eli5 How come if highly radioactive radium was on watches that the wearers didn’t succumb to radiation poisoning at the wrist?
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Even a cheap wristwatch has the clock face encased in metal and glass, which would have provided at least some sheilding. Compare and contrast with the Radium Girls, who had a more direct, prolonged exposure to the radium paint.
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It was usually a minuscule amount of radium-laced paint on the hands of the watch. The main reason why the watch factory workers (the “radium girls”) got radiation poisoning was because they worked with large quantities of radium paint on a daily basis. And since science was unaware of the radioactive effects at the time, they’d do stuff like lick the tips of their paintbrushes to taper the point, so they were actively ingesting it too.
Alpha radiation from radium consists of helium nuclei, which is a whole helium atom minus the electrons. These are (in relation to other forms of radiation) HUGE and easily stopped by almost anything, even a few centimeters of air are enough. You can shield yourself from alpha radiation literally by holding up a sheet of paper. So a watch or some thin glass cover would be sufficient.
I have a friend who has a uranium pellet encased in resin as a keychain, it was an official souvenir from visiting a nuclear power plant.
Radium is what we call an Alpha Emiter. It sends off large destructive alpha particles but those are easily blocked. Your outer skin is covered by old dead or dying skin cells that block virtually all of it.
The danger is if it gets inside your body like your lungs or stomach. Inside it will start tearing up the living cells of your organs.
The entire radium decay chain* emits only alpha and beta particles (and no gamma radiators)
Alpha particles are blocked by a piece of paper, so these are mainly dangerous if you get the material inside your body (if you for example lick a brush with radium paint on it) and beta radiation is slightly better at penetrating materials but is generally stopped by a very thin sheet of metal (like the metal body of a watch).
There is also very little radium in a watch (very thin layer of paint that contains radium and a material that absorbs radiation and then emits that energy as a photon, ie visible light).
*Radium collapses into radon->Polonium 218->Lead 214->Bismuth 214 etc etc. Each time it emits either a helium-4 particle (alpha radiation) or an electron/positron (beta radiation).
Think of a radioactive object as a person with a gun who randomly shoots bullets in different directions.
In our case, the radioactive object is the radium. It fires 3 types of bullets: alpha particles, beta particles, and gamma radiation.
Alpha particles are huge, so they tend to get stopped by almost anything they hit, from your skin or the back of the watch or the glass of the watch.
Beta particles are smaller and pass through more things: they can go a half inch through skin but something like a thin sheet of plastic can stop it.
Gamma rays can pass through the human body, ionizing (or changing the atomic makeup of atoms) as it goes.
With all that in mind, go back to the person with a gun shooting 3 types of bullets from the radium watch. Half the bullets are fired away from the body and won’t hit you. The other half are fired towards the back of the watch and that stops alpha and most beta particles.
So, you are left with gamma rays, which might kill a handful of blood cells or marrow cells as they pass. Not great, but not the end of the world.
Now, imagine you are painting the watches and, as part of your practice, you lick the radium coated brushes. Now, the little person shooting radiation bullets is inside your body! 1) Every direction it shoots, it hits your body. There’s no shielding and no firing off in a direction that won’t hit you. 2) It’s not hitting skin, it’s hitting organs and things inside of you. 3) Unlike a watch, you can’t take it off. It’s inside your stomach and passes out your bowels or gets integrated into your cells. 4) The dose of radioactive material gets higher over time as you consume more radium, much more than the tiny watch arm.
Even if you are ingesting radium, the dose only becomes really deadly over time: killing smallish numbers of cells per day, until the dose accumulates and starts killing more and more cells at a higher rate and possibly mutating DNA, leading to cancers.
Radium itself doesn’t glow. The watch dials were done with luminous paint containing only enough radium to get the paint to luminesce due to the gamma radiation emitted by the radium.
Radium mostly emits alpha radiation. Alpha radiation consists of accelerated helium cores and is the most devastating on our bodies, but also the easiest to shield. The casing of the clock shields *all* of the alpha radiation and I do mean *all* of it. But if you swallow radium, you don’t even have your skin to protect you, so the alpha radiation can hit your organs directly and will kill you in short order.
A company called Traser still manufactures tritium gas contained in small tubes indicating the numbers in the watch dial for the US Military.
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