Everything under the sun
Scientists at Virginia Tech explore the heart of the sun ... and other deep mysteries
By Catherine Doss, College of Science
Virginia Tech researchers are partners in long-standing international research projects that have brought the world closer to understanding the sun and the mysterious subatomic particles called neutrinos. Meanwhile, in an underground laboratory near the university’s campus, physics researchers in the College of Science are initiating new neutrino studies.
“Each of these experiments is at the crest of the advancing wave in neutrino science and will directly impact particle physics, stellar astrophysics, and cosmology for years to come,” says Raju Raghavan, a long-time international neutrino researcher who arrived at Virginia Tech in 2004 as professor of physics and director of the Institute of Particle, Nuclear, and Astronomical Sciences.
“Discoveries in this area of science can potentially lead to a better understanding of the origins of the earth, sun, and stars,” says Bruce Vogelaar, professor of physics at Virginia Tech, also a veteran neutrino researcher on the world scene. Vogelaar created the underground laboratory in the Kimballton limestone mine.
Heavy stuff. Let’s start with the basics.
Neutrinos are one of the fundamental particles of the universe. They are tiny, nearly massless, uncharged particles in a family of matter called leptons. Neutrinos exist in three distinct states or “flavors” — electron, muon, and tau forms.
Neutrinos originate from many sources: the center of the sun (which is the focus of this article), the interior of the earth, and from terrestrial accelerators and nuclear reactors. Neutrino experiments of the Virginia Tech group look at all these sources with different objectives for particle physics, astrophysics, and cosmology.
For a long time, scientists believed neutrinos didn’t have any mass, but recent discoveries have determined that they do indeed have a tiny mass, and their behavior is quite odd. For instance, if neutrinos have mass, they can change their flavor in a phenomenon known as oscillation.
Neutrinos are also very elusive because they interact so weakly with other particles. For example, 100 billion neutrinos from the sun pass through your thumb per second without leaving any trace. And yet scientists have long suspected their importance in the grand scheme of the universe.
“Neutrinos are kind of funny,” says Jonathan Link, assistant professor of physics who has recently joined Virginia Tech’s neutrino team. “I would compare them to a place kicker in football. You don’t see them very often but their role is critically important. Indeed, neutrinos played a central role in the Big Bang theory of how the world came into existence and, as the universe aged, helped to form the elements.”
Scientists believe that light elements, such as hydrogen and helium, were created during the Big Bang, but heavy elements, which are critical for life on earth, were created in later events called supernovas. These are catastrophic explosions following star collapse when the nuclear source of energy is no longer sufficient to withstand the inward pressure of gravity. Exactly how supernovas work in such an extremely complex process is still a mystery. But we do know that neutrinos play an important part in supernova explosions.
A Department of Energy (DOE) experiment carried out at the Los Alamos National Research Laboratory in the mid 1990s discovered that the oscillation in some neutrinos was inconsistent with the three neutrino flavors that we now know. The odd new neutrinos implied by the Liquid Scintillator Neutrino Detector (LSND) project are known as “sterile“ neutrinos because they don’t interact with the fundamental particles of the universe, except, perhaps, through oscillations with the other three flavors as implied by LSND. Since the controversial LSND result, theorists have shown that they can still solve many problems in physics, from supernova explosions to the mysterious dark matter that binds galaxies together.
“Because of the far-reaching consequences of this interpretation, the LSND findings cried out for independent verification,” Link says.
So in 1998, the DOE approved another major neutrino research project called MiniBooNE, (of which Link is a member) which would use neutrinos produced by the Booster accelerator at the agency’s Fermilab facility in Illinois. The experiment’s goal was to either confirm or refute the startling observations reported by LSND.
In an announcement that received worldwide attention in spring 2007, the DOE revealed that the MiniBooNE project was unable to detect the same oscillations that were observed in the earlier experiment. In other words, the latter experiment did not verify the former.
“The possibility of sterile-neutrino-induced oscillation observed by LSND seems to be ruled out,” Link says. “But there may still be sterile neutrinos with somewhat different properties.”
Detecting neutrinos is a challenging task and one that requires extreme patience and perseverance. Neutrinos can go through material equal to 200 Earths without interacting. But it is those very rare occurrences when they do interact that Link and his colleagues study.
“Of all the billions and billions of neutrinos that pass through a detector, we’ll count maybe a few hundred thousand,” he says. “What makes it even more difficult is that we’re searching for one particular type of neutrino that has changed flavor.”
For its observations, MiniBooNE relied on a 250,000-gallon tank filled with ultra-pure mineral oil, which was crystal clear. A layer of 1,280 light-sensitive photomultiplier tubes, mounted inside the tank, detected collisions between neutrinos made by the accelerator and carbon nuclei of oil molecules. Muon neutrinos — with Giga-Electron Volt energies, delivered in bursts that last 1.6 millionths of a second, five times per second — collide with carbon atoms in the mineral oil, producing muons, which then create cones of light that travel to the edges of the detector tank and are converted into electrical signals.
Another important announcement was made in 2007 regarding the findings of a path-breaking neutrino experiment called Borexino, in which Raghavan and Vogelaar have been active for more than a decade. For the first time, Borexino attacked the problem of detecting low-energy neutrinos — for example, those emitted by the sun.
The Borexino collaboration consists of more than 100 scientists from Virginia Tech, Princeton, and groups from Italy, France, Germany, Russia, and Poland. The 20-year project was funded in part by the National Science Foundation. In this collaboration, researchers observed tell-tale signals of neutrinos emitted by thermonuclear fusion reactions that power the sun. Deep within the sun’s interior, at approximately 15 million degrees, protons and light elements fuse in a series of reactions that convert hydrogen into helium and release about 25 million times more energy per gram than TNT, oil, or coal.
“While neutrinos take only about eight minutes to reach the earth, the thermal energy produced at the center of the sun only appears as sunlight some 40,000 years later. The difference lets us probe the rate of energy production over geological times,” says Vogelaar. “The only way to prove the validity of this model of solar energy generation is to observe the neutrinos.”
It was these low-energy neutrinos that the Virginia Tech team and their colleagues observed directly for the first time in the Borexino detector. Previous detector technologies were unable to discriminate low-energy neutrino signals from background radiation due to normal radioactivity in the environment. These include the detector itself and cosmic rays. It is to avoid the latter that the detector was shielded by placing it deep underground.
Raghavan’s early scientific work laid the foundations of the Borexino experiment, whose myriad technical demands were solved by innovations developed over the next 20 years by a worldwide research collaboration.
The basic constituent of Borexino is 1.3 million gallons of organic liquid scintillator. Borexino is installed in an underground lab deep within the snow-covered peaks of the Gran Sasso Mountain in Italy. “The basic problem in the detection of low-energy neutrinos is that it requires material purities several million times better than those normally achievable, even with the development of ultra-clean technologies for the semiconductor industry,” says Raghavan.
With colleagues from the University of Pavia, Italy, he invented new methods of purification and material characterization that explicitly showed for the first time that the solubility of heavy metals — such as radioactive uranium and thorium — in nonpolar liquids was a million times lower than earlier thought. He also showed how to avoid radioactive carbon-14 that cannot be removed chemically.
“These results on the laboratory scale showed the potential for low-energy neutrino spectroscopy in Borexino and paved the way to large-scale investments for the experiment,” Raghavan says. “These new technologies have also impacted commercial technology in areas such as photolithographics in microelectronics.
“By measuring the neutrino flux, one can understand the workings of the sun as well as neutrino masses and their flavor mixings. Observing neutrinos from the sun — literally looking into the center of the sun because neutrinos that weakly interact with matter easily travel through the sun — is a monumental task conceptually and technically because of the low energies (1 million times lower than in MiniBooNE),” says Raghavan. “Borexino made the first major breakthrough in detecting such neutrinos.”
Meanwhile, Borexino’s results provide proof of the theory regarding how low-energy neutrinos are produced. In stars the size of the sun, most solar energy is produced by a complex chain of nuclear reactions. Some of the steps along this sequence of events require the presence of the element beryllium, and physicists have theorized that these steps are responsible for creating about 10 percent of the sun’s neutrinos. Findings from Borexino confirmed this theory and may also help scientists understand other hypotheses of neutrino oscillation that have not yet been tested.
Now a new project to study particles from the sun has been created just minutes from Virginia Tech under a beautiful Southwest Virginia mountain, instead of thousands of miles away under a grand Italian mountain. “The Kimballton Underground Research Facility (KURF) provides shielding from cosmic rays, large cavern size, and easy access, features coveted by the physics community to develop next generation detectors to answer fundamental questions about our universe,” says Vogelaar.
The energy of the sun is predominantly due to the basic thermonuclear fusion of two protons, or hydrogen nuclei, by far the largest component of the sun. This solar reaction can be detected on Earth by the neutrinos that it emits, the so-called p-p neutrinos. Detecting them has been a holy grail of physics for 50 years, but it is extremely difficult because of their very low energy, even lower than the neutrinos observed by Borexino.
Raghavan invented the first method to attack the problem of detecting p-p neutrinos directly and developed a series of technical innovations crucial for its feasibility. This experiment is now called LENS (low energy neutrino spectroscopy). But LENS will not only observe the p-p neutrinos, it will measure the entire low-energy solar neutrino spectrum with high accuracy.
Vogelaar, KURF director, emphasizes that Kimballton will be a unique and powerful resource, not only for LENS and other university experiments, but also for a national and international community of researchers. Kimballton adds a major new dimension to Virginia Tech’s research in experimental neutrino science — a box seat for observing our universe with “neutrino” eyes,” helping fulfill part of the vision members of Virginia Tech’s neutrino research group have had for many years.