Super Multi-Coated Takumar 50/1.4 Yellowing

[RLangham]

I have a question about the Pentax Super Multi Coated Takumar 55mm f/1.8. I think this one is radioactive. One day I used the lens to view some negatives up close, like as a magnifying loupe, for a few seconds, until I realized it's radioactive. Can it cause damage to the eyes for a short exposure like this ? I was very close to it, but for a brief amount of time.

Not in the least. No more radioactive than a banana with its trace amounts of potassium-40, and much less radioactive than antique radium glow-in-the-dark paint like you would have on your average 50's pocket watch. You get more radiation from the sun.

Don't worry about it. I have used it for the same purpose a number of times since I usually use the nearest loose 50mm to hand as a loupe. My eyes are no worse than they were before.

 

[Ryan Oliveira]

Thanks !

 

[RLangham]

Thanks !

Of course. Just remember that it's easy to make something sound dangerous using a scary adjective unqualified. I've seen bottles of varipus chemicals that say "no chemicals" and drugs that say "drug-free recipe." It'd be hilarious if Pentax found some kind of legal loophole where they could likewise label their radioactive lenses "non-radioactive" to comfort the buyer.

 

[MattKing]

It probably wouldn't be ideal if you sat on the lens for extensive periods of time.
Of course, that applies to every lens, and certainly applies to bananas!
Welcome to Photrio.
(FWIW, the concern about lenses of that type relate mostly to the long term yellowing, and to the potential danger involved in long term exposure for those workers who manufacture them).

 

[AgX]

Not in the least. No more radioactive than a banana with its trace amounts of potassium-40,

Then put a common Geiger-Müller tube with the respective meter next to a banana. There will be no rise in counts. Then put that next to one of those lenses and the meter will go havoc.

Yes, that will not say much on radiotoxity, but it will be frightening nonetheless.

 

[AgX]

Can it cause damage to the eyes for a short exposure like this ? I was very close to it, but for a brief amount of time.

see my post here and the following 2:

https://www.photrio.com/forum/threads/canon-ltm-lenses-with-thorium.177672/#post-2315228


By the way, statements that repeatedly show up, telling you that such lenses only emit alpharadiation are nonsens. Just with a cheap meter one can demonstrate that.

 

[Donald Qualls]

@Ryan Oliveira My understanding is that you should be fine. I used my Super Takumar 50mm f/1.4 (a known thorium glass example) as a loupe occasionally for years.

First, the thoriated element isn't the rearmost one; there's one additional piece of glass behind it. Second, thorium is an alpha emitter -- and alpha is the leas penetrating form of radiation. Classic radiation safety documents claim a single sheet of paper will stop most alpha particles, where it takes a paperback to stop beta. That means the amount of radiation that can get through that glass element behind the thorium glass is near nil.

But "near nil" isn't "nil" -- which is why the caution against using the radioactive lenses as loupes. If they emitted enough radiation to be a short-term hazard, they'd most likely have been recalled or we'd have been warned to put them in safe storage decades ago, as radiation awareness climbed. Further, they'd have been of limited use as camera lenses, especially in the days of cloth shutter curtains, since they'd have had a tendency to fog film if the mirror was up (say, in early SLRs left uncocked after exposure).

So, overall, what I'd recommend is don't worry about it, but don't make a daily habit of it.

 

[Dan Fromm]

https://web.archive.org/web/2018101...k.net/~michaelbriggs/aeroektar/aeroektar.html

 

[Ryan Oliveira]

I thought I was crazy when I did this. Thanks for letting me know.

 

[AgX]

@Ryan Oliveira Second, thorium is an alpha emitter -- and alpha is the leas penetrating form of radiation. Classic radiation safety documents claim a single sheet of paper will stop most alpha particles, where it takes a paperback to stop beta. That means the amount of radiation that can get through that glass element behind the thorium glass is near nil.

See my post above on this.
The decay of Thorium is a complex one with various stages of decay with different radiations being emitted.

 

[Donald Qualls]

Yep. Page break meant I was typing on page 3 while reading page 2. None the less, the actual radiation level is quite low (though it may increase over time, if decay products are more active than the thorium), and most natural decay chains are mostly alpha; beta is less common and gamma very rare.

 

[reddesert]

It's not dangerous for putting to your eye occasionally. However, thoriated glass was not permitted to be used in eyepieces because of the possibility of long term exposure, for example someone using the camera hours per day for work. See for example: https://www.orau.org/ptp/collection/consumer products/cameralens.htm
from the Oak Ridge museum of radiation in consumer products.

 

[AgX]

Classic radiation safety documents claim a single sheet of paper will stop most alpha particles,

Well, that is what one reads everwhere on Alpha-radiation.

I just did a rough test on a radioactive lens.


Reduction in Geiger-Müller tube counts:

-) sheet of paper : zero reduction

-) 8 layers of kitchen aluminium-foil : reduction by 20%

-) 0.7mm sheet of steel : reduction by 70%


Metered at about 1cm distance from front-element of Steinheil Quinon 50mm 1.9 : about 3 microSievert/hour

(Take this last reading with a lot of salt as I am neither sure whether that meter still is in calibration, nor at which position the respective lens element is. At the rear side of the lens no extra radiation is to be measured.)

 

[Donald Qualls]

But how do we know what the counter is registering? Sure, most natural decay chains are alpha or alpha followed by beta -- but what you got may suggest that all you were reading was beta.

 

[AgX]

That is one of the major issues with radiometry, typically overlooked, as most meters in amateur hands do not tell what they actually are metering.

Well, and I myself did not tell either.
But instead I said above that there are various emissions, whereas one typically reads in context of lenses of alpha-radiation and the paper sheet. Clinging to this lens case, either the paper shielding thing is grossly exaggerated, or there is other radiation of relevant level (to be metered).
All I wanted to say is that we are in quite cloudy waters...

 

[Donald Qualls]

Suffice, then, to say that I'm pretty confident what you were measuring was beta. Alpha from direct thorium decay in a lens won't produce easily counted emissions even beyond the glass of the thorium element (you can measure alpha from the thorium salts in an old pressure lantern mantle; there the salts are pretty exposed, but in glass, virtually all the alpha is absorbed before it can leave the glass). Beta from secondary decays will build up as the lens ages and decay products accumulate -- since the newest thorium glass lenses are around fifty years old now, there ought to be enough beta emission to easily detect (as you've done -- BTW, I couldn't have done that, I've never owned a Geiger-Muller counter).

Your counter is actually counting ionization cascades in the detector tube -- theoretically, those can be produced by alpha, beta, or depending on the gas in the tube, possibly even by gamma and x-ray photons -- but few tubes are made to detect EM radiation, and the tube shell itself blocks almost all alpha (again, a few tubes are made with alpha-pass windows that can be covered and uncovered to get alpha counts by subtracting the closed count from the open count). In the end, almost all Geiger-Muller tubes are first and foremost beta detectors.

 

[reddesert]

But the upshot is, there is detectable radiation outside a thoriated-glass lens. It is likely not enough to have an effect unless you hold it to your eye 4 hours a day, or carry it in your pants pocket. But it isn't just a banana.

Understanding the actual radiation and decay products experimentally would likely require a radiation spectrometer, which is something no everyday person has lying around, but in practice, the decay products of Th-232 should be the same whether it's in a lens or not. There is the chain of decay to radium and radon and the ensuing gamma radiation. At least it should be embedded in the glass, so one won't breathe the radon.

On an intuitive level, I would treat the lens like I would a box of old Coleman lantern mantles (which also have thorium, but less by weight): I would be ok keeping it in my house, but I wouldn't store it under my bed.

 

[AgX]

Exactly.

Furthermore one should look into the very case. I referred to a case of official investigation into using a SLR with a hot Takumar lens, which turned out being rather benign in use.
Would one carry for instance a radioctive lens with frontal emission over months of use with the lens against one body, the radiation take up would be more substantial.

On the other hand, as you indicated, we are surrounded by various natural or manmade emitters of such energetic radiation, from Radon gas, over tiles or certain glassware to those lantern sockets.

 

[AgX]

...but few tubes are made to detect EM radiation, and the tube shell itself blocks almost all alpha (again, a few tubes are made with alpha-pass windows that can be covered and uncovered to get alpha counts by subtracting the closed count from the open count).

My detector is stated to meter Alpha-radiation too. But it cannot discriminate between kinds of radiation. And that renown sheet of paper was of no effect.

 

[Donald Qualls]

If there's no noticeable attenuation due to very light shielding, like a layer or two of paper, then the counts you're recording are most likely not due to alpha -- which is entirely sensible, since as noted almost none of that gets outside the thoriated element, even less beyond other elements or the lens housing.

In terms of radon, I'd be much more concerned about that in a house with a basement in the Appalachian mountains and foothills of the US Eastern Seaboard (like where I live in North Carolina). There are known thorium deposits under the limestone layers of this region, and the soil exudes radon -- which is enough denser than air to collect in low areas and, due to inhalation, is the second greatest risk factor for lung cancer in regions where it occurs. Further, if you live in a house with a basement or poorly ventilated crawl space in the radon zone, you owe it to yourself and your family to install a simple ventilation system (onr or two computer case fans exhausting air from the lowest point through drier ducts to the outside ought to be enough) to ensure the air isn't allowed to stagnate and build up concentration of radon (there are detectors, but they amount to a dosimeter -- simpler just to ventilate).

If I've correctly understood radiation safety standards, activity levels in microsieverts per day are very low risk. Some vacuum tube based ham radios and old analog color TV sets would emit more than that (in the form of soft x-rays), though distance from the source was usually sufficient for safety. A single CAT scan is several millisieverts -- or equivalent of several years of exposure to a Super Takumar or Super Multicoated Takumar lens with thorium glass element -- and my doctors, at least, aren't shy about giving me a couple of those (and several flat x-ray images) a year when needed. A single intercontinental airline flight (6-10 hours at roughly 10km altitude) gives about as much excess radiation exposure as carrying such a lens constantly for several months.

So, bottom line: the science says that you're at more radiation risk from long distance air travel, or living in a house with a basement in some regions, or smoking (in which one of the major risk factors is inhalation of alpha emitters in the smoke), than from normal storage, handling, and use of such a lens. All bets are off if you were to crush and snort, or swallow the thorium glass element from such a lens, however...

 

[AgX]

In terms of radon, I'd be much more concerned about that in a house with a basement in the Appalachian mountains and foothills of the US Eastern Seaboard (like where I live in North Carolina).

That is what I meant. Radon in significant concentrations only occurs in houses at certain locations, and there typically in the basement. Also radioactive tiles (glazings) or glassware only are to be found at some households. These thus only were meant as examples. I could add digesting radioactive food.
At my household only those lenses make the counter go above the radioactive noise, but then by magnitudes.

But there are people who do no travel by airplane and who do not smoke. Well, then one either could argue that the risk share of for instance of sporting such lens is higher for them, or that they are lucky to ommit some common risks... all a matter of perspective.

 

[Donald Qualls]

In terms of relative risk, I've been told by a medical doctor that if I don't smoke, but do ride a motorcycle, they wouldn't be aggressive in treating my high cholesterol (since I was much more at risk from the motorcycle -- well, mainly from other vehicles while riding the bike -- than from heart attack). Relative risk being what it is, even for people who don't smoke and don't live in radon regions, don't fly, don't ride motorcycles, and have in-range cholesterol and blood sugar, a radioactive lens is a pretty minor risk factor. For most of us, the most dangerous thing we'll do any given day is drive to and from work.

 

[Ryan Oliveira]

I'm curious to know about a daily exposure to the lens, let's say, if I handle the lens every day or take pictures with my camera all day, will it be advisable to stop that habit ?
I have my camera with the lens sitting on a shelf in my bedroom, above my bed, about 4 feet away. Does it pose any danger for myself with it every day "not that near me" close to where I sleep ?

 

[RLangham]

I'm curious to know about a daily exposure to the lens, let's say, if I handle the lens every day or take pictures with my camera all day, will it be advisable to stop that habit ?
I have my camera with the lens sitting on a shelf in my bedroom, above my bed, about 4 feet away. Does it pose any danger for myself with it every day "not that near me" close to where I sleep ?

I don't think there's any account of anyone having a problem

 

[Ryan Oliveira]

Alright, so I think I'm going to be fine with it. I got worried when I realized it was radioactive, and having it near me almost everyday made me concerned.