CNN.com article "New fabric touted as radiation-proof"

Thanks for your explanation. I found it very informative.

> Let me explain some physical principles of shielding.
Radiation that
> can damage living cells is usually divided into two categories,
charged
> particles (such as solar wind protons, alpha particles and others)
and
> photons (gamma and x-rays). Both interact mostly with electrons,
so any
> material that holds lots of electrons in a small volume will work
for
> stopping them. The materials we have for shielding here is
atomic, and
> atoms are roughly all the same size, around an angstrom (or 0.1
nanometer)
> in diameter. So which atoms have the most electrons and are not
too
> expensive? Pick the atoms with the highest number of protons,
because each
> neutral atom has the same number of protons and electrons. So of
the
> naturally occurring elements, uranium would be the best, except it
is
> radioactive, expensive and can be used to make bombs. So pick
some other
> abundant and non-radioactive elements. That would be (the highest
> non-radioactive element, which is) Bismuth, with 83 protons and
electrons,
> and a density of 9.8 gram/cc. Another good choice is Lead, with
82 protons
> and electrons, and a density of 11.4.
> Now not all electrons are the same for stopping gamma rays.
The more
> tightly bound ones work better for photons providing that the
gamma ray
> energy exceeds the electron binding energy. And more loosely bound
> electrons are better for stopping charged particles. But ignoring
these
> complicating factors, the best shielding in a small space is
provided by
> materials which have the highest mass density. The mass comes
mostly from
> the protons and neutrons, but the shielding comes from the
electrons.
> Now, how would plastic and organic compounds, with a density of
roughly
> 1, work. Okay for charged particles if they were maybe 10 times
as thick as
> a piece of lead, but not as well for gamma rays. One could try
and lower
> the mass to electron ratio by using lighter isotopes, such as
hydrogen 1
> instead of hydrogen 2, which we mostly do with out trying with
hydrogen, but
> most elements don't differ nearly so much and separating isotopes
will make
> the shielding very expensive.
> So without having seen any technical data on this "radiation-
proof"
> fabric, my guess is that it incorporates some higher elements into
it, but
> will still not be very good at stopping radiation or be very light-
weight
> either. The Earth's atmosphere is equivalent to about 76 cm of
Mercury
> (remember barometers?), which would be about 90 cm of lead. We
suspect we
> need such shielding for the long term.
> Sincerely, Jay Huebner
>
radiation-pr
> oof"
>
> It sounds a bit like the polyethylene that was used out on the
ISS. It
> does work to some extent- it cuts the radiation by a few percent,
and
> weighs comparatively little, but apparently it doesn't work as
well as
> was hoped.
>
> I'm beginning to think that we're never going to get around the
fact that
> more protection is always going to mean more mass.
>
> On the other hand, some people go too far with that. They advocate
> burrowing many meters into the interior of asteroids, and tout
protection
> from radiation as an advantage of that approach. I always tell
them that
> anything over 2 meters is overkill.
>
> Regards,
>
> Mike Combs
>
7050835
>
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