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Yellowing Pentax lenses

Discussion in 'AP Magazine Feedback & Suggestions' started by DaveS, Oct 28, 2017.

  1. DaveS

    DaveS Well-Known Member

    RE: yellowing in thorium containing Takumar lenses. My undetstanding is that this is radiation damage to the glass. Anecdotally this can be reversed by exposure to UV radiation.

    I have a yellow 35mm f/2 lens that chucks out a fair bit of beta radiation from the rear, something to be a little cautious about.
     
  2. Andrew Flannigan

    Andrew Flannigan Well-Known Member


    Have you measured it? I'm wondering what the count is.
     
  3. DaveS

    DaveS Well-Known Member

    I could do. If I get a chance when I go back after half term I'll put it in front of our geiger counter.
     
  4. Roger Hicks

    Roger Hicks Well-Known Member

    Usually high enough that you wouldn't want to use it as a telescope eyepiece (long periods close to the naked eye), which (believe it or not) has been known. Otherwise, unless you crunch it up and inhale it or use it for seasoning on your food, negligible. Keeping a large collection of hot glass under the bed isn't recommended either. Dr. A. Neill Wright covered it in some detail in the British Journal of Photography in 1987.

    Cheers,

    R.
     
  5. spinno

    spinno Well-Known Member

    Am I allowed a hot woman in the bed:eek:
     
  6. Roger Hicks

    Roger Hicks Well-Known Member

    Dear David,

    Whenever you can get one!

    Often best to marry her first and stay married to her, forsaking all other.

    Cheers,

    R.
     
  7. Learning

    Learning Ethelred the Ill-Named

    The culprit would seem to be Thorium 232 which produces alpha particles and has a half life of 1.4 x 10 to the 10 years. The alpha will not even leave the lens unless it is in an end element.
    The daughter of the Thorium 232 is Radium 228 which has a half life of only 6.7 yrs. and this emits beta radiation so forming Actinium 228 which rapidly decays to Thorium 228 as it emits another beta and a gamma. The Thorium 228 decays .......... and so on. If you follow the sequence with the aid of a table of radioactive elements then you will also find some gamma emittors. I gave up at Bismuth 212 where it gets more complicated. Beta and gamma should be detected.
    I have just looked at my old SMC Takumar 50mm f1.4. It is clearly yellowed enough to upset colour balance.
     
  8. Fishboy

    Fishboy Well-Known Member

    This might sound facetious but it isn't meant to be...but wouldn't using one of these 'yellowed' lenses with black and white film simply have the same effect as using a yellow filter?

    Cheers, Jeff
     
  9. Roger Hicks

    Roger Hicks Well-Known Member

    Dear Jeff,

    Yes.

    Cheers,

    R.
     
  10. Roger Hicks

    Roger Hicks Well-Known Member

    Which unfortunately it is in some lenses. Hence the warnings about telescope eyepieces. Nor should other radiation be ignored. When all this was first reported, I tried standing an (notoriously "hot") f/2.5 Aero Ektar on a sheet of 5x4 inch Polaroid 3000 and got an autoradiograph in less than 24 hours.

    Cheers,

    R.
     
  11. DaveS

    DaveS Well-Known Member

    OK

    As promised some findings

    With the rear element of the lens positioned approx. 10mm from the GM tube (Lind 72233 at 420V), averaging three sets of 10sec counts and rounding to whole numbers.

    Total: 94 counts per sec.

    With 1mm Al to remove the alpha: 49 counts per sec

    With 3mm Pb to remove all but gamma: 17 counts per sec

    Therefore we get : 45 counts per sec alpha, 32 counts per sec beta, and 17 counts per sec gamma.

    The alpha will be somewhat reduced as the GM tube was a significant distance from the rear of the lens, however I needed enough distance to insert the absorbers without risking the GM tube window.

    HTH.
     
  12. Andrew Flannigan

    Andrew Flannigan Well-Known Member


    So what does that come out to in millirem or millisieverts?
     
  13. DaveS

    DaveS Well-Known Member

    Don't know off the top of my head. I would need to investigate the energy abdorbed, rather than the bare count rate. This may be beyond the scope of the school level kit I have access to.

    Sorry.
     
  14. Learning

    Learning Ethelred the Ill-Named

    I wouldn't worry about it.The camera will absorb the alpha and beta; the gamm wil pass through the camera and one's head without any interaction.
    Dave, are you a secondary school physics teacher? If so, and you can carry on doing it then you are a better teacher than I. Well done.
     
  15. DaveS

    DaveS Well-Known Member

    Alas no, only the senior science technician, though I do have a chemistry degree. I'm also the deputy RPO.

    Having seen the paperwork and government directives teachers have to deal with put me right off the thought.

    Thinking about the lens, the alpha coming out of the rear would make it no-go as an eyepiece.
     
  16. Learning

    Learning Ethelred the Ill-Named

    Why "Alas"? I left teaching years ago.My sixth form students got good 'A' level results, but the rest of the job was hell. I was not cut out for it. Being on the autistic spectrum didn't help, but I drifted into teaching never the less. The days of the more than slightly eccentric physics teacher had gone.
    A senior science technician might have suited me but I would never have got the job because I have a PG Dip Ed.
    As to the lens to be used as an eyepiece,I would think that one could mount a microscope slide cover slip to absorb the alpha radiation.
    I am out of touch. To me, with an interest in music, RPO is the Royal Philharmonic; what is the RPO in a school? Advanced answer to your answer "I should have guessed":)
     
    Last edited: Nov 1, 2017
  17. spinno

    spinno Well-Known Member

    Radiological Protection Officer?
     
  18. DaveS

    DaveS Well-Known Member

    Got it in one squire!

    Our head of science / physics is the official RPO but she leaves the day-to-day work to me, ie logging sources in and out, doing the monthly stock check and anual leak test plus being in attendance during lessons. I'm also training up our other technician for when I retire. He's qualified to degree level too.
     
    spinno likes this.

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