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06-04-2016, 04:46 PM   #31
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QuoteOriginally posted by dcshooter Quote
the primary thorium isotope in the Thorium oxide salts used in the glass is Th232
You suggested that it was TH228 was dominantly responsible for the yellowing effect. My issue is that the discolouration of the lenses is not perfectly uniform, the edges of the lens are nearly always darker than the center of the glass element. If colour centers are indeed the mechanism in effect here then shouldn't the discolouration be more homogeneous?

QuoteOriginally posted by dcshooter Quote
Some amount of gamma emission occurs at after each decay in the TH228 decay chain
Wouldn't gamma emission events be statistically dwarfed by the amount of alpha/beta decay events?

QuoteOriginally posted by dcshooter Quote
you don't understand what it is that you are talking about.
oh I made no claim to be an expert on radioisotopes. I'm well prepared to admit to that..my ex -partner worked in medical radiology - she would have corrected me.

06-04-2016, 06:15 PM   #32
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QuoteOriginally posted by Digitalis Quote
the discolouration of the lenses is not perfectly uniform, the edges of the lens are nearly always darker than the center of the glass element.
The edges being somewhat thinner, I would expect them to appear lighter as a matter of course.

QuoteOriginally posted by Digitalis Quote
Wouldn't gamma emission events be statistically dwarfed by the amount of alpha/beta decay events?
That is a good question, the measurements done by Flickr user s58y using the Geiger-Mueller counter both with and without plastic caps went as follows:

Close in at front of SMC Tak 50/1.4...

w/o cap - 858 CPM
w/ cap - 778 CPM

Close in at rear of SMC Takumar 50/1.4...

w/o rear cap - 26838 CPM
w/ rear cap - 12512 CPM


I have been unable to find a source listing the relative energy distribution (eV) of alpha vs. beta vs. gamma in thorium oxide. As a constituent in the glass being darkened, it would be difficult to assign blame.


Steve

Last edited by stevebrot; 06-04-2016 at 06:24 PM.
06-04-2016, 08:08 PM   #33
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Color centers are formed by high energy photons (e.g. Gamma Rays, UV, X Rays) interacting with valence electrons. They are not caused by comparatively lower energy particle collisions, either with Alpha (neutron proton clusters) or Beta particles (electrons/positrons). Particle activation can lead to the creation of unstable isotopes, but that would lead to transmutation into other elements not activation of color centers, which are solely the product of valence energy states.

Reversible radiological browning of glass is well understood to be the result of color center formation, and it is commonly seen in x ray chambers and other environments subject to high photonic flux. While the gamma ray emissions associated with Th228 are low, because the half life is so short, nearly all the Th228 contained in the initial Thorium dopant has decayed over the decades these lenses have been in existence. Because the daughter products are also short-lived, they have released nearly all of the gamma radiation they are going to over that time, giving the glass a relatively high dose over time. The remaining Th232 will very gradually decay itself, producing more isotopes along the Th232 chain, but likely not in the levels seen in the earlier life of the lens.

There is another type of browning that can be caused by electron bombardment, but this results in the formation of alkali metal strands in glass through the reduction of metal oxide dopants in the glass. This kind of browning is sometimes seen in CRT tubes (which also suffer color center browning from x rays) However, unlike color center browning, since the molecules are chemically changed, it cannot be simply reversed by pushing electrons into stable valences.


QuoteOriginally posted by Digitalis Quote
You suggested that it was TH228 was dominantly responsible for the yellowing effect. My issue is that the discolouration of the lenses is not perfectly uniform, the edges of the lens are nearly always darker than the center of the glass element. If colour centers are indeed the mechanism in effect here then shouldn't the discolouration be more homogeneous?



Wouldn't gamma emission events be statistically dwarfed by the amount of alpha/beta decay events?



oh I made no claim to be an expert on radioisotopes. I'm well prepared to admit to that..my ex -partner worked in medical radiology - she would have corrected me.

Last edited by dcshooter; 06-04-2016 at 10:45 PM.
06-05-2016, 12:28 AM   #34
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QuoteOriginally posted by Digitalis Quote
You suggested that it was TH228 was dominantly responsible for the yellowing effect. My issue is that the discolouration of the lenses is not perfectly uniform, the edges of the lens are nearly always darker than the center of the glass element. If colour centers are indeed the mechanism in effect here then shouldn't the discolouration be more homogeneous?



Wouldn't gamma emission events be statistically dwarfed by the amount of alpha/beta decay events?
My 50/1.4 takumars are more yellowed in the center than the edges, the positive lenses have the most glass there.

And about the emission, I assume most alpha radiation stays trapped in the glass, only the particles emitted at the glass/air surface will make a change to come out, gamma rays you get from the complete volume of the glass.

06-06-2016, 10:28 AM   #35
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Meanwhile my supertak lost the significant amount of yellow with OttLite, but not all of it. Perhaps Ikea lamp would wokr faster? My copy was not lightly yellowed, plenty of dark amber cast.
I tested the lens, and put back under the lamp.
Even now I can tell that focus confirmation works better, more accurate. With fine adjustment I can shot in Av wide open, and get more accurate focusing results. Before I've been using that lens mostly at 2.8
Should have done it long time ago. It's like I got a brand new lens.
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