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Tritium is itself radioactive and used in watches to make a luminescent material glow. I think zinc sulphide was common and it would probably glow if lit up enough. Not sure you want to be near something that makes your watch dial markings glow visibly more than the radium or tritium that's in the paint, though.

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The tritium interacts with a phosphor through beta decay, so presumably if you hung a watch near it the phosphor would glow brighter with higher concentration of beta decay. Basically a sort of visual Geiger counter.

Zinc sulphide, from brief googling, seems like it would glow in anything so if this was a thing, it probably makes most sense for gamma radiation since the 'detector' isn't being massively lit up by that internally already.

Sure, but run the inverse-square law on the difference between a ~100 micron paint layer and a source that's ~1 meter away.

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