a proton, an electron, and a neutrino. Initially neutrinos were thought to be undetectable, but in 1945 Pontecorvo noted that a neutrino striking a chlorine nucleus could transform it into unstable argon-37 that emits, with a 34 days half-life, after a K-capture reaction, a 2.8 keV Auger electron allowing its direct detection: + → + Pontecorvo's 1945 paper credits the idea using carbon tetrachloride (CCl4) to the French physicist Jules Guéron. Experiments were conducted at Chalk River using the NRX as a neutrino source, but were unsuccessful, and were abandoned in 1949, after Pontecorvo had left. The experiment was unsuccessful because, unknown at the time, nuclear reactors produced antineutrinos instead of neutrinos. In what is now known as the Cowan–Reines neutrino experiment, Frederick Reines and Clyde Cowan detected antineutrinos in 1955, for which they won the Nobel Prize in Physics in 1995. The idea was taken up again in the search for solar neutrinos. Theoretically, the Sun produced neutrinos in the course of nuclear fusion reactions. Pontecorvo credited Maurice Pryce for this idea. The most common, the proton–proton chain reaction in which hydrogen is fused to form helium produces neutrinos that are not energetic enough to interact with chlorine. However, the much less common CNO cycle that produces carbon, nitrogen and oxygen does. In the late 1960s, Ray Davis and John N. Bahcall detected solar neutrinos in the Homestake Experiment, for which Davis was awarded the Nobel Prize in Physics in 2002. The experiment was the first to successfully detect and count solar neutrinos, but the number of neutrinos detected was between one third and one half of the predicted number. This became the solar neutrino problem. For a time, scientists contemplated the awful possibility that the Sun might have burned out. The problem had already been solved by Pontecorvo in 1968. In 1959, a powerful accelerator (that was never built) was being designed, and he began considering experiments that could be performed with it. He contemplated a project investigating muons. Julian Schwinger had hypothesised that particles experience the weak interaction through exchanging W bosons. The W boson would not be discovered until 1983, but a problem immediately surfaced. Gerald Feinberg pointed out that this implied that some interactions that had never been observed should occur, but conceded that this was only true if the neutrinos associated with electrons were the same as those associated with muons. In a 1959