The footprint characteristics of cosmic ray thermal neutrons

Abstract

Cosmic ray neutron (CRN) sensing is a non-invasive method for intermediate scale soil moisture measurements (Zreda et al., 2008). This method relies on the inverse dependence of aboveground epithermal neutrons (energy range from ∼0.5 eV to 100 keV) on the environmental hydrogen content in a footprint of 130-240 m radius and soil depths ranging from 15 to 83 cm (Köhli et al., 2015;Schrön et al., 2017). In terrestrial environments, most hydrogen is stored in water in soils. Therefore, it is possible to infer soil moisture content from the amount of aboveground epithermal neutrons. Secondary hydrogen pools, such as biomass, have a large impact on the measurement accuracy, especially when they are not constant in time. For reliable soil moisture estimation, these secondary pools thus need to be considered (Baatz et al., 2014;Bogena et al., 2013;. Recently, it was found that by additionally considering the thermal neutron intensity below ∼0.5 eV, aboveground biomass can be inferred using the ratio of thermal to epithermal neutrons (Jakobi et al., 2018;Tian et al., 2016).CRN sensors are currently installed in approximately 200 locations worldwide (Andreasen et al., 2017). Many of these locations are also instrumented with thermal neutron detectors. However, these extensive data sets are rarely explored because key properties of the thermal neutron signal, such as the footprint of thermal neutrons, are not well defined. Preliminary investigations suggest that the thermal neutron footprint is smaller than the epithermal neutron footprint and in the order of tens of meters (Bogena et al., 2020). For