Abstract
We present high-resolution data of the energy widths Γ of elementary excitations in superfluid 4He on the phonon branch 0.3Å−1 < Q < 0.6 Å−1 and both the energies and the widths on the roton branch 1.93Å−1 < Q < 2.6Å−1 at T = 0.98 K. In both Q-regions a broadening of Γ is clearly observed. The roton energies and group velocities observed in the present experiment are significantly smaller than those reported from previous experiments.