"KamLAND -Neutrino View from Underground"


Kunio Inoue

Associate Professor,

Graduate School of Science, Tohoku University


Where is the best place to see an astronomical object? One would imagine it should be in space just as the Hubble space telescope views objects. Going into space eliminates most obstacles and makes a signal very pure. Digging deep underground purifies the signal in another way. Mountain and/or ground rocks work as efficient absorbers for lights and cosmic rays and finally only neutrinos remain. Neutrinos penetrate everything almost freely but at a small probability it interacts with material in the detector. Preparing big mass around the detector increases neutrino detection probability. This is why underground experiment is good for neutrino detection. Still, neutrinos rarely interact and the detector must be very clean to eliminate background from radioactive impurities. Ordinary material contains order of 1ppm Uranium and Thorium but KamLAND contains only a trillionth of it. This world's cleanest environment reveals previously unseen neutrino sources.


First observation of neutrino was carried out 11 m away from the core of the detector in the Poltergeist experiment at Savannah River in 1956. Almost 50 years later, KamLAND can now observe neutrino from 183 km away because of it's special location and environment. The obtained result of the deficit of reactor neutrinos finally resolved the long-standing solar neutrino problem. It also provided a hint of geo-neutrinos relevant to a distribution of uranium and thorium in the earth. Those radioactive nuclei are a point of interest as a major source of heat generation in the earth. It determines the dynamics of the earth. Thus, when it is clearly observed, it starts a new field of science, "neutrino geo-physics." KamLAND is also aiming at observing low energy solar neutrinos in real-time. More sophisticated purification system is required to achieve the observation and vigorous research and development are going on now. In this year after we've solved the solar neutrino problem and understood the property of neutrinos, it will be an actual start of "neutrino astronomy".



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