after the photons from a positron annihilation event. This experiment was designed by Cowan and Reines to give a unique signature for antineutrinos, to prove the existence of these particles. It was not the experimental goal to measure the total antineutrino flux. The detected antineutrinos thus all carried an energy greater than 1.8 MeV, which is the threshold for the reaction channel used (1.8 MeV is the energy needed to create a positron and a neutron from a proton). Only about 3% of the antineutrinos from a nuclear reactor carry enough energy for the reaction to occur. A more recently built and much larger KamLAND detector used similar techniques to study oscillations of antineutrinos from 53 Japanese nuclear power plants. A smaller, but more radiopure Borexino detector was able to measure the most important components of the neutrino spectrum from the Sun, as well as antineutrinos from Earth and nuclear reactors. Radiochemical methods Chlorine detectors, based on the method suggested by Bruno Pontecorvo, consist of a tank filled with a chlorine containing fluid such as tetrachloroethylene. A neutrino occasionally converts a chlorine-37 atom into one of argon-37 via the charged current interaction. The threshold neutrino energy for this reaction is 0.814 MeV. The fluid is periodically purged with helium gas which would remove the argon. The helium is then cooled to separate out the argon, and the argon atoms are counted based on their electron capture radioactive decays. A chlorine detector in the former Homestake Mine near Lead, South Dakota, containing 520 short tons (470 metric tons) of fluid, was the first to detect the solar neutrinos, and made the first measurement of the deficit of electron neutrinos from the sun (see Solar neutrino problem). A similar detector design, with a much lower detection threshold of 0.233 MeV, uses a transformation which is sensitive to lower-energy neutrinos. A neutrino is able to react with an atom of gallium-71, converting it into an atom of the unstable isotope germanium-71. The germanium was then chemically extracted and concentrated. Neutrinos were thus detected by measuring the radioactive decay of germanium. This latter method is nicknamed the "Alsace-Lorraine" technique in a joke-reference to the reaction sequence. The SAGE experiment in Russia used about 50 tons of gallium, and the GALLEX / GNO experiments in Italy about 30 tons of gallium as reaction mass. The price of gallium is prohibitive, so this experiment is difficult