Autoradiograph of some sources

This image was produced with an x-ray film that was wrapped light-tight with aliminium foil. I put three radioactive sources directly on the wrapped film and left them there for three days. The film was then developed in the usual way. The three sources were (from left to right):

  1. an old watch with the glass removed, dial towards film (GIF/JPG)
  2. another watch, slightly less active, with front glass (GIF/JPG)
  3. a plastic box containing thorium nitrate (metal toy figure between film and box) (GIF/JPG)
The luminous paint used on old watches contains considerable amounts of radium and its decay products, making it a quite active alpha, beta and gamma emitter. Thorium is an alpha emitter, some of its decay products are beta emitters. The film probably is most sensitive to beta rays.

You can see that the front glass of a watch holds back quite a bit of radiation! The metal case absorbs nearly all, so there's no danger from the backside at least.

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Jochen Kronjaeger