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.