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In chemistry we learned about the penetrating power of three common types of radiation: alpha, beta, and gamma. Alpha can be stopped by paper, beta is stopped by a sheet of metal (I think) and gamma is stopped by lead. In the same unit, we talked a lot about nuclear weapons, but not once discussed the neutron radiation that causes the chain reaction in nuclear weapons. What kind of penetrating power does neutron radiation have in comparison to the other three we learned about?

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  • $\begingroup$ Have a look nrc.gov/about-nrc/radiation/health-effects/… $\endgroup$ – anna v Aug 3 '16 at 4:04
  • $\begingroup$ @CuriousOne That comment would make a good answer. $\endgroup$ – rob Aug 3 '16 at 4:15
  • $\begingroup$ @rob: I wasn't sure about the cadmium and had to look it up. :-) $\endgroup$ – CuriousOne Aug 3 '16 at 4:16
  • $\begingroup$ @CuriousOne Looking up things to put them into answers is encouraged; answering in comments is not. $\endgroup$ – rob Aug 3 '16 at 4:20
  • $\begingroup$ The section "Health hazards and protection" of the Wikipedia article "Neutron radiation" I found interesting; particularly its last paragraph. en.m.wikipedia.org/wiki/… $\endgroup$ – Farcher Aug 3 '16 at 5:17
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Neutrons are a lot harder to shield than other forms of radiation. The best way is by using light elements, ideally hydrogen, but water and plastic made from hydrocarbons will do fine because of their high hydrogen content. Such a shield doesn't actually absorb the neutrons, it merely slows them down until they move at thermal velocities inside the material. Once we have slowed the neutrons down, we can capture them in materials that have large absorption cross sections at thermal velocities.

http://environmentalchemistry.com/yogi/periodic/crosssection.html has a table of such cross sections for the elements in the periodic table. The interesting ones are at the bottom, and if we remove the rare and expensive (not to mention radioactive) ones, we end up with boron and cadmium as suitable absorbers. Boron has the additional advantage of being light.

Even if we optimize the choices of materials, a typical neutron shield for a reactor will be several meters thick and we still have to worry about activation and secondary nuclear reaction products. Neutron shielding in an actual facility is therefor complicated and there are no simple recipes.

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Neutron radiation is hard to shield because neutrons are not charged and interact only in reactions of the strong and weak interaction. The strong interaction has a short range and the weak interaction is weak compared to the other interactions.

Another problem with it is that neutron radiation tends to activate material quite stongly. Shielding neutron radiation and developing materials that can withstand it as long as possible is one of the major challanges of fusion research.

@CuriousOne mentioned some options for materials with high neutron capture cross sections. Another very interesting set of reactions are the one on lithium-6 and lithium-7: \begin{align} \rm {}^6_3Li + {}^1_0n &\to\rm {}^4_2He + {}^3_1H + 4.8\,MeV \\ \rm {}^7_3Li + {}^1_0n\text{ (fast)} &\to\rm {}^4_2He + {}^3_1H + {}^1_0n - 2.5\,MeV\end{align}

These two are very interesting for fusion because they produce tritium $^{3}_1\mathrm{H}$, an imporatant fusion fuel, by absorbing/moderating neutron radiation, a fusion product.

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