The SoMMet characterization of a Finapp Cosmic-Ray Neutron Sensor and its first real-world application

Cosmic Ray Neutron Sensing (CRNS) has been established as a reliable method for measuring Soil Moisture (SM) at an intermediate spatial scale, bridging the gap between point-scale measurements and satellite observations. While CRNS stations are increasingly included in meteorological and environmental monitoring networks, integration and intercomparison between different methods remain tricky.

Different technologies not only explore different scales of observations, but they do that through different physical methods, with possibly different responses to the same event. CRNS relies on the correlation of SM with the count of environmental neutrons, generated by cosmic rays and absorbed by hydrogen in water. While a standard conversion formula is widely used, it’s known to significantly deviate from experimental validation under extreme conditions of either dryness or wetness. For this reason, new formulas have been proposed and are in a phase of validation.

The SoMMet (Soil Moisture Metrology) project, funded by EURAMET (European Partnership on Metrology), was set up to develop metrological tools to enhance traceability and harmonization across different methods of SM observation. As part of the SoMMet project activities, various commercial CRNS probes were tested in SI-traceable reference neutron fields at participating national metrology institutes. The understanding of detector performance under laboratory conditions and the validation of Monte Carlo (MC) neutron transport modelling can be used to predict the detector response under real field conditions.

The development and validation of the specific MC model for the CRNS detector manufactured by Finapp has been recently published by the SoMMet Collaboration [1] and it introduces a new conversion formula. We will here review the SoMMet activities on characterization and MC model validation of the Finapp CRNS probe, performed in the reference neutron fields of Czech Metrology Institute (CMI) and Slovak Institute of Metrology (SMU) and consequent model verification at the Physikalisch-Technische Bundesanstalt (PTB), Germany.

As a first application to real-world conditions, we apply the SoMMet conversion formula to the datasets of two automated snow stations managed by the Office for Hydrology and Dams of the Civil Protection Agency of the Autonomous Province of Bolzano, Italy, equipped with Finapp CRNS sensors. The two sites (Pian dei Cavalli and Malga Fadner) are mountain sites at elevations above 2000 m, characterized by a very low soil bulk density and a very high water content, with presence of peatland in the footprint. The CRNS measurement was calibrated by the standard gravimetric campaign, but the standard conversion formula provides physically unrealistic results. The formula proposed by SoMMet is successfully applied.

[1] Z. Vykydal et al. (2025), Monte Carlo Simulation and Experimental Validation of the Finapp Model 3 Cosmic-Ray Neutron Sensor. Meas. Sci. Technol., in press, DOI:10.1088/1361-6501/ae2649

Aknowledgments: The project 21GRD08 SoMMet received funding from the European Partnership on Metrology, co-financed from the European Union’s Horizon Europe Research and Innovation Programme and by the Participating States.