Data are shown at Q = (0.5, 0, -1.5) (a–d) and Q = (0.15, 0, -1.15) (e–h). Solid lines for T = 1.5 K in a and b are fits as described in Fig. 3. Solid lines for T ≥ 10 K are fits to the following response function satisfying a detailed balance constraint: S(Q,ω) = Γ/π(1/(Γ2 + (ℏω - ℏω0)2) - 1/(Γ2 + (ℏω + ℏω0)2))1/(1 - exp (- β ℏω)). The temperature dependence of the relaxation rate, Γ, for the lower energy peak at both wavevectors is shown in the inset to a. The line corresponds to exponentially activated behaviour with Δ = 2.0 meV. Coloured areas below peaks indicate the assignment of different contributions to the spectra. Dashed lines indicate incoherent elastic nuclear scattering. Solid (dashed) horizontal bars in d and h indicate resolution (width of the low energy peak). Error bars show statistical uncertainty estimated as the square root of total neutron count measured at each point.