Basic understanding of instrument components

In the McStas formulation of a neutron scattering instrument, all objects apart from the neutron ray are referred to as components. This includes for instance

• Source The exit of a neutron production facility, where neutron rays of certain velocities are emitted into some portion of space.
• Monochromator (Idealized) crystal that is used to select neutrons of a single wavelength¹ $\lambda_0$ to probe the sample with (monochromator) or to analyze with (analyzer).
• Sample An object altering the neutron physical properties in some sense, examples used here are:
  • Vanadium. Scatters incoming neutron rays incoherently.
  • PowderN. Can be thought of as a large number of crystals, each scattering neutron rays according to Braggs law, thereby producing N concentric Debye Scherrer cones. This sample also has the posibility of adding inchoherent, eleastically scattered neutron rays.
• Monitors Objects monitoring or registering neutron ray characteristics. In the exercises below are used different types of detectors or monitors:
  • Monitor. Single monitor, detecting the number of neutrons flying through a plane. (User defined opening size).
  • PSD_monitor. Square monitor, detecting the number of neutron rays passing through a plane, divided into pixels. Square regions of a plane. (User defined resolution and opening size).
  • PSD_monitor_4PI. As PSD_monitor but shaped like a sphere.
  • L_monitor. Wavelength monitor, measuring the different wavelengths of the passing neutron rays. (L is for $\lambda$).
  • Monitor_nD. General monitor for detecting all sorts of physical properties of the neutron ray. In our cases used with options:
    • 'single' - as PSD_monitor but only one small square.
    • 'banana' - as PSD_monitor but shaped like a curved, horizontal band.
• Collimators Devices controling the direction and divergence of the neutron ray.
  • Collimator_linear A series of parallel absorbing neutron plates that limits the beam divergence. Typical values are $6'$ to $120'$.

More information on the McStas components is available by using the mcdoc program (You may need to set the BROWSER system variable to your webbrowser of choice):

• mcdoc -s , Shows a html list of all the components
• mcdoc Monitor.comp , Shows the documentation for a given component
• mcdoc -M , brings up the McStas manual in PDF format
• mcdoc -c , brings up the McStas component manual in PDF format