From an article in Physics World by Ken Croswell, August 29, 2014:
Physicists working on the Borexino experiment in Italy have successfully detected neutrinos from the main nuclear reaction that powers the Sun. The number of neutrinos observed by the international team agrees with theoretical predictions, suggesting that scientists do understand what is going on inside our star. (See Section 12.7, p. 322.)
Each second, the Sun converts 600 million tons of hydrogen into helium, and 99% of the energy generated arises from the so-called proton–proton chain. And 99.76% of the time, this chain starts when two protons form deuterium (hydrogen-2) by coming close enough together that one becomes a neutron, emitting a positron and a low-energy neutrino. It is this low-energy neutrino that physicists have now detected. Once this reaction occurs, two more quickly follow: a proton converts the newly minted deuterium into helium-3, which in most cases joins another helium-3 nucleus to yield helium-4 and two protons.
Neutrinos normally pass through matter unimpeded and are therefore very difficult to detect. However, the neutrinos from this reaction in the Sun are especially elusive because of their low energy. The measurement therefore took scientists by surprise.
The Borexino detector is a large sphere containing a benzene-like liquid that is located deep beneath a mountain at the Gran Sasso National Laboratory to shield the experiment from cosmic rays. Occasionally, a neutrino will collide with an electron in the liquid and the recoiling electron will create a flash of ultraviolet light that can then be detected.