Study of the properties of sustainable materials for shielding against ionizing radiations

In the world of high-energy physics, scientists build massive machines to smash particles together and study the fundamental building blocks of the universe. These experiments produce intense bursts of ionizing radiation, a form of energy powerful enough to damage living tissue and electronic equipment. To keep people and machines safe, researchers must surround these experiments with thick walls designed to stop this radiation. For decades, the standard material for these walls has been concrete. It is strong, cheap, and effective, but it comes with a heavy price tag for the planet. The production of concrete releases vast amounts of greenhouse gases and generates enormous quantities of waste. As the scientific community seeks to build more sustainable facilities, a critical question arises: can we replace the concrete blocks with materials that are kinder to the environment without compromising safety?

This question sits at the heart of the SUSHI project, a study led by physicist Julien Faivre at the University of Grenoble-Alpes in France. The project investigates whether sustainable materials, specifically rammed raw earth, can serve as effective shields against ionizing radiation. Rammed raw earth is an ancient building technique where soil is compressed into solid blocks. It is widely used in construction because it is abundant, requires very little energy to produce, and can be recycled. The researchers wanted to know if this humble material could stand up to the same radiation threats as modern concrete. To find out, they compared how photons—packets of light energy that make up a significant part of the radiation in these experiments—pass through blocks of concrete versus blocks of raw earth. They looked at how the thickness, density, and chemical makeup of these materials affect their ability to stop the radiation, ranging from low-energy photons to those with extremely high energy.

The researchers began by creating digital models of both materials. For the concrete, they used a standard recipe of cement mixed with sand and gravel. For the raw earth, they modeled a mixture of sand, gravel, and clay, which acts as a natural binder. They accounted for the fact that raw earth is porous and absorbs moisture from the air, so they included a small amount of water in their calculations to reflect real-world conditions. They then simulated how photons of different energies would interact with these materials. The goal was to see if the raw earth needed to be significantly thicker than concrete to provide the same level of protection.

The study revealed that for the most common energy levels found in these experiments, the chemical composition of the material matters very little. When photons have energies around one million electron volts, they interact with matter primarily through a process called Compton scattering. In this interaction, the photon bounces off an electron, losing a bit of energy but continuing on its path. The researchers found that in this energy range, the ability of a material to stop the radiation depends almost entirely on its density and how much hydrogen it contains, rather than the specific types of atoms it is made of. Since raw earth is less dense than concrete, it simply needs to be thicker to do the same job. The calculations showed that a block of raw earth would need to be about 20 percent thicker than a block of concrete to achieve the exact same reduction in radiation. This difference is consistent across a wide range of higher energies as well, where photons create cascades of secondary particles known as electromagnetic showers.

The situation changes slightly at lower energies, below one hundred thousand electron volts. Here, a different interaction called the photoelectric effect takes over, where the photon is completely absorbed by an atom. In this range, the specific types of atoms in the material do matter, and the presence of moisture in the raw earth becomes more significant. Because raw earth absorbs water, the amount of water inside a block can vary with the seasons. The researchers found that while this moisture has a negligible effect on high-energy radiation, it can change the shielding effectiveness by up to ten percent for lower-energy photons. However, even with these variations, the raw earth remains highly effective at stopping low-energy radiation, often blocking it much more completely than it blocks higher-energy radiation.

To ensure their digital models were accurate, the team conducted physical experiments using a radioactive source and a specialized detector. They measured how much radiation passed through actual blocks of recycled rammed earth with different densities. The results from these real-world tests matched their computer simulations closely, confirming that their models were reliable. The study concluded that density is the primary factor determining the difference between concrete and raw earth as shielding materials. While the raw earth requires that extra 20 percent in thickness, it offers a compelling alternative that generates far fewer emissions and uses less energy to produce.

The findings suggest that sustainable materials like rammed raw earth are a viable option for shielding against ionizing radiation in high-energy physics facilities. The research indicates that the trade-off is straightforward: a slightly thicker wall made of earth instead of concrete. This small increase in size is a manageable compromise for the significant environmental benefits gained. The author notes that while these results are encouraging, further studies are needed to test other types of radiation and to refine the understanding of how these materials behave over time. For now, the work provides a clear path forward for building the next generation of physics experiments with a lighter footprint on the planet.