Shielding of Ionizing Radiation

Shielding of Alpha Radiation

The following features of alpha particles are crucial in their shielding.

• Alpha particles are energetic nuclei of helium, and they are relatively heavy and carry a double positive charge.
• Alpha particles interact with matter primarily through coulomb forces (ionization and excitation of matter) between their electrons’ positive and negative charge of the electrons from atomic orbitals.
• Alpha particles heavily ionize matter, and they quickly lose their kinetic energy. On the other hand, they deposit all their energies along their short paths.
• The Bethe formula well describes the stopping power.

The stopping power of most materials is very high for alpha particles and heavy-charged particles. Therefore alpha particles have very short ranges. For example, the ranges of a 5 MeV alpha particle (most have such initial energy) are approximately 0,002 cm in aluminium alloy or approximately 3.5 cm in air. A thin piece of paper can stop most alpha particles. Even the dead cells in the outer layer of human skin provide adequate shielding because alpha particles can’t penetrate it. 

Therefore the shielding of alpha radiation alone does not pose a difficult problem. On the other hand, alpha radioactive nuclides can lead to serious health hazards when ingested or inhaled (internal contamination). When they are ingested or inhaled, the alpha particles from their decay significantly harm the internal living tissue. Moreover, pure alpha radiation is very rare. Alpha decay is frequently accompanied by gamma radiation which shielding is another issue.

See also: Shielding of Alpha Radiation.

See also: Interaction of Heavy Charged Particles with Matter

Shielding of Beta Radiation

The following features of beta particles (electrons) are crucial in their shielding.

• Beta particles are energetic electrons. They are relatively light and carry a single negative charge.
• Their mass is equal to the mass of the orbital electrons with which they are interacting. A much larger fraction of its kinetic energy can be lost in a single interaction than the alpha particle.
• Their path is not so straightforward. The beta particles follow a very zig-zag path through absorbing material. This resulting path of the particle is longer than the linear penetration (range) into the material.
• Since they have very low mass, beta particles reach mostly relativistic energies.
• Beta particles differ from other heavy charged particles in the fraction of energy lost by the radiative process known as the bremsstrahlung. Therefore for high energy, beta radiation shielding dense materials are inappropriate.
• When the beta particle moves faster than the speed of light (phase velocity) in the material, it generates a shock wave of electromagnetic radiation known as the Cherenkov radiation.

Beta radiation ionizes matter weaker than alpha radiation. On the other hand, the ranges of beta particles are longer and depend strongly on the initial kinetic energy of the particle. Some have enough energy to be of concern regarding external exposure. A 1 MeV beta particle can travel approximately 3.5 meters in the air. Such beta particles can penetrate the body and deposit dose to internal structures near the surface. Therefore greater shielding than in the case of alpha radiation is required.

Materials with low atomic number Z are appropriate as beta particle shields. With high Z materials, the bremsstrahlung (secondary radiation – X-rays) is associated. This radiation is created during the slowing down of beta particles while they travel in a very dense medium. Heavy clothing, thick cardboard, or a thin aluminium plate will protect from beta radiation and prevent bremsstrahlung production. Lead and plastic are commonly used to shield beta radiation. Radiation protection literature is ubiquitous in advising plastic placement first to absorb all the beta particles before any lead shielding is used. This advice is based on the well-established theory that radiative losses (bremsstrahlung production) are more prevalent in higher atomic number (Z) materials than low Z materials.

See also: Shielding of Beta Radiation.

See also more theory: Interaction of Beta Radiation with Matter

See also calculator: Beta activity to dose rate 

Shielding of Positrons

See first: Shielding of Beta Radiation – Electrons

The coulomb forces that constitute the major mechanism of energy loss for electrons are present for either positive or negative charge on the particle and constitute the major mechanism of energy loss also for positrons. Whatever the interaction involves a repulsive or attractive force between the incident particle and orbital electron (or atomic nucleus), the impulse and energy transfer for particles of equal mass are about the same. Therefore positrons interact similarly with the matter when they are energetic. The track of positrons in a material is similar to the track of electrons. Even their specific energy loss and range are about the same for equal initial energies.

At the end of their path, positrons differ significantly from electrons. When a positron (antimatter particle) comes to rest, it interacts with an electron (matter particle), resulting in the annihilation of both particles and the complete conversion of their rest mass to pure energy (according to the E=mc² formula) in the form of two oppositely directed 0.511 MeV gamma rays (photons).

Therefore any positron shield has to include also a gamma-ray shield. A multi-layered radiation shield is appropriate to minimize the bremsstrahlung. Material for the first layer must fulfill the requirements for negative beta radiation shielding. The first layer of such a shield may be a thin aluminium plate (to shield positrons), while the second layer of such a shield may be a dense material such as lead or depleted uranium.

See also: Interaction of Beta Radiation with Matter

Characteristics of Gamma Rays

Key features of gamma rays are summarized in the following few points:

• Gamma rays are high-energy photons (about 10 000 times as much energy as the visible photons), the same photons as the photons forming the visible range of the electromagnetic spectrum – light.
• Photons (gamma rays and X-rays) can ionize atoms directly (despite they are electrically neutral) through the Photoelectric effect and the Compton effect, but secondary (indirect) ionization is much more significant.
• Gamma rays ionize matter primarily via indirect ionization.
• Although many possible interactions are known, there are three key interaction mechanisms with the matter.
  • Photoelectric effect
  • Compton scattering
  • Pair production
• Gamma rays travel at the speed of light, and they can travel thousands of meters in the air before spending their energy.
• Since gamma radiation is very penetrating, it must be shielded by very dense materials, such as lead or uranium.
• The distinction between X-rays and gamma rays is not so simple and has changed in recent decades.  According to the currently valid definition, X-rays are emitted by electrons outside the nucleus, while the nucleus emits gamma rays.
• Gamma rays frequently accompany the emission of alpha and beta radiation.

Shielding of Gamma Radiation

In short, effective shielding of gamma radiation is in most cases based on the use of materials with two following material properties:

• high-density of material. 
• the high atomic number of material  (high Z materials)

However, low-density materials and low Z materials can be compensated with increased thickness, which is as significant as density and atomic number in shielding applications.  

A lead is widely used as a gamma shield.  The major advantage of the lead shield is its compactness due to its higher density. On the other hand, depleted uranium is much more effective due to its higher Z.  Depleted uranium shields in portable gamma-ray sources. 

In nuclear power plants, shielding of a reactor core can be provided by materials of reactor pressure vessel, reactor internals (neutron reflector). Also, heavy concrete is usually used to shield both neutrons and gamma radiation.

Although water is neither high density nor high Z material, it is commonly used as gamma shields. Water provides a radiation shielding of fuel assemblies in a spent fuel pool during storage or transports from and into the reactor core.

In general, gamma radiation shielding is more complex and difficult than alpha or beta radiation shielding. To comprehensively understand how a gamma-ray loses its initial energy, how it can be attenuated, and how it can be shielded, we must have detailed knowledge of its interaction mechanisms.

See also: Shielding of Gamma Radiation.

See also more theory: Interaction of Gamma Radiation with Matter

See also calculator: Gamma activity to dose rate (with/without shield)

See also XCOM – photon cross-section DB: XCOM: Photon Cross Sections Database

• Neutrons have no net electric charge. Therefore they cannot be affected or stopped by electric forces. Neutrons ionize matter only indirectly, which makes neutrons highly penetrating types of radiation.
• Neutrons scatter with heavy nuclei very elastically. Heavy nuclei very hard slow down a neutron, let alone absorb a fast neutron.
• An absorption of neutron (one would say shielding) causes the initiation of certain nuclear reactions (capture, rearrangement, or even fission), accompanied by many other types of radiation. In short, only neutrons make matter radioactive. Therefore with neutrons, we have to shield also the other types of radiation.

The best materials for shielding neutrons must be able to:

• Slow down neutrons (the same principle as neutron moderation). The first point can be fulfilled only by a material containing light hydrogen atoms, such as water, polyethylene, and concrete. The nucleus of a hydrogen nucleus contains only a proton. Since a proton and a neutron have almost identical masses, a neutron scattering on a hydrogen nucleus can give up a great amount of its energy (even the entire kinetic energy of a neutron can be transferred to a proton after one collision). This is similar to a billiard. Since a cue ball and another billiard ball have identical masses, the cue ball hitting another ball can be made to stop, and the other ball will start moving with the same velocity. On the other hand, if a ping pong ball is thrown against a bowling ball (neutron vs. heavy nucleus), the ping pong ball will bounce off with very little change in velocity, only a change in direction. Therefore, lead is ineffective for blocking neutron radiation, as neutrons are uncharged and can pass through dense materials.

• 

  Absorb this slowed neutron. Thermal neutrons can be easily absorbed by capture in materials with high neutron capture cross-sections (thousands of barns) like boron, lithium, or cadmium. Generally, only a thin layer of such absorber is sufficient to shield thermal neutrons. Hydrogen (in the form of water), which can be used to slow down neutrons, has an absorption cross-section of 0.3 barns. This is not enough, but this insufficiency can be offset by sufficient thickness of the water shield.

• Shield the accompanying radiation. In the case of cadmium shield, the absorption of neutrons is accompanied by strong emission of gamma rays. Therefore additional shield is necessary to attenuate the gamma rays. This phenomenon practically does not exist for lithium and is much less important for boron as a neutron absorption material. For this reason, materials containing boron are often used in neutron shields. In addition, boron (in boric acid) is well soluble in water, making this combination a very effective neutron shield.

Water as a neutron shield

Water, due to the high hydrogen content and availability, is effective and common neutron shielding. However, due to the low atomic number of hydrogen and oxygen, water is not a good shield against gamma rays. On the other hand, in some cases, this disadvantage (low density) can be compensated by the high thickness of the water shield.  In the case of neutrons, water perfectly moderates neutrons, but with the absorption of neutrons by hydrogen nucleus, secondary gamma rays with high energy are produced. These gamma rays highly penetrate matter, and therefore they can increase requirements on the thickness of the water shield. Adding a boric acid can help with this problem (neutron absorption on boron nuclei without strong gamma emission) but results in another problem with corrosion of construction materials.

Concrete as a neutron shield

The most commonly used neutron shielding in many nuclear science and engineering sectors is the shield of concrete. Concrete is also hydrogen-containing material, but unlike water, the concrete has a higher density (suitable for secondary gamma shielding) and does not need any maintenance. Because concrete is a mixture of several different materials, its composition is not constant. So when referring to concrete as a neutron shielding material, the material used in its composition should be told correctly. Generally, concrete is divided into “ordinary” concrete and “heavy” concrete. Heavy concrete uses heavy natural aggregates such as barites  (barium sulfate) or magnetite or manufactured aggregates such as iron, steel balls, steel punch, or other additives. As a result of these additives, heavy concrete has a higher density than ordinary concrete (~2300 kg/m³). Very heavy concrete can achieve density up to 5,900 kg/m³ with iron additives or 8900 kg/m³ with lead additives. Heavy concrete provides very effective protection against neutrons.

See also: Shielding of Neutron Radiation

See also: Interaction of Neutrons with Matter