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# Basic understanding of instrument components

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

- Source The exit of a neutron production facility, where neutrons of certain velocities are emitted into some portion of space
- Monochromator/Analyzer (Idealized) crystals used to select a neutrons of a single wavelength 1 to probe the sample with (monochromator) or to analyze (analyzer)
- Sample An object altering the neutron physical properties in some sense, examples used here are
  - Vanadium. Scatters incoming neutrons incoherently
  - Powder2. Can be thought of as a large number of crystals, each scattering neutrons according to the Bragg law, thereby producing two concentric Debye Sherrer cones. This sample also has the posibility of adding inchoherent, eleasticly scattered neutrons.
- Monitors Objects monitoring or registering neutron characteristics. In the exercises below are used different types of detectors or monitors:
  - Monitor. Single monitor, detecting the number of flying through a plane. (User defined opening size)
  - PSD_monitor. Square monitor, detecting the number of neutrons passing through a plane, divided into pixels. square regions of a plane. (User defined resolution and opening siz)
  - PSD_monitor_4PI. As PSD_monitor but shaped like a sphere.
  - L_monitor. Wavelength monitor, measuring the different wavelengths of the passing neutrons. (L is for )
  - Monitor_nD. General monitor for detecting all sorts of physical properties of the neutron. 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 to .

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

Next: Basic McStas Up: McStas neutron ray-trace tutorial Previous: Basic neutron scattering