these different tubes had in common was the fact that they were made of glass and were partially evacuated. But at the time they were being manufactured, they were simply known as X-ray tubes. Today, gas discharge X-ray tubes are commonly referred to as “cold cathode” tubes in order to distinguish them from “hot cathode” Coolidge X-ray tubes that employ a heated filament. After the discovery, new types were developed that were specially designed to produce X-rays. Prior to Roentgen’s discovery of X-rays in 1895, many different types of gas discharge tubes were already in use (e.g., Geissler, Crookes, Hittorf, Lenard tubes).
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