Obtain the de-Broglie wave length associated with thermal neutrons at room temperature $27^{\circ}$ Hence explain why a fast neutron beam needs to be thermalised with the environment before it can be used for neutron diffraction experiments.

Question

$\begin{array}{1 1} 145 \times 10^{-10}\;m \\ 14.5 10^{-10}\;m \\ 1.45 \times 10^{-10}\;m \\ 1.45 \times 10^{-10}\;cm \end{array} $