According to the law of conservation of energy, the total energy of the particles emitted during radioactive decay must equal the energy of the decaying atom. By precisely measuring the energy spectrum of those electrons, scientists can determine how much energy is missing, allowing them to infer the neutrinos’ mass. Peña spent more than a year developing and refining a flashlight-size prototype of the device in collaboration with machinists and fellow physicists. Peña designed a prototype copper microwave resonator to amplify the signals of electrons emitted as tritium decays, allowing researchers to measure them exactly and infer the neutrino’s mass. In May, Peña sent his final prototype to the University of Washington, where it was installed in July.