Improving neutron spectrometry measurement methodology to better understand soil moisture variability: application to an area subject to strong seasonal and daily variations

Secondary cosmic rays (CRs) are produced when primary CRs interact with atmospheric atoms, leading 
to the formation of a cascade of secondary particles such as neutrons, pions, protons, and muons, with 
energies ranging from a few dozen meV to over 1 GeV. Neutrons produced during the extensive air 
shower spreading is characterized by a high elastic scattering cross section with hydrogen nuclei. This 
latter effectively moderates neutrons by slowing them down, and composes different media in the 
atmosphere, such as water vapor, ice and liquid vapor. 
Neutron spectrometry is based on this singular ability of hydrogen to moderate neutrons. In addition of 
interacting with the atmosphere, cosmic neutrons also interact with the Earth’s surface. Some of them 
are scattered back to the surface and are referred to as albedo neutrons. This phenomenon is crucial for 
studying soil moisture with a Bonner sphere spectrometer. Indeed, previous studies on both neutrons 
monitors and Bonner spheres spectrometers highlighted the impact of soil water content on neutron fluxes, 
validating the use of these methods to monitor soil moisture. However, it has been established that 
atmospheric water vapor induces a significant decrease in neutron counts that requires consideration. 
For this study, an experimental platform was deployed at the Atmospheric Research Center in 
Lannemezan, France. This platform includes instruments monitoring the atmospheric column 
hygrometry (precipitations, mixing ratio) and pressure -provided by a 60 m high mast- and soil moisture 
variations measured by refractometric probes in a 120 cm depth pit. In addition, a BSS extended to high 
neutron energies is constantly measuring the neutronic natural environment near the pit and mast since 
September 2023. The Bonner sphere spectrometer consists of three high-density polyethylene spheres 
(3, 5, and 8 inches) and two polyethylene spheres with inner high-density metal shells (8 and 9 inches), 
each equipped with a 2-inch proportional counter. This instrument provides a valuable information about 
the detected neutrons by allowing the reconstruction of the full spectrum, from meV to GeV. Thus, this 
approach enables the study of the impact of different hydrogen pools across the four main energy 
domains (thermal, epithermal, evaporation, and cascade neutrons). 
To complement these experimental data, a simulation work was necessary. The URANOS (Ultra Rapid 
Neutron Only Simulation) code has been a reference for several years in the field of simulating the 
transport of atmospheric neutrons in the atmosphere and soils. It is based on the application of the Monte 
Carlo method, and allows to calculate physical quantities such as energy distribution, spatial distribution, 
and neutron interaction processes. To meet more accurately the needs of this study, a module 
specifically designed for Bonner Spheres has been developed, providing key information on the impact 
of the atmosphere on neutron counts measured by each sphere.  
In this study, we apply a new methodology to a set of experimental time series in order to reduce the 
impact of the atmosphere on neutron counts from the Bonner sphere spectrometer. We will finally 
compare the results to the same uncorrected time series.