In a neutron scattering soil moisture probe, fast high-energy neutrons emitted by the radioactive source ($^{241}\text{Am-Be}$) are thermalized (slowed down) primarily through collisions with:
Options:
(A) Hydrogen nuclei ($^1\text{H}$) whose mass is nearly equal to that of a neutron
(B) Silicon atoms in quartz sand
(C) Iron oxides in clay
(D) Lead shielding inside the housing
Answer Key:
A
Step-by-Step Solution & Explanation
Because a hydrogen nucleus (proton) has essentially the identical mass as a neutron, elastic collisions with hydrogen atoms (predominantly residing in soil water molecules) cause maximum momentum transfer, slowing fast neutrons to thermal velocities.