Soil moisture monitoring using cosmic ray muons

Abstract: Cosmic ray neutron flux measurements have emerged for the soil water content in many fields in recent years. According to the count of the cosmic ray neutrons above the ground scattered by the soil, the detection depth is very sensitive to the soil water content, atmospheric pressure, and hydrogen-containing substances on the surface. In this study, a novel approach was proposed to monitor the soil moisture using cosmic ray muons with a higher average energy (about 3GeV). The water content of the soil was inferred from the muon count of the detector. The depth of detection was improved to minimize the influence of surface environmental factors on the measurement, compared with the cosmic ray fast neutron. A CRY program was used to generate the energy and angular distributions of cosmic ray muons at sea level. A Monte-Carlo program (FLUKA) was selected to construct the soil model with the different moisture. The transportation process of cosmic ray muons in soil was simulated to determine the relationship between the muon count of the detector in the soil and the average water content of the soil above the detector. The soil was equally divided into the upper and lower layers with a total thickness of 80 cm in the model. The moisture of the upper layer of soil varied from 0.05 to 0.5 cm3/cm3, while the water content of the lower layer was maintained at 0.5 cm3/cm3. The variation of the upper layer's moisture was simulated with the FLUKA program. The results showed that the count of the detector was a linear function of the soil water content. The detection resolution of the soil moisture was inversely proportional to the square root of detecting time, which reached 0.1 and 0.05 cm3/cm3, when the detection time was 2 and 8 h, respectively, particularly with the placement depth of the detector of 80 cm. Furthermore, the situations of the water content of the upper soil varied from 0.1 to 0.5 with the placement depth of the detector (from 60 to 120 cm), and the sensitive radius of the soil model (from 5 to 8.5 m) were simulated with FLUKA, indicating that the detection resolution also varied in the detection time at various placement depths. The highest resolution accuracy (0.038 cm3/cm3) of soil water content was achieved at the placement depth of 90 cm under the same duration. Another identical detector was required on the soil surface to monitor the cosmic ray muon flux reaching the surface in practical measurements, in order to correct for the effect of atmospheric factors on the muon count of the detector placed in the soil. Correspondingly, an experiment was performed which suggested that the muon event count can be an indicator of the mass thickness, and the soil density. In summary, the cosmic ray muons can be widely expected to monitor the soil moisture with a moderate and flexible detection range without radioactive hazards.