I was recently reading some previous postings and web articles about certain lens glasses containing radioactive elements, and wondered if any of my collection might be 'hot'.<br>Initially, I was more curious than worried, but after doing some tests, I'm now definitely worried, and I think my findings deserve wider attention.<p>I have a sample of a 7" Kodak Aero-Ektar, which is a lens that has a big reputation for being potentially radioactive.<br>However, all the articles I've read state that the radioactivity is mainly in the form of alpha emmission, and shouldn't give much call for concern, since alpha particles are quite low energy, and easily stopped by any solid object. I'm told that alpha particles can only penetrate about 40 microns into human tissue.<br>Anyway, to cut to the chase: I just got our radiation protection officer to run a geiger counter and a dosimeter over the Aero-Ektar, and he found that the rear element was quite hot, giving about 200 counts/second. Worse yet, we discovered that it was mainly GAMMA emmission, since even an inch of perspex and a steel plate hardly affected the counts at all. The active element used in making the glass is Thorium, and a quick check of the table of its decay products confirmed that they're mostly gamma emmitters, after about 6 years of decay.<br>Dosimeter readings showed that within 1" of the rear lens surface, the dose was above the limit allowed for monitored radiation workers, and only fell to the publicly allowable safe limit at more than 6" from the lens.<br>I don't think I'll be using that lens as a paperweight, or handling it too much from now on. By all modern standards, that lens would be classed as downright hazardous, and not to be used without protective clothing!<br>I'll be testing the rest of my lenses as potential gamma sources as well in the near future.