Science Hall Colloquium

Anne Aaker

Tuesday, May 13, Kate Scholberg exposed the world of atoms and their mysterious parts to a handful of chemistry students and one newspaper reporter. An Associate Professor of Physics at Duke University, Scholberg gave a presentation entitled “Neutrinos from the Sky and Through the Earth”at a chemistry colloquium.
Scholberg, whose research takes her back and forth between Mozumi, Japan and the U.S., began her talk by presenting the million-dollar question: “What makes up the universe?” The answer hasn’t yet been solidified, Scholberg said, but “neutrinos are a piece of that puzzle.”
The existence of neutrinos was first suspected by Austrian physicist Wolfgang Pauli in the 1930s. Pauli found that, during radioactive beta decay of the nucleus, an electron broke away from the nucleus, splitting it into two parts and emitting less energy than was previously thought.
This low level of energy suggested to Pauli that there was energy “missing” from the decay — a brand new discovery in physics. Energy, as Scholberg said, should never be missing — so where did it go?
Pauli’s theory was that an invisible particle also came of the decay and made off with some portion of the energy as well. Later, Italian physicist Enrico Fermi named that invisible particle “the neutrino.”
The first time a neutrino was actually detected came almost 20 years after Pauli’s discovery, when a pair of American physicists named Frederick Reines and Clyde Cowan used a nuclear reactor to pick up on the neutrino in 1956.
A neutrino has no electric charge and almost no mass. The latter characteristic is one of great interest in the physics world, and is one of Scholberg’s main focuses in the field.
The various tests that have been done regarding neutrino mass have concluded that because the neutrino does steal some energy after radioactive beta decay, it must have a non-zero mass — i.e., a very tiny neutrino mass is possible, though it hasn’t quite been figured out.
One of the tests that can lead to more clues as to the mass of the neutrino takes measurements of its oscillations, which depend on the “flavor” of the neutrino; that is, which lepton family it belongs to. If neutrinos had no mass, they wouldn’t oscillate, so if the flavors do oscillate, it means that they must have mass.
Here, Scholberg introduced the water Cherenkov detectors, which allow researchers to observe, measure and experiment with neutrinos and their interactions.
Scholberg has a soft spot for the Cherenkov detectors because of the “Super Kamiokande” — the detector she works with in Japan. The “Super-K” stands 40 meters high with a 17-meter radius and is filled with 32 kilotons of ultrapure water.
Inside Super-K, thousands of photomultiplier tubes line the walls in order to prevent cosmic rays from getting inside and interacting with the neutrinos, which would flaw any measurements the lab makes.
Scholberg went on to tell the story of the accident that occurred in 2001 in Super-K: After replacing some of the photomultiplier tubes, the detector was being refilled with water and, completely unexpectedly, a photomultiplier tube imploded, sending out a shockwave.
This shockwave caused more tubes to implode, thus beginning a domino-effect disaster inside the detector. When it was over, nearly every, if not all, photomultiplier tubes were destroyed.
Reconstruction of Super-K took one year, and now the tubes have fiberglass/acrylic shells to prevent the spread of a shockwave, should a tube implode ever again.
Still, the research continued, and it was discovered through an experiment involving the measurement of neutrino zenith angles that neutrinos change their flavor as they travel through the earth.
What this means, Scholberg said, is that the neutrinos oscillated during their trip through the earth, thereby supporting the theory that neutrinos have mass, albeit a small one.
To further support that theory, an experiment was conducted to see whether neutrinos oscillate or not. If a certain number of neutrinos was sent across the earth and that number changed by the time they reached their destination, then oscillation was confirmed, because during oscillation, neutrino flavors were supposed to change and therefore some of the neutrinos would disappear.
The successful completion of the experiment yielded affirmative results: the oscillation theory was proved to be true, which means that neutrinos do have mass.
At the end of her presentation, Scholberg listed the next things on the agenda for scientists studying neutrinos. Discovering their absolute mass, symmetry properties, and exact parameters were just a few of the items on her list.
Though these infinitesimal parts of atoms are practically invisible, they could eventually help us understand the big picture, Scholberg said — and go a long way in answering questions we have about the universe.

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