The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an experiment at the Kamioka Observatory, an underground neutrino observatory near Toyama, Japan. It was built to detect electron antineutrinos. The experiment is situated in the old Kamiokande cavity in a horizontal mine drift in the Japanese Alps. The site is surrounded by 53 Japanese commercial power reactors. Nuclear reactors produce electron antineutrinos (ν
If neutrinos have mass, they may "oscillate" into flavors that an experiment may not be able to detect, leading to a further dimming, or "disappearance", of the electron antineutrinos (see neutrino oscillation). KamLAND is at a flux weighted average distance of ~180 km from the reactors which makes the experiment sensitive to the neutrino mixing associated with the large mixing angle (LMA) solution to the solar neutrino problem.
The KamLAND detector
KamLAND consists of an 18 m diameter stainless steel spherical vessel with 1,879 photomultiplier tubes mounted on the inner surface. Inside the sphere is a 13 m diameter nylon balloon filled with liquid scintillator. The scintillator consists of 1,000 metric ton of mineral oil, benzene and fluorescent chemicals. Outside of the balloon, non-scintillating, highly purified oil provides buoyancy for the balloon and acts as a shield against external radiation. Surrounding the stainless steel vessel is a water Cherenkov detector, which acts as a muon veto counter and provides shielding from radioactivity in the rock.
Electron antineutrinos (ν
To compensate for the loss in ν
Studying neutrino oscillation
KamLAND started data taking in January 2002, and with only 145 days of data, reported its first results (Eguchi et al., 2003). Without neutrino oscillation, the experiment expected to see 86.8±5.6 events, with 2.8 background events after all event cuts. However, only 54 events were observed. KamLAND confirmed this result with a 515 day data sample (Araki et al., 2005), when 365.2±23.7 events were expected in the absence of oscillation, while 258 events were observed (with 17.8±7.3 background events). This establishes antineutrino disappearance at the 99.998% significance level.
The KamLAND detector not only measures the total number of antineutrinos, but also measures their energy. The shape of this spectrum carries additional information that can be used to investigate the neutrino oscillation. Different oscillation hypotheses are investigated by fitting them to the data. Statistical tests show that the distortion of the spectrum is inconsistent with the no-oscillation hypothesis and is also inconsistent with two alternative neutrino disappearance mechanisms, namely the neutrino decay and decoherence models. However, the spectrum is consistent with neutrino oscillation and a fit provides the values for the Δm2 and θ parameters. Since KamLAND measures Δm2 most precisely and the solar experiments exceed KamLAND's ability to measure θ, the most precise oscillation parameters are obtained by combining the results from solar experiments and KamLAND. Such a combined fit gives Δm2 = 7.9+0.6
Geologically produced antineutrinos
KamLAND also published a recent investigation of geologically produced antineutrinos (so-called geo-neutrinos). These neutrinos are produced in the decay of thorium and uranium in the Earth's crust and mantle. (Araki et al., 2005)
Abe S, et al. [KamLAND Collaboration] (2008). "Precision Measurement of Neutrino Oscillation Parameters with KamLAND". Physical Review Letters 100 (22): 221803. Bibcode 2008PhRvL.100v1803A. doi:10.1103/PhysRevLett.100.221803. PMID 18643415. arXiv:0801.4589