A NEUTRON-SCATTERING STUDY OF THE IMPULSE APPROXIMATION IN SINGLE-CRYSTAL BERYLLIUM

The inelastic scattering of high-energy neutrons from a single crystal of beryllium has been measured in order to test the validity of the impulse approximation. Separate measurements were performed, all with the sample at room temperature, with the wave-vector transfer along (100), (110) and (001) using the time-of-flight spectrometer HET at ISIS. For wave-vector transfers >or approximately=35 AA-1, corresponding to an incident neutron energy of approximately=1 eV, the authors find that the scattering is centred at the recoil energy given by the impulse approximation. Also, in this regime, the width is independent of the orientation of the wave-vector transfer with respect to the crystal axes and has a value consistent with that expected from a harmonic model of beryllium. They show that their experimental criterion for the validity of the impulse approximation is in agreement with a numerical calculation of the asymptotic behaviour of the exact incoherent neutron dynamic structure factor.