Cosmic-ray neutrons: a proxy for environmental water content at the hectare scale
Dr. Jannis Weimar
INF 229, SR 108/110

Cosmic-ray neutrons are constantly produced by galactic cosmic rays and their progeny in the earth’s atmosphere and the upper soil layers. The above-soil flux of these neutrons within a specific energy regime inversely relates to the surrounding environmental hydrogen content, e.g. soil moisture. Moreover, the neutrons have a mean free path of several centimeters in soil and several ten meters in air and, thereby, transport the soil moisture information over large distances.

This area-averaged soil moisture sensing technique called CRNS (Cosmic-Ray Neutron Sensing) monitors soil moisture but also snow water equivalent and is deployed for flood and hydro-power management, micro climate modeling and within first pilot schemes related to agriculture.

Within an interdisciplinary DFG research unit, our group at the Physikalisches Institut contributes to the understanding of the method from two sides: Firstly, we use Monte Carlo codes to simulate the neutron production and transport processes at the land-atmosphere interface. These results are used to approximate analytical functions that help to interpret the neutron signal. Secondly, the limitations of standard cosmic-ray neutron detectors and the shortage of 3-He as a standard neutron converter have led to a gaseous 10-B-lined neutron detector design specifically tailored to the needs of CRNS. These detectors are deployed stationary or as a mobile version that can monitor soil moisture on up to a square kilometer per day.