The fusion reaction that powers our sun has been detected in real time for the first time with an instrument buried deep beneath a mountain in Italy detecting the resulting neutrinos.

Before this, measurements of solar energy output relied on photons reaching the earth from the sun from the same kind of fusion reactions, but those reactions happened one hundred thousand years ago -- the amount of time the photon energy takes to make its way through the sun's dense interior to burst from its surface and begin the journey toward Earth.

In the new experiment conducted by an international team of researchers working with the Italian National Institute for Nuclear Physics, solar energy has been measured almost from the moment of its generation, because the neutrinos detected need just 8 minutes to travel from the very core of the sun to Earth.

The amount of energy produced by the sun today, as measured using the neutrinos, is identical to what was determined by photon measurements which looked a hundred thousand year's into the sun's past, proving the energy output has remained the same for all that time, the researchers say.

"This is direct proof of the stability of the sun over the past 100,000 years or so," says team member Andrea Pocar of the University of Massachusetts, Amherst.

Within the sun's core, protons of hydrogen atoms, the sun's major constituent, collide with such force they undergo a fusion reaction, producing a nucleus of heavy hydrogen, an antielectron (a positron) and a neutrino.

Further reactions produce helium and other elements and more kinds of neutrinos, although the majority of neutrinos streaming out of the sun are from that initial proton-proton reaction initiating fusion.

They have proved the hardest of all the various solar neutrinos to detect on Earth, the scientists explain, because they have low energy levels that are similar to that of many radioactive decays that occur on Earth.

This causes detectors to have trouble, confusing a radioactive decay with a solar neutrino event.

That's why the Italian detector instrument, dubbed Borexino, is buried more than 4,500 feet below the Apennine Mountains.

The overlying rock can shield it from decay energy, while neutrinos pass easily through it and into the detector.

After 18 months of collecting data and a full year of analyzing it "to show it was not background [radiation] or a detector effect," the team came up with a figure for neutrino flow of 66 billion per square centimeter per second, very close to model predictions of 60 billion, as reported in the journal Nature.

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