NavList:

Here's another link confirming that the risk is manageably low:
https://hps.org/publicinformation/ate/q11972.html.

That article includes a link, which has yet another link, to this detailed article:
https://www.orau....radioluminescentinfo.htm
I loved the bit about early Radium painters in Switzerland, glowing at night as they walked down the streets, with hair sparkling like halos.

Note that the normal radiation from Radium is alpha particles (Helium nuclei) which are completely blocked by the cases of watches and sextants, too. Of course that helium is slowly building up inside your sextant, and eventually it will explode from over-pressure! Well, no, not even that. You'll get something like a microgram of Helium from a milligram source of Radium in a century.

The article notes that the real radiation risk comes not from the Radium initially but further down the decay chain which does produce penetrating gamma rays. Nontheless, under normal conditions it's a small fraction of background radiation even from a watch which is worn on the wrist all day long. Exposure from sextants for professional navigators decades ago, using sextants several times a day, would still be yet another small fraction of that small amount. For modern navigation enthusiasts, who might use a radioactive sextant once a week on average at the high end, it's another order of magnitude lower exposure.

Brad mentioned storing radioactive sextants in a safe. This won't help a whole lot since the dangerous radiation is gamma down the decay chain. The primary alpha that makes the glow and most of the beta radiation (which is small anyway) are stopped inside the instrument. But as we know, the gamma will penetrate a few feet of solid lead with little reduction. So now what??

There is actually a simple defense against gamma radiation in this case. These instruments are essentially point sources, and the sources are confined. They won't escape. Therefore the radiation intensity obeys the inverse-square law (which is why I suggested you would be ok unless you sleep with it under your pillow!). A radioactive watch about a foot from your body (average distance) increases exposure modestly above background --a few percent. Want to decrease that by a factor 100? Then store the sextant at least ten times further away. Store the sextants at least ten feet from all humans (and pets, if you worry), and the radiation exposure is essentially un-measurable. Still worried? Go to 100 feet (store them in your garage?). Every factor ten in distance reduces your exposure by a factor of 100.

While you're at, get rid of all rocks in or near your house. Many are natural gamma emitters. Seriously, lots of natural granite is radioactive, and all radiation is problematic, but it's a question of scale compared to natural background radiation. Here's some EPA advice on granite countertops: https://www.epa.gov/radiation/granite-countertops-and-radiation.

Want to avoid genuine, major sources of radiation? Never visit mountains, and never fly above 2500 feet! The atmosphere is our radiation-blocking security blanket (keeps out meteors, too!). Every time you fly on a normal commercial airline flight with a cruising altitude up near the tropopause you're exposed to roughly ten times the background sea level dose. Yet frequent flyers don't get cancer at measurably increased rates. More here: https://www.scientificamerican.com/article/air.... It's a lot of radiation, but its effects don't show up in the statistics. Still, you can't live up there permanently, or you will die from the radiation. As this article notes "it’s the radiation dose problem that is a major spoiler for Elon Musk’s goal of inhabiting Mars. An extended stay on Mars, with its extremely thin atmosphere, would be lethal due to the high radiation doses." 

Frank Reed