Earth Radioactivity Measurements with a Deep Ocean Anti-neutrino Observatory

TL;DR

This paper proposes a deep-ocean anti-neutrino detector to measure Earth's internal radioactivity and test for a natural nuclear reactor, building on KamLAND's experience to optimize design and placement for sensitive measurements.

Contribution

It details the design parameters, optimal location, and required sensitivity for a new deep-ocean detector to study Earth's internal radioactivity and potential natural nuclear reactors.

Findings

A 10 kilotonne-year exposure is needed for significant measurements.

Placement at 4 km depth near Hawaii is optimal.

Radon background must be reduced by a factor of 100.

Abstract

We consider the detector size, location, depth, background, and radio-purity required of a mid-Pacific deep-ocean instrument to accomplish the twin goals of making a definitive measurement of the electron anti-neutrino flux due to uranium and thorium decays from Earth's mantle and core, and of testing the hypothesis for a natural nuclear reactor at the core of Earth. We take the experience with the KamLAND detector in Japan as our baseline for sensitivity and background estimates. We conclude that an instrument adequate to accomplish these tasks should have an exposure of at least 10 kilotonne-years (kT-y), should be placed at least at 4 km depth, may be located close to the Hawaiian Islands (no significant background from them), and should aim for KamLAND radio-purity levels, except for radon where it should be improved by a factor of at least 100. With an exposure of 10 kT-y we should achieve a 24% measurement of the U/Th content of the mantle plus core. Exposure at multiple ocean locations for testing lateral heterogeneity is possible.