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Measuring Earth's uranium and thorium with neutrinos

Takeshi Iizuka, Associate Professor, Department of Earth and Planetary Science

Determination of uranium (U) and thorium (Th), radioactive elements that are heat sources in the Earth's interior, is one of the most important issues in geoscience, which aims to elucidate the evolution of the Earth over 4.5 billion years. However, the materials available to us in the Earth's interior are limited to rocks in the crust and uppermost mantle, and it is difficult to directly measure the amount of U and Th in the Earth's interior. Therefore, the amount of U and Th in the Earth has been estimated based on the assumption that the relative concentrations of difficult-to-volatile elements such as U and Th are equal between the Earth as a whole and primitive meteorite chondrites. This "chondritic earth model" is a fundamental model of geoscience that is also used to estimate the formation age of the earth, but its validity could not be rigorously verified.

Recently, the existence of U/Th has been measured using elementary particle neutrinos (terrestrial neutrinos) emitted during radiative decay of U/Th in the earth. In 2005, the Kamioka Liquid-scintillator Anti-Neutrino Detector (KamLAND: The Kamioka Liquid-scintillator Anti-Neutrino Detector) at the Kamioka Mine was used for the first time to measure U and Th in the deep Earth. (KamLAND: The Kamioka Liquid-scintillator Anti-Neutrino Detector) at the Kamioka Mine in 2005, data have been accumulated since then, and the accuracy of the flux measurement has improved to 15%. This accuracy provides constraints on the amount of U and Th in the Earth's deep interior and enables quantitative verification of the chondritic Earth model. However, this requires that the flux of ν originating from the crust around the detector be estimated independently with the same degree of accuracy. The probability of detecting earth ν is higher the closer the detection site is, and since the crust contains higher concentrations of U and Th than the mantle, roughly half of the detected earth ν is expected to originate from the U and Th of the Japanese island arc. Therefore, to estimate the amount of U and Th in the mantle, it is necessary to estimate the crustal ν by obtaining the U and Th distribution of the Japanese island arc. Therefore, geoscientists and particle physicists are currently collaborating to address this issue by combining chemical data of crustal rocks and seismic data.

Schematic cross-section of the Earth (right) and predicted origin of Earth neutrinos observed at KamLAND

Although the crust is more accessible than the mantle, the available deep crustal rocks are limited to capture rocks (rocks that were captured by fire during the ascent of magma), and their production areas are localized. On the other hand, seismic waves propagate at different speeds depending on the rock type, which enables us to estimate the rock types constituting the Japanese island arc, but does not reflect the composition of trace elements such as U and Th. Therefore, we determine the U/Th concentration distribution of each rock type from available rocks and combine it with the spatial distribution of rock types obtained from seismic wave data to obtain the U/Th distribution and estimate the crustal ν.

The problem in carrying out this interdisciplinary research between geoscience and particle physics is how to add quantitative errors to the estimation of crustal ν. Often in geoscience, only the most plausible models are presented, such as the chondritic earth model, without quantifying their uncertainties. In geophysics, on the other hand, the range of possible models is often presented quantitatively in the form of a probability density function. For quantitative error estimation of crustal ν, it is necessary to describe the Japanese island arc U-Th distribution model in terms of a probability density function, but the actual distribution of rocks of different compositions is extremely complex, making the formulation of such a model difficult. Once this problem is solved, quantitative constraints on the Earth's thermal history and chemical composition can be provided with unprecedented precision.

Published in Faculty of Science News, July 2022

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