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In any (deep) spacecraft, radiation shielding is paramount. Neutrons and gamma rays can only be stopped by putting mass (or distance) between source and target. It seems that one should put more lightweight elements on the outside of the hull for proper shielding.

I wonder if PV cells could provide this kind of neutron absorption/deflection layer and if so at what cost. My rationale is to generate a secondary usage to the inevitable mass. For a larger spacecraft it might e.g. be viable to partially rely on solar instead of nuclear power in order to reduce radiation exposure to the crew.

How does a typical solar cell compare as a neutron shield?

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  • $\begingroup$ Using PV cells in deep space is pointless, there's no enough light to generate electricity from. They'd be dead weight. $\endgroup$ – GdD May 19 '16 at 15:37
  • $\begingroup$ @GdD The space exploration definition of deep space is different than the astronomical one. For space exploration, deep space can be anything beyond Earth orbit. $\endgroup$ – called2voyage May 19 '16 at 15:40
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PV cells would not be a terribly good neutron absorber, mainly because PV cells are thin.

Neutrons penetrate deeply into materials because they are not charged - the main method of reducing neutron radiation is through physical collisions of neutrons. Atoms with low atomic weights remove the energy from neutrons most efficiently.

Water is a good neutron absorbing material because it includes a lot of hydrogen, which has a low atomic number. Water has a tenth value / thickness of 10 inches.

That means that water reduces the neutron radiation flux by 1 order of magnitude for every 10 inches of water present.

PV cells are going to be less effective at reducing neutron radiation, because they will have a higher average atomic number, and they will only be a few cm thick.

So PV cells will have a negligible neutron shielding effect.

Note that basic information about the tenth thickness for water is present in the next section of the Project Rho link that OP provided, labeled "Shield Rating."

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  • $\begingroup$ What if the frame/structural material of the PV cells is from B11? As I know, it is one of the best solid neutron absorber (He3 would be only better). $\endgroup$ – user259412 May 20 '16 at 7:39
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There is practically negligible neutron radiation in the deep space.

Gamma radiation exists, but it is also very small.

This problem happens only if the spacecraft has a nuclear power source (which is mostly RTG in todays practical designs). But if there is an RTG power source, there is no need for PV cells.

PV cells have very strict requirements to their chemical composition, and they are thick.

But:

  1. PV cells need to be semiconductors, thus the feasibility of a material to be used as PV cell has a requirement to the structure of its electron shell. They can be also very thick, only some wavelength of the visible light is enough. The only reason, why the current PV cells are thinner, that it is simpler (= cheaper) to produce some tenths of mm thin silicon layer as some micron.
  2. Neutron shielding and gamma shielding depends mostly on the nucleus. Unfortunately, effective neutron shields have mostly small nuclei, while the gamma shielding requires big (for example, lead).
  3. In the case of neutron shielding, there are big differences in the actually used isotopes, while the gamma cross section depends mostly simply on the proton number of the nuclei (and it is proportional to its fourth power).
  4. Both neutron and gamma shielding requires thick layers. In the nuclear technology, some cm thick lead layers are used.
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  • $\begingroup$ This paper seems to imply that there is some risk from neutron radiation, albeit that which is produced by interactions with materials. Do you have a reference for the claim that neutron radiation in deep space is negligible? $\endgroup$ – called2voyage May 19 '16 at 15:38
  • $\begingroup$ @called2voyage: Free neutrons have a half-life of about 15 minutes, per this reference I dug up in a similar situation. $\endgroup$ – Nathan Tuggy May 19 '16 at 17:36
  • $\begingroup$ @NathanTuggy That doesn't mean there is no risk. Again, the neutron radiation is produced by interactions with materials. If, for example, it is produced by interactions with materials on the spacecraft, 15 minutes is plenty of time to do damage. $\endgroup$ – called2voyage May 19 '16 at 17:38
  • $\begingroup$ @called2voyage: Well, there's self-evidently no useful shielding against collision-generated neutrons from PV panels, which need to be on the outside of the craft. $\endgroup$ – Nathan Tuggy May 19 '16 at 17:40
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    $\begingroup$ @NathanTuggy Agreed, but that is for the answer to explain to the OP. $\endgroup$ – called2voyage May 19 '16 at 17:41

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