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):
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.
- an old watch with the glass removed, dial towards film
- another watch, slightly less active, with front glass
- a plastic box containing thorium nitrate (metal toy figure between film and box)
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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