Tritium watch question

Factual Questions
1

Shout out to the intersecting set of watch geeks and physics geeks.

I needed a watch that had a bezel for timing things, and high luminescence that persisted all the way to dawn. This reduced the scope to, in essence, diving watches. I also strongly prefer mechanical watches.

So my final choice was a Ball, with tritium illumination. (The numbers on the dial and the hands are fitted with tiny glass tubes lined with chemicals that fluoresce in the presence of the tritium gas that fills the tube).

Now I gather that the radiation from the tritium poses no real risks, absent breaking open the watch and ingesting the tritium. Tritium decays by beta radiation, which is said to be very low energy (0.018 MeV).

It is said that 6 mm of air will stop this, and the watch is very chunky, electrons will not get through the steel casing, nor, I imagine, the sapphire crystal lens.

But it occurred to me that the beta particles themselves might not be the whole story wrt radiation. I know you typically need neutron radiation (which tritium does not do) to get any serious fission going, and that is not at all my worry. But is it possible for the betas to interact with other elements in the watch (Ti, Fe, C, Si, O, etc) to trigger some radiation which might be more problematic than the original beta from the T?

I imagine it is highly unlikely. I know of research done re tritium illumination in plastic watches, which indicates harmlessness. And the likelihood is that given that this watch is a honkingly solid lump of metal and Al oxide (because it is a diving watch) it is even less likely to be a problem. Further, ever since the disasters of radium illumination in the early 20C, it is unlikely that any watch manufacturer would dip their toe into radioactive illumination without being very sure it was safe.

But I am still curious. Could the betas excite a more worrying form of radiation?

2

Googling produces results that seem to all come down of the side of ‘No’.

Here’s a document that discusses the health factors associated with Tritium

3

I keep a vial of tritium four inches from my testicles at all times.

The short answer is no. Note that the potassium-40 in a banana decays with beta radiation at 1.33 MeV.

The energy in these electrons is just not enough to do any damage. It’s barely enough to light the phosphor (a CRT TV has electron energy of ~20 keV). Once it hits the phosphor, all the energy is turned into a harmless visible-light photon (and some heat). There’s nothing left to cause radiation damage.

4

Thanks for that. I had Googled, but lack the expertise to know whether I was asking the right question or understanding the answer correctly.

5

What you’re wondering about is whether other atoms nearby can be made unstable (radioactive) by absorbing the beta rays, i.e. fast electrons. The answer is generally no, because these electrons will be strongly repulsed by the electrons surrounding any nucleus, so no nuclei will absorb them (even assuming absorbing an electron would turn them radioactive, which is only rarely true).

Almost always secondary radioactivity comes from the emission of neutrons. Neutrons, being neutral, pass readily through the electrons around nearby atoms and are not repulsed by the positive charge on the nucleus (unlike alpha particles), and hence are not at all unlikely to penetrate nearby nuclei and be absorbed. And it is pretty common for the absorption of an extra neutron to turn an atom unstable.

6

You don’t necessarily need nuclear interactions to get secondary radiation. In fact, the glow of the hands is itself a form of secondary radiation. It’s not that the secondary radiation doesn’t occur; it’s that it’s a sufficiently low energy that it isn’t a problem.

7

The bottom-line answer to “Is it dangerous?” has been correctly answered (“No”). A couple of notes, though…

Bananas are safe (including safe to eat) because there isn’t much [sup]40[/sup]K in them, not because their energy is “low”. On the contrary, a 1-MeV electron is highly ionizing and will rip apart plenty of molecules. Electrons from [sup]40[/sup]K outside the body, of course, are easy to shield against.

20 keV is a massive amount of energy for lighting a phosphor. Atomic excitations have energies in the eV range, so a single 20-keV electron can excite thousands of molecules. Note that CRTs do emit a low level of x-rays induced by the electrons.

This watch will also emit x-rays due to bremsstrahlung radiation and a little from the knock out of low-lying atomic electrons. But, these x-rays will also be heavily attenuated by the watch material to essentially undetectable levels. (I mention it for completeness, not because it’s a worry.)

8

I illuminate mine with a string of LED Christmas lights.

9

Here’s a lecture of physics professor from UCBerkeley talking mentioning a tritium watch and how he wouldn’t wear it. He talks about it several times. Very interesting lecture about radioactivity for the curious.

I put the time when he first mentions it:

10

Which Ball watch? Can you include a link?

11

Sounds like a Neal Stephenson character.

12

But… but… he says exactly the opposite of this. He is actually wearing one, he says he loves his tritium watch (that quote is right at your linked timestamp), and he draws nice analogies between radioactivity on one side and fire and electricity on the other, noting that if all you knew about fire and electricity were that they could kill you, you’d be scared to cook anything or change flashlight batteries. He also explicitly mentions that the radioactivity in his watch can’t get out and he demonstrates it by getting no appreciable increase in readings from his Geiger counter when he places it against the watch.

13

Engineer Hydrocarbon NEDU.

Mine has a blue face.

14

Thanks, and that’s a very nice watch. On this page when you tap on the NEDU, the watch face changes to darkened with the tritium glowing: Welcome to BALL Watch - Engineer Hydrocarbon.