KamLAND-Zen Started Zero-Neutrino Double-Beta Decay Search

Thursday, December 15, 2011

Submitted by Prof. Werner Tornow After discovering reactor neutrino oscillations, determining the heat produced by radioactive decays in the interior of the Earth, determining the extraterrestrial high-energy anti-neutrino flux on Earth, and studying solar neutrinos with the KamLAND detector, on October 12, 2011 a subgroup of the original Japanese-US KamLAND collaboration commenced its search for the zero-neutrino double-beta decay of 136Xe. For this purpose, a 3.4 m diameter nylon balloon filled with about 400 kg of 136Xe absorbed in scintillator fluid and viewed by about 2000 phototubes was immersed in September into the original 1 kilo-ton KamLAND liquid scintillator detector (see Figure). Discovering zero-neutrino double-beta decay would allow the determination of the electron neutrino mass (known to be below about 300 meV) and would prove whether neutrinos are Dirac particles or Majorana particles, i.e., whether they are identical to their anti-particles.

The KamLAND detector is located in the Kamioka mine in the Japanese Alpes in the cave vacated by the Kamiokande collaboration once Super-Kamiokande became operational. A TUNL group, under Prof. Werner Tornow’s leadership,  has built the outer detector (also called veto detector) of KamLAND in the years 1999 - 2001. Even before then, Werner Tornow, together with Ludwig De Braeckeleer (at that time at Duke) and Giorgio Gratta from Stanford were responsible for the double-beta decay section of the original US-KamLAND proposal. Werner Tornow, now a Professor Emeritus, was on a week-long KamLAND remote shift (i.e., monitoring the experiment remotely from his office during the Japanese night) just before the official start of this double beta-decay search.  This new important phase of Kamland is called "KamLAND-Zen" . After two years of running, the collaboration hopes to reach a sensitivity to the neutrino mass of 80 meV. Future plans for after 2013 call for increasing the 136Xe mass to 1000 kg and for replacing the liquid scintillator fluid with a “brighter” scintillator which provides a better energy resolution. This will help to distinguish the tail of the abundant two-neutrino double-beta decay energy spectrum from the potential zero-neutrino events of interest.

Photo: KamLAND Zen balloon