EAPSI: Understanding the Composition of the Earth by Charactering the Radioactivity of Rocks

Wipperfurth Scott A, Greenbelt MD

Investigators

Abstract

The KamLAND detector in central Japan is the longest running and largest detector measuring neutrinos emitted from the Earth. Neutrinos, or geoneutrinos when from the Earth, are incredibly small, chargeless particles emitted from natural radioactivity within rocks. These particles have a very small chance of hitting anything, and can thus travel easily from anywhere inside the Earth to the surface. Counting the number of geoneutrinos passing through the KamLAND detector allows us to know the supply of radioactivity within the Earth, even though we cannot access rocks from the deep Earth. This project aims to investigate the rocks closest to the detector, as the geoneutrinos emitted by these rocks have a larger effect on the measured signal vs those farther away. The estimated signal from Japanese rocks in this study will be subtracted from the measurement by KamLAND, leaving only the signal from the inaccessible deep Earth. The final signal is indicative of the amount of radioactivity within the Earth. This work is being conducted in conjunction with Professor Kunio Inoue at Tohoku University, who is the Director of the Research Center of Neutrino Sciences at Tohoku University and principal investigator of KamLAND. The radiogenic power of the Earth is not well constrained, with estimates ranging from low (10 TW) to high (30 TW) heat production. Measurement of the flux of geoneutrinos from the Earth thus acts as a proxy for the heat production within the Earth. KamLAND (Japan) and Borexino (Italy) are the only two operating detectors. Determining the mantle signal, an important objective, requires subtraction of the crustal flux from the measured flux at these detectors. Unfortunately, discrepancy exists between model estimates of the crustal flux at KamLAND. This project will create a high-resolution, 3D model of the nearest 300 km of crust surrounding KamLAND (est. ~ 50% of total signal). All available geochemical (U and Th) and geophysical (seismic, cross-sections, borehole, heat flow, and gravity) data will be integrated into a single coherent and self-consistent model. Consolidation of units defined on a geologic map of Japan will provide simplified reservoirs, which can be defined 3-dimensionally by the assembled geophysical data in Central Japan. To these reservoirs we will characterized the U and Th concentrations, and their uncertainties, from the assembled geochemical data. This 3D model, with defined physical structure and radioactive element concentration, will be used to calculate a new near-field, crustal geoneutrino signal at KamLAND. This award under the East Asia and Pacific Summer Institutes program supports summer research by a U.S. graduate student and is jointly funded by NSF and the Japan Society for the Promotion of Science.