For two summers it has rained far too little. The ground lacks water. Cosmic radiation can help to find out how dry the earth is. In the future, Cosmic Ray Neutron Sensing is intended to also show how much water plants contain.
It’s stunning: eternities ago, many millions of light years away, stars died, and today, their explosion helps to determine the moisture of the earth on our planet. More precisely, the cosmic rays generated by stellar explosions help (1). When these hit the earth’s atmosphere, neutrons are produced (2). A part of them hurtles to the surface at high speed and penetrates to a depth of almost one metre. The neutrons bounce off heavy atoms in the ground, such as silicon (3). When they hit water molecules, however, they give off part of their energy to the hydrogen’s light atomic nuclei. This slows down the neutrons (4). The more often they collide with hydrogen atoms, the slower they become. For many of them, the energy is at some point no longer sufficient to leave the ground (5). “This is why there are far fewer so-called fast neutrons travelling over moist soils than over dry ones,” explains Jannis Jakobi from the Jülich Institute of Bio- and Geosciences (IBG-3).
This difference can be detected with a special method called Cosmic Ray Neutron Sensing: “We use neutron detectors in meadows and fields as well as a mobile measuring device in a van (6). In passing, so to say, we use the device to count the fast neutrons (7) near the ground, thus determining the soil moisture relatively easily and over a large area,” explains Jakobi. In the future, the researchers want to measure the water content of plants in the same way, since their hydrogen atoms also slow down the neutrons. “For this purpose we use another detector (8) to measure so-called slow neutrons (9) above the ground. These have had eight to twelve collisions with hydrogen atoms,” says Jakobi. “From the ratio of slow to fast neutrons, we want to draw conclusions about the water content and biomass of trees and field plants.” Such information might help farmers to optimise irrigation, for example.
Photo: Yasemin Bas, Video:UK Centre for Ecology & Hydrology, Grafic: SeitenPlan GmbH