Nonexponential nuclear decay

Yuri Litvinov and coworkers at the GSI heavy-ion research institute in Darmstadt, Germany, have attracted much interest and puzzlement with their recent observation of sinusoidal modulation in the decay of two heavy nuclear species. The group produced single-electron ions of praseodymium-140 and promethium-142 and observed their decays over several minutes as the ions circled inside GSI’s ion-storage ring. Recording K-capture decays, in which the lone remaining atomic electron is ingested and a neutrino spat out, the experimenters found that, for both ¹⁴⁰Pr and ¹⁴²Pm, the expected exponential decay curves exhibit seven-second modulations with amplitudes of 20%. Exponential decay is the hallmark of a system that doesn’t know how old it is. But Eugene Wigner and Victor Weisskopf pointed out long ago that unstable quantum systems could exhibit departures from exponential decay at very early and late times. Litvinov and company tentatively attribute their observed oscillations to interference between...