Researchers detect low-energy neutrinos, probe energy production in sun's center

At approximately 15 million degrees, protons — the nuclei of hydrogen atoms — and light elements can fuse to form new nuclei. Several such steps eventually convert the hydrogen in the sun into helium, releasing about 25 million times more energy per gram than TNT, oil, or coal.

“While the neutrinos, which are uncharged elementary particles, only take about eight minutes to reach the earth, the thermal energy produced at the center of the sun only appears as sunlight some 50 thousand years later, after diffusing to the sun’s surface,” said Bruce Vogelaar professor of physics and leader of Virginia Tech’s research team for this project.

“The only way to prove the validity of this model of solar energy generation is to observe these neutrinos which easily travel right through the sun because of their weak interaction with matter,” Vogelaar said. “Of special interest are those neutrinos from the decay of 7Be, a critical step in the energy chain of the sun.”

It is these neutrinos that the Virginia Tech team and their colleagues have observed directly for the first time in the Borexino detector, located under the Gran Sasso peak in the Apennine mountain range about 100 miles east of Rome. Borexino is a massive detector that contains some 350,000 gallons of organic liquid. Its central region detects neutrinos by seeing the light given off when a neutrino collides with an electron, using some 2,200 photosensors arrayed around the detector.

“The sun emits copious amounts of neutrinos in a wide range of energies,” Vogelaar said. “About 10 billion pass through your thumbnail each second.”

In the last decade, the much rarer high-energy fraction (one part in ten thousand) has been seen in many experiments, he said. The vast majority of the flux, however, is at much lower energies and had not been directly observed until now. This is because previous detector technologies were unable to discriminate low-energy neutrino signals from formidable backgrounds due to radioactivities normally present in the environment. These include the detector itself and cosmic rays. To avoid the latter, the detector was shielded by placing it deep underground at Gran Sasso. The Borexino Collaboration has developed and employed a new technology that virtually eliminated even trace contaminations, allowing successful measurement of the low-energy solar neutrinos.

The required purities are unprecedented — several million times lower than levels normally achievable, even with the development of ultra-clean technologies for the semiconductor industry. Another major problem with detecting low-energy neutrinos was the inescapable carbon in the detector’s organic liquid, which normally contains a million times more radioactive 14C than tolerable for Borexino. 14C is normally used in radiocarbon dating studies.

Raju Raghavan, professor of physics at Virginia Tech and formerly with Bell Laboratories, made the first breakthrough in methods for reducing radioactive contamination sufficiently as well as discovering how to avoid the radiocarbon. With colleagues from 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 non-polar liquids were a million times lower than thought earlier, and thus suitable for Borexino. Since radiocarbon cannot be chemically purified from normal carbon, Raghavan side-stepped the problem by postulating that petrochemicals derived organic liquids ought to contain much less radiocarbon than normal, due to their residence deep in the earth for geological times. Raghavan and colleagues from the University of Toronto developed a method to show this was the case, and that indeed, the purities reached Borexino levels, which are parts per million billion.

“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 said. “These new techniques have also impacted commercial technology needed today,” For example, he solved the sodium contamination problem in photolithographic chemistry in the fabrication of chips in the microelectronic industry using these techniques.

Showing that these results were valid at the ton, and then kiloton, scales was accomplished over the next 10 years by the Borexino collaboration, including exhaustive field tests using a five-ton prototype detector constructed in Gran Sasso.

The Borexino collaboration consists of more than 100 scientists, post-doctoral fellows, and students from Tech and Princeton University in the U.S., and groups from Italy, France, Germany, Russia, and Poland. In addition to Vogelaar and Raghavan, other members of the Virignia Tech team were Henning Back (currently at NCSU), Christian Grieb, Steven Hardy, Matthew Joyce, Derek Rountree, and. Szymon Manecki, along with several undergraduates. The collaboration is led by Gianpaolo Bellini of the University of Milan, Italy. Essential support for the 20-year effort was provided by the Laboratori Nazionali del Gran Sasso, the INFN (Italy), the National Science Foundation, and other funding agencies in Europe and Russia.

“The scientific and technological achievement of Borexino is a testament to the value of international collaboration and the ingenuity and tenacity of the Borexino collaboration over 20 years to achieve the present success.” Vogelaar said. “We expect that information on the 7Be solar neutrinos will clarify the sun’s energy cycle in great detail and throw light on the nature of the neutrino itself”