Reactor Operator Practice Exam

The main idea is how neutron energy defines their speed class. Cold (slow) neutrons are characterized by low kinetic energy, which also means longer de Broglie wavelengths. In practice, cold neutrons have energies in the meV range, and a widely used upper limit for this category is about 0.01 eV. That boundary sits below thermal neutrons, which are around 0.025 eV at room temperature, so cold neutrons are indeed slower than thermal ones. If you plug in the energy–wavelength relation E = h^2/(2mλ^2), you’ll see that energies around 0.01 eV correspond to wavelengths of a few angstroms, consistent with the long-wavelength, low-energy nature of cold neutrons. The other values are too high: 0.1 eV, 0.5 eV, and 1 eV lie in ranges associated with epithermal to faster neutrons, not cold ones.