A geoneutrino experiment at Homestake - eScholarship

A N. Tolich, geoneutrino experiment at Homestake Y.-D. Chan, C.A. Currat, M.P. Decowski, B.K. Fujikawa, and J. Wang R. Henning, K.T. Lesko, A.W.P. Poon, K. Tolich, Nuclear Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA Physics Department, University of California at Berkeley, Berkeley, California 94720, USA Department of Physics, Stanford University, Stanford, California 94305, USA ^Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA (Dated: June 1, 2006) A significant fraction of the 44TW of heat dissipation from the Earth's interior is believed to originate from the decays of terrestrial uranium and thorium. The only estimates of this radiogenic heat, which is the driving force for mantle convection, come from Earth models based on meteorites, and have large systematic errors. The detection of electron antineutrinos produced by these uranium and thorium decays would allow a more direct measure of the total uranium and thorium content, and hence radiogenic heat production in the Earth. We discuss the prospect of building an electron antineutrino detector approximately 700 m in size in the Homestake mine at the 4850' level. This would allow us to make a measurement of the total uranium and thorium content with a statistical error less than the systematic error from our current knowledge of neutrino oscillation parameters. It would also allow us to test the hypothesis of a naturally occurring nuclear reactor at the center of the Earth. I. INTRODUCTION Thanks in part to Ray Davis' pioneering neutrino experiment[1] located in the Homestake mine, more is now known about the interior workings of the Sun than the Earth. The KamLAND collaboration has recently investigated electron antineutrinos originating from the interior of the Earth[2]; however, the sensitivity achieved was limited by a large background from surrounding nuclear power reactors. A similar experiment located deep underground to reduce cosmic-ray backgrounds, and away from nuclear power plants, could reach a sen­ sitivity that would allow constraints to be placed on our current knowledge of the Earth's interior. The idea of using electron antineutrinos, zVs, to study processes inside the Earth was first suggested by Eder[3] and Marx[4]. U, Th, and K decays within the Earth are believed to be responsible for the majority of the current radiogenic heat production, which is the driv­ ing force for Earth mantle convection, the process which causes plate tectonics and earthquakes. These decays also produce zVs, the vast majority of which reach the Earth's surface since neutrinos hardly interact with mat­ ter, allowing a direct measurement of the total Earth radiogenic heat production by these isotopes. TABLE I: Estimated concentration and mass of U, Th, and K in the major Earth regions. It is assumed that there is no U, Th, or K in the Earth's core. The concentration of radiogenic elements in the mantle is obtained by subtracting the isotope mass in the crust from the Bulk Silicate Earth (BSE) model. The masses are obtained from Schubert et al.[8]. Region Total mass Oceanic crust [10] Continental crust [11] Mantle BSE[6] Concentration kg] U[ppb] Th[ppb] K[ppm] ments. Table I shows the estimated concentration of U, Th, and K in the different Earth regions. Models of Earth composition based on the solar abun­ dance data[6] establish the composition of the undifferen­ tiated mantle in the early formation stage of the Earth, referred to as Bulk Silicate Earth (BSE). Table I in­ cludes the estimated concentration of U, Th, and K in the BSE model. The ratio of Th/U by weight, between 3.7 and 4.1[7], is known better than the total abundance of each element. The rate of radiogenic heat released from U, Th, and K decays are 98.1//Wkg , 26.4//Wkg , and 0.0035//Wkg ^], respectively. Table II summarizes the total radiogenic heat production rate in the Earth regions based on the mass and concentrations of these elements given in Table I. For comparison, the rate of mantle heating due to lunar tides is a negligible ~ 0.12 TW[9]. The regional composition of the Earth is determined by a number of different methods. The deepest hole ever dug penetrates 12 km of the crust [5], allowing direct sam­ pling from only a small fraction of the Earth. Lava flows bring xenoliths, foreign crystals in igneous rock, from the upper mantle to the surface. The regional composition of the Earth can also be modeled by comparing physical properties determined from seismic data to laboratory measurements. Our current knowledge suggests that the crust and mantle are composed mainly of silica, with the crust enriched in U, Th, and K. The core is composed mainly of Fe but includes a small fraction of lighter ele- The radiogenic heat production within the Earth can be compared to the measured heat dissipation rate at the surface. Based on the rock conductivity and temperature gradient in bore holes measured at 20,201 sites, the esti-