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Introduction of Thorium Oxide into Super Takumar Lenses
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PostPosted: Tue Dec 13, 2011 11:02 am    Post subject: Reply with quote
Yawn...


Wink


PostPosted: Tue Dec 13, 2011 12:16 pm    Post subject: Reply with quote
peterqd wrote:
Yawn...

It may be a bit boring, but I for one would like to know whether there's an increased risk of cancer before using this lens for hours!

s58y wrote:
Close to the rear element (with rear lenscap off), the reading was about 15000 counts/minute (CPM) measured with a typical thin-window 1.75-inch pancake probe (alpha+beta+gamma). With the plastic cap over the probe, the reading was about 8000CPM. Most gammas and quite a few of the betas should reach the probe with the cap on, but few, if any, alphas would get through. The background count averages about 25-40CPM.

The measurement with the rear cap on is surprisingly high! Maybe the subsequent beta decays are very fast? Hmmmm...


PostPosted: Tue Dec 13, 2011 12:32 pm    Post subject: Reply with quote
noctilux wrote:
peterqd wrote:
Yawn...

It may be a bit boring, but I for one would like to know whether there's an increased risk of cancer before using this lens for hours!

OK, I didn't direct that at you so please don't take it personally, . It's just that this subject has been discussed ad nauseum in the past.

And, be honest, do you really expect anyone to believe what they read about radioactivity on a photo discussion forum? I'm content in the knowledge that there has never been a reported case of a photographer contracting cancer as a direct result of using particular lenses. If it was that dangerous his films would have become fogged long before that, and don't you think some health and safety committee somewhere would have banned them long ago?


PostPosted: Tue Dec 13, 2011 1:17 pm    Post subject: Reply with quote
peterqd wrote:

It's just that this subject has been discussed ad nauseum in the past.

I see your point -- until we find some real expert with a definite answer, this discussion is just going to go on and on.

You're probably right; authorities would have banned it. But it would be nice to know exactly why it's not dangerous! If it were only alpha radiation, it would be easy to explain: Alpha radiation becomes harmless after 4cm or so. But s58y measured significant non-alpha decay, so that explanation is apparently not enough.
The answer is probably that the level measured by s58y is very low, but it would be nice to have confirmation.


PostPosted: Tue Dec 13, 2011 4:44 pm    Post subject: Reply with quote
noctilux wrote:

The answer is probably that the level measured by s58y is very low, but it would be nice to have confirmation.


If you think of it is as a game of chance (i.e. the “no safe amount of radiation” view), then any amount is potentially dangerous and can never be confirmed otherwise. Of course if you do get cancer, then no-one can ever prove if it was due to a radioactive lens, a high-altitude flight, radon in your house, or something completely different.

Personally I would argue that things ought to be in perspective. For example, Finland has significantly higher background radiation levels than the world average, yet one of the highest life expectancies. Moving to a different (lower radiation) location would have a cumulative effect orders of magnitude greater on my dose of radiation than using radioactive lenses for photography every day (which I don't do, sadly). Similarly for airplane flights, eating bananas, etc. It seems pointless to worry about this one thing (lenses) in the context of all the (constant, impossible to avoid) radiation around us (not to mention other risks; traffic, crime, natural disasters, many of which are probably way more likely to damage us than lenses). But, again, if viewed as a game of chance then each must make up their own mind on what level of risk is acceptable.

(Of course the “no safe amount” is only one theory, alternative theories suggest that the body may indeed be able to repair low doses. This might explain why people in Ramsar aren't all dying. However, I do not have the knowledge or expertise to argue this case further.)


PostPosted: Tue Dec 13, 2011 7:51 pm    Post subject: Reply with quote
You can assume that natural background radiation is safe because evolution has killed the type that can't take it.
Human made radiation can be a different thing.
Now anybody who has Takumars can send them to me. I will take care of their proper use... I mean disposal Very Happy

With that said... it's crazy to think that government bureaucracy would protect us from possible health hazards.

Look at Chelyabinsk, Former Soviet Union:
http://www.cultureunplugged.com/play/6080/Chelyabinsk--The-Most-Contaminated-Spot-on-the-Planet


PostPosted: Tue Dec 13, 2011 9:32 pm    Post subject: Reply with quote
kds315* wrote:
OMG, will this "radioactive lens" thing ever end - PLEEEZZZE have mercy Smile


+1,000,000


PostPosted: Wed Dec 14, 2011 5:55 pm    Post subject: Reply with quote
Reading comments on this thread through a typical computer monitor delivers rays that completely negate any prior exposure to thorium glass radioactivity. So it's vital to keep this thread posted prominently and constantly growing. The negative energy of commenters who hate this topic (but why do they read this stuff then?) adds to this curative effect.


PostPosted: Wed Dec 14, 2011 6:46 pm    Post subject: Reply with quote
ramiller500 wrote:
Reading comments on this thread through a typical computer monitor delivers rays that completely negate any prior exposure to thorium glass radioactivity. So it's vital to keep this thread posted prominently and constantly growing. The negative energy of commenters who hate this topic (but why do they read this stuff then?) adds to this curative effect.


Laughing Laughing Laughing


PostPosted: Thu Dec 15, 2011 2:39 am    Post subject: Reply with quote
Lightshow wrote:

...
The second issue -- subsequent decay -- ought to be negligible, for the following reason: The rate of radioactive decay is proportional to the amount of particles that are radioactive. Since Thorium 232 decays very slowly (14*10^9 years half life), its decay products are few and should thus have a low decay rate.

All in all: Thorium should indeed emit mainly alpha radiation which is dangerous only if it is unshielded and closer than about 4cm to the body. If the lens is not inhaled, consumed or snuggled continuously, all should be OK. Smile


1) Supposedly, refined thorium (unlike refined uranium) approaches secular equlibrium in just a few decades, since the sum of all the half lives of the thorium decay products is less than 10 years. Once in secular equilibrium, all of the stages in the decay chain should be decaying at about the same rate, despite the greatly differing half lives. There are around 10 stages in the decay chain, so I'd expect most of the radioactivity coming out of an old lens to come from the decay products, not from the thorium 232 itself.

2) There are gamma emitters in the thorium decay chain. Perhaps the most famous/infamous of these is Thallium 208, which emits a very "hard" gamma ray (2.6? MEV) It's also unclear how much alpha would escape the lens, especially if the radioactive element is not the frontmost of rearmost one.

3) Note that this Pentax lens is far less radioactive than an old luminous-dial watch I measured (there's a YouTube video of it on the uyt384 channel, BTW), and you're supposed to wear the watch on your wrist all day long.

4) Note that my readings come from a simple pancake probe. This geiger counter is a survey meter and is used to detect the presence of radioactive stuff. It is not a dosimiter designed to measure how hazardous the radiation is. This pancake probe is poor at picking up gamma rays, for example. Much more interesting would be measurements of these lenses using a real gamma-ray scintillator/dosimiter, compared to background (which is supposed to be much higher with a scintillator).

5) I am not an expert on radioactivity, so you need to take everything I say here with a grain of salt, and do your own research.


PostPosted: Thu Dec 15, 2011 3:58 am    Post subject: Reply with quote
I live near a large nuclear reprocessing facility and have a load of friends who work there, including a few who work on monitoring the other workers for radiation, I asked one of them about radioactive lenses and he laughed at me, said there was no point trying to measure the emissions as they would be insignificant.


PostPosted: Thu Dec 15, 2011 8:41 am    Post subject: Reply with quote
And on that note I think this thread has run its course.
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