Kamioka Liquid Scintillator Antineutrino Detector - The KamLAND Detector

The KamLAND Detector

KamLAND consists of an 18 m diameter stainless steel spherical vessel with 1,879 large (50 cm diameter) 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 3.2 kiloton cylindrical water Cherenkov detector, which acts as a muon veto counter and provides shielding from cosmic rays and radioactivity in the rock.

Electron antineutrinos (ν
e) are detected via the inverse 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, E_ν = E_prompt + 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 n> is the average neutron recoil energy, which is only a few tens of keV. The neutron captures on hydrogen ~200 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 experiments. The KamLAND experiment uses a 1 kt 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.