Don't eat yellow lenses!

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

A thousand apologies to Elliott for not giving him the correct 
answer.  I went home last night and looked at my Kodak (S/N ~ 
040000) and by golly the lenses _were_ a weeee bit yellow.  Very 
pale, but it's there.  So I brought the camera to work with me and 
found a freshly-calibrated pancake Geiger counter.  The lens is 
indeed radioactive.  Here's what I got.  At 1" from the lens I got 
1300 counts per minute.  At 3" I got 400 counts per minute.  At 6" 
I got 100 counts per minute.  Background was nil.  Going around to 
the back of the camera, I didn't get anything detectable.  Putting 
the Kodak case over the lenses, the counts dropped to 50 right 
against the case.  The pancake is quite a bit larger in diameter 
than the lens in its aluminum barrel (so only a part of the 
pancake is being illuminated when it's close up) therefore I'm not 
surprised at the deviation from the inverse square law.

I don't know how many of you do this 8-), but I wouldn't recommend 
carrying this camera with no case and the lenses against your chest 
for a really long period of time.

Hey Dr. T!  Here's _one_ reason to have a camera case!

The Geiger counter's dial suggests that I'm getting a half a 
mRad/hour right at the lens but I don't know what the particle
spectrum is so I won't go that far.  Is there a Doctor in the house?  
Specifically one with a specialty in nuclear medicine?  8-)

I think we used to be allowed 5 rems per year and now it's down to 
maybe 1, and that's whole-body.  This lens would be very weak in 
whole-body mode.  (A rem is a rad times the quality factor which 
is about one for beta or gamma and ~20 for alpha and other numbers 
for other things.)  So assuming we're looking at, say, gamma here, 
you'd get a year's worth (a rem) _locally_ (not whole-body) in 
2000 hours or three months.  That should put it in perspective.

One thing's for sure, this isn't a really high energy particle if 
it's getting more or less stopped by 2 or 3 mm of leather case.  
Nevertheless, I wouldn't eat the lenses (a new form of pica?).

I hear what they used in lenses was thorium and a high occurrence 
long-life isotope of that would probably be 232 which emits some 
alphas (4Mev), electrons (75Mev), and x-ray/gammas (10-100Kev).  
A separate check with an alpha meter showed nada so the counts 
aren't coming from alphas.  The count efficiency on the Geiger 
counter used is around 1 or 2 % at low energy and 35% at high 
energy.

Well, enough meandering blather from me; I'm not going to worry 
about it but I've said that before and been wrong and besides I 
don't know what I'm talking about so let's see what the Doctor 
says.

Back to the discussion of polonium-powered camera obscurae!

John B


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End of PHOTO-3D Digest 1341
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