Response characterization of Cosmic-Ray Neutron Sensors in neutron metrology reference fields

Abstract. Soil moisture is recognized as an Essential Climate Variable (ECV) by the World Meteorological Organization (WMO), as it plays a key role in hydrology, agriculture, and climate by regulating land–atmosphere interactions such as evapotranspiration, groundwater recharge, and surface runoff. Despite its importance, existing monitoring methods operate at different spatial and temporal scales and remain poorly harmonized, leading to inconsistencies among datasets. The SoMMet (Soil Moisture Metrology) project addresses this gap by developing robust metrological tools to ensure consistent and comparable soil moisture observations.

Within this framework, the neutron response functions of commercial Cosmic Ray Neutron Sensing (CRNS) detectors (Hydroinnova CRS1000, Hydroinnova CRS2000/B, and StyX Neutronica S1) were characterized in radionuclide-based neutron reference fields (²⁵²Cf, moderated ²⁵²Cf, and ²⁴¹Am-Be), covering the full energy range relevant to CRNS applications. The experimental campaign was carried out at two neutron metrology laboratories: PTB (Physikalisch-Technische Bundesanstalt, Germany) and CIEMAT (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas, Spain). Monte Carlo simulations of the detector response were performed using MCNP and adjusted based on experimental data obtained at both facilities.

Independent validations were conducted at PTB and CIEMAT using monoenergetic neutron reference fields (from 24 keV to 5 MeV) and radionuclide reference sources, yielding agreement within 12 % between the experimental data and the validated simulations. These results provide the first benchmark data for CRNS detector models under reference conditions, representing a key step toward establishing robust SI-traceable calibration frameworks and harmonized deployment in soil moisture monitoring applications.