Filament burnout is tungsten vaporizing off the filament until the wire breaks
Key takeaways
- The filament thins as tungsten vaporizes, then breaks.
- Long periods at high mA vaporize more tungsten.
- The deposit on the enclosure adds filtration and can cause arcing.
- Bushong names tungsten vaporization with deposition as the most common cause of tube failure.
- Bushong describes a gassy tube separately from a broken filament.
A filament burns out because tungsten slowly vaporizes off it until the wire is too thin and breaks. The result is called an open filament. It is the same kind of failure as an old light bulb burning out (Bushong 11th ed., PDF p.138).
Keeping a filament hot is its job. Heat releases electrons, which is called thermionic emission, and that same heat slowly drives off tungsten atoms, even in normal use. Long periods at high mA heat it more and vaporize more.
Where does the tungsten go?
It plates the inside of the enclosure, and that causes two more problems:
- Filtration. The coating on the window adds to inherent filtration, which tends to rise as a tube ages.
- Arcing. The deposit upsets the tube’s electrical balance. Tube current can change abruptly, and current can arc from the filament to the enclosure. This usually ends the tube suddenly.
Bushong calls tungsten vaporization with deposition the most common cause of tube failure, and arcing from it the most frequent cause of abrupt failure (PDF pp.127, 138). Metal enclosures hold up better than glass here. Some tungsten also comes from the anode when it overheats.
Is a gassy tube the same thing?
No. A gassy tube has gas inside the enclosure instead of a vacuum. X-ray production falls and the tube can fail. Bushong describes this apart from a filament that thins and breaks, and it’s a separate item in our Image Production guide. The two aren’t unrelated, though: Bushong says that if tungsten vaporized easily, the tube would become gassy quickly (PDF p.127).
An earlier version of this page wrongly taught vacuum loss as the cause of burnout. We corrected it.
Is it just normal wear?
Vaporization happens with normal use, so some wear can’t be avoided. You can limit it:
- Don’t hold prep longer than you need. Bushong says to push a two-position exposure switch to its final position in one motion, which minimizes the time the filament is heated and prolongs tube life (PDF p.132).
- Avoid long runs at high mA where a lower station will do the job.
Anode damage such as pitting and cracking is its own failure, and so is bearing wear. A tube rating chart guards against overloading the anode. Our page on tungsten anodes covers why the metal is used at all.
To go further, read the Equipment Operation and QA chapter and try the equipment practice questions.
Frequently asked questions
What causes x-ray tube filament burnout?
Tungsten vaporization. The filament runs hot enough to release electrons, and tungsten atoms slowly leave it too. The wire thins and eventually breaks.
What is the most common cause of x-ray tube failure?
Bushong names tungsten vaporization with deposition on the inside of the enclosure. The deposit upsets the tube's electrical balance and can lead to arcing.
What is a gassy tube?
A tube with gas inside the enclosure instead of a vacuum. X-ray production falls and the tube can fail. Bushong describes this apart from a filament that thins and breaks, and never says gas breaks a filament.
How can you extend filament life?
Limit time spent at high mA, and don't hold prep longer than needed. Bushong says to push a two-position exposure switch to its final position in one motion, which minimizes filament heating and prolongs tube life.
Sources
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