What is a thermal neutron? What is its importance?

Hint: To solve this question, we have to remember the basic concept of wave nature of matter. A thermal neutron is a free neutron with a kinetic energy of about 0.025 electron volts. Thermal neutrons are spread both by nuclei and unpaired electrons, providing a perfect probe for fine-grained magnetic materials to study atomic and magnetic structures.

Complete step-by-step solution:
Thermal neutrons can be characterized as any free neutron (the neutron not bound inside an atomic nucleus) that has average motion energy (kinetic energy) corresponding to the average energy of the ambient material particles. Relatively slow and low-energy, thermal neutrons exhibit properties that make them attractive in certain chain-reaction applications, such as large cross-sections in fission.
The thermal energy per particle is about 0,025 volt — a volume of energy that corresponds to a neutron velocity of about 2,000 meters per second and a neutron wavelength of about meter (or about two angstroms). Since the wavelength of thermal neutrons corresponds to the natural spacing between atoms in crystalline solids, thermal neutron beams are ideal for investigating the structure of crystals, particularly for locating positions of hydrogen atoms, which are not well located by X-ray diffraction techniques. Thermal neutrons are also needed in the naturally occurring uranium-235 and artificially created plutonium-239 and uranium-233 to induce nuclear fission.

Note: Whenever we ask such type of questions, we have to remember that thermal neutrons are produced by slowing down more energetic neutrons in a substance called a moderator after that they have been ejected from atomic nuclei during nuclear reactions such as fission.