Radiation shielding is imperative as radiation can be a serious concern in nuclear power facilities, industrial or medical x-ray systems, radioisotope projects, particle accelerator work, and a number of other circumstances. Containing radiation and preventing it from causing physical harm to employees or their surroundings is an important part of operating equipment that emits potentially hazardous rays. Preserving both human safety and structural material that may be compromised from radiation exposure are vital concerns, as well as shielding sensitive materials, such as electronic devices and photographic film.
The process of regulating the effects and degree of penetration of radioactive rays varies according to the type of radiation involved. Indirectly ionizing radiation, which includes neutrons, gamma rays, and x-rays, is categorized separately from directly ionizing radiation, which involves charged particles. Different radiation shielding materials are better suited for certain types of radiation than others, as determined by the interaction between specific particles and the elemental properties of the shielding material.
General Radiation Shielding Properties
Radiation shieldingis based on the principle of attenuation, which is the ability to reduce a wave’s or ray’s effect by blocking or bouncing particles through a barrier material. Charged particles may be attenuated by losing energy to reactions with electrons in the barrier, while x-ray and gamma radiation are attenuated through photoemission, scattering, or pair production. Neutrons can be made less harmful through a combination of elastic and inelastic scattering, and most neutron barriers are constructed with materials that encourage these processes. The main types of radiation encountered in industrial projects include:
- Gamma and X-rays Shielding:These are forms of electromagnetic radiation that occur with higher energy levels than those displayed by ultraviolet or visible light.
- Neutron Shielding:Neutrons are particles that have neither a positive nor a negative charge, and thus provide a wide range of energy and mass levels that must be blocked.
- Alpha and Beta Particles: Alpha particles are positively charged helium nuclei, and are relatively easy to block, while beta particles are negatively charged electrons that are more difficult to shield against.
When it comes to protecting against radiation, the basic radiation protection principals or radiation safety tips involve time, distance, and shielding. Time, in this case, means to limit exposure to the minimum amount possible. Distance means staying as far from radiation sources as possible as a best practice. The intensity of radiation generally follows the inverse square law, meaning that it falls off with the square of the distance from the source. Moving twice the distance away from a source of radiation reduces the intensity of exposure by a factor of 1/22 or one fourth the value. Beyond time and distance, making use of effective shielding is the other approach to managing exposure to radiation.
But what materials protect against radiation? The most common ones used include lead, concrete, and water - or a combination of these. Below
X-ray and Gamma Radiation Shielding Materials
In most cases, high-density materials are more effective than low-density alternatives for blocking or reducing the intensity of radiation. However, low-density materials can compensate for the disparity with increased thickness, which is as significant as density in shielding applications. Lead is particularly well-suited for lessening the effect of gamma rays and x-rays due to its high atomic number. This number refers to the number of protons within an atom, so a lead atom has a relatively high number of protons along with a corresponding number of electrons. These electrons block many of the gamma and x-ray particles that try to pass through a lead barrier, and the degree of protection can be compounded with thicker shielding barriers. However, it is important to remember that there is still potential for some rays making it through shielding and that an absolute barrier may not be possible in many situations.
Alpha and Beta Shielding
What is required to shield alpha particles? While density remains an important characteristic for blocking alpha and beta radiation, the thickness is less of a concern. A single centimeter of plastic is sufficient for shielding against alpha particles, as is a half-inch of paper. In some cases, lead is ineffective in stopping beta particles because they can produce secondary radiation when passing through elements with a high atomic number and density. Instead, plastic can be used to form an efficient barrier for dealing with high-energy beta radiation. When negatively charged beta particles hit a high-density material, such as tungsten, the electrons are blocked, but the target which the barrier is intended to protect can actually become irradiated. In the air, beta particles of the highest energies can travel up to two meters or more
Neutron Shielding
Lead is quite ineffective for blocking neutron radiation, as neutrons are uncharged and can simply pass through dense materials. Materials composed of low atomic number elements are preferable for stopping this type of radiation because they have a higher probability of forming cross-sections that will interact with the neutrons. Hydrogen and hydrogen-based materials are well-suited for this task. Compounds with a high concentration of hydrogen atoms, such as water, form efficient neutron barriers in addition to being relatively inexpensive shielding substances. However, low-density materials can emit gamma rays when blocking neutrons, meaning that neutron radiation shielding is most effective when it incorporates both high and low atomic number elements. The low-density material can disperse the neutrons through elastic scattering, while the high-density segments block the subsequent gamma rays with inelastic scattering.
Radiation Shielding Design and Selection Considerations
There are several factors that influence the selection and use of radioactive shielding materials. Considerations such as attenuation effectiveness, strength, resistance to damage, thermal properties, and cost efficiency can affect radiation protection in numerous ways. For example, metals are strong and resistant to radiation damage, but they undergo changes in their mechanical properties and degrade in certain ways from radiation exposure. Likewise, concretes are strong, durable, and relatively inexpensive to produce, but become weaker at elevated temperatures and less effective at blocking neutrons. Here are some important considerations for radiation shield material selection:
- Neutron attenuation is a function of the effective cross-section of the shielding material which is a gauge of the probability that the incident neutrons will be reduced in their energy level as a result of undergoing a nuclear reaction.
- Also, consideration should be given to secondary radiation effects from the shield material itself as a result, for example, of the absorption of gamma rays being generated from the neutron absorption process. Material selection plays an important role in reducing the risk of secondary radiation generation by choosing one that will not become radioactive.
- Energy absorption in shielding material can result in the generation of heat, so choosing materials with acceptably high coefficients of thermal conductivity is another factor to be given consideration.
- Material selection needs to assess the impact of the radiation absorption on the properties of the materials being used, and how those changes will impact the shield’s performance.
- The acquisition cost, weight, fabrication methods, as well as transportation costs, installation costs, waste involved, and ultimate scrap value of the material are also relevant points to consider when deciding on what materials should be used.
Lead Radiation Shielding Products
Given that lead is a heavy element (heavier than around 80% or so of the other elements found on the periodic table), it is a common choice for use in fabricating radiation shielding products. Lead is fabricated into different product forms to provide radiation shielding and protection, and which includes these types:
- Lead Sheets, Plates, Slabs, & Foils
- Lead Shot
- Lead Wools
- Lead Epoxies
- Lead Putties
- Lead Bricks
- Lead Pipe
- Lead-clad Tubing
- Lead-clad Pipe
- Lead Sleeves
- Lead Glass
- Lead-Polyethylene-Boron Composites
Lead can also be added to concrete or cinder blocks for use in wall construction. By adding unperforated sheets of lead to the blocks and extending the sheet beyond the edge of the concrete block and overlapping shield of lead can be embedded in a wall to form an effective radiation barrier utilizing a continuous lining of lead sheet. A similar approach can be used to create lead shielded doors and door frames. As with the wall construction, it is important to overlap the lead that is used in the door frame with the lead that is used in the wall construction to provide a continuous lead barrier that will function as an effective shield.
For applications such as viewing windows in X-ray rooms, lead glass can be used and added in several layers as a means of producing an effective radiation barrier. As an alternative to lead glass, lead-filled acrylic sheet materials are available which have had lead added to the acrylic resin during the fabrication process.
Lightweight Radiation Shielding Products
There are lightweight radiation shielding products that have been developed to afford individual protection and personal radiation shielding. One such product in the form of flexible fabric is called Demron®, which can be fabricated into hazmat suits, Demron® blankets, Demron® tents, and other personal protection products such as tactical vests. Testing by the United States Department of Energy (DoE) has demonstrated the effectiveness of the material to reduce the levels of high energy alpha and beta radiation as well as to reduce low energy gamma radiation. The lightweight flexible nature of these types of products makes them ideal for individual wearable protection, with the additional benefit of being easy to clean, maintain, and store.
Summary
This article summarized the material used in radiation shielding and some of the product forms available. For additional information on different types of radiation, as well as the physics involved in radiation shielding, please visit the Health Physics Society. To learn more about additional topics, visit our other guides or the Thomas Supplier Discovery Platform, where you will find suppliers of products for radiation shielding, including doors, cabinets, enclosures, and radiation shielding equipment.
Copyright notice:
Demron® is a registered trademark of Radiation Shield Technologies, Coral Gables, Florida.
Sources:
- https://www.raybar.com/
- https://www.epa.gov/radiation/protecting-yourself-radiation
- https://www.globecomposite.com/basics/3-different-types-of-radiation-shielding-materials
- https://blog.universalmedicalinc.com/3-different-types-radiation-shielding-materials/
- http://hps.org/publicinformation/ate/cat47.html
- http://www.lancsindustries.com/
- https://www.eichrom.com/wp-content/uploads/2018/02/Gamma-Ray-Attenuation-White-Paper-by-D-M-rev-6-1-002.pdf
- https://www.nrc.gov/about-nrc/radiation/health-effects/radiation-basics.html
- https://www.canadametal.com/wp-content/uploads/2016/08/radiation-shielding.pdf
- http://nuclearconnect.org/know-nuclear/science/protecting
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