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

Message 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, andatoms 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 neutralatom 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 photonsproviding 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 wouldplastic 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 willmake 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 stillnot 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

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