Researchers used one of the most sensitive neutrino detectors ever built to observe for the first time neutrinos created by the keystone proton-proton (pp) fusion process in the Sun's core.

The pp reaction is the first step in a series that is responsible for 99 percent of the Sun's power. Solar neutrinos are created in nuclear processes and radioactive decay of different elements. These particles fly of the Sun at breakneck speeds, hitting every square inch of the Earth, UMass reported.

The detector allowed researchers to look directly at the source of the majority of the Sun's energy producing process, the chain of reactions going on in its core.

"By comparing the two different types of solar energy radiated, as neutrinos and as surface light, we obtain experimental information about the Sun's thermodynamic equilibrium over about a 100,000-year timescale," researcher Andrea Pocar said.

"As far as we know, neutrinos are the only way we have of looking into the Sun's interior. These pp neutrinos, emitted when two protons fuse forming a deuteron, are particularly hard to study. This is because they are low energy, in the range where natural radioactivity is very abundant and masks the signal from their interaction," he continued.

The Borexino instrument is the only detector on Earth capable of observing the entire neutrino spectrum at once. Neutrinos are seen in three types of "flavors"; the ones from the Sun's core are of the electron flavor, but as they move away they change between the "muon" to "tau" flavors.

Observing pp neutrinos was not part of the original National Science Foundation-sponsored Borexino experiment, but is a lucky bonus.

"It's a little bit of a coup that we could do it," Pocar said. "We pushed the detector sensitivity to a limit that has never been achieved before."

The findings were published in the current issue of Nature.