What happens to fast neutrons emitted from the nuclear gauge when they interact with hydrogen atoms in water?

When fast neutrons emitted from a nuclear gauge interact with hydrogen atoms in water, they undergo a process known as thermalization. This process involves the transfer of kinetic energy from the fast neutron to the hydrogen atoms, causing the neutrons to lose energy as they collide with and are scattered by these atoms.

As fast neutrons collide with hydrogen atoms, the significant mass similarity between the neutron and the hydrogen nucleus allows for effective energy transfer. After several collisions, neutrons slow down significantly and can ultimately reach thermal energies, which are in the range of the thermal motion of the surrounding molecules. This slowing down makes the neutrons behave similarly to the thermal energy of the matter in which they are embedded, hence the term "thermalized."

Thermalized neutrons have much lower energy levels and are more likely to be captured by nuclei of various elements or participate in nuclear reactions, making them an essential aspect of neutron detection and analysis in various applications involving nuclear gauges.