A Effect of High-Energy Particles on Lead

[RJ Emery]

Neil Comins in The Hazards of Space Travel states a 40-inch thick wall of lead would be needed to shield interplanetary travelers from the solar wind, coronal mass ejections, and cosmic rays. When lead is constantly bombarded with high energy charged particles, what changes occur to the lead?

 

[trurle]

Most significant effect is highly penetrating bremsstrahlung X-rays secondary radiation. Therefore, lead is out of favor for spacecraft radiation shielding. Aluminum have same efficiency as lead at gram-per-gram basis stopping primary radiation, but produce much more benign secondary emission. Also, aluminum can double as structural wall of spacecraft ..
Overall, 3-5mm aluminum shield do a decent job for probes, reducing doses about ~10 times for GEO spacecraft . Terrestrial levels of protection (which is likely implied in your citation) would need about 2 meters of aluminum though.

P.S. Average interplanetary dose for unshielded person in absense of flares is about 60-100 Rad/year. It is actually survivable - and even hardest solar storms are survivable with light (<1cm Al equivalent) shielding. Most of Mars colony ship projects are unshielded as whole, with light to medium radiation shields sporadically applied only to sleeping berths which double as "storm shelters". Most spaceship design use no dedicated shielding at all, relying instead on shielding by hull and strategically placed equipment.

 

[mfb]

When lead is constantly bombarded with high energy charged particles, what changes occur to the lead?

The environment in space is far away from radiation levels where changes to the shielding material would be a concern. You need to go to nuclear reactors for that.
A common type of radiation damage (outside of semiconductors/life) is materials getting brittle as their crystal structure gets more and more damaged and more and more other atoms get into the material.

 

[Vanadium 50]

states a 40-inch thick wall of lead would be needed to shield interplanetary travelers from the solar wind, coronal mass ejections, and cosmic rays.

I am quite suspicious of this calculation. This is the number so that the density of lead per square inch is the same as the density of Earth's atmosphere per square inch. In short, it calculates what we already have, not what we need.

 

[gleem]

The calculation neglects the dependence of atomic number for the various cross sections of interactions that can take place.

 

[DEvens]

It depends on the type of radiation. And the energy level. And the time scale and the type of effects you care about.

It was mentioned up thread that radiation can damage the mechanical structure of a solid. It takes a lot to do that. And lead tends to be kind of a blob, so it takes a lot of damage to make much difference to the properties of lead. But, eventually, you could turn it to a powder.

For beta or alpha, almost anything is good shielding. Alphas can be stopped by just about anything only a few mm. Betas by a few cm. Don't eat that, and you are probably good.

For gamma radiation you need a little more. The 40 inch of lead would probably be good for just about anything you'd see in terms of gamma in space, at least not too far from Earth.

There's lots of other things in space. Solar wind contains protons. Protons probably get stopped by quite thin shielding.

There are also some neutrons. This gets very complicated.

https://en.wikipedia.org/wiki/Isotopes_of_lead
Lead has a bunch of isotopes. It can catch a neutron and go to the next higher isotope. Except for Pb204 that will get you a beta decay to Bismuth. Which probably brings you back to Lead when it catches a neutron. Or Thallium after it decays, which again bounces around with Lead and Bismuth.

But lead can also have a neutron knocked off. The process is sometimes called "spalling" You start with one neutron at 1 MeV and wind up with two neutrons at 0.4 MeV or so. This can actually increase the dose for the person shielding behind it. So, in those cases where there is a neutron source in the radiation, you probably want to not be using lead as your shield.

Which brings us to cosmic rays.

http://explorecuriocity.org/Portals...ackgrounder -Cosmic and Neutron Radiation.pdf
These are very high energy particles. When they hit things they tend to produce showers of radiation. If they go by without that you are generally better off. So having a bunch of shielding is not necessarily the best possible situation. Probably what you want is to have only as much shielding as required for the background. If you try to have a lot of shielding the tendency is for the cosmic rays to kick up a lot of secondary stuff that spoils the benefits.

In our atmosphere, a lot of these shower things happen at high altitude. So the atmosphere is there to catch a lot of it. But some make it to the ground.