Let’s start this discussion by saying that although crazy things are possible we aren’t trying to build for every crazy thing.
For example, the Oh-My-God particle describes a cosmically-sourced particle moving so fast that it hits with the force of a baseball. Despite the enormous power and theoretical possibility of being hit by a barrage of the things, they are so rare that we aren’t going to consider them.
Space dust is another example. The faster you go, the more space dust starts to feel like bullets. I’m leaving it to you to deal with shielding against that sort of debris… we’re only talking about radiation here.
Ultraviolet Radiation - No Problem
Ultraviolet radiation - the 10-125nm area - ionizes air molecules and is biologically hazardous. On Earth we are generally shielded by our atmosphere, but in space that is not the case.
Fortunately UV radiation is blocked or reflected by any material likely to enshroud your spacecraft. Quoting Wikipedia, “standard summer fabrics have UPF of approximately 6, which means that 20% of UV will pass through”. In contrast, Earth’s atmosphere blocks about 98-99% of UV radiation. The logical extension here is that if your spaceship’s hull were comprised entirely of t-shirts, with just 3 layers you would block more UV than Earth’s atmosphere (99.2% vs 98%), and at 5 layers you could block 99.968%.
To be safe I would probably move up from t-shirts to sweaters… or, you know, pretty much any metal of non-trivial thickness.
Alpha Radiation - No Problem
Alpha particles consist of two protons and two neutrons. They are charged and so interact with matter strongly, which is both a good and bad thing. On the bad side it means that standing next to an alpha particle emitter can be very hazardous, however on the good side the fact that they do react with matter so readily means they are also blocked very readily.
The standard statement on Alpha radiation is that, in general, a standard piece of paper is sufficient to shield you. So unless you plan on making your spaceship out of ultra-thin paper, I think you are OK on this one.
Beta Radiation - No Problem
Beta radiation is basically an energetic electron. Being smaller it can be more penetrating than alpha radiation, and is also capable of causing bodily harm. However, any non-trivial amount of metal shielding (eg. 1-2mm of aluminum) will stop this radiation from penetrating.
Gamma Radiation - How Much Weight Do You Wanna Bring?
Photons less than 3x10^-11 meters. Bad ju-ju.
The trick with gamma radiation is that the type of shielding is almost (but not quite) irrelevant. What matters most is the mass of material that gamma rays pass through, not the makeup of that mass. For example, a lead shield only provides a 20-30% improvement over a lighter metal like aluminum or something like soil on a per-kg basis. Obviously though, lead (or, better, tungsten) is more compact, which may have advantages in terms of ship construction.
To identify how much shielding you need, you’ll need to consult the Half Value Layer index for your target material. This index identifies how thick the layer of material needs to be to reduce the gamma radiation allowed through by half. The HVL for a few materials for gamma radiation from Cobolt-60 are:
- Concrete: 60.5mm
- Steel: 21.6mm
- Lead: 12.5mm
- Tungsten: 7.9mm
- Uranium: 6.9mm
Next you need to identify what amount of gamma radiation is normal for your environment and what you consider “safe”. To tell the truth I could not find a good number for the amount of normal background radiation in space, nor even good peak estimates. On the safety side though, we do know that when we are talking about short-term exposure anything less than 15rem is basically undetectable and that, discussing long-term exposure norms, the average American receives ~0.62rem.
Not knowing the background radiation amount is a problem, but I have a solution: we cheat. I’m going to make the not-so-wild assumption that a 1.2 megaton nuclear blast releases more gamma radiation at 2km (500 rem / 5sv) than you are going to get from peak background radiation in space. Though I could not find any hard numbers about what background levels are in space, this assumption seems to hold with what I found. So let’s just find out what it would take to protect one against that level of radiation all the time.
To reduce 500rem to <0.62rem, you should plan on 10 HVL’s (takes it down to 0.49rem). More is better though. That would mean shield thicknesses of:
- Concrete: 605mm (23.8”)
- Steel: 216mm (8.5”)
- Lead: 125mm (4.9”)
- Tungsten: 79mm (3.1”)
- Uranium: 69mm (2.7”)
Personally though, I’d add another layer to be safe.
As a reminder, the key here is MASS. You can use water and only take a 20% performance hit on a per-mass basis, but 1 cubic centimeter of Tungsten weighs a LOT more than 1 cubic centimeter of water. You can choose other materials like water, but you the thickness required will increase sharply when using non-metals or even lighter metals.
This is really a duplicate to Gamma radiation. Effectively, if you elect to block Gamma radiation with a dense metal, it’s going to deal with X-ray radiation as well.
In short, using Concrete as the example, a 1GVp X-ray has the same HVL requirements as gamma radiation from Cobalt-60 decay (44.45mm). Lead, however, goes down from 12.5mm with Gamma radiation to 7.9mm with X-rays.
Here’s where things get interesting.
To protect against Gamma radiation and maintain ship slimness you want a heavy metal shield… but that’s totally inappropriate for a Neutron shield. Heavy nuclei have a very hard time slowing down a neutron, let alone absorb a fast neutron.
At the same time, having no charge they ionize matter only indirectly and have great penetrating power.
Your best bet for Neutron radiation is to use a high concentration of light elements, like hydrogen, which can absorb neutrons quite effectively. Water with Boric Acid is ideal, though probably not useful in constructing a long-term vessel.
To make a long story short here, see the link below from the Space SE, which recommends 1m of water be used for shielding when in Earth’s orbit. It’s either 1m of water or a lot more of something else, when discussing long-term travel.
Shielding So Far
So far we have identified that to shield the ship you need:
- Alpha Radiation: Literally anything of any thickness
- Beta Radiation: Any non-trivial amount of metal is ideal
- Gamma Radiation: 125mm of lead (more examples in section)
- X-Ray Radiation: Covered by Gamma Radiation shield
- Neutron Radiation: 1m of water
With that summary being stated though, you don’t need both 125mm of lead AND 1m of water; in reality the answer could be a compromise between the two (as long as you compromised on the amount of lead and not on the amount of water).
For the sake of safety factors though, let’s say that you keep both. Presuming that you don’t want the water outside the hull and that you don’t want it uncontained INSIDE your hull either, you could happily get away with an 80mm outside hull of lead, a cavity for 1m of water, and a 45mm internal containment wall.
If you have other material lining your ship (and I presume you would, on average) then you can shave off some additional size that way or leave the measurements as-is and enjoy some additional safety margins.
This section covers visible light, infrared, microwaves, radio waves, VLF and ELF. In general, this is all radiation on the electromagnetic spectrum which your shielding as currently described could handle with ease. Visible light isn’t going to be a substantial problem, and otherwise the ship is going to act like a faraday cage.
One challenge you may have is heat acquisition due to infrared (and the other frequencies to a lesser degree). Object cooling in space can be quite difficult, but how that can be dealt with is really another topic… though one I would suggest looking into.