Gas Discharge Tubes and Cold Cathode X-ray Tubes

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. If the tube had an anticathode, it was the target. If the tube had no anticathode, the anode almost always served as the target. When the electrons struck the target, x-rays were emitted. 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. The higher the applied voltage, the higher the energy of the x-rays, and the more penetrating they became. For diagnostic imaging, more penetrating x-rays were preferred. For therapy, less penetrating x-rays were desired.

Several problems had to be overcome when designing the tubes. 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.

Making the target more massive was the primary method used to prevent it from overheating. Adding one or two special appendages to the tube (regulators or regenerative devices) helped control the gas pressure inside the tube.

The primary ways that x-ray tubes differ involve the construction of the target and the design of the regulator.

Gas Discharge Tubes - Some Details

The physics of a gas discharge tube are complicated, and I only understand a little. Corrections and suggestions for improvement to the following discussion are welcome.

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. Free ions are always present due to interactions with of the gas cosmic rays and background gamma rays.

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. 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. Upon striking the cathode, some of the positively charged gas ions dislodge electrons which join the other electrons accelerating towards the anode. The fundamentals of this process are illustrated in the next series of drawings. Of course, in reality many more electrons and gas ions would be involved than are pictured.

For every electron striking the target, a positive gas ion strikes the cathode.

As long as a high enough potential is maintained between the two electrodes, the cathode ray beam (and canal ray beam) is self-sustaining and the region between the electrodes contains a mix of free electrons, positively charged gas ions, and neutral gas molecules (both excited and unexcited).

The positive gas ions travelling to the cathode are less likely than the electrons travelling to the anode to ionize the gas. However, over time these ions might damage the cathode when they strike it if the tube was operated at high enough currents and voltages. This damage involves "sputtering," a process whereby some of the atoms of a material are knocked free when struck by ions. Electrons striking the target don't have sufficient mass to cause significant sputtering.

As the electrons travel to the anode, they excite, as well as ionize, some of the residual gas molecules. This causes the gas to glow. Ionization and excitation will not occur in the gas close to the cathode because the electrons have not had enough time to gain the required kinetic energy.

Typical Cold Cathode X-ray Tubes

The following diagram illustrates the key components of a typical heavy anode cold cathode x-ray tube.

Cathode

The original cathodes were flat plates that emitted a broad beam of cathode rays (electrons). The resulting x-rays issued from the extended region of the target upon which the cathode rays impinged. Such x-rays produced relatively poor radiographic images. It was soon recognized that better results would be obtained by focusing the cathode rays on a small region of the target. If the resulting x-rays issued form a small point on the target, the images produced by such x-rays would be 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" and their cathodes described as "cups." The earliest type of x-ray tube with a focussed cathode was the Jackson tube. Professor H. Jackson of Kings College, London is usually credited to being the first to use a cupped cathode in an x-ray tube. Nevertheless, William Crookes had employed focussed cathodes in his "Crookes tubes" in the days before x-rays were discovered.

The cathode was almost always made of aluminum because aluminum is less prone to "sputtering" than other metals. Sputtering is a volatilization of the metal that occurs when it is struck by gas ions. The volatilized metal, which deposits on the glass walls of the tube, has a tendency to absorb some of the gas in the tube. This increases the tube's resistance to the flow of current and ultimately shortens its life. If sufficient metal deposits on the tube's glass walls, the latter might darken. In general, a visibly darkened tube is likely to have too high a vacuum (too low a gas pressure) to be usable.

In almost all cold cathode x-ray tubes, the cathode cup (aka mirror) is positioned near the circumference of the spherical bulb portion of the tube. Unfortunately, I have never seen the reason for this clearly explained. The position of the cup along along the glass arm that leads to the bulb certainly affects the voltage required to generate a given intensity of x-rays: the farther along the glass arm (i.e., the closer to the bulb) the lower the required operating voltage and the lower the energy of the x-rays produced. The reason, as Hittorf noted, is that a static charge will accumulate on glass (e.g., the glass arm) located near an electrode. The greater this charge, and proximity of the glass to the electrode, the more resistant the electrode becomes to the emission of electrons.

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