Coolidge X-ray Tubes

Paul Frame, Oak Ridge Associated Universities

Introduction

Without a doubt, the single most important event in the progress of radiology was the invention by William Coolidge in 1913 of what came to be known as the Coolidge x-ray tube.  Nevertheless, despite its clear superiority, the Coolidge tube did not immediately replace cold cathode tubes  -  the latter continued to be manufactured into the 1920s and saw routine use into the 1930s. In fact, there were instances of cold cathode tubes being employed in radiology as late as the 1960s!

If you are interested in the details of Cooludge tube production, the 1920 paper Manufacture of the Coolidge X-ray Tube by Robinson and Moore is worth a read.

The characteristic features of the Coolidge tube are its high vacuum and its use of a heated filament as the source of electrons. There is so little gas inside the tube that it is not involved in the production of x-rays, unlike the situation with cold cathode gas discharge tubes.

The operation of the Coolidge tube is as follows. As the cathode filament is heated, it emits electrons. The hotter the filament gets, the greater the emission of electrons. These electrons are accelerated towards the positively charged anode and when the electrons strike the anode, they change direction and emit bremsstrahlung, i.e., x-rays with a continuous range of energies. The maximum energy of the x-rays is the same as the kinetic energy of the electrons striking the anode.  In addition to the x-rays produced at the focal spot of the anode, some undesirable x-rays (stray radiation) are produced by electrons striking other tube components.

The key advantages of the Coolidge tube are its stability, and the fact that the intensity and energy of the x-rays can be controlled independently.  Increasing the current to the cathode increases its temperature.  This increases the number of electrons emitted by the cathode, and as a result, the intensity of the x-rays.  Increasing the high voltage potential difference between the anode and the cathode increases the velocity of the electrons striking the anode, and this increases the energy of the emitted x-rays.  Decreasing the current or the high voltage would have the opposite effects.  The high degree of control over the tube output meant that the early radiologists could do with one Coolidge tube what before had required a stable of finicky cold cathode tubes. As a bonus, the Coolidge tube could function almost indefinitely unless broken or badly abused.

The Shape of the Tube

The basic shape of the original Coolidge tube consisted of a spherical bulb with two cylindrical arms extending out on opposite sides: the cathode arm and the anode arm.  The arms increase the dimensions of the tube so as to avoid electrical arcing from one end to the other. The length of the arms also helps prevent stray electrons on the inside of the tube from getting close to the tube ends.

The bulb is large so as to increase the surface area through which the tube radiates heat (to limit the glass temperature). In addition, its large size reduces the amount of tungsten that can be deposited per unit area, and it keeps the glass away from the strongest portion of the electrostatic field  -  the longitudinal electrostatic stresses in tubes with a conductive layer of tungsten deposited on the inner surface could negatively affect a tube’s operation.  If the tube incorporates another mechanism by which heat can be dissipated (other than simple radiation through the glass envelope), the bulb can be made smaller. 

The hard glasses (e.g., Pyrex) that began to be used in the 1930s had a sufficiently high dielectric strength and heat resistance that the bulb could be eliminated and the tubes made completely cylindrical.  This was advantageous because shielding a cylindrical tube was a simple proposition.

The Anode

Tungsten is the most commonly used target material in the anode because it has a high atomic number which increases the intensity of the x-rays, and because it has a sufficiently high melting point that it can be allowed to become white hot. During operation, the tungsten target can get as high as 2,700 degrees centigrade.  In many cases, the tungsten target is surrounded by copper  -  the high heat capacity of copper improves the dissipation of heat.

The area over which the electrons from the cathode strike the anode is referred to as the focal spot. The cooler the anode can be kept, the smaller the focal spot can be and the greater the image detail that is possible. If a high x-ray output is required, a larger focal spot would be needed to mitigate the temperature increase.

In the early tubes the angle of the target was usually 45 degrees (see figure below left).  Later tubes often employed the so-called line-focus principle in which the target angle was closer to 20 degrees (see figure below right) . This reduced the effective area of the focal spot (as viewed from the perspective of the object being x-rayed permitted) without significantly affecting the area of the anode bombarded by the electron beam from the cathode.   In other words, it permitted high loading (x-ray intensity) without having to sacrifice image detail.

Some degree of cracking and pitting of the focal spot is normal, especially in tubes with a large focal spot. Fortunately this doesn't affect the tube's performance unless the damage is so severe that the ability of the anode to dissipate heat is compromised. At least in some cases, a tungsten-rhenium alloy has been used in the construction of the anode to reduce the cracking and pitting, as well as reduce the evaporation of tungsten onto the tube envelope.

The Cathode

The cathode of the Coolidge tube incorporates a wound tungsten filament that emits electrons when heated.  There are two general configurations of the filament. Either the spiral tungsten wire takes a circular/conical form (typical of the "Universal" tubes) or it is shaped like an elongate coil (the so-called Benson design).

The filament is located in a cylindrical chamber or slot machined into in the cathode. As a rule, the focal spot is similar in size and shape (either round or elongate) to this opening.

The size and shape of the focal spot is partly determined by the filament's position relative to the other components of the cathode. The focal spot size can be reduced by positioning the filament deeper in the slot (or opening) in the cathode, or increased by moving the filament closer to the top of the opening.

Dual focus tubes employ cathodes with two filaments: one small, the other large. Each filament is positioned in its own slot. The smaller filament produces a small focal spot for fine focus work. The larger filament, which produces a broader focal spot, is employed when faster exposures (higher intensities) are required.  These dual focus tubes usually have a switch at the end of the cathode arm that is used to select the desired focal spot. 

A step down transformer is used to reduce the line voltage from 110 volts to the 12 volts required for the tube filament.  The current to the filament is adjusted with either a Rheostat (resistance) or Inductance type regulator. An ammeter is employed to measure the current used to heat the filament  -  the current serves as a measure of the intensity of the x-rays that are being produced.

To help prevent cold cathode discharges that can result in erratic performance of the tube, the electrodes are smooth and rounded. Such discharges are more likely when very high voltages are used with tubes in which the anode and cathode are closely spaced.

The Glass

Although the envelopes of x-ray tubes can be made of metal, glass is more popular because it is a good insulator, it is vacuum tight, it has a relatively high melting point, it does not seriously attenuate the x-rays, and it can be fabricated into a wide range of shapes.  The early tubes (e.g., pre 1930) were made from soft glass containing sodium or cerium. Later tubes tended to use hard borosilicate glass even though it was more difficult to work with. Borosilicate glass has the advantages of a greater dielectric strength and higher melting point. The latter means that the tubes could be more effectively degassed and evacuated. Because borosilicate glass walls were thicker than those made from softer glass, a window had to be created by grinding down that region of the glass wall that the x-rays needed to penetrate.

With use, the clear colorless glass of an x-ray tube can become colored. Such a color change is either due to the absorption of radiation energy, or the deposition of tungsten on the inside surface of the glass. The absorption of x-ray energy by electrons in the glass promotes them to a higher energy state (the conduction band). The now mobile electrons move to positively charged impurities in the glass where they become trapped. Since these trapped electrons are at a higher energy level than they were prior to the exposure to radiation, the absorption spectrum and the color of the glass is affected.  The resulting color depends on the type of the impurities: the softer glass of the older tubes (e.g., 1920s vintage) tended to turn purple, while the harder glass of the more modern tubes (e.g., post 1930) tended to take on an amber color. 

Just as tungsten can be deposited on the inside surface of an incandescent light bulb, tungsten can be deposited on the inside surface of the envelope of an x-ray tube. If this deposition is significant, the tube was probably operated beyond its rated capacity.  In extreme cases, the deposition can be sufficiently great that it gives the glass a mirrored appearance. A problem with an excessive accumulation of tungsten is that the inside of the glass becomes conductive and allows electrons to leak across the glass surface.  This makes the tube more susceptible to puncture.

In addition, the accumulation of electrons on the inside of the glass can distort the flow of electrons from the cathode towards the anode.  It can also produce a static charge that attracts dust onto the outer surface of the tube. If this dust absorbs moisture from the air, the outer surface of the tube becomes conductive and this increases the possibility that sparks will be produced that can puncture the tube’s envelope. Tubes designed for therapy required thicker glass than diagnostic tubes because the higher voltages at which they operated produced a greater potential difference between the negatively charged inside of the glass and the outside of the glass. If the glass was too thin, the charge would break through the glass and puncture it. 

When operated at low voltages, the glass envelope might fluoresce. The color of the fluorescence depends on the type of the glass: soft and hard glass usually exhibited a green and blue fluorescence respectively.

The wire penetrating the glass seals at the end of the tube is what was known as Dumet wire, a copper-coated alloy of nickel and iron that had the same coefficient of expansion as the glass.

Shock-Proof Systems

The electrical cables providing high voltage and current to the early (pre 1930 or so) x-ray systems used bare uninsulated wire.  The reason for this was that insulated wire could duplicate a choke-coil and limit the filament current. As a result, early radiologists had to be very careful to avoid electrocution.  The first completely shock-proof x-ray system seems to have been a dental x-ray unit developed in 1921 by William Coolidge  -  the high voltage system and tube was immersed in a grounded metal tank filled with oil.

Immersing the tube in oil not only improves cooling, it reduces the potential for leakage current across the tube surface, and this means that the tube can be made shorter. It also reduced the effect of altitude and humidity on tube performance (altitude and humidity affect the cooling capability of air).

References

E. C. Jerman. Modern X-Ray technic. Bruce Publishing Co. St. Paul. 1928.

R.L. Eisenberg. Radiology – An Illustrated History. Mosby. St. Louis. 1991.

W.D. Coolidge. The development of modern roentgen-ray generating apparatus. Am. J. Roentgenology. 24: 605-620. 1930.

M.J. Gross, Z.J. Atlee. Progress in the design and manufacture of x-ray tubes. Radiology 21:365-377; 1933.

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Last updated: 06/24/09
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