Just FYI, If you were using a Geiger-Mueller tube it is not going to detect alpha. You need an instrument specifically designed for alpha detection to detect alpha particles. However, with your "Geiger Counter" you may be able to detect the accompanying gamma or any beta radiation, if the detector has a beta window, that maybe emitted by radioactive daughters in the the decay chain.

Quote Originally Posted by One_DaveT View Post
Tonight I just learned about the use of thorium in lenses up until 1980's. This prompted me to go dig out my 60's vintage civilian defense geiger counter, which I bought years ago after some goof ball medical technician made a wise crack about the half life of the liter of barium I had just quaffed for a cat scan. (fwiw, the drinkable stuff is from a non-radioactive isotope).

Anway, I put my old lenses to the test. I can say after scanning for alpha and beta particle that I believe the following lenses are Thorium-free:

Nikkor-N 24/2.8 pre-AI
Nikor-S.C 50/1.4 pre-AI
Nikkor-P 105/2.5 pre-AI
Nikkor-Q C 200/4 pre-AI

FWIW, I checked a few other lenses:
Zuiko OM 50/1.4
Minolta Rokkor 200/3.5
Minolta Rokkor 50/1.7

After reading that Thorium in lenses goes back to 1937, I checked my Exakta lenses: Steinheil 135/3.5, Westenar 50/2.5..... nope.

In hindsite, after reading wikipedia, it doesn't seem like much of an issue anyway. It says alpha particles from thorium can't even penetrate human skin, so even if I did have a thorium lens, it likely wouldn't be a concern unless I breathed the pulverized dust of a broken lens. Given that the use of it in commercial lenses stopped around 1980 makes me wonder if three-mile island incident in 1979, might have made marketing thorium lenses impossible, and lead to the end of it's use.

Interesting diversion for the evening though.