The First Direct Observation of Double-Beta Decay

The possibility of double-beta (ββ) decay was recognized in the mid 1930s, and by 1939 something of its potential to reveal fundamental properties of the neutrino was known as well. But experimental observation of the phenomenon was out of reach. Despite numerous attempts over the next 30 years, and strong suggestions of its existence from geochemical experiments, ββ decay had not been observed to occur in the laboratory. Our group at UC Irvine took up the search in the early 1970s, first with a cloud chamber and then with a time-projection chamber. The following narrative traces a discovery process that unfolded over the ensuing 15 years—a sequence of setbacks and false starts punctuated by occasional victories and, ultimately, by a definitive laboratory observation of two-neutrino ββ decay, in August 1987.