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Aligning a crystal using the Neutron Laue Diffraction Pattern Simulator for HRC(BL12)

[Web tool] Neutron Laue Diffraction Pattern Simulator for HRC(BL12)

Step 0: Neutron Laue diffraction measurement

When you start a neutron inelastic scattering experiment using a single crystal at HRC, you need to measure neutron Laue diffraction pattern using a white beam.

An intensity map showing neutron counts at each detector pixel, I(d,p), is obtained by the HANA software. (Please contact instrument scientist for more details on the HANA software.)

In this tutorial, we will use datasets below: these are Laue diffraction patterns of a tetragonal crystal.

Run014190.png : intensity map measured at omega=0 deg.
Run014193.png : intensity map measured at omega= –30 deg.

Sample

Lattice constants : a = b = 8.499 A, c = 5.528 A, alpha = beta = gamma = 90 deg.
Space group : P-4 21 m
(Nominal) horizontal scattering plane : (H,0,L)

Step 1: Input parameters

In the "Setting" tab, you need to input the following parameters at least.

Lattice constants: Input the above lattice constants, and click the "Set lattice" button.
Reflection condition: If the crystal has a general reflection condition for hkl, select it from the "Reflection condition" pull-down menu. (for the P-lattice we are going to simulate, select "none".)
Orientation: In an ideal case, there are (at least) two high-symmetry axes are on the horizontal plane. For the present case, a* and c* axes are supposed to be on the horizontal plane. Thus, we should input u=(1,0,0) and v=(0,0,1). Then, click the "Set orientation" button.
Vertical offset: There is some ambiguity in height-difference between the sample position and the center of detector tubes, and thus we need to assume a small offset value. Normally, this value should be smaller than 40 mm. For the present case ~20 mm should be fine.

Step 2: Load observed intensity map

Moving to the "Simulation" tab, you can load a png file of the intensity map from "Load observed Laue pattern" section.

The png file is supposed to have dimensions of 800(W) x 500(H) pixels. First, let us try "Run014190.png".

Step 3: Rotate the crystal in the simulation

In the "Free rotation" section , you can rotate the crystal in the simulation (e.g. by clicking the "rotate z+" button). The step of the rotation can be changed using textboxes on the left.

The current orientation of the a*, b*, and c* axes are shown in the Sample orientation viewer (left bottom panel).

The viewpoint in the Sample orientation viewer can be changed by using slidebars in the "Camera angle" section.

When the data "Run014190" was measured, the c* axis was about 10 degrees away from the direction of the incident neutron beam, as shown in a picture on the right. Please try to find the correct orientation in which the simulated and observed diffraction spots exactly coincide with each other.

> More detailed info on the sample rotation (in Japanese).

(In actual experiments, you should know rough orientation of your crystal. Probably, you put a mark on top of the refrigerator. It would be easy to find the correct orientation by starting from the rough estimation of the initial orientation. )

Step 4: Find omega_offset angle

Once you find the right orientation, you can determine the "omega_offset" angle, which is necessary to convert the omega angle of the instrument to the psi angle for MSlice software.

When the data "Run014190" was measured, the omega angle was 0. In the "Sample rotation" section, you should put "0" in the textbox for the current omega angle, and click the "set" button.

Now you can simulate Laue diffraction patterns for any omega angles. You can directly specify the omega angle using a textbox in the subsection labeled "omega rotation".

For example, by inputting "–30" in the textbox, you can simulate the diffraction pattern which should be measured after rotating the omega angle from 0 to –30. You can actually load the data "Run014193.png", which was measured at omega= –30. Please check if the simulated and observed Laue spots still coincide with each other.

Step 5: Adjusting Ei max

You may find that there are a number of observed Laue spots which do not coincide with the simulations. One possible reason is that there is a limit for Ei (the energy of the incident neutrons) in the simulation. As we increase the limit value, there will be more points on the screen, which would consume a lot of CPU power.

But, if you want to check the diffraction pattern with larger Ei (actually, the HRC instrument provides high-energy neutrons up to about 700 meV), you can change the limit value by using a slidebar in the "Incident Neutron Beam" section in the "Setting" tab.

This website is maintained by T. Nakajima Lab (ISSP-NSL)

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