Simulation Project powder: Monochromator

The monochromator is placed just after the guide exit (0.5 m downstream). You can assume it is a flat piece of PG crystal with a reflectivity of 80 %. You will use the (002) reflection (\(\tau = 1.8734\) Å\(^{-1}\)) to scatter the neutrons. The height of the monochromator is 0.120 m and the width is 0.200 m. The mosaicity of the PG crystal is 30' in both directions. Use the McStas component monochromator_flat.

You should select the "DMC standard" neutron wavelength of 2.4 Å. Calculate the scattering angle, \(2\theta\) and place a monitor at the sample position, 1.8 m from the monochromator centre. The monitor should have the typical sample dimensions, 2 cm high and 1 cm wide. Now, scan the monochromator orientation to obtain the maximum signal at the sample position (which is kept fixed in space, at an angle calculated from the Bragg law). Comment on the outcome.

Simulate the energy distribution on the sample position and explain. From this point on you can limit the simulations to first order scattering from the monochromator.

Using the best setting found above, simulate the intensity, divergence and the energy distribution at the sample position. Do the same for the other "DMC standard" wavelength of 4.2 Å. Comment on the results.