Excitation of Rotons in Helium II by Cold Neutrons

The nature of collective excitations in helium II has been investigated by means of the interaction of these excitations with cold neutrons. The experiment demonstrates the existence of excitations with long mean free path compared to their wavelength, confirming the theoretical ideas of Landau and Feynman. The dispersion relation for the excitations in the energy region of the "roton minimum" is directly measured and gives the following Landau parameters: $\Delta =8.1\mathrm{}\pm 0.4°$K, $\frac{P_0}{\hslash }=1.9\mathrm{}\pm 0.03$ ${\mathrm{A}}^{-1\mathrm{}}$ and $\mu =0.16\mathrm{}\pm 0.02m_{\mathrm{He}}$.