[pathdoc]

That's odd, because I'm sure I've seen this sort of thing done somewhere, whether here or mflenses.com or YouTube, but SOMEONE pulled a thoriated lens apart and even de-yellowed HALF the thoriated element (or the group; I don't think he separated the elements) to show it was the glass.

[micromacro]

If de yellowing really improves the sharpness of supertak, I'm in. Found my old OttLite lamp, spent 5 minutes to secure the lens. Hope it will work.

[HoustonBob]

The LEDS in these lights are (in all probability) cheap GaN/InGaN LEDs - which have a peak emission in the UV spectrum - A dedicated monochromatic UV light source will be more efficient. UV leds would be great for this approach.




I have been awaiting for you to give hard evidence of this. Until you do find compelling evidence that elevates your concept beyond a mere theory: my opinion stands. Molecular chains in polymers require less energy to break, do you not agree? The data I have been able to find on this phenomena is limited to crystal structures, not amorphous glass structures from what I have been able to find. The amounts of energy to deform the molecular bond of a material in its glass state is tremendous. You DO know they vitrify high level nuclear waste in glass do you? If there was even a suggestion that emitted radiation itself could cause a breakdown of the material that is used to contain it is troubling indeed.

I suspect it is the gamma emission which causes the yellowing. Gamma rays are easily powerful enough to have an effect on any chemical bonds - regardless of the material.

[Digitalis]

Mild gamma emission from daughter isotopes in the Th228 decay series. The halflife of this isotope is less than 2 years

1.9116 years to be exact, however TH228 is an Alpha emitter.

I suspect it is the gamma emission which causes the yellowing. Gamma rays are easily powerful enough to have an effect on any chemical bonds - regardless of the material.

Indeed Gamma rays are strong enough to do this, there is a problem: TH228 and its daughter radioisotopes are alpha/beta particle emitters.

Sources:

http://www.ld-didactic.de/software/524221en/Content/Appendix/Th232Series.htm
https://ehs.ucsd.edu/rad/radionuclide/Th-228.pdf
Table of Isotopes decay data
https://groups.nscl.msu.edu/hira/pdf/alpha_source_cal.pdf
http://www.nucleide.org/DDEP_WG/Nuclides/Th-228_com.pdf
http://www.radiochemistry.org/periodictable/gamma_spectra/pdf/th228.pdf
The radioactive series of thorium-232
http://gonuke.org/ComprehensiveTeachingToolkits/Radiation%20Protection/ChSCC...cay-series.pdf

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Last edited by Digitalis; 06-04-2016 at 10:17 AM.

[stevebrot]

Indeed Gamma rays are strong enough to do this, there is a problem: TH228 and its daughter radioisotopes are alpha/beta particle emitters.

I believe you are incorrect. The gamma-induced creation of "color centers" in thoriated lenses is well established* as is gamma emission from the thorium 232/thorium 228 decay sequences.

http://gonuke.org/ComprehensiveTeachingToolkits/Radiation%20Protection/ChSCC...cay-series.pdf

...specifically, figure 3...the gamma emissions are energetic release and not nuclear decay products.

In addition, the most commonly referenced survey of radioactive lenses was done using both a Geiger counter and a gamma-specific scintillation probe.

https://www.flickr.com/photos/s58y/6802092736

As for the efficacy of the IKEA JANSJÖ series desk lamp, the proof of the pudding is in the eating. I have used my JANSJÖ to clear both my Auto Rikenon 55/1.4 and ST 55/1.8. I am with you in regards to doubts about UV output from those lamps. That being said there are online sources that indicate high intensity light of a broader spectrum may be capable of dislodging valence electrons in some glass to disrupt the color centers.


Steve

* To broaden a little further, it is not just optical glass. There is an entire industry dedicated to coloring plain and doped glass with gamma radiation.

[stevebrot]

Incidentally, the primary thorium isotope in the Thorium oxide salts used in the glass is Th232, which has a very long half life of 1.40 × 1010 year (i.e. it is incredibly mildly radioactive) so those who point to the decay sequence of 232 rather than the naturally co-occurring 228 as the culprit for yellowing are doubtlessly mistaken.

This is technically true, though in practice both Th232 and Th228 would have been present in the original "dope" along with a full complement of daughter products from both isotopes. Some participation from Ac228 (very hot stuff) in gamma emission cannot be ruled out.


Steve

* Th232 decays to Th228 via Ra228 and Ac228...

[stevebrot]

What has me shocked is that no one has done this demonstration before.

That's odd, because I'm sure I've seen this sort of thing done somewhere, whether here or mflenses.com or YouTube, but SOMEONE pulled a thoriated lens apart and even de-yellowed HALF the thoriated element (or the group; I don't think he separated the elements) to show it was the glass.

That would be PF user @glasbak who posted his results...

Radioactive Pentax Takumar lenses.


Steve

[pathdoc]

That would be PF user @glasbak who posted his results...

Yep, that's the one I remember.

[Digitalis]

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?

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?

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.

[stevebrot]

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.

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

[glasbak]

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.

[micromacro]

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.