Despite significant progress in understanding the origin and composition of the Earth, many mysteries remain. A crucial aspect of this understanding is the accurate measurement of the planet's internal heat budget, which is driven primarily by the natural radioactive decays of potassium ( $^{40}$K) and the elements in the decay chains of uranium ($^{238}$U) and thorium ($^{232}$Th). Direct measurement of this radiogenic energy output is essential for a complete understanding of the Earth's thermal history and planetary dynamics.
Geoneutrinos, electron antineutrinos emitted in the β-decay of these radioactive isotopes within the Earth, were first proposed in the mid-twentieth century as a valuable tool for exploring its interior. $^{238}$U, $^{232}$Th and $^{40}$K release geoneutrinos and heat in a fixed proportion as they decay. Due to the weak interaction of geoneutrinos with matter, they can travel through most matter without interacting, providing valuable information about the deep interior of the Earth when detected. Measuring the geoneutrino flux at the surface allows for the estimation of the uranium, thorium and potassium content of the mantle and the radiogenic heat production within it. Unfortunately, present state-of-the-art detection techniques based on Inverse Beta Decay on free protons only permit the detection of $^{238}$U and $^{232}$Th geoneutrinos having an energy above 1.8 MeV, leaving $^{40}$K-geoneutrinos (whose endpoint is at 1.3 MeV) impossible to detect. In order to explore this low energy region of the geoneutrino spectrum, considered today as being practically impossible, detection via neutral-current interactions such as elastic scattering has been proposed, however, solar and radioactivity backgrounds remain challenges limiting its feasibility.
In this study, we propose a novel approach to detect $^{40}$K geoneutrinos, using the unique antimatter signature of antineutrinos to reduce the otherwise overwhelming background and detect this rare signal. Our approach employs the novel LiquidO detection technique, which allows identification of positron and tolerance to reduced scintillator transparency to overcome the limitations of current approaches. We present a list of suitable isotope targets together with a complete experimental methodology to achieve the first detection of potassium geoneutrinos. The scientific and technological development foreseen for detecting these low energy antineutrinos, together with their relevance in Earth sciences, makes the $^{40}$K geoneutrino measurement the “holy grail” for neutrino geoscientists, potentially providing a significant breakthrough in understanding the Earth's internal heat budget and thermal history.
