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Rocks and neutrons

The Spallation Neutron Source.

The world below our feet can be as important as the one we see around us. It provides many things we need, including drinking water, but it also carries contaminants that poison that water.

Much of what we know about underground contamination comes from scientists such as geochemist David Wesolowski and his group at ORNL. Using unique tools such as the neutron scattering capabilities at ORNL’s Spallation Neutron Source and High Flux Isotope Reactor, Wesolowski and his colleagues see rocks very differently from the way most of us do.

“You can think of rocks as like a sponge,” he said, “with pores ranging from very small, nanometer size, to large cracks and pores you can see with your eyes.”

When you look at how water flows through rock, it’s important to know what gets trapped and what flows through: these are concepts known as porosity and permeability. As it turns out, neutrons are great at answering those questions. Neutrons penetrate deeply into matter, giving scientists a detailed, three-dimensional picture of rock sections, even ones that are contained within a pressure vessel to simulate conditions deep below ground.

One especially revealing technique involves deuterium, also known as heavy water. The hydrogen atoms in deuterium have a proton and a neutron in their nuclei, unlike those in common water, which have a proton but no neutron. While the two forms of water behave the same, they are easily distinguished in a neutron-scattering experiment.

If you flood a rock with regular water, let it drain and then flood it with deuterium, neutron scattering shows you which pores are connected and which are not.

“The neutrons give you a picture of the pore structure,” Wesolowski said. “If those pores are filled with water, and that water can be replaced by heavy water, then you get a different scattering.”

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