KamLAND

Objective

Detect electron antineutrinos from Japanese nuclear reactors at an average baseline of 180 km, probing the solar-sector oscillation parameters with a terrestrial source.

Method

1 kiloton of linear-alkylbenzene liquid scintillator in a 13 m transparent nylon balloon, surrounded by non-loaded buffer oil and an outer water Cherenkov veto. Inverse beta decay on hydrogen gives a prompt positron signal followed by a delayed 2.2 MeV gamma from neutron capture.

Key results

• 2002: first observation of reactor antineutrino disappearance consistent with solar-sector oscillation parameters.
• 2003: confirmation of the MSW-LMA oscillation solution by terrestrial measurement, selecting the parameter region that solar neutrino experiments had pointed to.
• 2005: first detection of geoneutrinos from U and Th decay chains in the Earth's crust and mantle.
• Ongoing precision measurements of Δm²₂₁ and θ₁₂, dominating the world-average values.

Significance

Established the terrestrial complement of solar neutrino measurements, confirming oscillation as the explanation for the solar neutrino problem and fixing the parameters with high precision. KamLAND also opened the field of geoneutrino observation, providing a direct probe of radiogenic heat in the Earth.

A detector with dual purpose

KamLAND — the Kamioka Liquid-scintillator Antineutrino Detector — occupies the cavern that previously housed the original Kamiokande experiment. A 13 m transparent nylon balloon holds 1 kt of linear-alkylbenzene liquid scintillator loaded with the fluor PPO, giving ~250 photoelectrons per MeV and allowing a sub-MeV detection threshold. The balloon is suspended in an outer volume of buffer oil, itself contained in an 18 m stainless-steel sphere instrumented with 1,879 photomultiplier tubes for 34% photo-coverage. An outer 3.2 kt water Cherenkov detector provides cosmic-ray veto.

Japanese commercial reactors contribute approximately 80% of the unoscillated antineutrino flux, distributed across the home islands with a population-weighted mean baseline of ~180 km.

The reactor oscillation

At 180 km, reactor antineutrinos at the few-MeV scale have traversed about one full solar-frequency oscillation length. The ${\bar{\nu }}_e$ survival probability develops a clear minimum and partial recovery in the observable energy range, producing a characteristic spectral modulation.

KamLAND’s 2003 first result (see the paper) reported a 38% reduction in the event rate — incompatible with no oscillation at 99.95% confidence — and a matching spectral distortion. Combined with solar data, the fit selected the MSW-LMA region with $\Delta m_{21}^2\sim 7\times 10^{-5}{\text{ eV}}^2,{\tan }^2{\theta }_{12}\sim 0.4$

Geoneutrinos

Beyond reactor antineutrinos, KamLAND made the first detection of geoneutrinos in 2005 — antineutrinos from the beta decay of ${}^{238}$U and ${}^{232}$Th chains in the Earth’s crust and mantle. The measurement constrains the radiogenic contribution to Earth’s heat flow and, combined with Borexino, supports the bulk-silicate-Earth model.

Later phases and future

After the Fukushima nuclear accident in 2011, Japanese reactor power fell dramatically and has recovered only partially. KamLAND’s reactor flux is now substantially below the pre-2011 level, but the detector has repositioned itself around geoneutrinos, solar neutrinos (through the KamLAND-Zen double-beta-decay program), and low-energy astrophysical physics.

KamLAND-Zen loads the inner balloon with xenon dissolved in the scintillator and searches for neutrinoless double beta decay in ${}^{136}$Xe. Current limits on the half-life exceed $2\times 10^{26}$ years, corresponding to effective Majorana mass limits of ~40 meV.

Significance

KamLAND is the prototype for JUNO’s much larger liquid-scintillator approach. Its technical lessons — scintillator purity, photon statistics, energy calibration, background control — directly informed the JUNO design specifications. The experiment continues to run in its KamLAND-Zen configuration and remains a central instrument for low-energy neutrino physics.