As described in The Cosmos, 4ed, Section 12.7, the solar-neutrino and other experiments have been detecting subsidiary nuclear interactions, but could not reach the energy range of the most fundamental process that fuels the sun and stars like it–the interaction of two protons. Scientists of the Borexino project in Italy has announced that they have finally detected this fundamental process. Here is an abridged version of their press release of August 27, 2014:
Scientists working on the neutrino experiment in the Italian National Institute for Nuclear Physics (INFN) Gran Sasso Laboratories have managed to measure the energy of our star in real time: the energy released today at the center of the Sun is exactly the same as that produced 100,000 years ago. For the first time in the history of scientific investigation of our star, solar energy has been measured at the very moment of its generation. The study was published on August 28, 2014, in the journal Nature.
Borexino has managed to measure the Sun’s energy in real-time, detecting the neutrinos produced by nuclear reactions inside the solar mass: these particles take only a few seconds to escape from it and eight minutes to reach us. Previous measurements of solar energy, on the other hand, have always taken place on radiation (photons) which currently illuminate and heat the Earth and which refer to the same nuclear reactions, but which took place over a hundred thousand years ago: this, in fact, is the time it takes, on average, for the energy to travel through the dense solar matter and reach its surface. The comparison between the neutrino measurement now published by Borexino and the previous measurements concerning the emission of radiant energy from the Sun shows that solar activity has not changed in the last one hundred thousand years.
The Borexino detector, installed in the INFN underground Laboratories of Gran Sasso, has managed to measure the flux of neutrinos produced inside the Sun in the fusion reaction of two hydrogen nuclei to form a deuterium nucleus: this is the seed reaction of the nuclear fusion cycle which produces about 99% of the solar energy. Up until now, Borexino had managed to measure the neutrinos from nuclear reactions that were part of the chain originated by this reaction or belonging to secondary chains, which contribute significantly less to the generation of solar energy, but which were central to the discovery of certain crucial physical properties of this “ephemeral” elementary particle, the neutrino.