Kamioka Liquid Scintillator Antineutrino Detector
Coordinates: The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an experimental device that was built at the Kamioka Observatory, an underground neutrino detection facility near Toyama, Japan. Its purpose is to detect electron antineutrinos. The device is situated in a drift mine shaft in the old KamiokaNDE cavity in the Japanese Alps. The site is surrounded by 53 Japanese commercial nuclear reactors. Nuclear reactors produce electron antineutrinos (ν
e) during the decay of radioactive fission products in the nuclear fuel. Like the intensity of light from a light bulb or a distant star, the isotropically-emitted ν
e flux decreases at 1/R2 per increasing distance R from the reactor. The device is sensitive up to an estimated 25% of antineutrinos from nuclear reactors that exceed the threshold energy of 1.8 megaelectronvolts (MeV) and thus produces a signal in the detector.
If neutrinos have mass, they may oscillate into flavors that an experiment may not detect, leading to a further dimming, or "disappearance," of the electron antineutrinos. KamLAND is located at an average flux-weighted distance of approximately 180 kilometers from the reactors, which makes it sensitive to the mixing of neutrinos associated with large mixing angle (LMA) solutions to the solar neutrino problem.
|This section does not cite any references or sources. (May 2014)|
The KamLAND detector's outer layer consists of an 18 meter-diameter stainless steel containment vessel with an inner lining of 1,879 photo-multiplier tubes, each 50 centimeters in diameter. Its second, inner layer consists of a 13 m-diameter nylon balloon filled with a liquid scintillator composed of 1,000 metric tons of mineral oil, benzene, and fluorescent chemicals. Non-scintillating, highly-purified oil provides buoyancy for the balloon and acts as a buffer to keep the balloon away from the photo-multiplier tubes; the oil also shields against external radiation. A 3.2 kiloton* cylindrical water Cherenkov detector surrounds the containment vessel, acting as a muon veto counter and providing shielding from cosmic rays and radioactivity.
Electron antineutrinos (ν
e) are detected through the beta decay reaction (ν
e + p → e+ + n ), which has a 1.8 MeV ν
e energy threshold. The prompt scintillation light from the positron (e+) gives an estimate of the incident antineutrino energy, Eν = Eprompt + <En> + 0.9 MeV, where Eprompt is the prompt event energy including the positron kinetic energy and the e+–e− annihilation energy. The quantity <En> is the average neutron recoil energy, which is only a few tens of kiloelectronvolts (keV). The neutron is captured on hydrogen approximately 200 microseconds (μs) later, emitting a characteristic 2.2 MeV γ ray. This delayed-coincidence signature is a very powerful tool for distinguishing antineutrinos from backgrounds produced by other particles.
To compensate for the loss in ν
e flux due to the long baseline, KamLAND has a much larger detection volume compared to earlier devices. The KamLAND detector uses a 1,000-metric-ton detection mass, which is two orders of magnitude larger than the previous largest experimental device. However, the increased volume of the detector also demands more shielding from cosmic rays, which explains why the detector must be placed underground.
Studying neutrino oscillation
KamLAND started to collect data on January 17, 2002. The first results were reported using only 145 days of data. Without neutrino oscillation, 86.8±5.6 events were expected, 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, when 365.2±23.7 events were expected in the absence of oscillation, but only 258 events were observed (with 17.8±7.3 background events). These results establish 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 neutrino oscillation and different oscillation hypotheses. Statistical analyses show that the distortion of the spectrum is inconsistent with the no-oscillation hypothesis and with two alternative neutrino-disappearance mechanisms, namely the neutrino decay and de-coherence models. However, the spectrum is consistent with neutrino oscillation and a fit provides the values for the Δm2 and θ parameters. Given that 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
−0.5×10−5 eV2 and tan2θ = 0.40+0.10
−0.07 , the best solar neutrino oscillation parameter determination to date.
Geologically produced antineutrinos
KamLAND also published an investigation of geologically-produced antineutrinos (so-called geoneutrinos). These neutrinos are produced in the decay of thorium and uranium in the Earth's crust and mantle.
- Eguchi K, et al. [KamLAND Collaboration] (2003). "First results from KamLAND: evidence for reactor antineutrino disappearance". Physical Review Letters 90 (2): 021802–021807. arXiv:hep-ex/0212021. Bibcode:2003PhRvL..90b1802E. doi:10.1103/PhysRevLett.90.021802. PMID 12570536.
- Araki T, et al. [KamLAND Collaboration] (2005). "Measurement of neutrino oscillation with KamLAND: evidence of spectral distortion". Physical Review Letters 94 (8): 081801–081806. arXiv:hep-ex/0406035. Bibcode:2005PhRvL..94h1801A. doi:10.1103/PhysRevLett.94.081801. PMID 15783875.
- Araki T, et al. [KamLAND Collaboration] (2005). "Experimental investigation of geologically produced antineutrinos with KamLAND". Nature 436 (7050): 499–503. Bibcode:2005Natur.436..499A. doi:10.1038/nature03980. PMID 16049478.
- Abe S, et al. [KamLAND Collaboration] (2008). "Precision Measurement of Neutrino Oscillation Parameters with KamLAND". Physical Review Letters 100 (22): 221803. arXiv:0801.4589. Bibcode:2008PhRvL.100v1803A. doi:10.1103/PhysRevLett.100.221803. PMID 18643415.
- Eguchi K, et al. [KamLAND Collaboration] (2004). "High sensitivity search for ν
e's from the sun and other sources at KamLAND". Physical Review Letters 92 (7): 071301–071305. arXiv:hep-ex/0310047. Bibcode:2004PhRvL..92g1301E. doi:10.1103/PhysRevLett.92.071301.