The Kamioka Liquid Scintillator Antineutrino Detector (KamLAND) is an electron antineutrino detector at the Kamioka Observatory, an underground neutrino detection facility in Hida, Gifu, Japan. The device is situated in a drift mine shaft in the old KamiokaNDE cavity in the Japanese Alps. Although located in the Kamioka Observatory, which is part of the University of Tokyo, this project is conducted by a team at Tohoku University. The site is surrounded by 53 Japanese commercial nuclear reactors. Nuclear reactors produce electron antineutrinos ( ν ¯ e {\displaystyle {\bar {\nu }}_{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 {\displaystyle {\bar {\nu }}_{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.
KamLAND Detector
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 (1325 17" and 554 20" PMTs). Photocathode coverage is 34%. 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 from the surrounding rock. Electron antineutrinos (νe) are detected through the Inverse beta decay reaction ν ¯ e + p → e + + n {\displaystyle {\bar {\nu }}_{e}+p\to e^{+}+n} , which has a 1.8 MeV ν ¯ e {\displaystyle {\bar {\nu }}_{e}} energy threshold. The prompt scintillation light from the positron ( e + {\displaystyle e^{+}} ) gives an estimate of the incident antineutrino energy, E ν = E p r o m p t + < E n > + 0.9 M e V {\displaystyle E_{\nu }=E_{prompt}+<E_{n}>+0.9MeV} , where E p r o m p t {\displaystyle E_{prompt}} is the prompt event energy including the positron kinetic energy and the e + e − {\displaystyle e^{+}e^{-}} annihilation energy. The quantity < E n {\displaystyle E_{n}} > 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 {\displaystyle {\bar {\nu }}_{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 over twice the size of similar detectors, such as Borexino. However, the increased volume of the detector also demands more shielding from cosmic rays, requiring the detector be placed underground. As part of the Kamland-Zen double beta decay search, a balloon of scintillator with 320 kg of dissolved xenon was suspended in the center of the detector in 2011. A cleaner rebuilt balloon is planned with additional xenon. KamLAND-PICO is a planned project that will install the PICO-LON detector in KamLand to search for dark matter. PICO-LON is a radiopure NaI(Tl) crystal that observes inelastic WIMP-nucleus scattering. Improvements to the detector are planned, adding light collecting mirrors and PMTs with higher quantum efficiency.
Results
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