Inside the magnetron the tungsten cathode contains 2% thorium 232 which is radio active this is what sets off the radiation scanners at the scrap yards.
The image below is thoriated tig welding electrodes which are much larger than the microwave cathode. The welder who has to sharpen the tip has much more exposure to this radio active material than you will ever experience.
I would think any welding shop disposing on the electrode nibs in the scrap bin would also have a rejected load, finding that nib would be like looking for a needle in a hay stack.
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