Reactor Anti-Neutrino Disappearance

Reactor anti-neutrino disappearance, measured by a detector in Japan, supports the idea that neutrinos oscillate from one type to another and that they possess mass. Nuclear reactors produce several things: heat, electricity, spent fuel rods, and neutrinos. The neutrinos (or, to be more exact, electron anti-neutrinos) are a result of fission reactions inside the reactor core. But some of the electron antineutrinos, once they're underway and moving through the Earth, manifest one of the weirdest phenomena in all of physics, namely the ability to exist as a composite of several sub-species. That is, what we call a neutrino is really several (perhaps three) neutrinos in one. At any point along its trajectory the generic neutrino might (if you were to capture it just then) appear as an electron neutrino, but farther along it might look like a muon neutrino, in which case it would elude detectors tuned to detect only electron nu's.

The Kamioka Liquid Scintillator Anti-Neutrino Detector (KamLAND) sets out to sample this odd mode of being. The apparatus, basically a huge reservoir of optically-active liquid viewed by numerous phototubes, looks for interactions in which an incoming nu strikes a proton, creating in their stead a trackable neutron-positron pair. KamLAND resides in an underground lab beneath Toyama, Japan. It is a sort of telescope peering not at galaxies in the sky; instead it stares through a block of terrestrial crust looking for the neutrino warmth cast off by a constellation of 69 reactors in Japan and Korea.

Taking into account the laws of physics governing the reactions in the reactor cores, the known power ratings for the reactors, their aggregate reactor-detector distances, and the duration of the experiment (145 days), one would expect seeing 86 true events, whereas the actual number was 54. The researchers conclude that the disappearance of events is due to neutrino oscillation.

This result is not merely a confirmation of oscillation research carried out with solar nu's at such detectors as Super Kamiokande in Japan and the Sudbury Neutrino Observatory (SNO) in Canada (see Update 586). For one thing KamLAND studies anti-neutrinos rather than neutrinos. Furthermore, the production of neutrinos in a reactor is much closer at hand and better understood than is the case for the sun. The KamLAND finding also serves to narrow the theoretical explanation of the neutrino's split personality. (Eguchi et al., paper submitted to Physical Review Letters; text and background information on Stanford KamLAND page)