Block 4 Nuclear Science Practice Exam

Fast neutrons are slowed most effectively by collisions with light nuclei, because a neutron loses the most energy in a hit with a nucleus similar in mass to itself. Hydrogen nuclei fit this perfectly, so materials rich in hydrogen provide many opportunities for rapid slowing of fast neutrons. Once the neutrons are slowed to thermal energies, they are much more likely to be captured by certain absorbers; boron, in particular boron-10, has a very high neutron capture cross-section and efficiently absorbs these thermal neutrons, converting them to charged particles with minimal dangerous gamma emission.

So, a hydrogen-rich material such as polyethylene or water, especially when doped with boron, combines effective moderation with strong absorption, giving superior shielding for fast neutrons. In contrast, heavy metals like lead mainly shield gamma rays and don’t moderate fast neutrons well; concrete and aluminum lag behind as neutron moderators/absorbers compared with a dedicated hydrogenous, boron-enhanced shield.