XRD tubes rely on the ability to precisely regulate the flow of electrons from the filament to the anode in order to create x-ray emissions. That requires a completely evacuated envelope to avoid having the high-voltage short to ground. The resulting symptom is that the high voltage generator will shut down almost instantly when the high-voltage potential is applied.
This particular tube looked fine on the outside and didn’t even look all that old so it was a little surprising to find it behaving like the vacuum in the envelope had been compromised. However, after two tests, it was definitively bad so it was set aside for disposal. The first step is simply to remove the head of the tube which is little more than a mounting flange and cooling water distribution device, but when it was removed, the back side of the anode is cooled directly with water and showed some of the most extreme pitting we’d ever seen!
The next step is to break the ceramic envelope off the metal body of the tube. It was full of water which was obviously the cause of the problem. The pitting must have broken through to the vacuum envelope, which then sucked in water until it was 100% full. Hence the lack of any “sloshing” while we handled the tube.
Cross sectioning the tube anode showed just ow extensive the pitting was. The actual hole was so small that I could only guess where it had been, but it was definitely there.
We disassemble the x-ray tube on XRD systems during every preventative maintenance procedure and definitely would have caught this before it failed completely. It was most likely caused by either very low quality tap water being used in the recirculating chiller or some additive designed to prevent scale or algae.
