UMass faculty research solar neutrinos

By Melanie Muller

Shedding light on the operations and properties of the sun, an international team of researchers – including University of Massachusetts Assistant Professor Andrea Pocar and Assistant Professor Laura Cadonati – have used a high precision neutrino detector buried deep under the mountains of Italy to measure the once elusive 7Be solar neutrinos.

Courtesy ganehsa.isis/Flickr

Neutrinos are elementary particles of matter that exist throughout the universe. To detect them, the team used what Cadonati called the “‘cleanest’ detector on earth,” the Borexino liquid scintillator, a machine that measures the low energy level of neutrinos.

With this equipment, the team has mapped the neutrino interaction rates to an unprecedented degree of accuracy. Their results were published on Sept. 30 in the journal Physical Review Letters, as an article titled “Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino.”

The Borexino instrument makes spectrally resolved measurements of 7Be neutrinos – a specific type of neutrino that is easy for the scintillator to measure – at the low energy level of less than 1 MeV, which is a measurement of the energy of electron volts, according to the Physical Review article, allowing for solar neutrino flux measurements with a “total uncertainty of less than 5 percent.” Neutrinos, the lightest fundamental particles, are produced in great quantities by the sun and continually bombard the earth.

According to Cadonati and Pocar, measurements of low energy particles like the 7Be neutrinos are normally difficult to measure because radioactive contaminants and cosmic rays create background energy that complicates the readings. The Borexino scintillator overcomes these difficulties by limiting radioactive contamination. The design purity of the scintillator fluid reduces radioactive activity. The detector core is protected from cosmic rays by the mountain rock above it, steel and layers of water.

The Borexino project began in late 1980s, according to Cadonati, when Raju Raghavan, a recently deceased professor from Virginia Tech., proposed that “neutrinos could be detected in a very pure organic liquid scintillator.” The original formulation of liquid in the detector included Boron, giving the instrument its name. Due to the delicacy of the solution, despite starting the project in 1997, the instrument was not finished until 2007.

Since then, according to Pocar and Cadonati, Borexino has produced several physics papers each year and “for the recent publication we had to collect sufficient data and perform delicate calibration operations in order to reach the high precision result we have published.”

Cadonati has worked on the project since she was an undergraduate studying in Milan, Italy. Candonati started work in Gran Sasso taking while taking a year off from her University with a fellowship to work in a laboratory on her undergraduate thesis project.

“I chose Borexino and the undergraduate laboratory in Gran Sasso because I was fascinated by these elusive particles from the sun that were holding many secrets,” she said.

As an undergraduate, she was a member of the team working to design the nylon sphere that would contain the scintillator, and she was in charge of measurements of stresses on the nylon membrane that would surround it. She also worked with a Monte Carlo simulation to determine effects radioactive contamination could have on the detection potential of the device.

Cadonati credits these experiences with helping her decide to pursue a career in science and to attend graduate school. She continued working on the project during her graduate school years, and after a hiatus when she studied “an even more elusive signal,” gravitational waves, she resumed working on the Borexino project again which was just beginning to take data in 2007. Since then she has been in charge of one of the data analysis teams.

Pocar became involved in the Borexino project as a graduate student at Princeton University in 1998, after having heard about it from friends while pursuing his undergraduate in Milan.

Pocar said he was attracted to the study of neutrinos, fundamental particles “whose behavior seemed to hold the answers to many interesting questions about the nature of mass and fundamental interactions.”

He worked on a team which built and installed the nylon containment vessel. In particular, he designed a radon filter, which allowed the nylon spheres to be built inside a “radon-suppressed clean room, the first of its kind,” said Pocar.  The filter reduced radioactive contamination of the sphere surfaces even further. The elimination of such trace radioactivity is key, according to Pocar and Cadonati, in giving the Borexino the ability to make accurate measurements of low energy particles.

During installation he said worked on “implementing other ultra-clean fabrication techniques” and integrity of the installed nylon spheres. He spent a year at the Gran Sasso labs while working on his PhD, where one of his projects involved the commissioning of a system that filled the detector with scintillator fluid.

After working on another international project, in which he is still involved to investigate whether neutrinos are their own anti-particles, Pocar began collaborating on the project with Cadonati in 2007 when he came to UMass. In 2009, he was involved in the calibration of Borexino and followed “the data analysis performed by our students.”

Both Cadonati and Pocar enjoy the experience of working on an international team, and believe this kind of collaboration is necessary in their particular field of work.

“Personally, I enjoy working with different groups around the world and find it very healthy for science,” said Pocar. “It was my need to experience a new culture of doing research that brought me to the U.S. in the first place.”

According to Cadonati, Borexino and other projects she works on “are addressing some of the big open questions for the understanding of nature at a fundamental level, and these projects require joining efforts not only across institutions but across countries and funding sources.”

In the future, Pocar and Cadonati hope to continue improving their measurements of solar neutrino flux so that they can investigate a set of reactions called the “carbon-nitrogen-oxygen cycle” which are connected to the atomic composition of large stars. They would like to further lower the energy threshold to enable measurements of other neutrinos and neutrino properties. A secondary side interest is investigating if the data provides unexpected evidence of “exotic physics phenomena,” such as decaying electrons and violations of the Pauli Exclusion Principle.

The Borexino program was made possible by funding from INFN (Italy), NSF (USA), BMBF, DFG, and MPG (Germany), NRC Kurchatov Institute (Russia), and MNiSW (Poland), according to the authors of “Precision Measurement of the 7Be Solar Neutrino Interaction Rate in Borexino.”

Melanie Muller can be reached at [email protected]