Archived posting to the Leica Users Group, 2007/11/07

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Subject: [Leica] OT: Fixing radioactive lenses
From: hoppyman at bigpond.net.au (G Hopkinson)
Date: Wed Nov 7 17:24:29 2007
References: <27347376-1AD0-40B7-B526-32A6572F1A6D@comcast.net> <200711072256.lA7MuoDn068946@server1.waverley.reid.org>

Marc, firstly obligatory disclaimer. I am no nuclear chemist.
A quick Wiki shows half-lives for various isotopes from 25.5 hours to REALLY 
REALLY long (3 times the age of the earth)!
I have seen it used in large optics in an aviation application with no 
functional issues despite many years of operation in very
hostile environments (kinetic heating, vibration and more). I recall that 
the same equipment also contained a large mass of depleted
uranium (ballast). Maybe there was some synergistic radioactive voodoo going 
on there.

Back on Len's lens, I wonder if this is a coating, adhesive or lubricant 
affected rather than the element used within the particular
glass compound?
Weren't there some Leica lenses also using this glass type?

Cheers
H0 P2y


-----Original Message-----
From: lug-bounces+hoppyman=bigpond.net.au@leica-users.org 
[mailto:lug-bounces+hoppyman=bigpond.net.au@leica-users.org] On Behalf Of
Marc James Small
Sent: Thursday, 8 November 2007 08:54
To: Leica Users Group
Subject: Re: [Leica] OT: Fixing radioactive lenses

At 05:10 PM 11/7/2007, Leonard Taupier wrote:
 >I have an old Pentax 50/1.4 Super Takumar that has, over time,
 >yellowed due to one of the elements containing Thorium, a mildly
 >radioactive element. The addition of thorium makes a very high
 >refractive index glass, and a very good lens. This lens is probably
 >one of the most famous lenses with this element but I'm sure there
 >were more.

Len

I love you like a brother, but be conscious 
here.  The yellow cast may well result from 
decaying coatings.  Disassemble the lens, remove 
the lens coatings, and then contemplate the 
result.  If an element is yellow, then, yes, your 
original thesis may be correct.

It takes thousands of years for radioactive 
elements to decay.  I am baffled at the thought 
that the passage of only a few decades can lead 
to some change in the color of glass 
elements.  Maybe I am a babe in the woods -- you 
guys have locked my heels over the years on 
things ranging from the utility of Tilley hats to 
the vibrant necessity of US television -- but I 
just do not see this as change as a reality.

Do we have any nuclear chemists on the List who 
can add anything to the discussion?  Jerry 
Lehrer, I realize, will tell us of his experience 
at the Brooklyn Home For Broken Boys or whatever 
it is called, back in 1921 <he grins>, but I 
really would like to know the straight skinny 
from guys who regularly have worked with thorium or with thorium glasses.

And, Jerry, you STILL owe me those promised 
serial numbers from your studio Rollei.  <he grins, yet again>

In short:  can Thorium glass really change color 
in a few decades?  We hear this all the time.  But is it real?

Marc


msmall@aya.yale.edu
Cha robh b?s fir gun ghr?s fir!



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In reply to: Message from len-1 at comcast.net (Leonard Taupier) ([Leica] OT: Fixing radioactive lenses)
Message from marcsmall at comcast.net (Marc James Small) ([Leica] OT: Fixing radioactive lenses)