Environmental Neutron Spectrometry: Continuous outdoor measurement with the PTB Bonner sphere spectrometer NEMUS-UMW

After three years, the European research project SoMMet (Soil Moisture Metrology) has come to an end. One of PTB’s (Physikalisch-Technische Bundesanstalt) tasks within this collaboration with 17 other institutes was to develop the Bonner sphere spectrometer (BSS) system NEMUS-UMW, capable of performing continuous, automated neutron spectrometry under outdoor conditions. PTB now plans to continue these activities by identifying new scientifically interesting sites for such measurements.The BSS NEMUS-UMW uses 11 proportional counters to detect the neutron component of secondary cosmic radiation. By varying the sizes (3" to 10" in diameter) of the polyethylene moderating spheres surrounding the counters, and by using copper or lead shells in the larger spheres, the system covers an energy range from 10⁻⁹ MeV to 10³ MeV. Using the known response functions of the individual spheres, the neutron energy spectrum can be unfolded. The system was calibrated in the PTB neutron reference fields and is therefore capable of determining outdoor neutron spectra and radiation levels in absolute units of neutron fluence rate.During SoMMet, the BSS NEMUS-UMW was deployed at the test field site ATB Marquardt (Potsdam, Germany). In collaboration with the University of Potsdam and Heidelberg University, surrounding field and soil parameters were monitored, and the measured neutron-spectrum time series was used to benchmark URANOS-based neutron simulations.In January 2026, the BSS NEMUS-UMW was installed on the PTB premises in Braunschweig (Germany), where it has also been used to study the impact of heavy snowfall on neutron radiation in early 2026.This presentation introduces the BSS NEMUS-UMW setup and data analysis, including corrections for environmental influences, and compares measurement results with simulations.