Radiation Shielding: Rocks as a Concrete Replacement

Today’s blog post is sponsored by Horiba – a leading manufacturer of spectroscopy and material analysis solutions.

What is Ionizing Radiation?

Ionizing radiation is a type of radiation where the energy released travels in the form of electromagnetic waves or particles. Ions are compounds that have a different number of electrons than protons. The energy emitting from ionizing radiation can be enough to remove an electron from an atom, also known as ionizing.

Although radiation can be toxic, there are natural sources of ionizing radiation, which include soil and water. Radon is the element found in soil and water that is the cause of this radiation. Besides naturally occurring radiation, medical treatments are the most common forms of radiation that humans are exposed to. 

Gamma and X-Rays

This article will specifically discuss gamma and X-ray radiation which are examples of high frequency ionizing radiation. Gamma rays are dangerous since they have very high energies and can penetrate through the entire body, which is deleterious to all bodily functions. This is seen first hand with the Chernobyl nuclear power plant disaster where gamma radiation was emitted in enormous quantities. Within three months of the accident 31 people died from radiation exposure. Years later, 20,000 people under the age of 18 were diagnosed with thyroid cancer. X-rays are similar to gamma rays but are emitted from different parts of the atom and are lower in energy. Both gamma and X-rays are emitted from nuclear power plants, cancer treatments, and even security scanners. 

In medicine, an equivalent dose is used to express a measure for radiation. Sieverts (Sv) is the most common unit. The U.S Nuclear Regulatory Commission set the safety limit at 50 Sv per year. For reference, a banana has 0.0001 mSv, a set of dental X-rays is 0.005 mSv, and a full CT scan has 20 mSv. Because these medical instruments can have such high levels of radiation, radiation shielding materials are vital. Radiation shielding is a way to “shield” the body from radiation. The skin can only protect against low energies of radiation, while materials like concrete can protect against higher energy radiations such as gamma rays. 

Radiation Shielding Materials

Because radiation can be so harmful, is it important that the buildings and rooms containing the radiation are built with the proper materials. Although in low increments radiation is harmless, constant exposure can lead to life threatening conditions, which is why radiation shielding is so important. Brick and concrete are the most common materials for buildings such as dental and healthcare practices, however researchers at King Abdulaziz University in Saudi Arabia looked at rocks as replacements. In a paper titled “The use of rocks in lieu of bricks and concrete as radiation shielding barriers at low gamma and nuclear medicine energies” compared the radiation shielding capabilities of rocks to concrete and brick. Their reasoning is that rocks are high-density materials and should therefore act as a more effective barrier.

The radiation shielding capabilities against different rays and particles. Photo courtesy of Marshield.com

Usually, concrete is used as a radiation shielding material. It is a popular building material because it is cheap, strong, and easily moldable. It is common for radiation shielding because of its high density and water content, making it a good barrier against radiation such as gamma rays. Research is still ongoing about the optimum thickness and shape that materials used for radiation shielding should be. The photo above shows how effective different materials are to different radiations, with concrete having the largest shielding effect.

In the study, four different rock samples were used, all with different amounts of iron, silicon, aluminum, carbon, calcium, and oxygen. Brick and concrete were also tested as comparisons. Concrete is mostly comprised of calcium, carbon, and oxygen, also known as limestone. Different levels of gamma rays were emitted at the rocks, and the total mass attenuation coefficient was measured. The higher the coefficient, the better radiation shield the material is. A software called WinXCom was used to measure the constants. This software is based on the cross-sections of photons at these low gamma energies.

Six different radiation energies were emitted at the materials, from 20 keV to 364 keV. These are kiloelectronvolts and are units of ionizing radiation energies. X-ray machine energies range from 100 eV to 100 keV. The rock samples outperformed the brick and concrete samples at radiation shielding for 20keV, 30 keV, 40 keV, and 60 keV. The rock sample with the highest amount of Iron performed the best at these energies. At 364 keV concrete performed better; however diagnostic radiation energies usually fall below 100 keV.

An analyzer for gamma ray spectroscopy can be found here.

Because rocks outperformed brick and concrete at lower energies, they can be a material replacement for places such as hospitals with X-rays and CT scans. Rocks are also cheap and plentiful, making this swap not only healthier, but also more economical. 

Previous Research into Radiation Shielding

The paper titled, “Determination of gamma ray shielding parameters of rocks and concrete,” was an inspiration for this study. Rocks such as feldspathic basalt, compact basalt, volcanic rock, and pink granite were compared to the gamma ray shielding abilities of concrete. These rocks are made of mostly oxygen and have trace amounts of Iron, very different from the iron heavy rocks tested in the discussed article. The mass attenuation constant was also measured in this study, and it was found the tested rocks also had higher shielding abilities than concrete. All of these tested rocks are volcanic rocks however, which may make it harder for them to become mainstream building materials. 

Research about building materials for nuclear facilities is crucial for the future of renewable energy, and therefore the climate. About 10% of all the world’s energy comes from nuclear power. The Nuclear Energy Agency, under the Organization for Cooperation and Development, has proposed a sustainable development plan in order to move away from fossil fuels. This plan involves nuclear capacity increasing by 55% by 2040. This will require many more power plants that will need to have the highest shielding and strength properties, while also being cheap. Rocks of differing elements may be the answer to this.