| | | X-ray window. This is described as an x-ray tube window.  It has a crude edge around it, which I thought might be just the way they are (since the x-ray beam may be exiting though just a small area in the middle).  However reader Marten Kooistra from the Netherlands thinks it looks more like someone cut it out of the tube with a screw driver or something.  He sent the following picture of a tube he had with a similar window, showing what a window like this looks like in place:
 
  Source: eBay seller rubbleshop
 Contributor: eBay seller rubbleshop
 Acquired: 4 November, 2002
 Price: Donated
 Size: 0.5"
 Purity: >99%
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