Exercise: Insert a monochromator

1. Keeping your current components, insert a Monochromator_flat component (use mcdoc to get the needed parameters) and a new set of PSD and L_monitor after the monochromator. You should now add two new input parameters of your instrument, e.g. OMM (Omega Monochromator) and TTM (Two Theta Monochromator) for rotation of the monochromator and the remaining part of the instrument so that the orientation of monochromator is as portrayed in Figure 3.

   Figure 3: Illustration of the monochromator orientation

   
   These two instrument input parameters should be added to the DEFINE line of the instrument (start of file). Remember to add an Arm()'s at the rotation point.
2. Given $\lambda$ = 4Å, and knowing that for the monochromator $Q=1.8734$ Å$^{-1}$ (Pyrolytic Graphite), use Bragg's law to determine the correct Bragg angle (i.e. OMM/TTM) for the monochromator for the $n=1$ reflection.
3. Do a scan of OMM a couple of angles around this value to verify the finding, keeping TTM fixed. This will be achieved by setting the OMM value to 'min,max' (replace by relevant numerical values) in the Run Simulation window, and selecting the Scan check button in that same window. Enter the number of steps to compute (e.g. 10). Check the position of the peak on the PSD and the wavelength on the L_monitor.
4. What should $Q$ be set to to get the $n=2$ reflection at exactly OMM=45$^\circ$ (TTM=90$^\circ$)? Adjust $Q$ for the monochromator and verify the calculation by a scan, check the wavelength.
5. Determine the Bragg angle for the $n=1$ reflection in this setting of $Q$, and verify by scaning OMM. Set OMM to this value
6. Before you go on, change the minimum and maximum wavelengths of the source to a narrow interval around the wavelength you select, e.g. 4Å. (No need to produce neutrons that will not be scattered at the monochromator...)