>>3821800>which in some cases have more aggressive decay mechanisms.The only issue along the chain is Radon. And even then the gamma emission is noted as "significant" only in comparison to the other decays which emit pretty much no gamma radiation.

In any case, the half life of the intermediary atoms is longish by human standards, so think about that:

Once 4 Radium-228 isotopes become available, it'll take ~5 years to convert 2 to Thorium-228.

Once you have 2 Thorium-228 isotopes, it'll take ~2 years to convert 1 to Radon-220.

Once you have Radon-220 (and the ones below it), you have your tiny gamma radiation.

Now the elephant in the room: How often do you get those 4 Radium-228 isotopes?

Well if you had 8 Thorium-232 isotopes, it'll take 14billion years to get 4 Radium-228 isotopes.

So the already insanely long half life of Thorium-232 is very slowed down considerably (by human life standards) when by the decay of the products along the chain until you reach the "dangerous" Radon-220.

In other words, the rate of production of Radon-220 from the initial pure Thorium is very small, and hence the rate of energy emitted through gamma radiation very, very small.

Of course it's all probabilistic and proportional to how much Thorium you start with, double the amount means double the Radon in the same timespan.

Now combine all that with the fact that Thorium was used at up to 30% concentration in *some* elements of the lens, and you get a sense of the scale.

People have run the numbers, and long story short, for a 70kg photographer using a Jena Tessar lens, for a "typical" 240 hours/year of holding the camera to your face.

Long story short, per year that lens will eat up 0.17% of your total radiation "allowance" to stay within safe limits.

>source: https://www.diva-portal.org/smash/get/diva2:652338/FULLTEXT01.pdf