The goal of this short example is to simulate the behavior of a very simplified ROCK beamline that includes a bending magnet as the source, a channel cut for energy selection and an absorption sample. Several monitors will be placed between these equipments in order to show the evolution of the front of the beam across the beamline.
## Source
Let's start with inserting a bending magnet as the source. We want the energy spread to be white going from 4 to 40 keV. Make sure to choose an adequate E0 (center of emitted energy spectrum) and dE (half width of emitted energy spectrum). The storage ring electron energy (Ee) is equal to 2.75 GeV and the magnetic field (B) of the bending magnet is equal to 1.72 T.
According to these parameters the characteristic energy is : `Ec = 0.665*Ee*Ee*B = 8.65 keV`.
Now insert an energy monitor after the source. You can also place a PSD monitor there.
:runner: Run the simulation and observe through the energy monitor the white source.
## Double Crystal Monochromator (channel cut)
To select an energy we need to insert a channel cut double crystal monochromator. There are different ways of doing this. In this example we will use **two** Bragg_crystal component**s****to materialise the channel cut**.
Add an input parameter Etohit=8 in the DEFINE INSTRUMENT line.
From the Bragg law we can deduce the calculated_angle : `calculated_angle = asin(m*lambda/(2*d))`. Where d is **the interplanar distance** (d = 3.1355 Angström for Si 111) and m the Bragg order (here m = 1). Lambda is the incident wavelength. To easily convert E(eV) to lambda(Angström) use the relation : `E(ev) = 12398.42 / lambda(Angström)`.
In the DECLARE section, declare the calculated_angle variable by adding the line: `double calculated_angle;`
Do the calculation in the INITIALIZE section. The angle needs to be in degrees so use RAD2DEG to convert radians into degrees.
The channel cut is made up of two parallel crystals placed at a certain attack angle. To do this :
- Insert a Si 111 crystal in between the source and the monitors. Choose 2 as the crystal_type, 2 being the diamond type. We can keep the rest of the parameters as default.
Make the crystal rotate `-calculated_angle`**(anti clockwise ?)** around the x axis.
Now position an arm on top of the crystal and make it rotate another `-calculated_angle` around the x axis. Make sure to use the PREVIOUS keywords.
- Place **a** second **Si 111** crystal and it's following arm. This time the rotation will be done in the opposite direction.
Translate the second crystal in the z direction 0.05 m (default length of the crystals) away from the first crystal.
The vertical separation between the two parallel crystals is equal to 0.01 m so translate the second crystal in the y direction 0.01 m away from the first crystal.
Make sure to translate (AT) the second crystal relative to the first crystal, use `RELATIVE first_crystal_name`.
:runner: Run the simulation. Look at the energy monitor now, does the cc work?
## Monitor_nD (versatile monitor)
After the CC, on top of the existing energy monitor, insert a Monitor_nD. We will use it to show the vertical energy distribution on the monitor. To do so:
After the CC........... To do so: ==> d'abord dire ce que l'on veut faire, je remplacerais plutôt cette phrase par : **(In order to show the vertical energy distribution insert a monitor_nD after de channel cut, on top of the existing energy monitor To do so : )**
Copy paste `char e_repartition_options[512];` in the DECLARE section and `sprintf(e_repartition_options,"energy bins=500 limits=[%g %g], y bins=500",Etohit-0.02*Etohit, Etohit+0.02*Etohit);` in the INITIALIZE section.
Now insert the Monitor_nD with `options=e_repartition_options` as one of the parameters.
Run the simulation. Explain the result of the monitor_nD.
*Tip*: You can insert another Monitor_nD before the CC, just after the source. Change the limits of the monitor so that the energy ranges from 4.0 to 40.0 keV.
## Absorption sample
Finally, let us add an absorption sample after the monitor_nD. We will use the Absorption_sample component. We want the box in a box configuration. Choose zinc as the material for the inner box and copper for the outer box. Make sure the sample isn't too thick and remember to add the densities of the materials used (8.96 g/cm³ for Copper and 7.14 for Zinc).
Add an energy monitor right before the sample.
:runner: Do a scan in energy from 8 to 10 keV.
Remember that the source is white, the energy scan is actually just the scan of the CC's attack angle, it's the energy we select with the CC.
Do you notice anything?
*Tip*: Pressing `l` to view the plots in log scale can make things more obvious.
## Post-scriptum
This short example of a very Simplified ROCK beamline did not include many core elements of the real ROCK beamline such as:
- the toroidal collimating mirror (called M1 **on this beamline**)
- the harmonic rejection mirror (called M2a)
- the harmonic rejection and vertical focusing curved mirror (called M2b)
- the vertical and horizontal slits
If you want to play and/or improve the more realistic ROCK beamline that comprises the elements listed above you can find it here: [ROCK](https://github.com/McStasMcXtrace/McCode/blob/master/mcxtrace-comps/examples/SOLEIL_ROCK.instr)
Happy simulating!
# Archives
Work done before, kept for archiving purposes only.
