thorium in pentax

[j_buck]

<p>hello!<br>

i have a question about Thorium in Pentax 67 system<br>

i know that Thorium is in the old 105mm f2.4 lenses, but somewhere i was reading that it also might be in the gals of the eyepiece magnifier, the prism viewfinder and the other view magnifiers ...<br>

anyone has any info on this?</p>

[User_4136860]

<p>http://www.photo.net/classic-cameras-forum/00cYCS</p>

[PapaTango]

<p>Ben, thanks for the link. Benoit's posting there and examining the original document he summarizes makes the matter quite clear for those that are interested in facts. It will not help those that read too many posts from the "OMG" reactionaries who see everything today as a threat that needs addressed. The bottom line is there is no threat to health from using the thoriated lenses that common photographers will encounter. Nor is there any evidence that ANY camera manufacturer went to the unnecessary expense of including such glass in a viewfinder objective. It simply would not have made a whit of difference in increasing performance.</p>

<p>I have seen this same "OMG" routine in my horology and ham radio hobbies. There the bogeyman is Radium. That material is considerably radioactive in how it was used and applied. Yet, if one keeps the material contained--and uses precautions when servicing and removing the materials--there is little to no threat. But the reactionaries convinced the clueless to throw away perfectly good watches as "cancer sources." Trash out perfectly good electronic equipment. So on and so forth. </p>

<p>Facts are that the exposure to ultra high frequency radio emission from cell phones is far more problematic than a room full of isotope containing lenses, watches, and WWII era electronics. </p>

[hjoseph7]

<p>I have a couple of old Thorium coated Pentax lenses and I read all of the articles about Thorium. Some which were a little scary. It's good to know that Pentax also manufactures lenses for medical equipment not just cameras, some which use to contain Thorium. I'm pretty sure the medical community would be up in arms by now if there was really any danger. Just in case, my Thorium coated lenses are nowhere near where I sleep...</p>

[rodeo_joe1]

<p>I agree with Patrick that it's highly unlikely that thorium containing glass would be used in eyepieces and prisms - too expensive to produce and totally unnecessary for such applications.</p>

<p>However, I can't agree that Thorium glass is harmless. The problem is that Thorium, which is itself an alpha particle emitter, decays over time into daughter isotopes that are gamma emitters. Alpha particles are low energy and easily stopped by, say, an aluminium lenscap or similar thin piece of low-density material. Gamma particles OTOH have a high energy and will easily penetrate through a thick sheet of lead, or practically any other material. That's where the danger lies, in old lenses (>25 years) where much of the Thorium has decayed and the glass becomes a gamma emitter.</p>

<p>This situation can usually be spotted without access to radiation detection equipment, simply by looking at the colour of the glass. Old Thorium glass develops an amber or brownish tint due to the structure of the glass itself being damaged by the emitted particles. Old and absolutely colourless glass is a good indication that no Thorium has been used in its manufacture.</p>

<p>Due to the comparative "unstoppabability" of gamma particles I would be very wary of any old and browned lenses. It's definitely wise not to live in close proximity with one.</p>

<p>Incidentally, Thorium isn't used as a coating on the glass, but is incorporated in the bulk glass melt itself.</p>

[glen_h]

<p>natural thorium, Th-232, has a 1.4e10 year half life, alpha decay, as noted.</p>

<p>http://www.nndc.bnl.gov/chart/chartNuc.jsp</p>

<p>Decay is to Ra-228 (subtract 2 from Z, 4 from A) <br>

Ra-228 decays beta with 5.75y half life. After 20 or so years, it reaches equilibrium,<br>

being produced about as fast as it decays.</p>

<p>Ra-228 decays beta to Ac-228 which has a 6.15h half life. Again it will be in equilibrium pretty fast.</p>

<p>Ac-228 beta decays to Th-228 which has a 1.9y half life, again equilibrium.</p>

<p>Th-228 alpha decays to Ra-224 which has a 3.6d half life.</p>

<p>Ra-224 alpha decays to Rn-220 which has a 55s half life.</p>

<p>Rn-220 alpha decays to Po-216 which has a 0.145s half life.</p>

<p>Po-216 alpha decays to Pb-212 which has a 10.6h half life.</p>

<p>Pb-212 beta decays to Bi-212 which has a 60m half life.</p>

<p>Bi-212 decays either alpha or beta, to Po-212 or Tl-208</p>

<p>Po-212 alpha decays to Pb-208, Tl-208 beta decays to Pb-208.</p>

<p>Pb-208 is stable. </p>

<p>After 20 or so years, all those will be in equilibrium, so all decaying at about the rate of the original Th-232.</p>

[sterling_bjorndahl]

<p>While I agree with most of Patrick's post, there is one statement I disagree with: "Facts are that the exposure to ultra high frequency radio emission from cell phones is far more problematic...."</p>

<p>Citation needed. That's not ionizing radiation. Back when ionizing radiation was discovered it didn't take long to learn that it had a profound effect on tissue. Some bad judgement calls were made on how to use that effect, but everyone quickly agreed that there was an effect. That's not the case with non-ionizing radiation from cell phones, where there may be a small number of studies indicating a small effect - but given how many years of exposure humans have accumulated so far with no alarming spikes in age-adjusted effects across the population, and the lack of solid theoretical grounds for indicating that there should be some effect (going back over 100 years), I am convinced that we have a solid understanding of radio waves at those frequencies, and that you are far, far more in danger of harm if you go out in the sunlight without adequate protection against UV radiation. The sun is going to kill you from radiation long, long before your cell phone will.</p>

<p>More on topic, though is Rodeo Joe's concerns about gamma emitters. There is some gamma radiation to be sure, but the primary decay mechanism in the Thorium chain is alpha and beta particles, not high energy photons. The detailed study in NUREG-1717 out of the U.S., table 3.1.7 shows that in the Thorium decay chain, gamma radiation exposure is 5 to 7 orders of magnitude less than the total radiation (assuming exposure in air). The gamma radiation is 1/100,000th to one ten millionth of the total radiation for each product in the thorium decay chain.</p>

<p>According to Wikipedia, in the U.S. the estimated average background radiation is 3.1 millisievert per year, depending on where you live. According to NUREG-1717 the additional exposure for an average photographer would be 0.007 millisievert per year (60 hours of use per year). For a "serious" photographer (180 hours of use per year) the additional exposure would be 0.02 millisievert per year. That's their definition, not mine; if you want the details, google NUREG-1717 or go to http ://www.nrc.gov/reading-rm/doc-collections/nuregs/staff/sr1717/nureg-1717.pdf. A single air flight between New York City and Los Angeles is an additional 0.04 millisievert, or twice what a "serious" photographer would get from the lens in a whole year. Tobacco users inhale alpha emitting particles; smoking a pack of cigarettes a day for a year increases your radiation exposure by a whopping 0.36 millisieverts, or about 10% more than background radiation alone.</p>

<p>People who are more concerned about radiation from the lens than they are about a single trans-continental air flight should attempt to understand the math better.</p>

<p>This assumes the lens remains intact. If it gets ground down or polished and particles get into the air and from there into one's lungs, that would probably be very bad for the lungs.</p>

[PapaTango]

<p>@Joe, my cell phone comment was a bit of snarky humor in the vein of the "OMG" reactionaries. As we know, ionizing and non-ionizing radiation are two different cats with associated risks. My brain is already cooked, so any thermal stimulation at this juncture is moot. The connection of cell phone radiation and sperm motility is rather interesting--mine are so old that they require tiny walkers so that is not much interest either. Studies otherwise have differing results--frankly we have absolutely no understanding at this point on the long term ramifications on genetic effects to make any cogent statements. Time will indeed tell, as they say...</p>

<p>It appears that even considering decay products, thoriated lenses made for consumer photography do not pose such a risk that we need to avoid them. Truly, the amount of exposure is minimal and is exceeded by many other activities and general environmental exposures. As Sterling points out, the likelihood of real issues only would come if we grind the lenses down and inhale the dust. Somehow I do not see a significant number of people doing that and a need to sound the alarm. And even that does not pose the same risk of carcinogenic outcomes amongst all populations.</p>

<p>I have however become convinced that it is time that I remove the collection of smoke detectors I keep under my pillow. My plan is to bury them in the back yard along with my asbestos pants...</p>