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Care of Thorium lenses
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PostPosted: Tue Jul 09, 2013 7:50 pm    Post subject: Care of Thorium lenses Reply with quote
I'm posting this here because it seems to me this falls into the broad category of MF "lens care".

Technically, I suppose it could better be stated that this falls into the category of MF "lens owner care"! Very Happy

Whichever, I was rooting around eBay pursuant to my Aero Ektar when I happened upon this auction:

Click here to see on Ebay

The lens here has nearly twice the focal length of mine. It is a really large MF lens. But what caught my attention was this warning:

"Typical " Tee - color " of rear lens cell is caused by radioactivity.

You should store it in a remote place and you should not stay closer than 6 feet longer than 60 hours a year. Please
browse the web for information."

This lens has more thorium than mine and a whale of a lot more than a lens of normal size. But this is the first time I have seen a time and distance-specific warning about exposure to any thorium lens.


PostPosted: Wed Jul 10, 2013 1:04 pm    Post subject: Reply with quote
Even supposing the amount of radioactivity emitted by this lens was a hazard (and I doubt it is), thorium is an alpha emitter so even a paper bag will be sufficient to absorb 100% of the emitted radiation (which will become harmless helium). There is the question of how any daughter products will decay; the decay chain for Thorium-232 is via alpha and beta emitters (no gamma emitters). So, if you want to be ultra cautious, upgrade your paper bag to a tin box and rest easy.

Mark


PostPosted: Wed Jul 10, 2013 4:41 pm    Post subject: Reply with quote
SXR_Mark wrote:
Even supposing the amount of radioactivity emitted by this lens was a hazard (and I doubt it is), thorium is an alpha emitter so even a paper bag will be sufficient to absorb 100% of the emitted radiation (which will become harmless helium). There is the question of how any daughter products will decay; the decay chain for Thorium-232 is via alpha and beta emitters (no gamma emitters). So, if you want to be ultra cautious, upgrade your paper bag to a tin box and rest easy.

Mark


Not quite true. Majority of decay chain from thorum to lead is alpha emission, with a couple of beta emissions, which act to change the atomic number of the atoms in each decay step, but the difference in energy between each step is accomplished by gamma emission. The average geiger counter measurement is going to be counting the gamma rays, actually, because, as you said, even the window over the sensor in a counter is going to block the alpha particles.

The Ebay listing is simplistic at best. The seller gives a time/distance number, but what arbitrary dose limit are they using to determine this? All in all, I don't think it matters much, unless you are grinding the glass and breathing or swallowing it where the alpha or beta emissions could do damage internally, or holding the doped elements up to your eye for extended periods where the gamma rays could do damage, then this kind of glass doesn't introduce much danger. Just don't put the lens in your pocket and carry it around 24/7 Wink


PostPosted: Wed Jul 10, 2013 5:14 pm    Post subject: Reply with quote
JC wrote:
SXR_Mark wrote:
Even supposing the amount of radioactivity emitted by this lens was a hazard (and I doubt it is), thorium is an alpha emitter so even a paper bag will be sufficient to absorb 100% of the emitted radiation (which will become harmless helium). There is the question of how any daughter products will decay; the decay chain for Thorium-232 is via alpha and beta emitters (no gamma emitters). So, if you want to be ultra cautious, upgrade your paper bag to a tin box and rest easy.

Mark


Not quite true. Majority of decay chain from thorum to lead is alpha emission, with a couple of beta emissions, which act to change the atomic number of the atoms in each decay step, but the difference in energy between each step is accomplished by gamma emission. The average geiger counter measurement is going to be counting the gamma rays, actually, because, as you said, even the window over the sensor in a counter is going to block the alpha particles.

The Ebay listing is simplistic at best. The seller gives a time/distance number, but what arbitrary dose limit are they using to determine this? All in all, I don't think it matters much, unless you are grinding the glass and breathing or swallowing it where the alpha or beta emissions could do damage internally, or holding the doped elements up to your eye for extended periods where the gamma rays could do damage, then this kind of glass doesn't introduce much danger. Just don't put the lens in your pocket and carry it around 24/7 Wink


Yes, I quite agree regarding the gamma rays. I can post a reference if anyone is interested (would have to dig it out).

But the alpha particle emissions from these things are of virtually no concern whatsoever as a practical matter. I'm a very long time Aero Ektar owner. It's the gamma radiation, and only that radiation, that has ever been a worry.

Bear in mind, too, the amount of thorium comes into play. With the tiny little thorium lenses most MF lens owners own there's not very much of it. The Aero Ektars are another matter entirely . . . . a whole nother other matter!!! And that 1217mm bruiser I posted in the OP is the mother of thorium lenses. Very Happy


PostPosted: Wed Jul 10, 2013 9:43 pm    Post subject: Reply with quote
JC wrote:
SXR_Mark wrote:
Even supposing the amount of radioactivity emitted by this lens was a hazard (and I doubt it is), thorium is an alpha emitter so even a paper bag will be sufficient to absorb 100% of the emitted radiation (which will become harmless helium). There is the question of how any daughter products will decay; the decay chain for Thorium-232 is via alpha and beta emitters (no gamma emitters). So, if you want to be ultra cautious, upgrade your paper bag to a tin box and rest easy.

Mark


Not quite true. Majority of decay chain from thorum to lead is alpha emission, with a couple of beta emissions, which act to change the atomic number of the atoms in each decay step, but the difference in energy between each step is accomplished by gamma emission.


Yes, of course this is correct. Thanks for putting me right.

Mark
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