[stevebrot]

Per the dose analysis cited above, In the USA, 10 CFR 40.13(c)(7) set a regulatory exemption for up to 30% Thorium by weight in new photographic lenses until August 27, 2013, at which point the standard was lowered to 10% (which incidentally is where the dose study estimates lenses like the Takumar are). This is the law as it stands today.

And the laws for Japan?


Steve

[vyoufinder]

...

The non-radioactive elements of the Pentax-K 50mm f/1.4 are literally drop-in replaceable with those of the radioactive SMC Tak of the same specs with zero change in performance, aside from the effects of Thorium browning, which weren't a consideration for lenses that were never conceived of as potentially being used for 40+ years....

There's nothing magic about the thorium. It was just the most economical way to hit the specs produced by designers at the time.

I was thinking that this could be the reason for my SMC K 50mm 1:1.4 is radioactive. I've not seen it reported anywhere as being radioactive but my yellowing finally cleared up with UV light, so I'm pretty sure mine is. I haven't actually tested it (yet.) I've been meaning to ask my friend at the rock shop about testing them. I don't know the best device to test with. Anyone care to suggest?

I'm not selling my radioactive lenses. It's no coincidence that they're some of the best. I may handle/store/etc. them differently, however. I might have as many as 50 highly radioactive lenses.

[Digitalis]

People in general are terrible at assessing risk. They are even worse when it deals with something with a scary reputation like radioactivity.

To be fair we seem to lack the inherent capacity to sense radiation*, we have senses that can tell us when a predator is darting at us - and the capacity to react to such a threat. But we lack the capacity to sense when a wave of gamma rays hits us.

*Anecdotally: The physicist Harry Daghlian who inadvertently triggered a criticality incident with the famous "demon core" was quoted to have experienced a metallic taste in his mouth when one of the neutron reflective tungsten carbide blocks around it slipped and caused the core to undergo a critical excursion - unfortunately the cumulative Neutron/Gamma dose of radiation Harry copped was about 3.1 Gy. Exposure to radioactive thorium in a Takumar utterly pales in comparison to this - but it is interesting.

[Alex645]

I don't know the best device to test with. Anyone care to suggest?

I went to my high school's chemistry/physics department and borrowed a geiger counter. Science teachers dig this stuff. You can set it to different ranges.

My guess is that your lens is made from thoriated glass and you'll notice the readings are affected by distance from the geiger counter sensor to the lens. If you are getting a reading through metal (I used a lead lined film x-ray bag) then that's gamma. If there is no reading, but with a cardboard or paper barrier you get one, then those are beta. If you still don't get a reading, but do so with nothing but air, then those are alpha particles.

In my huge collection, I found one Kodak Instamatic with an Ektar lens and two 50mm Takumars with alpha, beta, and gamma readings.

[cdd29]

slightly off topic but I've seen (including in this thread) where one can clear up a yellowing lens with UV light. Obviously there are many variables (how yellow, exact glass composition, etc.) Roughly how long would it take to clear one? Mine has a little bit of yellow to it. I've wrapped the sides and bottom in aluminum foil and place it on the patio table, dash of my car, etc. when I can. Three days an not much of a change.

[dConvert]

I was thinking that this could be the reason for my SMC K 50mm 1:1.4 is radioactive. I've not seen it reported anywhere as being radioactive but my yellowing finally cleared up with UV light, so I'm pretty sure mine is. I haven't actually tested it (yet.) I've been meaning to ask my friend at the rock shop about testing them. I don't know the best device to test with. Anyone care to suggest?

Well the notes from my radiological protection supervisor training state that Semiconductor/Scintillation Detector should be used for solid samples while the GM tube is for gas. this could be because the scintillation detectors are considerably more sensitive.

I'm not selling my radioactive lenses. It's no coincidence that they're some of the best. I may handle/store/etc. them differently, however. I might have as many as 50 highly radioactive lenses.

Not a chance! You may have 50 radioactive lenses, but unless they've been contaminated or neutron bombarded you won't have even one that's highly radioactive.
I don't think any lenses have been produced that would be classes as moderately radioactive in the view of legislation. If it were it could not be legally sold to the general population.
It's almost certain that they would only be classified as low level.


FWIW in the UK the transport index for radioactively contaminated materials is 0 for anything up to 5µSv/hr, (~44mSv/Yr) this gives it the lowest category. It's possible that lenses fall below this being classed as exempt but this determination is based on the activivity in Bq/g & I don't have that data for doped lenses.