. Neutrinos are produced inside the Sun as a byproduct of nuclear fusion reactions in its core, but both the amounts and energies of the neutrinos depend on the element fused and the detailed model of the solar interior. Here we show the neutrino flux predicted by the Standard Solar Model. The largest flux of solar neutrinos is found at low energies; they are produced by the main proton-proton, pp, reaction in the Sun’s core. Less abundant, high-energy neutrinos are produced by a rare side reaction involving boron 8. The shading denotes the detection range for the gallium, chlorine and water experiments, with detection thresholds marked by the vertical lines. The gallium experiment can detect the low-energy pp neutrinos, as well as those of higher energy; both the chlorine and water detectors are sensitive to the high-energy boron-8 neutrinos.
. The original solar neutrino detector located 1,500 meters underground in the Homestake Gold Mine, near Lead, South Dakota, to filter out strong signals from energetic cosmic particles. The huge cylindrical tank was filled with 100,000 gallons of cleaning fluid. When a high-energy solar neutrino interacted with the nucleus of a chlorine atom in the fluid, radioactive argon was produced, which was extracted to count the solar neutrinos. This experiment operated for more than 25 years, always finding fewer neutrinos than expected from the Standard Solar Model. (Courtesy of Brookhaven National Laboratory.)
. Calculated and measured solar neutrino fluxes have consistently disagreed over the past two decades. The fluxes are measured in solar neutrino units, or SNU, defined as one neutrino interaction per trillion trillion trillion atoms, or ten to the thirty-six, per second. Measurements from the chlorine neutrino detector (small dots) give an average solar neutrino flux of 2.55 ± 0.25 SNU (lower broken line). The most recent theoretical calculations (large dots) predict a flux of 8.0 ± 1.0 SNU (upper broken line) for the Standard Solar Model. Other experiments have also observed a deficit of solar neutrinos, suggesting that either some process prevents neutrinos from being detected or the method by which the Sun shines differs from that predicted by current theoretical models.
. This neutrino detector has been built a kilometer underground in a Japanese zinc mine. The huge stainless-steel vessel, 40 meters tall and 40 meters wide, has been filled with 50,000 tons of highly purified water. Its walls are lined with 11,000 light sensors, called photo-multiplier tubes, that pick up a flash of light generated by electrons recoiling from neutrino collisions in the water. (Courtesy of Yoji Totsuka, Institute for Cosmic Ray Research, University of Tokyo.)
. The central spherical flask of this neutrino observatory is 12 meters in diameter and contains 1,000 tons of heavy water so that all three types of neutrinos can be detected. The flask is surrounded by a geodesic array of 9,500 photo-multiplier (18m diameter) tubes to detect the flash of light from the interaction of a neutrino with the heavy water. (Courtesy of Kevin Lesko, Lawrence Berkeley National Laboratory.)
. Neutrinos from the Sun travel through more than 2,000 meters of rock, entering an acrylic tank containing 1,000 tons of heavy water. When one of these neutrinos interacts with a water molecule, it produces a flash of light that is detected by a geodesic array of photo-multiplier tubes. Some 7,800 tons of ordinary water surrounding the flask blocks radiation from the rock, while the overlying rock blocks energetic particles generated by cosmic rays in our atmosphere. The heavy water is sensitive to all three types of neutrinos.