About the Detector 4

Between the stainless steel spherical tank and the detector cavity, 3000m^3 (3000ton) of ultrapure water is filled and the spherical tank sinks in it completely. Also, the ultrapure water is surrounded with 255 20-inch PMTs and serves two important functions. One is to detect cosmic-ray muons traversing it. Even under the rock overburden of 1000-meter, cosmic-ray muons traverse the detector volume the average of once every 3 seconds. Cosmic rays traversing the detector produce Cherenkov light in the ultrapure water outside of the spherical tank. Cosmic-ray muons can be detected by recording the light with the PMTs in the water. Signal events during a certain period after cosmic-ray muons traverse the detector are rejected, because they might be induced by muons.

Another role of the ultrapure water layer is absorbing the radioactivity coming from the walls and moderating the fast neutrons produced by cosmic rays in the rock. The uranium concentration in the rock or dirt is about 1ppm(ppm = parts per million = 0.0001%). An atom of uranium gradually decays emitting gamma, beta or alpha rays. Since Gamma rays have high penetration power, there is a possibility that they pass through the wall of the stainless steel spherical tank and enter into the central liquid scintillator. In this case, gamma rays interact with the scintillator and would make false neutrino signals. A 1-kg rock generates approximately 10 gamma rays per second. Therefore, several ten million of gamma rays are generated per second in the rock surrounding the detector cavity. Since most of these gamma rays are, however, absorbed in the ultrapure water between the cavity walls and the spherical tank, and the buffer oil in it, gamma rays reaching the balloon are sufficiently suppressed. The KamLAND detector consists of a series of concentric spherical shells and external backgrounds progressively diminish towards the inner side.   Next

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