Nonexponential nuclear decay
DOI: 10.1063/1.4796928
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 two different neutrino mass eigenstates. If true, such relatively inexpensive nuclear-physics experiments could greatly ease a high-priority task in particle physics: determining the masses and mixing angles of the three distinct neutrino states. Theorist Harry Lipkin supports the GSI group’s interpretation. He points out that the seven-second period is of the right order of magnitude for what’s already known of the tiny neutrino-mass splitting. But an increasing number of theorists argue that the neutrino-interference explanation is not only wrong in detail; they claim it violates fundamental tenets of quantum mechanics. Theory aside, Paul Vetter and colleagues at Lawrence Berkeley National Laboratory report that they have failed to reproduce Litvinov’s result. But the LBNL experiment was done with neutral atoms in a solid matrix, which, says Litvinov, complicates the simple two-body decay kinematics that renders the delicate effect visible. (Y. A. Litvinov et al., Phys. Lett. B 664 , 162, 2008 http://dx.doi.org/10.1016/j.physletb.2008.04.062 ;
