While radiation is constantly present both in our environment (natural background radiation) and in our bodies, it is incredibly important to limit radiation exposure. As we've touched on previously in our recent article, "Time, Distance, & Shielding: Key Radiation Protection Principles," following these three pillars of radiation safety is crucial.
While time, distance, and shielding make up the 3 key principles of radiation protection, in this article we'll dive deeper into the shielding materials that work best in keeping radiation dose “ALARA” (as low as reasonably achievable).
As always, we hope you find this article informative, and we look forward to hearing your feedback!
The United States Nuclear Regulatory Commission (NRC) defines radiation shielding as the "reduction of radiation by interposing a shield of absorbing material between any radioactive source and a person, work area, or radiation-sensitive device." Using or inserting the proper shield can greatly reduce or eliminate the dose received.
Sources of radiation can be shielded with solid or liquid material, both of which absorb the energy of the radiation. Different types of ionizing radiation interact in different ways with the shielding material(s) being used, which we'll explore further below.
Although radiation exists throughout both our environment and bodies, humans must do all they can to reduce unnecessary and excessive radiation exposure. Preventing exposure is critical, as radiation can damage the DNA in human cells.
High doses of radiation can cause Acute Radiation Syndrome (ARS) and/or Cutaneous Radiation Injuries (CRI). An increased dose of radiation also can potentially cause cancer in the future.
The barrier used depends on the type of radiation it is trying to block. The necessity of shielding in radiation is based on the principle of attenuation. Attenuation is the degree to which a radio wave or ray’s effect can be blocked or bounced with the use of a barrier material.
This process of radiation attenuation can be achieved by placing an absorbent substance between the source emitting radiation and an individual, workspace, or radiation-sensitive instrument. The radiation shielding calculations and properties of any material are presented in terms of the linear attenuation coefficient.
The effectiveness of shielding depends on stopping power, which varies both with the type and energy of radiation and the shielding material used. Different shielding techniques are therefore used depending on the application, and the type and energy of the radiation. In nuclear and materials physics, stopping power is defined as the retarding force acting on charged particles (typically alpha and beta particles) due to interaction with matter, resulting in loss of particle kinetic energy.
The short answer is simply "no." Specific materials are useful in protection against specific types of radiation. These same materials may not be effective in shielding any other type of radiation.
Shielding is necessary for both directly ionizing radiation and indirectly ionizing radiation. Indirectly ionizing radiation carries no electrical charge and produces ionization by first setting free a charged particle. The two indirectly ionizing radiations of concern are 1) gamma rays and x-rays, and 2) neutrons.
Since lead is an extremely heavy element (heavier than almost 80% of the other elements found on the periodic table), it’s a common choice for fabricating radiation shielding products.
Lead is a corrosion-resistant and malleable metal. Lead's high density (11.34 grams per cubic centimeter) makes it an effective barrier against x-ray and gamma radiation. Other key features include its significant flexibility, exceptional stability, and high atomic number. Finally, lead is available in a variety of forms, which makes it the best choice for shielding x-rays and gamma rays.
Pure lead is commonly blended with resins and fillers to create a flexible lead vinyl film that can be worn as a radiation shielding material. The lead layers are piled to the required thickness and inserted into the radiation shielding fabric to produce the desired lead thickness.
For classic lead radiation shielding clothing, there are 3 standard levels of lead equivalent shielding: 0.25mm, 0.35mm, and 0.5mm. Lead equivalent products (which are composites that are much lighter than lead) also use these levels. Examples of such products include EarthSafe, Xenolite, and Demron®.
Lead can also be added to concrete or cinder blocks for use in wall construction. In x-ray facilities, walls surrounding the room with the x-ray machine may contain lead shielding such as lead sheets, or the plaster may contain barium sulfate (a dense compound proficient in absorbing gamma radiation).
X-ray operators generally view the target through a lead glass screen. If it's necessary for them to remain in the same room as the target, they also commonly wear lead aprons.
Lead can be fabricated into different product forms to provide radiation shielding and protection - the most common of which are:
Alpha particles are the least penetrating type of radiation. Even the most energetic alpha particles can be stopped by a single sheet of paper. Alpha particles cannot penetrate the outer layer of dead skin, so they don't pose a risk to humans when the source is outside of the body.
Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. Low energy beta particles can be shielded simply by an outer layer of clothing in most cases. However, in cases where high-energy beta particles are emitted, shielding must be accomplished with low atomic weight materials (usually plastic, wood, water, or acrylic glass). This is done to shield out the Bremsstrahlung radiation produced. Bremsstrahlung radiation is the radiation given off by a charged particle (most commonly an electron) due to its acceleration caused by the electric field of another charged particle.
The effectiveness of a shielding material in general increases with its atomic number (called Z), except in the case of neutron shielding. Neutron radiation is more easily shielded by neutron absorbers and moderators like boron and cadmium. Water and hydrocarbons (like polyethylene and paraffin wax) are also used for neutron radiation shielding.
Neutron radiation is not as readily absorbed as charged particle radiation, which makes this type or radiation highly penetrating. In a process called neutron activation, neutrons are absorbed by nuclei of atoms in a nuclear reaction. This often creates a secondary radiation hazard, as the absorbing nuclei transform to the next-heaviest isotope (many of which are unstable) and then become radioactive and decay.
There are several factors that influence the design, selection, and use of shielding for radioactive material. Considerations such as attenuation effectiveness, strength, resistance to damage, thermal properties, and cost efficiency can affect these choices. While metals are strong and resistant to radiation damage, they undergo changes in their mechanical properties and can deteriorate in certain ways from radiation exposure.
On the other hand, concrete is strong, durable, and relatively inexpensive to produce, but it becomes weaker at higher temperatures, and is less effective at blocking neutrons.
Over time, lightweight radiation shielding products have been developed to provide protection and personal radiation shielding. One such product that is commonly used is called Demron®. This flexible fabric can be forged into hazmat suits, blankets, tents, tactical vests, and other personal protection products. Testing by the United States Department of Energy has demonstrated that the material is effective in reducing the levels of high energy alpha and beta radiation, as well as reducing low energy gamma radiation. The lightweight and flexible nature of these types of products makes them ideal for wearable individual protection. They are also easy to clean, maintain, and store.
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