[LargeFormat] Van Ripper didn,t know

[Michael Briggs]

On 29-Feb-2004 Richard Knoppow wrote:

>> You probably got the glass patents from me.  They are
> Morey, US Reissue 21,175
>> and Eberlin and De Paolis, US 2,241,249.
>>

>   I didn't mean the patent for the Aero-Ektar, I have that.
> I mean the patent for the rare-earth glasses from the NBS.

Those numbers are above, from my previous message.   

>   I doubt very much if Thorium was used in any consumer
> lenses at all. Lanthanum was used in lots of them but
> Lanthanum is not radioactive. Most of the radiation from
> Thorium is alpha particals which die away quickly with
> distance.

Thorium was used in MANY consumer lenses.  I have measured it in several
different consumer lenses.  For 35 mm photography, these include a 50 mm f1.4
Pentax screw mount Super-Takumar and pre-AI 35 mm f1.4 Nikkors.  For medium and
large format photography, lenses that I have measured radiation from include
Schneider Repro-Clarons and some Xenotars.   The articles I mentioned in the
British Journal of Photography have almost three pages of tables listing
radioactive lenses (but some of the entries are duplicates, measurements of
several samples of the same lens).  There are additional crediable reports on
the internet, e.g., some Canon and Leica lenses.

For many years Lanthanum glass was really Lanthanum/Thorium glass, so if a lens
manufacturer wanted a high-index, low-dispersion glass, they had to use a glass
with thorium.  I am not sure when optically equivalent glasses without thorium
became available.

As far as I know, thorium glass is no longer produced nor used in any current
lens.  Production seems to have ceased circa 1980.

Thorium and its daughters emit many forms of high-energy radiation. By
now, lenses containing thorium are old enough that the radioactive decay
products (daughters) of thorium have built up, so their radiation is also
present.  As you say, the alphas die away quickly -- most are absorbed in the
glass and if any manage to get out will be absorbed in a short distance of air.
However, thorium daughters also emit photons (x-rays and gamma-rays) which are
unlikely to be absorbed by air.   The falloff in this radiation is from the
inverse square law -- they photons spread out as they get farther from the
source.

http://www.euronuclear.org/info/encyclopedia/d/decaybasinnatural.htm has a
diagram of the decay chain of natural thorium (Th-232).  The diagram shows
which steps emit alphas and beta-minus (electrons), but not shown is that some
of the steps also emit gamma and x-rays.

--Michael