[Florin_Andrei]

> This is due to cosmic rays. On the surface of the earth, the effect is mostly abated by the atmosphere - except for neutrons.

So, if we had more hydrogen (either free or compounds) in the air this would not be the case, right?

The column of air on top of your head is equivalent (in terms of mass) to a column of water 10 meters tall, with the same base section area. But the composition is quite different, of course - the only major component they have in common is oxygen.

[gh02t]

Cosmic rays have energies in the tens to hundreds MeV range where pair production is the dominant attenuation mechanism. The probability of a photon inducing pair production in a material is roughly proportional to the square of the proton number, ergo cosmic rays don't give a shit about hydrogen (which has the smallest proton number possible). Even if the atmosphere was pure hydrogen gas at STP, the average distance traveled by a 20 MeV cosmic ray would be around 17km. http://physics.nist.gov/PhysRefData/XrayMassCoef/ElemTab/z01...

Hydrogen is pretty good at moderating neutrons down to thermal energies (eV range, ie room temperature) via elastic scattering, but gasses don't really have enough density to do a very good job. If you really want to protect something from neutrons you just coat it with boron. A mm coating of the stuff will keep out pretty much any common source of neutrons.

[Florin_Andrei]

Right, I was thinking about the neutrons. Maybe something like methane would be more efficient than pure hydrogen. Anyway, it's just a thought.

It's very surprising to see how efficient boron is. I thought neutron shields (paraffin, water) are supposed to be very thick. Maybe boron does the job via a different mechanism?

[gh02t]

Boron is almost exclusively an absorber, so the boron nucleus captures the neutron and it basically disappears. Hydrogen is primarily a moderator, so it reduces the energy via elastic scattering, but a significant number of very low energy neutrons still escape (some absorption to produce deuterium also occurs). Thermal neutrons can still cause damage, but they're easy to block with a subsequent thin layer of lead or something like that.

Boron's probability to capture a neutron is astronomically high, that's why you can get away with so little. Environmental sources of neutrons are actually pretty rare normally and most neutrons you do see will be pretty low energy and won't have a huge amount of penetrative power. A thin layer of boron will pretty much stop them. Pyrex (like the stuff baking dishes are made of, which is borosilicate glass - glass with boron added) is actually commonly used as control material in nuclear reactors.