Simulation Project tripleaxis: The resolution function

You may have noticed that your two phonon peaks are not identical, e.g. for 2.0 meV. The reason for this is hidden in the "resolution function", which is a measure of the precision of the spectrometer. In more precise terms, it is the distribution \(\hbar \omega\) and \(q\) of the scattered neutrons that actually hit the detector for one particular setting. The "resolution ellipse" is the contour in \((\hbar \omega, q)\)-space, where the transmission has fallen to half the value of the nominal point.

In experiments, the resolution function at \(\hbar \omega = 0\) can be measured by e.g. elastic scattering from single crystals, but direct measurements of the inelastic resolution function cannot be performed. Often, the resolution is calculated from the analytical Popovici approximation, by means of the program RESCAL.

The resolution function may, however, be calculated by McStas. Replace your sample by component res_sample and replace your detector by res_monitor. The sample then scatteres neutrons inelastically into a focusing direction and energy interval determined by you, and the monitor records the values of \(\hbar \omega\) and \(q\) of the neutrons that hits. After the simulation, the outcome can be shown with the McStas tool mcresplot.

Determine your resolution function at both 2.0 meV inelastic peaks and discuss why the peak shapes and peak intensities were not equal.