Introduction to Gas Discharge Tubes and Cold Cathode X-ray Tubes
A wide variety of gas discharge tubes were in use prior to the discovery (in 1895) of x-rays, e.g., Geissler, Crookes, Hittorf, and Lenard tubes. What these different tubes had in common was the fact that they were made of glass and were partially evacuated. A negatively charged cathode and a positively charged anode were located at opposite ends of tube. Depending on the type of tube, the residual gas might, or might not, be air. Geissler tubes, for example, usually employed noble gases.
The application of a high voltage, provided by a static machine or induction coil, caused any free ions (electrons and positively charged gas molecules) in the gas, such as those produced by cosmic rays, to migrate to the electrodes. Many of the electrons (originally referred to as cathode rays) accelerating towards the end of the tube where the anode was located picked up sufficient kinetic energy to cause further ionization of the residual gas. The resulting positively charged gas molecules (known as anode or channel rays) were accelerated towards the cathode. Upon striking the cathode, they dislodged electrons which joined the other electrons accelerating towards the anode.
|Cold Cathode X-ray Tubes
Following the discovery of x-rays, the design of these gas discharge tubes was altered so as to maximize and control their x-ray output.
The following diagram illustrates the key components of these so-called cold cathode x-ray tubes.
The original cathodes were flat plates that emitted a broad beam of cathode rays (electrons). The resulting x-rays issued from an extended region of the target upon which the cathode rays impinged. Such x-rays produced relatively poor images. It was soon recognized that better results would be obtained by focusing the cathode rays on a small region of the target. Because the resulting x-rays issued form a small point, the images produced by such x-rays were much sharper. To achieve this, the cathode was curved so that the focal point for the cathode rays was located on the target. For this reason, these tubes were often referred to as "focus tubes." The cathode was almost always positioned on the circumference of the central spherical portion of the tube. In almost all cases, the cathode is made from aluminum because it is less prone to "sputtering" than other metals. Sputtering is a volatilization of the metal - this volatilized metal will deposit on the glass of the tube and darken it.
In the first tubes used for x-ray production (e.g., Crookes and Hittorf tubes), the anode was located off to the side at one end of the tube. The cathode rays streamed past the anode and struck the glass end of the tube which served as the source of the x-rays. Unfortunately, glass could not hold up under prolonged use. One early solution was to position the anode (a flat metal plate) at the far end of the tube opposite the cathode. In this configuration, the anode served as the target of the cathode rays and the source of the x-rays. However, in the vast majority of the early x-ray tubes the target was an auxiliary anode, known as the "anticathode," that was positioned between the anode and cathode.
The term "anticathode" was coined by Silvanus Thompson to refer to the target upon which the cathode rays (electrons) impinged. Sometimes the anode and anticathode were one and the same, but in most of the early tubes they were distinct entities. The anticathode, electrically connected to the anode, usually came in from the side of the tube at a 45 degree angle. The earliest anticathodes were simply flat metal (e.g., platinum) plates. Later versions were more massive to facilitate heat dissipation. In many cases the anticathode consisted of two different metals bonded together: one metal, with a high atomic number (e.g., platinum, osmium, molybdenum, tungsten), served as the actual target for the cathode rays and source of x-rays, while the other metal (copper) helped dissipate heat.
There was no clear understanding as to why it was useful to have a separate anticathode (in addition to the anode). It seems that it was assumed to prolong the life of the tube and/or reduce variations in the gas pressure. In later tube designs (e.g., 1910+), the positions of the anode and anticathode were often reversed so that the anode came in from the side and the anticathode came in from the end of the tube opposite the cathode. Eventually, it was recognized that no useful purpose was served by employing both an "anode" and a separate "anticathode." The device that had been referred to as the anode was eliminated and the "anticathode" (i.e., the target) now became known as the anode.
Regulating Gas Pressure
For proper operation, the gas pressure inside the tube needed to be on the order of 0.2 to 0.5 mm Hg. A tube with higher pressures was too "soft" while a tube with a lower pressure had too "hard" a vacuum. Initially, outgassing from some of the tubes metal components (especially the aluminum) would soften the tube, but over time it was common for the gas pressure to decrease as the residual gas in the tube was adsorbed on the tube’s walls and other components. This hardening of the tube reduced the intensity of the x-rays. At the same time, the x-rays became more penetrating (higher energy).
While the long term trend was towards a hardening of the tube, the pressure in the tubes could vary in an unpredictable fashion over the short term. For example, during use the gas pressure could increase temporarily as the glass wall of the tube heated up and released some of the adsorbed gas.
In general, two methods were employed to correct for the hardening of the tube:
Copyright 1999, Oak Ridge Associated Universities