Neutrons not only reveal the nanostructure of ordinary materials, they can also reveal secrets about the early universe.

Neutrons make up matter. They also make excellent detectives. The uncharged subatomic particle is an ideal probe for spying on the inner workings of materials at the most minute level, allowing scientists a window into the atomic structure and composition of matter. Essentially, neutrons reveal the nanostructure of biomolecules for the discovery of new drugs, magnetic materials for better computer memories, credit cards—even clothing. They can even reveal secrets about the origin of the universe.

In 1946, physicist Clifford Shull pioneered a technique called neutron scattering. A beam of neutrons fired at a target material scatters as the neutrons bounce off atoms in that material. Based on the change in their direction, Shull discovered he could model the positions of atoms in the target material in a very similar fashion as William Henry Bragg and William Lawrence Bragg did earlier for X-ray scattering. Knowing where atoms are and how they interact with one another is critical to understanding the properties of electronics, molecules used in drugs and even window glass. It’s also critical to making materials more efficient.

Shull, a 1994 Nobel Prize winner in physics for this work, recognized early on that neutron scattering would be invaluable to researchers in a variety of scientific disciplines. It provides information not easily discovered using other techniques such as electron microscopy or X-ray diffraction.

Once Shull announced this work, performed at Oak Ridge National Laboratory (ORNL), in Oak Ridge, Tennessee, the United States immediately built more neutron sources to study materials. But time passed, and the facilities were not updated. Meanwhile, the Europeans and the Japanese began to construct and maintain newer neutron scattering facilities. In the past 15 to 20 years, these facilities surpassed those built at U.S. National Laboratories ORNL, Brookhaven National Laboratory in Upton, New York and Argonne National Laboratory in DuPage County, Illinois.

Meanwhile, a growing world neutron science community began to recognize the need for both continuous reactor-based neutron sources and pulsed accelerator-based sources. Pulsed neutron sources produce short, intense-pulse neutron beams that enable scientists to study a wide range of scientific problems and perform real-time analysis of the scattered neutrons. The use of accelerator based pulsed neutron sources was pioneered at Argonne and the first of such sources was built using an old accelerator (Intense Neutron Pulse Source).

In the late 1980s, U.K. neutron scientists made significant advances in accelerator-based technology, and by the 1990s, the U.S. Department of Energy (DOE) was itching to reclaim the United States’ prowess as a leader in the field. Consequently, DOE committed to building its own Spallation Neutron Source (SNS) in partnership with six National Laboratories. Ground was broken in 1999 for a that was to produce neutron beams in very intense pulses, with a performance 50 to 100 times better than the best sources. By 2006, the $1.4 billion complex opened at ORNL and quickly became the world’s most powerful accelerator-based, pulsed neutron source.

SNS, still ramping up to full capacity, already attracts researchers from a variety of scientific disciplines and countries. SNS aims to “provide research opportunities unavailable anywhere else in the world,” said Ian Anderson, SNS director. If funding holds, the facility should host at least 25 instruments.

Although the spallation process involves smashing protons, or positively charged subatomic particles, into a neutron-rich target to deliver neutrons, the process at SNS begins with negatively charged hydrogen ions consisting of a proton orbited by two electrons. Scientists send the ions into a linear accelerator, pulling them along so that they reach very high energies—up to 90 percent of the speed of light. The ions then pass through a foil that strips off each ion’s two electrons, turning the particle into a proton. The protons pass into a ring where they accumulate in bunches. Each bunch is then released from the ring as a pulse.

SNS currently operates at 670 kilowatts, which is the power of the proton beam that pulses and then slams into a container of liquid mercury, splaying trillions of neutrons at each go. Then the neutrons are slowed to different energies and sent down different beam lines to instruments that house materials scientists want to study on an atomic scale, just as Shull did with his early nuclear reactor neutron source.

People often think that neutron scattering is complex and leads to basic science irrelevant to their lives. “But that’s not true,” Anderson responded. Neutrons contribute fundamentally to improving medicines and drug delivery and to understanding defects in aircraft wings, engines, turbine blades and even electronics’ efficiency. Neutron scattering plays a role in meeting today’s high demand for less expensive, stronger and lighter new materials that perform well under severe conditions, as well as designer drugs and genetic engineering, which are revolutionizing medicine and health care. SNS probes biomolecules to design drugs to more accurately target the specific cells they aim to kill or switch off. Commercial and military aircraft and space probes demand lighter metal structures and stronger welds for increased speed and fuel efficiency. Automobiles require more plastics, materials that can withstand higher temperatures and lightweight components to be more fuel efficient and emit less pollution. Electronic devices like cell phones and computers require smaller and faster components and ever-increasing storage capacity through development of advanced magnetic materials.

Even more powerful neutron sources give researchers more detailed snapshots of the inner-workings of physical and biological materials so they can begin to restructure materials at the atomic level to meet societal needs. At SNS, the first instrument to receive neutrons was BASIS, a backscattering spectrometer that shines neutrons onto a material. From the way the neutrons are scattered, scientists can learn about a material’s composition and the distance between atoms. This is a “great clue” as to what is going on at the subatomic level in materials, said Ken Herwig, an instrument specialist working with BASIS. Neutrons sent down the 84-meter beam line to the instrument also can probe how subatomic structures are moving inside a material. BASIS, for example, can measure the tiniest energy changes in a neutron—a change as small as 1/100 degree Fahrenheit.

One way to think about it is to picture the atoms and neutrons as billiard balls. “The atoms are moving on one end of the billiard table and the neutrons are directed toward them along a defined direction and at a determined speed,” Herwig said. “When the neutron billiard ball collides with the atomic billiard balls, it may change direction and speed. If I can measure the change of this direction and speed, I learn something about how the atom billiard balls are moving.”

By knowing how atoms move, scientists can determine a lot about the invisible interactions between the particles, which they then can use to make predictions about how the macromaterial with which they are working will respond to certain stresses or how it will react in the human body. “The science that BASIS addresses is wide ranging, with the basic information researchers gather about the materials’ atomic world leading to more efficient energy usage, stronger materials and improved medications,” Herwig said.

At SNS, one instrument does not look at materials, but studies neutrons themselves, reported Geoffrey Greene, University of Tennessee physicist who heads the Fundamental Neutron Physics Beam Line (FNPB) team. This instrument is the only of its kind at the neutron source. “[With it,] we’ll look at the internal structure of the neutron and how ones not paired to protons—free ones—decay,” he said.

Neutrons are stable in most nuclei, he explained, but when knocked from a nucleus in an SNS neutron beam, the subatomic particles live for only about 15 minutes. Neutrons have no electric charge but may still show a balance between internal positive and negative charges, deemed a “neutron electric dipole moment,” Greene said.

Studying unbound neutrons and precisely measuring their lifetime will give physicists and astronomers clues about the beginnings of the universe. The research specifically hints at the abundance of each of the early chemical elements generated in the first few minutes of the big bang and sheds light on the amount of normal matter—as opposed to antimatter, dark matter and dark energy—in the cosmos, Greene said.

BASIS and the FNPB are among 10 instruments that are already running at SNS. The SNS instrument scientists are “working every day to bring more online soon,” Anderson said. Ideally the facility could host 25 instruments and shoot protons at the mercury target with a beam power of 3 to 4 million watts, which, he noted, would match any up-and-coming neutron facility for decades to come.