Prospecting for lunar Helium-3 with a radio-frequency atomic magnetometer

TL;DR

This paper addresses the challenge of prospecting for lunar He-3 by proposing a direct, in-situ measurement method using a radio-frequency atomic magnetometer to detect the dipolar magnetic field from thermally polarized He-3 spins in regolith. The approach combines regolith heating, pre-polarization in a strong field, and a compact rf-sensor in a lower holding field to enable detection of He-3 at around 5 ppb within about 5 minutes using a small, lightweight setup. The key contribution is a quantitative prospecting framework showing that a 200 g sample and a magnetometer with ~1 fT/√Hz sensitivity can achieve this detection threshold, with total apparatus mass <5 kg, volume ~1 L, and power around 1 kW. The practical impact is a low-cost, deployable instrument suitable for rover-mounted prospecting campaigns that could substantially improve the economic feasibility of lunar He-3 mining by identifying high-abundance regions.

Abstract

Mining $^3{\rm He}$ from lunar regolith has attracted significant interest in recent years due to the scarcity of $^3{\rm He}$ on Earth and its diverse applications, from cryogenics and medical imaging, to nuclear physics and future nuclear fusion. Given the stringent technical and economic challenges of mining lunar $^3{\rm He}$, precise prospecting is essential. Here we propose a prospecting methodology based on a radio-frequency atomic magnetometer, which can detect the dipolar magnetic field of thermally polarized $^3{\rm He}$ spins. With a 200 g regolith sample and an rf-magnetometer with sensitivity $1~{\rm fT/\sqrt{Hz}}$ we can detect $^3{\rm He}$ with abundance 5 ppb within a measurement time of just 5 min. The associated apparatus is lightweight and significantly more cost-effective than alternative measurement techniques. The proposed prospecting method is readily deployable and could substantially improve the technical and economic feasibility of mining lunar $^3{\rm He}$.

Figures (1)

• Figure 1: Schematic apparatus for measuring the abundance of $^3{\rm He}$ in regolith. Components are not drawn to scale. A regolith sample of mass $m_{\rm s}={\rm 200~g}$ is heated, and the extracted gases flow through a cold trap at $T_p=100~{\rm K}$. Then follows a polarizing magnetifc field $B_p=1~{\rm T}$, where $^3{\rm He}$ is thermally spin-polarized by flowing through a porous material designed to match diffusion time with longitudinal spin-relaxation time. Finally, the gas enters the measurement cell inside a smaller and homogeneous holding magnetic field $B_h=10~{\rm G}$. A free induction decay is induced, and the resulting oscillating magnetic dipolar field produced by $^3{\rm He}$ spins is detected by the radio-frequency magnetometer.