[LargeFormat] radioactive lenses

[Michael Briggs]

Hello all,

I have been researching the Aero-Ektars.   Soon, when I have time, I will write
up the results of my research.   Unfortunately much of the information on the
internet is wrong including some of the information on the link given below.
Regrettably too many people are guessing and their guesses are being accepted as
fact.  I have based my conclusions on patents and research articles from the
1940's and on laboratory gamma-ray spectral measurements.   I am by profession
a gamma-ray astrophysicists and so have training and experience in radioactivity
and laboratory measurements.

I insert a few comments below.

On 24-Nov-01 Clive Warren wrote:
> At 3:35 pm -0800 21/11/01, Les Newcomer wrote:
>>there was a thread on the graflex helpboard about this and the best
>>information was this link
>>
>>http://people.smu.edu/rmonagha/mf/radioactive.html
> 
> Thanks for the URL Les.  Have read through the information there and 
> was surprised by the potential levels of radiation from the Aero 
> Ektars!


> To summaries, Thorium emits Alpha particles. The resulting "daughter" 
> products of the radioactive decay of Thorium produces both alpha and 
> beta particles.
This statement is correct but seriously wrong if one takes that obvious
interpretation Thorium and its daughters emit only alpha and beta particles.
When some of the daughters decay into further daughters by alpha or beta decay,
the new daughter nuclei are created in an excited state.   These nuclei reach
their ground state almost instantly by emitting one or more gamma-rays.  There
are also X-rays from atomic transitions caused by the nuclear transitions. 
This is very important because it is very easy to shield against alphas, fairly
easy to shield against betas and quite difficult to shield against X- and
gamma-rays.  Not realizing the presence of X- and gamma-rays could cause
someone to seriously underestimate the amount of shielding needed to absorb the
radiation.  Without careful evaluation of shielding, the best plan is probably
the simplest: keep distance between people and Aero-Ektars except when the
lenses where in use.

> It seems that the decay process of Thorium results in 
> the glass becoming progressively more radioactive over time, as the 
> more highly radioactive decay products build up in the glass. 
> Eventually a more highly radioactive equilibrium will be reached. So, 
> over the years when you are expecting the lens to become less 
> radioactive they are actually becoming hotter -  more radioactive 
> rather than less!

The second longest half-life in the Thorium decay chain is 6.7 years (the
longest belongs to Thorium).  This means that equilibrium between Thorium and
its daughters was reached in these lenses many years ago.

> 
> I guess that the radiation is not that harmful, however also found 
> the following:
I am still working on my safety evaluation of the lenses.   My PRELIMINARY
results are that hour long exposures are safe but a year long exposure would
cause a substantial increase in a persons radiation dose.  I do NOT guarantee
the correctness of these calculations.


> 
> '.....Aero Ektar that is very impressive in girth and weight. One of 
> the elements in the rear group is yellow/brown in color and will peg 
> the meter on a geiger counter, if placed within 6"'
> 
> Hmmmmm..... better keep mine away from stored film then......

 
> Richard Knoppow stated he had read 'suggestions that the color 
> correction of the (Aero Ektars) is optimized toward the red since 
> aerial cameras are nearly always used with yellow filters and often 
> used with IR material. If this is true the lens might exhibit some 
> blue fringing'.

Elsewhere Richard states that the lenses were designed for night-time aerial
reconnaissance photography.  The documents from the period support this
statement. Based on the intended use, I think it unlikely that the Aero-Ektars
were designed to only be used with a filter.
 
> 
> The coverage of the 7" lens is 4 1/4 x 4 1/4.
The coverage of the 7" / 178 mm lens is 5 x 5 inches.   That is the meaning of
the "5X5" written on the edge of the lens.

--Michael