Free neutrons in Oak Ridge. They are drawing hundreds of scientists from all over the world to the High Flux Isotope Reactor (HFIR) and Spallation Neutron Source (SNS) at Oak Ridge National Laboratory.
“We are overbooked,” said Hans Christen, director of ORNL’s Neutron Scattering Division, in this month’s lecture to Friends of ORNL. One reason is that a few other neutron sources in the world are temporarily shut down.
Another reason is that the SNS and HFIR offer scientists some of the brightest of neutron beams (a million billion neutrons striking a tiny area every second), providing high resolution and sensitivity. It’s like observing dust floating in room air only when a beam of sunlight shines through a window.
The free neutrons are free in several ways. They are free of charge in the sense that they have no electrical charge like protons, their positively charged cousins with which they are confined in atomic nuclei. Because they have no charge, they can penetrate deeper into material than other probes such as X-rays and electrons.
Christen said they are free of charge for scientific users of the instruments costing millions of dollars that receive neutrons flowing from the beamlines of HFIR and SNS, provided the researchers publish the results of their experiments in the open literature.
He explained that the free neutrons had been liberated from nuclei in uranium-238 atoms bombarded by neutrons in HFIR or from the nuclei of mercury atoms in a steel vessel at the SNS target station, which are subjected to accelerated pulses of protons.
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For every proton striking a mercury nucleus, at least 20 neutrons are freed and then channeled into a beamline. They are aimed at a sample – a solid or liquid material in an instrument, ranging from plastics to proteins. Neutrons may bounce off a sample’s atomic nuclei at angles that can be measured. Or they may pass through the sample or get absorbed by it.
Detectors of the scattered neutrons that have different velocities and arrive at different times help researchers learn the positions of various atoms or molecules, indicating a material’s structure. They also reveal the ways the atoms behave, even when the samples are subjected to increases in temperatures, pressures or the strengths of magnetic fields.
As Christen points out, although the neutrons have no charge, scientists working at HFIR and SNS know they have great value. The detailed knowledge they uncover by probing materials has many practical applications. The findings have led to advances that, according to the neutrons.ornl.gov website, reduce the cost and improve the efficiency and safety of products we use, such as cell phones, engines, batteries and drugs.
In his talk, Christen focused on several recent examples. At ORNL neutrons are being used to improve the results of advanced manufacturing. The goal is to understand the structures of materials after or while they are produced by 3D printing.
“Because neutrons penetrate deeply into materials, we can look at 3D-printed steel components of a turbine blade that has a complex structure,” Christen said. “We can determine the quality of a cooling channel built within the 3D-printed steel component.”
Another application relates to food security in the context of climate change. “We need to understand how plants respond to drought and create food crops that better take up what little water is available. This is important for much of the world.”
Since neutrons penetrate through metal so well, he said, scientists can study the behavior of a living plant in soil by placing it within a metal enclosure in front of a neutron beam. “Because water is so absorbent of neutrons, scientists can see how much water is taken up by a plant after it is subjected to drought conditions.”
Lithium batteries have a high capacity and hold an electric charge longer than other battery types. But because of the high demand for them for smartphones, electric vehicles and energy storage to fill in for renewable energy sources when the sun’s not shining and the wind’s not blowing, scientists are trying to design better materials for lithium-ion batteries so they hold their charge even longer and not catch fire.
ORNL’s neutron sources are providing information that could help improve battery materials and designs, Christen said. “Because researchers can see lithium ions so well with neutrons, they can put a working battery on a neutron beam line, charge and discharge the battery and watch the growth of dendrites that might short-circuit the battery. With neutron imaging we can watch the formation of dendrites in three dimensions as we cycle the battery.”
ORNL’s neutron sources are contributing to the development of new drugs to fight viruses by providing data on the structure and other properties of proteins. “Neutron diffraction studies reveal how proteins interact with each other and with drug molecules,” Christen said. “Our protein structure information is in an open database, and all drug makers in the world have access to that data from Oak Ridge.”
A study using the steady-state neutron beams of the 85-megawatt HFIR has revealed at the molecular level how a key protein of the COVID-19 virus links up with the human protein interferon-stimulated gene 15, enabling the virus to evade the body’s immune response. The goal is to provide information that will aid pharmaceutical companies in making drug molecules that prevent the virus from replicating.
Christen mentioned thermoelectric materials, such as tin sulfide, which can absorb waste heat and convert it to electricity. “These materials have specific properties that influence the transport of heat,” he said. “Scientists use neutrons to map these properties and compare the results to theoretical calculations to make sure we understand precisely how waves propagate through the materials.”
Plastics that can be recycled or reused could be a source of the next generation of polymers for medical or electronic applications, Christen noted.
“For polymer production, you need pure acetylene, so you must separate it from ethylene, say, in a chunk of porous material with catalysts in its holes. Neutrons can be used to detect the intermediate molecules that form during the catalyzed reaction. That information helps scientists develop better catalysts for making polymers.”
He gave other examples of neutron studies of novel materials at ORNL that could lead to better superconductors, sensors, and computer security.
Asked if any future research at the SNS might win a Nobel Prize, Christen said an experimental instrument being built here to detect and measure for the first time the electron dipole moment (tiny electric charge) of a neutron would have a good chance. “If the neutron didn’t have that property, we would likely have the same amount of matter and antimatter in the universe, which would annihilate each other, making it impossible for humans to exist.”
Christen and his colleagues are looking forward to an accelerator upgrade at the SNS to double the power of the protons that dislodge neutrons from target tungsten nuclei by spallation in another planned addition, the second target station. Those enhancements will allow the SNS, which currently hosts 19 beam lines, to have more than twice as many instruments.
“We also are working on plans to extend the life of HFIR,” he said. “The pressure vessel eventually will have to be replaced to extend the life of the reactor to the end of the century.”
Also on his wish list is a plan to enable HFIR and SNS users to take advantage of the high-performance computing capabilities of the supercomputers (Frontier and Summit) available at ORNL for analysis of their data. “A closer integration of our users with our advanced computing abilities would be helpful,” he said, adding that one goal is an infrastructure that allows scientists to load their computer codes on Oak Ridge computers for the use of many researchers.
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