Kamioka Liquid Scintillator Antineutrino Detector

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_e) in the decays 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 as 1/R² for increasing distance R from the reactor. The experiment is sensitive to the estimated ~25% of antineutrinos from nuclear reactors that exceed the threshold energy of 1.8 MeV and thus produce a signal in the detector.

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 1879 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 tons 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_e) are detected via the inverse β-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, E_ν = E_prompt + <E_n> + 0.8 MeV, where E_prompt is the prompt event energy including the positron kinetic energy and the e⁺e^- annihilation energy. The quantity <E_n> is the average neutron recoil energy, which is only a few tens of keV. The neutron captures on hydrogen ~200μs 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 experiments. The KamLAND experiment uses a 1 kton detection mass, two orders of magnitude bigger than the previous largest experiment. However, the increased volume of the detector also demands more shielding from cosmic rays, which effectively means that the detector has to be placed underground.

Recent results

tudying 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 recently confirmed this result with a 515 day data sample (Araki "et al.", 2005), when 365.2±23.7(syst) 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 $Delta\; m^2=7.9^\{+0.6\}\_\{-0.5\}\; imes10^\{-5\}eV^2$ and $an^2\; heta=0.40^\{+0.10\}\_\{-0.07\}$, the best solar neutrino oscillation parameter determination to date.

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)

References

* arxiv|id=0801.4589

* [http://www.nature.com/nature/journal/v436/n7050/pdf/nature03980.pdf Full text at Nature]

* arxiv|archive=hep-ex|id=0406035

* arxiv|archive=hep-ex|id=0310047

* arxiv|archive=hep-ex|id=0212021

External links

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