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Protection against Neutron Radiation

Neutrons don’t have a net electrical charge. Thus electrical forces cannot be impacted or halted. There are three major characteristics of neutrons that are important for nuclear shielding:

  • Neutrons only indirectly ionize materials, making neutrons very penetrating radiation types.
  • Neutrons scatter extremely elastically with heavy nuclei. Heavy nuclei slowing down a neutron and capture a fast neutron alone.
  • The absorption of neutron enables some nuclear reaction, followed by other kinds of radiation, to start (capture, rearrangements or even fission). In summary, only neutrons make stuff radioactive. Therefore, we also have to protect the other kinds of radiation with neutrons.

The finest materials to protect neutrons must be able to:

Slow neutrons down

Only substances containing lightweight hydrogen atoms including water, polyethene and concrete may achieve the first point. There is just one proton in the nucleus of a hydrogen core. 

A proton and a neutron with nearly similar weights may give up a large amount of energy by scattering a neutron on a hydrogen nucleus (even the entire kinetic energy of a neutron can be transferred to a proton after one collision). It’s like a billiard. 

Because a cue ball and a billiard ball have the same masses, the cue ball may stop, and the other ball begins with the same velocity. However, if a table tennis ball (neutron vs heavy core) throws against a bowling ball, the ping pong ball bounces off with minimal alteration, just a change in direction. Blasting neutron radiation is thus very useless since neutrons are dissolved and may quickly go through dense objects.

Absorb neutron that has slowed down

Capturing materials with higher neutron capture cross-section (thousands of barns) such as boron, lithium or cadmium may readily absorb thermal neutrons. 

In general, just a small coating of this absorber is enough to protect thermal neutrons. The hydrogen needed to slow down neutrons is absorbed via barns 0.3. This isn’t adequate. 

However, the thickness of the water barrier may overcome this inadequacy.

Shield the radiation associated with it

For the cadmium shield, neutron absorption is accompanied by significant gamma-ray emissions. An additional shield is thus needed to attenuate gamma radiation. 

This phenomenon does not occur realistically for lithium and is considerably less significant for boron as a material for neutron absorption. 

This is why boron-containing materials are frequently employed in neutron shields. Furthermore, boron is highly water-soluble, making the neutron shield very effective.

Water as a barrier of neutrons

Water is adequate and typical neutron shielding owing to its high hydrogen concentration and availability. However, water is not a good barrier against gamma radiation because of the low hydrogen and oxygen atoms. 

In certain instances, however, this disadvantage may be offset by the large water shield thickness. The water moderates neutrons completely for neutrons, but high-intensity secondary gamma rays are generated by the absorption of neutrons by hydrogen nuclei. 

These gamma rays penetrate matter significantly and may thus raise the thickness requirements of the water barrier. Boric acid may assist this issue (neutron absorption on boron nuclei without significant gamma emission) but lead to further corrosion concerns of building materials.

Concrete as Neutron Shielding

The shield of concrete is the most frequently utilized neutron shielding in various nuclear research and engineering fields. Béton also contains hydrogen; however, unlike greater density of water concrete (ideal for secondary gamma shielding), it doesn’t need maintenance. 

Because concrete is a combination of many elements, its composition is not consistent. Therefore the material utilized in its design must be appropriately indicated when discussing concrete as a neutron shielding material. 

Concrete is often classified into “regular” concrete and “heavy” concrete. Heavy concrete utilizes heavy natural compounds such as barites, magnets or produced particles such as steel balls, iron, steel punch and other additions. Heavy concrete is used as a natural aggregate. 

These additions result in a greater density of heavy concrete than regular concrete (~2300 kg/m3). Hefty concrete with iron additions may reach a thickness of up to 5,900 kg/m3 or lead additives up to 8900 kg/m3. Heavy concrete offers excellent neutron shielding.