From: zowie@daedalus.stanford.edu (Craig "Powderkeg" DeForest)
Subject: Re: Radioactive Cat: How Dangerous is She? <LONGISH>
Organization: Stanford Center for Space Science and Astrophysics
	<67qnr=j@dixie.com>
Date: 8 Sep 92 01:48:17

In article <yfpnc!g.badboy@netcom.com>, Jay Keller writes:
   How much exposure to radiation will my family get after the cat comes
   back from the radioisotope treatment?  I was told the cat would be
   hospitalized for a few days to allow the radiation to subside.   
   But I have 2 small children and this is a very friendly cat...

   ...

   What questions should I ask of the radiologist?

I got out my handy Chart of the Nuclides, and phoned up to the Reed Reactor
Facility (portland, OR; (503) 777-7222) with a couple of health physics 
questions.

The main radioiodine treatment I've seen is I-131 (for thyroid destruction).
This is made by bombarding Tellurium with neutrons in a reactor, making
Te-131, which beta-decays into I-131.  

As jfreter@triton.unm.edu said, I-131 is a beta emitter: it gives off
high-energy electrons, turning into Xenon-131, a stable, inert
species.  (also emitting a gamma ray or three in the process.)  John
De Armond <jgd@dixie.com> said something about gammas and shielding of
I-131 patients.  The idea here is that the I-131 produces beta rays,
fast-moving electrons.  If you try to shield them with lead, they will
be suddenly stopped (when they slam into the metal).  Since the way to
make photons is by shaking electric charges (eg in a radio antenna),
they [the betas] will give off photons, in this case gamma rays, on
impact.  In general betas are absorbed by a few inches of air, or a
few millimeters of plastic or wood, or a few tens of microns of metal.

[Alpha particles are much heavier -- they are helium nucleii -- and much
slower, for a given energy.  They are typically stopped by a few inches
of air, or a sheet of paper, or your epidermis.  So long as you don't
ingest an alpha emitter, you're fine -- but, with iodine, there aren't 
any alpha particles involved.]

I-131 has a half-life of about eight days, so it would take 80 days
for it to be reduced to 0.1% of its original activity level.  The
*biological* half-life (time to be removed from the body by normal
metabolism; independent of isotope) of iodine is 138 days in humans,
and probably somewhere near that for cats, so the radioactive decay is
the important factor here, not feline metabolism.

The long half-life means that, *IF* they are putting I-131 into your
cat, you probably do not want it in your home for at least six weeks!
(unless there's a rather low concentration in the cat...)

There *are* several useful isotopes of iodine that could be used:
I-130 through I-135 are all beta emitters, with reasonably short 
half-lives.  I-130 lasts only 12 hours, for example.  Any of these
isotopes would allow you to hug your cat sooner.

Jfreter pointed out that direct, whole-body exposure from your cat is
probably going to be small.  Ask your radiologist, but this is probably
the case, since the idea is to concentrate the isotope in the cat's
thyroid.

One thing to consider is that, even after your cat isn't very
radioactive, it's still a living being, metabolizing and excreting its
radioidine.  The stuff will be present mainly in its urine, and
probably in its other excreta (feces, saliva, etc.).  Radioiodine in
your environment will be absorbed into *your* thyroid, presenting a
much larger hazard [from contamination] than direct exposure from the
cat.  If the cat is to be in your house during this time, you might
consider giving your family iodine tablets, to protect your own
thyroids: by saturating your system with iodine, you prevent uptake of
the unwanted radioiodine.

Some important questions, then, for the radiologist are:

* What isotope and dosage will my cat get?  What is the half-life of the
treatment?

* What will the exposure rate be at the surface of the cat, after it's
returned to the house?  One foot away?  Five feet away?

* Will there be significant amounts of iodine released from the cat?  Should
my family take iodine tablets after the release of the cat? 

Please post and let us know what happens!



--
zowie@banneker.stanford.edu  "Tell your senator to fund the MSSTA program!"

 The world would be much better if more physicists read Strunk and White, 
 and if more fuzzies read Tipler.   *Everyone* should read and understand
 at least one economics text.  No exceptions.       -- zowie