Cost-effective Thermal Neutron Proportional Counter (TNPC) for space weather monitoring and soil moisture sensing

In recent years, Cosmic Ray Neutron Monitors (CRNMs) have gained significant attention for their dual applications in space weather monitoring and soil moisture sensing. However, conventional CRNM designs rely on expensive and limited ³He gas or highly toxic BF₃ gas, making them challenging to manufacture and deploy. Furthermore, the geometry and dimensions of the conventional CRNMs have not been systematically optimized for either application. This research presents a safe and cost-effective Thermal Neutron Proportional Counter (TNPC) developed by the Radiation Physics Laboratory at Oklahoma State University. The TNPC features a boron-lined proportional counter that uses natural boron (20% ¹⁰B) instead of the more commonly used yet expensive isotopically enriched boron (>90% ¹⁰B). The TNPC uses commercially available materials and a simple manufacturing approach to lower costs while maintaining adequate thermal neutron detection efficiency. Extensive FLUKA simulations (a computer-based Monte Carlo particle transport code) were conducted to optimize the design of the neutron thermalizer employed in the TNPC and evaluate its performance. These simulation results led to a modular neutron thermalizer design that allows the TNPC to be adaptable to detect high-energy secondary neutrons for space weather monitoring and epithermal secondary neutrons for soil moisture sensing. In addition to its core neutron detection function, the TNPC incorporates a muon counter, which provides an in-situ data correction and normalization method for both applications by utilizing the correlation between ground level cosmic-ray-induced neutron and muon fluxes. Multiple prototypes were manufactured, tested, and deployed in real-world environments for space weather monitoring and soil moisture sensing. Preliminary data from laboratory and field tests validate the TNPC’s operational stability and adaptability, attesting to its potential as a cost-effective solution for its widespread deployment in these applications.