There are many types of particle radiation and many settings in which these phenomena occur. This page is focused on shielding from neutron radiation.
Radioactive isotopes, such as Californium-252 (Cf-252) and others, spontaneously emit neutrons and other subatomic particles. This radiation is harmful to humans and animals and damages objects and structures, so shielding with physical barriers is needed to mitigate radiation exposure. Shielding materials are used during shipping, storage, and daily use to contain isotopes in specialized enclosures or behind walls.
Why is Neutron Shielding Needed?
Cf-252 freely emits what are known as fast or fission energy neutrons. These fast neutrons have very high kinetic energy levels, close to 1 MeV (1.6×10−13 J) and a speed of ~14000 km/s. As a result, they move quickly and can penetrate most material easily, causing great harm to people and animals. Adverse health effects are seen especially in soft tissues and at the cellular level, in which cell function is altered or cellular reproduction is disabled entirely.
Fast neutrons can also penetrate solid objects, contaminating them and causing structural damage to equipment in the surrounding area. The damage occurs because neutrons move so quickly and transfer massive amounts of kinetic energy to the material they encounter at the atomic level. The material itself becomes radioactive after ongoing exposure.
Neutron Shielding Applications
Shielding is required any time a neutron source is in use or could come in contact with people. This includes shipping and transportation, short- or longer-term storage, as well as when the isotope is in use for nuclear or industrial applications.
Shielding containers and structures usually consist of a mix of materials that work together to block neutrons from escaping. Examples include:
- Steel shipping/storage containers filled with water-extended polymer resin (WEP)
- Shielding walls that incorporate concrete and panels filled with water
- Other steel boxes or enclosures with WEP resin or concrete fillings
Neutron Shielding Materials and Properties
The goal of shielding is to prevent neutrons from reaching people or objects in their path and to mitigate bodily harm and damage to equipment or structures. In order to stop a neutron, as much of its kinetic energy as possible must be transferred to another particle of similar mass. This reduces the speed at which the neutron travels or stops it outright, a process called thermalizing.
At the atomic level, hydrogen is the most effective material for shielding neutrons. This is because neutrons and protons have a similar mass, so when a neutron hits a single proton (e.g., a hydrogen nucleus), kinetic energy is transferred and the neutron slows or stops. Therefore, materials containing high levels of hydrogen tend to be the best to use for fast neutron shielding.
Heavy metals, such as lead, are not used in neutron shielding. The high density of lead means its nuclei are very heavy, far heavier than neutrons. As a result, instead of transferring their kinetic energy upon collision with lead nuclei, neutrons do not stop and continue to pass through the material.
Another important element in shielding is boron, which readily absorbs neutrons.
Shielding containers and walls are made with materials with high hydrogen content, since it has a superior ability to slow neutrons. Examples include:
- Water – usually a very thick vertical layer of water is needed in a shielding wall (e.g., 12-18 inches thick); boron may also be added to water to absorb neutrons
- Concrete – thick layers of concrete provide large amounts of hydrogen as well as physical structure and strength for shielding walls; boron can also be added to the concrete mixture
- WEP – this inert resin is lightweight, durable, and excellent at stopping neutrons due to its high hydrogen content; boron may be mixed into WEP resin as well
Paraffin wax is another common material for neutron shielding, however, it melts around 99 °F/37 °C (compared to WEP, which resists heat to 500°F/260 °C ), may form voids or empty gaps inside its enclosure, and could leak while in liquid form if its enclosure is not properly sealed.
Learn More About Neutron Shielding
When working with neutron-emitting sources, safety, and therefore shielding, are paramount. At Frontier Technology Corporation, we provide encapsulated Cf-252 neutron sources as well as the shielding solutions to safeguard your employees and your facility.
Please contact us to learn more about our custom shielding walls, type A shipping containers, as well as our encapsulated Cf-252 neutron sources.