Enhanced water Cherenkov detector for soil moisture detection

Abstract: This work evaluates the ability of a water Cherenkov detector to measure thermal neutrons and explores its application to soil-moisture monitoring. We study a NaCl-doped detector and model its response to (i) monochromatic thermal neutrons and (ii) the natural thermal-neutron flux expected from dry soil at the elevation of Bucaramanga, Colombia. The ambient flux is estimated with the ARTI framework, and the detector response is simulated with MEIGA in Geant4. Doping with NaCl introduces additional capture channels on $^{35,37}\mathrm{Cl}$ and $^{23}\mathrm{Na}$; in particular, $^{35}\mathrm{Cl}$ has a thermal-neutron absorption cross section up to two orders of magnitude larger than hydrogen, boosting the capture signal. Our results indicate that water Cherenkov detectors can detect thermal neutrons with practical sensitivity under field conditions, enabling their integration into precision agriculture networks for soil moisture sensing. More broadly, this approach extends the cosmic-ray detection range of Cherenkov detectors using non-toxic, low-cost materials.