Beryllium X-ray window
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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%