Title: Experimental Study of Geoneutrinos with KamLAND
Sanshiro Enomoto, for the KamLAND Collaboration
Research Center for Neutrino Science, Tohoku University
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 suggest that KamLAND observes
geoneutrinos at a rate of 30 events/10^32-protons/year from the 238U decay
chain, and 8 events/10^32-protons/year from the 232Th decay chain. With
7.09×10^31 proton-years of detector exposure and detection efficiency of
0.687±0.007, the 'rate-only' analysis gives geoneutrino candidates.
Assuming a Th/U mass concentration ratio of 3.9, the 'rate + shape'
analysis gives the 90 per cent 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.