The Neutron Catcher
By Catherine Longmire
If Oak Ridge were the fictional Land of Oz, the Spallation Neutron Source could certainly qualify as its Emerald City. Built atop a green and gently rolling site called Chestnut Ridge, the SNS is an impressive array of gleaming offices, modern labs, and sophisticated scientific equipment. Together they provide powerful tools for capitalizing on a particle far too small to see with the naked eye: the neutron.
Dr. Geoff Greene is one of the wizards behind the scenes here. By virtue of a joint appointment, he is both a physics professor at UTK and a researcher in the Oak Ridge National Laboratory Physics Division. A gifted ambassador for neutron research, Greene spent a snowy February morning giving an SNS tour and explaining his work. Clad in a hardhat and jeans, his ORNL badge hanging from an orange UTK lariat, he broke down the science of this world-class particle accelerator. It takes a big investment to study such a tiny entity, but the scientific gains are huge.
A Small Particle with Big Benefits
At first, neutrons may seem like ordinary subatomic particles. Neutral by nature, they typically lead quiet lives, sharing a home inside the nucleus with protons. When free to strike out on their own, however; they can become valuable assistants in decoding the intricate structure of materials. They are highly sensitive to hydrogen atoms, for example, which are abundant in biological molecules. They can also efficiently identify other light atoms among heavy systems, illuminating the architecture of advanced materials like superconductors or high-performance magnets. Medicine, transportation, and manufacturing all benefit from the neutron’s sensitivity.
To draw on this power, the SNS provides a multitude of neutrons for scientists to use and study. It generates a beam of hydrogen ions; then accelerates that beam to a billion electron volts. Along their rapid journey, the ions are stripped of their electrons and become protons. The resulting proton beam makes several hundred laps around a ring, where the protons group into bunches. The ring expels these bunches in pulses, which strike a mercury target that in turn produces pulses of neutrons. These neutrons are then guided to a number of experimental beam lines.
The SNS provides the world’s most intense pulsed neutron beams for scientific research and industrial development. And the more neutrons scientists have at their disposal, the more opportunities they have to understand and develop materials, and, in Greene’s case, answer fundamental scientific questions.
The Big Bang and the Left-Handed Universe
SNS scientists use the neutron to probe materials and get them to show what they’re made of. Greene and his collaborators, however, have a different agenda. They see the neutron not as a tool, but as an object worthy of study on its own merits. Observing a neutron’s life and demise offers hints to puzzles in cosmology, particle physics, and astrophysics.
The neutron’s lifetime, for example, sets the timescale over which chemical elements were produced during the Big Bang. The neutron might also have some answers as to what happened to all the anti-matter that appeared when the universe was brand new. Theory holds that for every bit of matter there is a corresponding bit of anti-matter. If that were so, the universe would ultimately have no matter at all, because in equal amounts the two annihilate one another.
”For some reason, there was little more matter than anti-matter,” Greene said. “The question is why. The simplest explanation is that there’s a process that prefers matter over anti-matter. However, such a process would represent totally ‘New’ physics.”
It’s believed that such a process would also generate something called the neutron electric dipole moment (EDM), which predicts that the positive and negative charges within a neutron are slightly displaced with respect to one another. Finding the EDM would confirm that matter somehow got the upper hand when the universe was born.
As informative as a neutron’s lifetime is the way in which it disintegrates, called beta decay. Not only does this process provide a simple blueprint for all radioactive decays, it may also explain why the universe shows a bias to lean left. This is related to parity—the principle that an original and its mirror image follow the same laws of physics. That principle is violated, however, in weak interactions (one of the four forces of physics). By looking at the beta decay of the neutron, scientists may be able to see if there’s a little “left-handedness” remaining from the universe’s explosive origins.
Building a Better Neutron Trap
Answering these questions means getting a good look at neutrons, and that requires slowing them down. To that end, Greene will use the Fundamental Neutron Physics Beam line (FNPB), designated as Beamline 13. Located in the cavernous SNS target building, it’s one of 24 paths that branch off from the main beam. Hardhats, steel, and concrete are the norm here, although the beamlines are brightened up with shades of yellow, blue, red, orange, and purple. (Greene wanted Volunteer Orange for his line, but it was already taken. He settled for blue.)
He and his collaborators have spent countless hours on this site, installing the system where he will catch and study neutrons. The FNPB splits its beam into two separate lines: one for cold neutrons and one for “ultra-cold” neutrons. The first will slow neutrons to the point that at any given moment, thousands per cubic centimeter will be available for observation. This line is dedicated to beta decay and parity violation experiments. The ultra-cold line goes a step further, slowing neutrons to a pace of a few meters per second and trapping them for hundreds of seconds at a time. Here is where the search for the electric dipole moment will be carried out.
While the simplicity of the neutron is what appeals to Greene, working with just one component of an atom doesn’t come without challenges.
“You gain by going to a simpler system, but you pay a price,” he said. “In order to get any information out of it, you have to perform extremely precise experiments, and these are difficult, time consuming, and expensive.”
Those experiments require a carefully-planned design, much of it concealed underneath the FNPB’s 18”-thick, blue concrete roof. The primary shutter that opens to receive neutrons from the main SNS beam comprises 30 tons of steel. There are secondary shutters for the cold and ultra-cold lines. There are choppers to insure that only neutrons of a given velocity get through. There are neutron “guides” to direct the beam and a special “beam stop” of lithium to stop the beam. There’s a data acquisition system to record neutron lifetimes and decay. And there are hundreds of tons of shielding to ensure both that the area is safe and that it won’t disturb neighboring experiments.
“Planning takes a big portion of our time. The entire beamline had to be designed from scratch,” Greene said. “A lot of our components are fabricated in the (UTK Physics) Machine Shop, where we can get expert attention to our rather special requirements.”
The cold line was completed a year ago. The installation of the ultra-cold line was completed in early February, a month ahead of schedule. This is a point of pride for Greene, who set up a detailed construction plan for the project in 2004 and has taken on multiple responsibilities over the past few years to get the project running. He has been, in his own words, “project manager, chief plumber, and babysitter,” and is also a veteran of fundraising, politics, and of course, groveling, adding with understated humor that “groveling is perhaps the most important aspect.”
Greene began hatching plans for this work when he was still a researcher at Los Alamos National Laboratory, before he had even been offered a position to join ORNL and the university.
“We started thinking about this before they did the groundbreaking for the SNS,” he said. “It’s been a long haul.”
The Envy of the Southeast
A key reason Greene has been able to invest so much of his time and talent in the SNS is because of his joint faculty appointment, which began in 2002. He’s a regular faculty member at UTK; however, by agreement with ORNL, the national laboratory pays half of his salary, which frees him from one-half of a regular teaching load and allows him more time for research.
"Joint faculty members are the most important conduits for collaborative efforts between UTK and ORNL,” said Dr. Soren Sorensen, head of the UTK Physics Department. They often originate research collaborations, he explained, and find research opportunities for students at either the national laboratory or the university.
“They are familiar with the different traditions and missions at UTK and ORNL and can find ways to enhance our interactions by using the strengths of each institution,” Sorensen added. “The joint faculty program is the bedrock below most of the UTK/ORNL interactions."
All joint faculty members claim either the national laboratory or the university as their home institution. For Greene, UTK is home, and he takes his professorial commitment seriously. He serves on departmental committees, has taught both undergraduate and graduate classes, and brings students into his research program to work toward graduate degrees. He has also built a collaboration of more than 30 other universities and national laboratories.
Greene said there’s no doubt the accessibility to and proximity of ORNL strengthens the university’s science and engineering departments.
“UT is the envy of the other schools in the Southeast because of that connection,” he said.
He also sees great potential in Governor Phil Bredesen’s plan, announced in January, to create 200 UTK faculty positions staffed by researchers at ORNL. The goal is to establish a world-class graduate program in energy sciences and engineering.
“They won’t be teaching 200 elementary classes,” Greene said of the proposed positions. “They’ll be advising graduate students, and that greatly expands the range of activities that our students in the sciences can explore. It increases opportunities for graduate students, which makes UT more attractive. The fellowships sweeten the pot.”
The fellowships he referred to are the UTK-ORNL Distinguished Fellowships, debuting this fall. This graduate program will allow students to work jointly with faculty members on campus and research scientists at ORNL. The fellowships are highly competitive and offer an annual stipend of $30,000 for as many as five years.
This concerted effort will attract high-caliber students, and to Greene, that’s what builds a great school.
“Better students are, ultimately, what makes a better university,” he said.
And for students who share his fascination with the neutron, opportunities abound on Chestnut Ridge.
“We’ve got years of really good science to do,” Greene said.
No ruby slippers required.
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