those pesky gammas in yr Kodak (LONG)

[P3D <gosfield@xxxxxxxxxxxxxxxxx>]

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ABSTRACT: 1) Low level radiation has never been shown to cause a
measurable effect in man.  All low level effects are extrapolated from
animal experiments and epidemiology of survivor populations (Hiroshima,
Chernobyl) 2) Geiger counters (survey meters) are inappropriate/innacurate
for the measurement of biologically significant dose rates (neat idea
though) for technical reasons. 3)calculations and current regulations for
occupational exposure permitted to radiation workers, presented below,
show no significant effects likely from any reasonable use of yr. Kodak. 
Just don't wear it pressed up against your gonads continuously for more 
than 25 days at a time (for those of your who REALLY LOVE your Kodak.)

Long boring Mime attachment follows--------------escape here--------



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I enjoyed the $^% out of John's post, since it made me think
of my days as a nuclear scanner.  Now that the destruction
of the medical profession has deprived me of the opportunity
to practice my nuclear craft, i don't often have the chance to 
talk about this stuff with anyone.  But i do have more time for
stereo photography. 

In discussing the 'danger' of Kodak lenses, I hope we are basically all
joshing around.  I doubt that the lenses of a Kodak can add any detectable
radiation risk to your life.  This is mostly because the dose is so low
that even if it caused an effect in some member of the population, the
spontaneous rates of cancer, mutation, birth defects, etc. from other
causes are so much higher, as are the rates of these problems attributable
to 'natural' radiation (cosmic rays, radon in the earth, uranium in cement
, etc.) that you would never be able to assemble a large enough population
of people to do a study of sufficient power to find the miniscule
contribution of even all diagnostic nuclear medicine tests, let alone the
Kodak and all other photographic lenses
ever made.  So much for epidemiological evidence. 

As a matter of fact, I don't think there has been any study in humans yet
showing GENETIC damage to offspring from radiation.  The increased
incidence of birth defects in the unfortunate victims of Chernobyl
(including downstream Europeans) are from direc t effects of radiation on
the fetus in utero.  Descendants of Hiroshima victims don't have any
increased incidence of defects due to ancestral radiation exposure. 
There is a current study being debated, as John pointed out to me by 
email, which purports to find differences in certain DNA sequences 
between Chernobyl area inhabitants and a comparison population of British 
people.  Whether this is truly a 'controlled' study, and how to 
demonstrate the differences are due to radiation rather than other 
environmental insult are controversial.  Radiation damage to DNA looks 
precisely like breakage and repair from any other cause.

There is, of course, no "ABSOLUTELY safe" level of radiation which has
zero statistical likelihood of causing cancer or genetic defects. This is
called the 'stochastic' kind of effect, meaning dose related
likelihood/probability of effect. It is so hard to detect the effects of
low dose radiation that the model for low dose effects has changed during
my very short (4 yr) nuclear career.  It started out linear down to 0
dose, then went to linear-quadratic, now is back to linear (they
reevaluated Hiroshima and some other stuff. ) As i said earlier, most of
the estimated effects are derived from measured effects on bone cancer
induced in mice from known exposure. 


The 'Nonstochastic' kind of effect is the kind where there is a safe
threshold of exposure, below which no effects take place.  This includes
direct radiation effects like bone marrow damage, cataract formation, etc. 
People who die of 'radiation poisoning' have died of nonstochastic
effects.  Low dose radiation is defined as 10 rem, below which no effects
have ever been demonstrated in humans. 

I was taught not to trust Geiger counters for any biological quantitative
measure of radiation dose, although they are quite good for the detection
of radiation.  As you know, they make their clicks by amplifying the
response to any single decay event of sufficient energy to cause
ionization, but any kind of decay event above threshold is amplified
equally, as you implied in your post.  So gamma equals beta equals alpha
from the point of view of the Gieger-Muller tube.  In addition, I was
taught that the "dose rate" calibration is suspect, since the correspondence
between event rate as measured by the tube, and energy dose, is not a
simple one. 

Current regs for radiation workers occupational exposure, as of January
1993 revision of the Code of Federal regulations, Part 20-industrial
radiation exposure): 

5 rem per year, whole body', or 3 rem per calendar 'quarter,
75 rem per year to the extremities, or 18.75 rem per quarter
30 rem/yr for the skin, or 7.5 rem/quarter

Research study dose limits:  
Maximum dose to any "non-sensitive"organ in a research study:  5 rads
Maximum 3 rads to any sensitive organ (testes, eye lens)
Max dose per year 15 rads, 5 rads for a sensitive organ
use 1/10 of these doses for patients under 18 years age

Assuming John's measurement of dose rate is correct:  Radiation damage is
decreased by fractionating the dose (intermittent exposure allows your
body to repair some of the damage before it has a permanent deleterious
effect), so instead of getting your 5 rem whole body dose (pretty
meaningless for this geometry) in 41.67 days, you could reduce the danger
by spreading it out over time.  If you wear it low on your leg, like a
gunslinger, you can have a whole 75 rem per year, and you can wear yr hot
Kodak 625 whole days in a row, so you are safer :^). And of course you are
allowed a whole 30 rem/year to your skin, so make sure you keep your Kodak
next to your skin ;^ ) Don't wear it next to your gonads, though, since
you would get your yearly dose in 25 days if you never took it off
(depending on how much lead there is in your underwear)
:^)

The half value layer (absorbs half the radiation) of lead for 140 KeV
Technetium-99 gamma rays, like the ones we use for diagnostic scans, (more
energetic than the Thorium-232 gamma) is 2.55mm.  I don't know what the
equivalent half value layer is for Kodak case leather. Realist case 
leather is thicker, so maybe George could modify Realist cases for 
owners of hot Kodaks.

Now here is how really to do the measurement:  stick the camera (caseless)
in a NaI crystal detector well counter (scintillation counter used for
measuring doses) and count in the various energy windows available to you,
so you can pick the ones where there is some chance of the gamma ray being
energetic enough to actually make it through the case or your clothing. 
Actually I suppose this is a joke, since well counters usually have a
smaller bore than the size of a Kodak. 

Or else put the camera in front of a single crystal (NaI) scintillation
counter (like the kind we use for measuring radio-Iodine absorption in
thyroids) and measure in the energy windows of choice. 

This will allow you to get a true dose rate of relevant gamma rays.

I still think there is no real risk, and I doubt the relevance of the
Geiger measurements you made (neat idea, though), but I have only used
Geigers as survey meters and very gross indicators of contamination.  What
we really need is a radiation physicist , since I am a mere medico, and
don't even have access to the equipment right now.  Maybe if I get really
bored I can take my Kodak in to one of my friends' nuclear suites, and
stick it in front of the single crystal detector.  But there ain't no
nasty yellow in my Kodak lenses, (i checked with the shutters open, over a
5000K light table) so maybe I don't have any of that awful nookyouler
energy in my lenses. I guess i shoulda used a spectrophotometer probe, 
but i don't have one.

I have actually asked on the Nuclear Medicine listgroup if someone can 
provide us with a good compact simple explanation in non-technicalese, as 
to why the Geiger-Muller measurements of dose rate are suspect.  That 
will allow us all to relax about the flux of gamma rays from our hot 
Kodaks.  I suspect the real reason we can relax is that the number of 
sufficiently energetic rays reaching us is vanishingly small.

Wow--you read the whole thing.  Now you know more about radiation dangers 
than most of the people in America.

Best wishes,

ted


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