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This image of a De Havilland Comet cockpit shows a radiation hazard symbol on each of the three centre panels, next to the (to me unknown) blue, green, and grey indicators. Why are they there?

I can't find any resources to shed light on it, but then again, I'm no aviator myself so I probably looked in the wrong places.

enter image description here Source: Wikipedia

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    $\begingroup$ That could be a warning that the instrument markings use a radioactive paint that would stay illuminated at night with no cockpit lighting. The PA23's (mid-1950s vintage) original instrumentation had radioactive instrument markings, if I recall. $\endgroup$ – acpilot Nov 28 '20 at 1:56
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Instrument markings back then were painted with luminescent paint containing radium, so they would glow in the dark continuously without having to be energized by a light source.

You can buy luminous watches today with the same feature that use tiny vials containing tritium gas for each marking (much less radioactive than radium and produces negligible amounts of radioactivity, so the watch can't hurt you). These also glow 24/7, but tritium has a shorter half-life and they lose about half their brightness every 7 years or so.

The radium paint was mostly a danger to the women who painted the instrument faces, who would lick the brush tips to get a fine point and could potentially ingest enough radium to develop radiation poisoning.

The radioactivity warning signs may be perhaps a little bit of overkill since the actual radioactivity emissions aren't that high and most of the risk is from ingesting radium directly; it's advised to avoid opening an instrument and poking around without appropriate precautions (If people knew the amount of radiation they're exposed to just being out in the natural environment, they'd probably want to move into a lead house. I recall a story about a 3 Mile Island Nuclear employee who was setting off the radiation detectors at work while coming IN to work, not while leaving. From some kind of radiation exposure while off work.) .

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    $\begingroup$ Ra226 is not significantly decayed after 70 years (halfatime around 1600 years IIRC). I have a WWII compass and it is still quite radioactive. However, maybe you are confused by it not being very bright. That is NOT because of the radiactive source, it is because of the luminophor. The alpha particles from Ra226 are strong enough to destroy the luminophor crystals and hence significantly reduce the light output. The largest danger with radium is swallowing it. Alpha radiation does not penetrate the skin but can be very harmful when staying inside the body for a long time. It makes radon too. $\endgroup$ – Vladimir F Nov 28 '20 at 10:50
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    $\begingroup$ BTW on my compass, some of the luminous markings are not even covered by any glass. $\endgroup$ – Vladimir F Nov 28 '20 at 10:55
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    $\begingroup$ @Stefan Only an "aviationist" would rename alpha and beta particles "alpha and bravo particles" as in your link:) $\endgroup$ – alephzero Nov 28 '20 at 15:58
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    $\begingroup$ @alephzero I didn't even notice that, and I'm working with radioactivity, not aviation... $\endgroup$ – Stefan Nov 28 '20 at 16:32
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    $\begingroup$ @JohnK if the worker forgot to take off the dosimeter, it could be radiation from a CT scanner, a baggage x-ray machine, or from cosmic radiation from high altitudes (e.g. flying in an airplane) $\endgroup$ – Stefan Nov 28 '20 at 20:08

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