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We present the first measurement of the coherent dynamic structure factor S(Q, ω) of solid 3 He. At a density of 13.25(10) cm 3 mol −1 , we observed longitudinal phonons, using high-resolution inelastic x-ray scattering at momentum transfers 6 nm −1 < |Q| < 23 nm −1 . Their dispersion relation was determined along the c-axis in single crystals of hcp 3 He, and the spectra were described with a single-phonon model. For comparison, hcp 4 He at the same density was studied with the same technique. The comparison, including all available 4 He data from neutron scattering experiments, results in a phonon frequency ratio ω 3 /ω 4 of 1.11(5). Special attention is given to the scattering contribution due to multi-phonon excitations.

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