This letter demonstrates that there are some mistakes in the article of Roshan et al (Pramana – J. Phys. 90(3): 30, 2018) in dealing with the scattering of neutrons on $\alpha$-particles in three-dimensional space. In fact, there is no one-to-one correspondence between the incident neutron energy and the scattering angle of the α-particle even if they have definite velocities before scattering. Instead, the situation is complex, and all the scattered $\alpha$-particles will emit in a cone, which is called the velocity cone. At each scattering angle, which is smaller than the apex angle of the velocity cone, there are two groups of $\alpha$-particles with different kinetic energies.