Question

Obtain the de Broglie wavelength associated with thermal neutrons at room temperature (27°C). Hence explain why a fast neutron beam needs to be thermalised with the environment before it can be used for neutron diffraction experiments.

Answer 1

T = 27 + 273 = 300K

Energy of the neutron at temperature, T,

Thus, thermal neutrons have wavelength of the order of interatomic spacing
(≈10^-10m) and therefore they are suitable for diffraction experiments. Fast neutrons will possess wavelength quite small as compared to interatomic spacing. Hence for neutron diffraction experiments, fast neutron beam needs to be thermalised.