A frequent question posed on a manufacturer of x-ray tubes relates to environmental conditions which pose a threat to a long lived x-ray tube. This application note sets out the basic conditions under which an x-ray tube will perform to its maximum life expectancy, as well as identifies known areas of concern.
Modes of Failure
The most frequent mode of failure of an x-ray tube is the failure to adequately dissipate the heat generated during normal operation. It is well known that 99%+ of the kinetic energy imparted on the electron beam is lost in the form of heat at the anode target. Thus, a 50W x-ray tube will produce roughly 49.8W of energy in the form of heat just through the conversion process. Add to this the thermal energy produced by the helical tungsten filament and one can readily see that heat dissipation is a major factor.
The failure mechanism, with respect to the x-ray tube itself, due to inadequate cooling can take on two forms; the first is simple sublimation of the anode target material. In converting the anode target material directly from a solid to a gas (sublimation), the resulting vapor rapidly degrades the internal ultrahigh vacuum necessary for proper operation. This loss of ultrahigh vacuum results in a failure of the x-ray tube to withstand the high voltage gap between the cathode electron source (helical tungsten filament) and the target anode. The x-ray tube begins to short circuit, or arc, which in turn liberates more gas, which in turn further degrades the internal vacuum, which finally results in an x-ray tube which no longer functions.
The second failure mode due to improper heat dissipation is the liberation of damaging ions. If the x-ray tube anode is allowed to surpass the vapor pressure point of the target material, than a liberation of ions occur. In turn, these liberated ions are attracted back toward the helical tungsten filament and begin to erode the filament through an ion scrubbing process. This can result in a premature failure of the filament, which manifests itself as a broken filament, or open circuit.
Prevention of both of these failure modes is made possible by ensuring that the x-ray tube is not allowed to overheat. This means careful monitoring of the cooling circuit with fault protection in the event of a cooling system failure. Oxford Instruments now offers integrated thermal protection in its packaged x-ray tubes to prevent this type of failure.
Thus, the acceptable operating temperature range of an x-ray tube relates to the cooling design. Each x-ray tube is slightly different with respect to its tolerance of an allowable temperature range, with some as low at 50 degrees C, while others will tolerate temperatures of 100 degrees C. It is highly recommended that any design integrating an x-ray tube be carefully measured for operating temperature range ensuring the chosen range does not allow the x-ray tube to overheat.
Most x-ray tubes possess either a Beryllium exit window or simply allow the x-rays to transmit directly through the glass envelope. The choice of selecting an x-ray tube with or without a Beryllium exit window will depend on the application, with factors such as low energy flux and mounting requirements influencing the decision. That said, if your x-ray tube contains a Beryllium exit window, you must pay special attention to its particular environment requirements. The Beryllium exit window is comprised of high purity Beryllium metal, typically 125 microns thick. In its metal form, Beryllium is highly soluble in polar solvents. Examples of polar solvents include water, alcohol and acids. Therefore, it is essential that you do not expose the Beryllium exit window to these agents for prolonged periods of time, as they will destroy the window, which in turn will compromise the internal high vacuum of the x-ray tube and cause the x-ray tube to fail. Should your Beryllium exit window need to be cleaned, use a cotton swab and acetone (a non-polar solvent).
Finally, as the exit window of the x-ray tube is typically exposed to a sample chamber environment, ensure that the samples to be analyzed do not outgas polar solvents, as this is a frequent failure mechanism of x-ray tubes. As you might suspect, environments containing high water vapor content (high humidity) should also be avoided as condensation of water on the Beryllium exit window will dissolve the Beryllium metal and cause subsequent failure. Should you wish to operate your x-ray tube in these less forgiving environments, contact Oxford Instruments about coating your Beryllium window with a polymer protective material. This material may adversely affect performance, particularly below 10KV, but will protect the fragile Beryllium window against corrosive damage.
As one might suspect, the operation of an x-ray tube with a Beryllium exit window in an environment comprised of hydrocarbons will depend on the polarity of the molecule in question. Typically most aliphatic hydrocarbons, and many aromatic hydrocarbons are safe with respect to the Beryllium exit window. In fact, the x-ray tube spends the better part of the first 5 days of its life submerged in high voltage transformer oil, a low polarity hydrocarbon. Most damage to the Beryllium exit window from hydrocarbons comes from precipitation of the hydrocarbon on the Beryllium window simply degrading the transmission characteristics. In many cases, this material can be cleaned off with acetone (a non-polar solvent) and a cotton swab. Remember, Beryllium is highly toxic and unprotected hands should never touch the window nor should the window be cleaned if it is broken or fragmented.
Another consideration is the operation of an x-ray tube made with a Beryllium exit window in the presence of a Helium environment. Typically found in x-ray spectroscopy, Helium, as a very small atom, has a high transmission rate through the Beryllium window. As such, careful attention to the design must be considered when using an x-ray tube in this environment. At a minimum, only Beryllium exit windows of at least 125 microns should be considered, unless the Beryllium window is coated with a highly transparent diamond-like protective coating.
Should your design require operation of an x-ray tube with a Beryllium window in a vacuum environment, it is important to remember the Beryllium window material is brittle and as such is highly susceptible to damage caused by cycling the Beryllium window between atmospheric pressure and vacuum environments typical for analytical analysis. Utilization of a secondary chamber is highly recommended such that the x-ray tube operates at sub-atmospheric pressures without cycling for each sample introduction. Contact Oxford Instruments for details on an appropriate design should you wish to operate the x-ray tube in a vacuum environment.