How can you look inside a solar cell at an atomic level? Or see exactly where energy is being lost inside a battery? How about to see inside a fragile artefact without opening it? When you want to look into matter at a high resolution, neutrons could be your best bet. Neutrons can easily pass through things which makes them perfect for penetrating deep into material and seeing what’s inside—and they won’t damage anything in the process. Emil Rofors is a second year PhD student in Nuclear Physics at Lund University who studies neutrons. He is researching ways of catching and counting neutrons to build high speed neutron detectors.
The tricky thing about neutrons is they like to go straight through the things you put in front of them. Neutrons can tell you a lot about the materials you let them pass through, but to read this information you need to be able to stop the neutrons and count them. There are a handful of elements on earth that are able to do that, and the most popular one is helium-3. Helium-3 is an isotope of helium with one less neutron than normal. It’s very expensive to produce, but fortunately, you can buy it. Unfortunately, most helium-3 is a by-product of decommissioning hydrogen bombs so there’s not a lot of it available. And after 9/11 the United States, which has the most helium-3, no longer sells it. In this “helium crisis”, two new isotopes have been found to be effective for counting neutrons: boron-10 and lithium-6.
When the neutrons interact with these elements in neutron detectors, they get converted into charged particles. For a nuclear physicist, these types of particles are much easier to detect and there are a myriad of ways to do so. Emil is assembling detectors in different configurations to find which is most efficient. He also needs his detector to be fast since Lund (where he studies) will be the site of the new European Spallation Source (ESS) which will be the strongest neutron source in the world. “It will produce billions more neutrons per second than other facility and there isn’t a detector today that can count that fast. Detectors were built for other facilities to suit their neutron rates. But now we are making a much faster detector,” explains Emil. Having access to a stronger neutron source like ESS will offer scientists new possibilities. “It’s like trying to look for something with a bad flashlight. You need to search for so long. This will be like a brighter flashlight. It will show things more clearly, but we will have to be able to deal with that strong signal.”
Neutron detectors are an essential component of neutron imaging. The object is placed in front of a beam of neutrons generated by spallation. Emil says to think of spallation as “The opening shot in billiards. When you shoot a strong beam of protons into metal, you can make the neutrons that sit in the metal detach and fly away, like the balls on a billiard table.” The neutron detector is behind the object to absorb the neutrons and convert them into a data signal. The detector has a lot of advanced electronics in it as well as a piece of lithium or boron to stop the neutrons. It’s hooked up to a computer that processes the data from millions of neutrons and turns it into an image that looks a lot like an x-ray image. The imaging process can take anywhere from seconds to months depending on what is being imaged.
Emil’s research will be used to design one of the neutron detectors in ESS, and the people at ESS are eagerly awaiting their results so they can be implemented straight away.“I haven’t made my big finding yet, but there are small steps that you take all the time. Okay, we’ve seen that this material gives this response to this energy of neutrons. By itself, that is a very small piece of information. But together with 100 similar discoveries we now have the best solution to this problem.”