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Plastic Radiation Shielding

Apparently, plastic can make for an effective radiation shield.
(06-14-2013, 04:51 AM)Sim Koning Wrote:

Apparently, plastic can make for an effective radiation shield.

It has been known since at least the 1940s that materials with a high hydrogen content, such as paraffin and plastics, are very effective at attenuating alpha and beta radiation, and thermal neutrons, but much less so in the case of x-rays and gamma radiation. Apparently, this has not changed appreciably in the intervening seven decades. According to the specifications given at for a polyethylene impregnated with gadolinium, 7.6 millimeters of plastic attenuates 1 MeV gamma radiation by 12.3%. While additional thickness would result in greater attenuation, at some point the thickness becomes prohibitive. Radiation with energies greater than 1 MeV (most cosmic rays, for example) will be attenuated even less. So, in general, while plastics can make effective radiation shields, likely design trade-offs will limit their usability in that regard.

"I'd much rather see you on my side, than scattered into... atoms." Ming the Merciless, Ruler of the Universe
Water ice contains a lot of hydrogen, and so does waste water. A spaceship can keep its water supply and waste water or grey water (lightly contaminated, used water) in tanks or shells on the outside of the craft. You've still got to protect against cosmic rays one way or another.

Note that most biont clades in OA and almost all aioid clades will have some resistance to radiation damage, and quite often some form of efficient repair system or systems as well. If you are expecting to venture into space, you are better off going prepared.
Ice is darn handy stuff and is likely to be used in lots of ways, often in applications that allow it to 'multi-task' in various roles. If you've got to lug mass around space anyway, you might as well maximize the use of it.

Lots of ships will probably be designed so that the crew quarters are in the center while the fuel tanks and such are on the outside to provide additional shielding.

This is not to say that plastic won't have a role. Hydrocarbons in various forms are also darn useful in a nanotech equipped civ. Perhaps plastic might play a role in smaller craft or spacesuits or devices that don't readily allow for lots of ice based shielding.

It's notable that alpha, beta, proton and neutron radiation are quite effectively shielded by low atomic weight materials such as carbon, also. I believe the reason is that in the case of these radiations, the closer the nuclear mass is to the mass of the incoming particles the more momentum is transferred. This is IMHO relevant, because of the likely use of carbon-based nanomaterials for structural components means that you're lugging around lots of carbon anyway.

To stop gamma and X-rays a reasonably small amount of lead or similar might do, at least in environments similar to that around Sol. The real problem is heavy ions in cosmic rays; although lead might stop those quite well you then get lots of secondary particles. For that reason, IMHO if you're going to line your ship with something heavy the best place for it is probably on the outside.

One more point: For habitats and long-duration ships in low-tech (by OA standards!) polities, you're going to need quite a lot of crushed mineral (i.e. dirt) to grow your crops (and leisure plants such as trees) in. A couple of metres of dirt would double quite well as a shield.

Just my 2 cents.
There is a soil-substitute in OA known as Garden Paste that presumable reduces the need for minerals in the crop-growing substrate. In a habitat or long-duration ship that grows its own food, the majority of that food would probably be vat-grown like algae; much of the rest will be grown aeroponically or using a minimal amount of Garden Paste, and only a small amount will be grown in the traditional way using mineral-based soil. So there might not be as much rock on a habitat as one might expect.

(Currently Garden Paste is one of the aspects of the scenario which needs expansion; currently the article takes the form of an advertisement or instruction sheet from a Garden Paste manufacturer, which is interesting as ephemera, but it needs more detail.)

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