I am very blessed to have been recently given a wonderful collection of historically-significant vintage cameras and lenses. Included in this collection were some amazing Pentax/Takumar lenses; upon doing some research, I learned that some of these may have been manufactured with Thorium, a radioactive isotope. Whoa!
I got in touch with Hope College's physics department, and Dr. Paul DeYoung generously offered his time to help me test these lenses with a Geiger counter. Turns out that only one of my lenses was radioactive -- the one pictured below, a Super-Tak 55/2, which measured hot at about 1.5 mR/hr Beta/Gamma (~15 mR Alpha) off its rear element. (Front element was not as hot.) Whoa! Hot enough to be used as an X-ray device when coupled with some old Polaroid film. Whoa!
Geeked as a physics professor would be upon seeing something like this (he had never heard of radioactive lenses before), Dr. DeYoung proceeded to give me an exciting crash-course lesson in nuclear physics, and showed me a bunch of other radioactive stuff he had in the lab, including a early, orange-colored Fiesta plate which was made with trace amounts of Uranium. Whoa! We had a great time playing around with Geiger counters!
Thorium was routinely used in lenses in the 60s up until the 80s to help the refractive properties of glass and to help reduce chromatic aberrations. It also allowed the glass to require less curvature, which made the manufacturing process faster and cheaper.
Anyway, this is awesome, not only because this is a cool (hot?) piece of photographic history, but also because of the great liberal arts essence of Hope College that made learning about this possible!! It's great for all of our students! I now have even more more to talk about in my photography seminar that I'm teaching this fall...not that I needed yet another topic to discuss. One semester is far too short. I can talk about this stuff for endless hours...my wife can attest to that. :)
Rest assured -- I will not be storing this beautiful lens in our home. :) Happy Shooting!