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The current estimate for how much shielding is needed for a long-term habitat on the Moon is a minimum of 700 g/cm2 of regolith, and 1000 g/cm2 for the levels at sea level on Earth. (The radiation dose on the lunar surface is not really known, so this is an estimate based on highly complex modelling.) That same estimate is mentioned in this video on civil engineering on the Moon:

I've been designing such habitats for a project to create a virtual lunar colony online. One area uses a configuration designed to allow in lots of natural light from above while still keeping the overall radiation low enough that a person can spend lots of time there. In that process I've been using the assumption that what they mean is that at least 700 g/cm2 in all directions yields this result. Today it occurred to me that it could instead mean that case over a hemisphere. At any rate, the only scenario where exactly the same amount of material stands between you and the outside environment is when you are standing dead center in an empty sphere with walls of uniform thickness, all of the same material (and you are also a dimensionless point, if one is especially a stickler about it).

Up to now I have been subtracting directions in which there is material of greater than 1000 g/cm2 from the total radiation dose, and I have been adding on higher radiation doses for the directions where there is less, as a proportion of all directions. It seems to be the best way to estimate dose within a given design. I also consider the proportion of time that a person might spend in an area with a given dose.

If these estimates aren't based on that simplification of a sphere, those results are off. If it is a model of a half-sphere, then I need to double the numbers. Or maybe it is modeled on something much more complex, like a point under a plane of that much material.

Does anyone know what assumption is used?

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    $\begingroup$ You may be overthinking this. If you build a box with walls thick enough to give 700 g/cm2 of material, the radiation dose will be acceptable anywhere in this box. $\endgroup$ – Hobbes May 2 '16 at 7:52
  • $\begingroup$ @Hobbes Yes, but a box doesn't have windows, or places where natural sunlight enters. If people are ever to live on the moon, they will need those things, preferably lots of them. Living in a box is no fun, especially when you can't go outside without a very bulky and clumsy spacesuit. For instance, a series of beams laid east-west across a large trench, with a direct view of the sky in between, lets in enough sunlight for plants to grow, and gives something of a feeling of being outside. But what would the dose be there? How do you size the beams to lower it? $\endgroup$ – kim holder May 2 '16 at 14:28
  • $\begingroup$ A bunch of beams with space between would reduce the exposure by the % of the roof that was beams. You're starting from .25 Sv/yr, you're not getting a reasonable amount of light through without letting in a lot of radiation. $\endgroup$ – Loren Pechtel Jun 2 '16 at 0:04
  • $\begingroup$ You can have windows without radiation, though--don't let the windows see the sky. Rather, the windows look upon mirrors. Since the radiation can't turn a corner you can build a path light goes through but which provides shielding. $\endgroup$ – Loren Pechtel Jun 2 '16 at 0:05
  • $\begingroup$ @LorenPechtel - Yes, i've used that concept in different areas, but feel there is value to a space where lots of natural sunlight comes in from above over a large area, so it feels a bit like the outdoors. I added a link in the question to a description in the project's forum, it hasn't yet been added to the main page. Here it is again. $\endgroup$ – kim holder Jun 2 '16 at 1:27
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From the PDF you linked:

Large-scale radiation shielding will be required, beginning in phase two, to protect humans spending time in lunar structures. The most commonly suggested type of radiation shielding is a regolith cover because it includes the advantages of in-situ resource utilization, as well as provides meteoroid and thermal shielding.

That sounds to me like they're assuming you're building on the Moon's surface, with a layer of regolith around and on top of your structure. A half-sphere would satisfy that.

Given the nature of light on the Moon (no atmosphere, so very little indirect light), you'll have to treat windows differently than you would on Earth. Direct sunlight will be much too bright (and full of UV) so would have to be shielded, and if your structure has no sun-facing window it'll be dark inside. To have usable illumination from the Sun you'll need light pipes and filters.

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  • $\begingroup$ It sounds like that to me too, though that paper took the radiation figures from another paper. It does seem like the most likely assumption, but i'm not sure. Ordinary glass blocks UV. The beams in the design i mentioned in the comment above block a third to a half of the sunlight, and some of the rest can be diffused with frosted surfaces. I absolutely don't agree that light pipes or more complex filters are necessary - though light pipes are a great solution in a lot of contexts. (In fact i'm working on light pipes for a different area right now.) $\endgroup$ – kim holder May 2 '16 at 15:59
  • $\begingroup$ 700g/cm2 translates to a layer of regolith 5 m thick, so you'd want to at least give the surface of those beams a high-albedo finish to get some light in when the sun's not directly overhead. $\endgroup$ – Hobbes May 2 '16 at 16:31
  • $\begingroup$ Yes, i thought about that. The gaps between them are long enough that a lot of sunlight enters over most of the day - the site is very near the equator and the gaps are oriented East West. And passive heat control is another important issue. I think they are going to be white on top and light matte colors on the sides to reduce glare. $\endgroup$ – kim holder May 2 '16 at 16:50

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