Simulation Project SANS-2: Velocity selector

Insert a velocity selector, centered 0.2 m after the guide. Use the default parameters, except for the aperture size (which you adjust to be "large enough"). Another exception is the number of blades which is 68, and the rotation, which you use as an input parameter of the instrument.

Adjust the rotation speed to select neutrons of \(\lambda =10\) Å. Perform the simulation and verify that the correct wavelength is chosen. Measure the wavelength spread, \(\Delta \lambda / \lambda\), where \(\Delta \lambda \) is the FWHM of the wavelength distribution.

In order to do more efficient simulation form now on, you can adjust the emitted wavelength band in the source according to the band that passes the velocityselector.

McStas hint

The wavelength monitor performs simple statistics when you perform a single plot from the plot menu. The plotted width is the statistical (or Gaussian) variance. In case the distribution is Gaussian, the FWHM is found by multiplying with \(2\sqrt{2 \ln(2)}\).