Experimental Study of Geoneutrinos with KamLAND

Physics

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8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8130 Heat Generation And Transport, 9800 General Or Miscellaneous

Scientific paper

It is widely accepted that radiogenic heat contributes a large part to the Earth's heat budget. Due to the direct relation between the number of radioactive decays in the Earth and the neutrino flux from the decays, neutrinos are expected to provide fruitful information on Earth's energetics. The Kamioka liquid scintillator anti-neutrino detector (KamLAND), which consists of 1000 tones of ultra-pure liquid scintillator surrounded by 1879 photo- multiplier tubes (PMT), is the first detector sensitive enough to detect geoneutrinos. Earth models [1] suggest that KamLAND observes geoneutrinos at a rate of 30 events/1032-protons/year from the 238U decay chain, and 8 events/1032-protons/year from the 232Th decay chain. With 7.09×1031 proton- years of detector exposure and detection efficiency of 0.687±0.007, the 'rate-only' analysis gives 25+19-18 geoneutrino candidates [2]. Assuming a Th/U mass concentration ratio of 3.9, the 'rate + shape' analysis gives the 90% confidence interval for the total number of geoneutrinos detected to be from 4.5 to 54.2. This result is consistent with predictions from the Earth models. The 99% C.L. upper limit is set at 1.45×10-31 events per target proton per year, which is 3.8 times higher than the central value of the model prediction that gives 16 TW of radiogenic heat production from 238U and 232Th. Although the present data have limited statistical power, they provide by direct means an upper limit for the Earth's radiogenic heat of U and Th. [1] S. Enomoto et al., Earth Planet. Sci. Lett., Submitted, 2006; [2] T. Araki et al. (KamLAND Collaboration), Nature, 436, 2005.

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