Orienting Crystals with the Laue Camera
The NeutronOptics Laue Camera is designed for orienting crystals.
A central collimator collects the beam transmitted through the crystal in forward scattering, or allows
the incident beam to pass through the camera before being reflected by the crystal in backscattering.
In either case, many Bragg spots are obtained from the different wavelengths in the white beam.
Backscattering is used when the crystal is too absorbing for forward scattering,
even though intensities are lower due to the X-ray "form factor".
This camera uses twin CCD units to increase efficiency, extend the image field, and avoid the shadow from the central collimator. The separate images are automatically stitched together to produce a single larger image after correcting each for its relative position, angle and lens aberrations. This custom software uses the free ImageJ package, which can handle the most complex images as well as simple snapshots from cameras usually included with mobile phone deals.
Our Laue camera can be supplied with scintillators, windows and collimators suitable for either neutrons or X-rays,
as shown in the photo. (Click the images to enlarge them).
Setting up the Laue Camera
First read the general instructions for standard NeutronOptics cameras. The only difference is that the Laue camera has two CCD units, so you have two power supplies, two exposure controllers and two USB video capture devices. Prefer to use different USB hubs for these EzCAP devices and install the EzCAP driver before plugging them in.You will be using the unlimited exposure option since typical exposure times for backscattered Laue are ~5 minutes. But first try a shorter exposure with no beam to check that everything is working - you should see background noise. Then try forward scattered Laue, where exposure times are much shorter. (Remove the collimator plug-ins so that the beam passes through the camera after being scattered by the crystal).
Remember that the camera itself integrates and stores the image(s). Start exposing both CCDs by simultaneously pressing the red buttons on the two controllers, and then press these two buttons again to stop integration and store the images (red lights come on). These images remain stored and you can read them out at any time using ImageJ via its capture macro. You may need to re-read the images after swapping the video cables so that the acquisition software adds the images from the two CCDs in the correct order.
Facing the camera, CCD#1 is on the left, and should also be on the left in the display.
Over-expose CCD#1 to check this order.
Using ImageJ with the Laue Camera
Download the ImageJ AMCap macro to your \ImageJ\macros\toolsets folder. Click the >> icon in the ImageJ toolbar and select the AMCap capture tool.
Right-click the resulting camera icon to bring up the options dialogue opposite. Depending on installed software and versions, fewer options may be shown.
Select the cameras to capture eg "USB 2861 Device", resolution, image directory, file name and format. Set the Capture Time to "0" to take snapshots (prefered) or a number of seconds to integrate using every Nth frame (e.g. if your camera is set to integrate over 32 frames, set N=32).
Set the lens corrections K1, K2 and camera rotation. When there is a second camera, set its rotation, X,Y offsets and X,Y scale.
These parameters will be supplied for each camera; if ever you need to re-determine them, you can use a pinhole calibration screen in place of the scintillator plate.
Left-click the camera icon to obtain the image, watching progress in the tool bar. Then re-read the AMCap instructions !
When you have captured and saved your Laue diffraction pattern, you can index it to
determine the crystal orientation using the Orient Express indexing programme.
The OrientExpress program for Laue indexing
OrientExpress is a Windows application for automatic Laue indexing, developed by Alain Filhol at the
Institut Laue-Langevin (ILL) with Jean Laugier
and Bernard Bochu at LMGP Grenoble. It is described on the
CCP14 crystallographic software
site and can be
downloaded with example files.
Under the Data/File menu select "EXAMP1.DAT", which contains
lattice dimensions for a ruby test crystal (Al2O3) together with the characteristics
of the camera (distance to crystal, angle between the beam and the camera normal,
wavelength range, and camera screen dimensions). Note that this is transmission Laue data, so the angle is 0o;
it would be 180o for backreflection Laue. Al2O3 is given on hexagonal axes, but the
programme can convert to rhombohedral axes, for which indexing is easier. You can look up lattice
dimensions on the Inorganic Crystal Structure Database (ICSD).
Measuring the Laue peak coordinates
Under the Coordinates/Image menu select "examp1b.jpg" or a Laue pattern you saved from your camera.
Select the Origin/Circle menu from the new "Laue" window that opens. Click and drag the red circle to define
the beam axis of the Laue pattern (not always coincident with the centre of the beam hole) then click "OK". Select the Scale/Enter menu and enter the number of pixels/cm; 44.25 for the
examples or 50 for
the 150x120mm NeutronOptics Watec camera. Check these numbers at the top left of the screen. Now click "Measure coord."
and click on up to 10 spots (right click to remove a spot). It is best to select only 5-6 spots that are at the
intersections of arcs of spots (zonal lines) which usually have small h,k,l indices. Finally, click "Save coord." to a file.
Note that the laue spots fall on ellipses, indicative of transmission Laue geometry; in backscattering geometry the spots
fall on hyperbolas, the
intersection of the scattering cones with the detector plane.
Indexing the Laue pattern & crystal orientation
On the OrientExpress main menu, click "Indexation search" and simply click "OK" in the dialogue that opens.
If indexing succeeds, a red Laue pattern will be super-imposed on your measured image corresponding to one of
possibly several solutions (click on the different solutions if there are several). A correct solution will
reproduce the observed spots (plus a lot more) and most importantly, the intersecting arcs of spots. If there
is no good solution, check the centering, scale and cell parameters or select fewer Laue spots only on arc intersections.
The menu Coordinates/Modification modifies the coordinate list or centre/scale image constants.
Refining the fit to the Laue pattern & orientation
On the OrientExpress main menu, select Matrix/Refinement and "No additional coordinates" then click "Refine". Try refining again after adding more Laue spots. The resulting file "Report.txt" will list the (h,k,l) of all the Laue spots used for the refinement as well as their position on the pattern, their deviation from the calculated pattern, and the refined distance between the camera and crystal.Rotating the Laue pattern & simulation
On the OrientExpress main menu, select Simulation/Laue and then select the rotation axes under the Options of the simulation window. Dragging the mouse in this window will rotate the Laue pattern around the selected axes. You can also choose the Indexation option and click on Laue spots to identify them, increasing their size as necessary. Even if you can't index the pattern, you can simulate it given the Ox,Oy,Oz crystal orientation axes by entering these under the Matrix/Acquisition menu then using the Simulation/Laue/Options menu as above to adjust the crystal orientation.Steffen Weber has also programmed a simple interactive Laue pattern simulator in Java.
Re-orienting the crystal on new axes
On the OrientExpress main menu, click "Reorientation" and enter the crystal orientation eg (1,1,1)
that you want either parallel to the beam axis OX, or parallel to the
vertical camera axis OZ, then click calculate. You will obtain a number of combinations of rotations
phiX, phiY, phyZ that will achieve that. Choose the solution most convenient for your goniometer.
Finally you can click on one of these solutions to display the expected Laue diagram, and you can
check your new orientation by comparing that with a new measured diagram.
Indexing backscattered Laue patterns
For backscattered Laue patterns, the "Angle" in the data file becomes 180o as in the "examp6.dat" file. File "grolland.bmp" is a backscattered pattern from ruby where we have measured 7 spots (below left) which allows us to find a unique solution (below right).
Note that we have chosen spots at the intersections of multiple arcs indicative of low order reflections in symmetry directions. Try removing the last 3 spots; you will still find a solution. Try adding spots at the intersections of fewer arcs; a solution is more problematic. Use only a few low order reflections to find a solution, then refine it.
These brief instructions should be supplemented by the OrientExpress "Help" file. The Laue patterns shown here are the X-ray scattering examples provided with the OrientExpress software, not those obtained with our cameras.
For Windows-7, install OrientExpress in write-enabled directories, not "Program Files".
LaueGen - Daresbury Laboratory Laue Software Suite
More experienced crystallographers will also want to use the Daresbury Laboratory Laue Software Suite and in particular the LaueGen program for Laue image display, simulations, auto-indexing and orientation refinement.