Simulation Project tripleaxis: Phonon sample

You should now simulate lattice vibrations (phonons) in a single crystal sample of Pb: cubic lattice constant \(a=4.95\) Å, scattering length \(b=9.4\) fm, and velocity of sound \(c=10\) meV Å. Keep the incoming neutron energy from the monochromator at 5 meV for now.

Position an energy monitor 1.26 m away from the sample (use e.g. an interval of 10-20 meV) and position the crystal at the (200) Bragg reflection by a proper value of the sample rotation OM. Try to turn OM \(2^\circ\) away from this reflection and measure the energy of the neutrons that now leave the sample in the direction of the analyzer. Comment on the result.

Change the temperature of the phonon sample, repeat the simulation and comment on the result.

McStas hints

In this project you will use the sample phonon_simple, which is documented in the component manual. When unrotated, the crystal is oriented with the (001) axis (lying in the scattering plane) perpendicular to the beam and the (100) axis along the beam direction. The strength of the phonons depend of the temperature which can be set in the component.

To optimize simulation time and avoid second order scattering, you may like to limit your simulated wavelength interval in the source component. Take care that you do not make the interval too narrow!