a, An iodine-124 atom can decay with a half-life of 4.2 days to an atom of tellurium-124, through a process called electron capture. The nucleus of the iodine-124 atom captures an electron from the electron shells that surround it. A proton (circled) in the nucleus is converted into a neutron, and a neutrino is emitted. b, A xenon-124 atom cannot decay by electron capture, because of the law of energy conservation. However, it can decay with an extremely long half-life to a tellurium-124 atom, through a process known as two-neutrino double electron capture. The xenon-124 nucleus captures two electrons from the surrounding electron shells, which results in the conversion of two protons (circled) into neutrons, and the emission of two neutrinos. The XENON Collaboration2 has measured the half-life of this process to be 1.8 × 1022 years — about one trillion times the age of the Universe.