UH Physics Part of International Collaboration Making Observation

Measuring what is happening inside the sun is no easy task.

The detection of neutrino particles from the main reaction that powers the sun was named a Top 10 physics breakthrough for 2014 by Physics World, the member magazine for the Institute of Physics. University of Houston’s Department of Physics is part of the Borexino Collaboration, the international group that made the observation.

“The number of neutrinos that we measured is the same as predicted by the standard solar model,” said Ed Hungerford, the M.D. Anderson Chair of Physics and lead of UH’s Borexino activities. “This means that our understanding and theories of the reactions inside the sun are consistent with what was measured.”

The sun’s energy mainly is due to proton-proton fusion which occurs in the core of the sun. This fusion process creates neutrinos which essentially pass through matter undetected.

“The neutrino energies are small, and they don’t interact very much either,” Hungerford said. “Because they interact weakly, they actually get to the solar surface within five or six minutes, and within another eight minutes, they reach Earth. However, energy in the form of light takes a few hundred thousand years to reach the solar surface, and the light which is finally emitted cannot be directly connected to the reactions which produced it.”

Specialized Detector Built under Mountain

Interior of Borexino detector with view of photomultiplier tubes.To measure these low-energy neutrinos, scientists use a detector with an extremely low background, and one that is well shielded from cosmic rays. The Borexino Collaboration detector at the Laboratori Nazionali del Gran Sasso in Italy was built in 2006 in a tunnel 3,800 meters under a mountain.

“The detector consists of a big spherical tank filled with a scintillating fluid. When the neutrinos enter the tank and strike an electron, the electron recoils and produces a flash of light,” Hungerford said. “These flashes are counted by 2,000 photomultiplier tubes which line the walls of the tank.”

In addition to observing solar neutrinos, the Borexino Collaboration has also investigated geo-neutrinos – neutrinos coming from the Earth’s molten core. Background subtraction for this measurement requires knowledge of neutrino emission from all the world’s nuclear reactors. Other experiments will look for abnormal behavior of neutrinos and for the existence of proposed sterile neutrinos – that is, neutrinos which do not interact with anything.

One of Five U.S. Universities

UH’s Borexino team includes Hungerford, two postdoctoral fellows Stephano Davini and Anton Empl, and electronic technician George Korga. Hungerford became involved with the Borexino Collaboration in 2010 and also uses data from the detector for a search for dark matter.

Only five U.S. universities are involved in the collaboration which also involves researchers from Italy, Germany, Russia, France and Poland.

A panel of six Physics World editors and reporters selected the top-10 breakthroughs. Criteria for judging included the:

• Fundamental importance of the research
• Significant advance in knowledge
• Strong connection between theory and experiment
• General interest to all physicists

The Borexino Collaboration findings were published in Nature. The collaboration is funded by multiple national agencies including the National Science Foundation and the National Institute for Nuclear Physics. Hungerford’s participation is funded by the National Science Foundation.

- Kathy Major, College of Natural Sciences and Mathematics