Aaron Couture: Capturing neutrons in the desert

Experimental nuclear astrophysicist Couture uses the Laboratory’s particle accelerator to find new ways of studying how neutrons change the evolution of the elements

April 18, 2023

Aaron Couture2 Opt
Aaron Couture.

By Aaron Couture

I do my best work when I maintain an uncomfortable balance between two things: studying fundamental nuclear astrophysics—what happens in stars—and applying nuclear physics to answer mission-relevant questions—understanding what happens during nuclear explosions and within nuclear reactors. I say “uncomfortable” because either of these things could keep me busy night and day. I say “best” because each provides insight and tools to help the other advance—this combination is what makes science at Los Alamos unique and positions the Lab to do work that can be accomplished nowhere else.

Sixteen years ago, I came to Los Alamos to work at the Los Alamos Neutron Science Center, or LANSCE, because I was attracted to the idea of using neutrons to study nuclear physics, and LANSCE has neutrons, lots of neutrons. Not many other places do. So, when an opportunity came to me, via my mentor and colleague René Reifarth, to join the Lab as a post doc, it was an easy decision to make.

Pretty much all the elements, once you get past helium, come from stars. Lighter elements are formed in stars by nuclear fusion, whereby two nuclei fuse into one larger nucleus. Heavier elements are formed in stars by neutron capture, whereby a nucleus absorbs neutron after neutron until, by way of a process called beta decay, the nucleus gains a proton and turns into the next element on the periodic table. But how… exactly? My career so far has been spent working to understand how different combinations of elements and particles react when they are put together inside nuclear weapons, inside nuclear reactors, or inside stars—we, as scientists, need to understand the reactions that are happening.

A free neutron is a pesky, ill-behaved particle. Unlike neutrons that are bound in an atomic nucleus, a free neutron only exists for about 15 minutes before beta decaying into a proton, an electron, and an antineutrino. During those 15 minutes it can cause all kinds of trouble, ambling along and interacting at a nuclear level with almost whatever it finds. Because the neutron is electrically neutral, there is no electric field to push it away from an atom. These encounters with atomic nuclei can have a variety of consequences. If the neutron has a lot of energy, it can knock another neutron (or several) out of the nucleus, changing the isotope of the nucleus. Or, the neutron can just bounce off the nucleus, carrying on in another direction much like a ball on a billiard table. Finally, sometimes a neutron is captured by the nucleus, making the nucleus into a new isotope of the same element, just a bit heavier than before. For example, an iron-58 atom may capture a neutron to become iron-59.

Neutron capture, with all its complexities, is how heavy elements are synthesized within stars. But neutrons are also a powerful tool for probing and understanding physical systems here on Earth. For example, through neutron diffraction—or the scattering of neutrons off a sample of material—the atomic or magnetic structure of the material can be inferred. In addition, because they can turn stable materials into radioactive ones, neutrons can be used to create certain important radionuclides such as molybdenum-99, used in medical imaging, or cobalt-60, used in cancer treatment. Neutron radiography is another important application, where neutrons can be used to penetrate a container allowing scientists to peer inside, much like x-rays, except neutrons can easily get through materials that x-rays can’t. It’s not good for looking inside people, but very good if you want to know what is inside a steel cask.

> Read the rest of Aaron’s story in the Laboratory’s science magazine, 1663.