Contextual detection of muons and neutrons as a way to self-referenced Cosmic Ray Neutron Sensing

As Cosmic Ray Neutron Sensing (CRNS) gains increasing importance in Hydrology and other fields related to measurement of water in various form (notably Soil Moisture, Snow Water Equivalent, Biomass Water Equivalent), its large-scale application risks to be hindered by the complex calibration of site-specific parameters.

The success of CRNS comes from its ability to straightforwardly determine the amount of water content in soil and snow, overcoming limitations in scale representativity of more traditional technologies like point probes and satellites. A single, autonomous probe placed over the ground is in fact capable of delivering a real-time estimation of the Soil Moisture (SM) on a large scale (hectares) and in depth (tens of cm in soil, meters in snow).

CRNS is based on the fact that neutrons strongly interact with hydrogen, of which water is rich, therefore providing a correlation between the number of neutrons backscattered by the soil and the water content. The neutrons source is provided by the interaction of cosmic radiations with the atmosphere. Advantageous as it is to use a naturally available source, it comes with the drawback of having a natural variability that needs to be account for, which is usually done by refferring to a public network of cosmic neutrons observatories (the Neutron Monitor DataBase, NMDB). A second critical hurdle is calibration, requiring the determination of a site-specific reference value for the neutrons count rate through a complex campaign of soil sampling and analysis.

Finapp developed a patented detection technology characterized by the unique capability of contextually detecting muons, another kind of particle generated by cosmic rays. Since muons are not backscattered by the soil, their measurement is only related to the incoming flux. Years of measurements provide evidence of the correlation of local muons flux variations with the general trend of incoming neutrons as measured by NMDB stations.

Not only this offers the possibility to monitor the incoming flux variations locally without the need to rely on an external entity, but it furthermore suggests that the expected reference neutrons rate on a given site should be proportional to the measured muons flux. Although additional parameters will still need to be included to account for Additional Hydrogen Pools in the soil, this approach can greatly simplify the calibration procedure.

We will therefore present the state-of-the-art of the use of muons as a reference in CRNS and in particular early results of the novel muons-based calibration approach.