Sample Alignment

The Theory

Sample alignment can be a challenging process, particularly if the sample “gets lost” at any point. It is important to remember that there is a method to the madness, and you can always start from the beginning to be sure of where the sample lies. With a better understanding of the sample geometry it is of course possible to skip steps and align faster. However, cutting corners can also get you lost, so when in doubt take your time.

In this experiment, for a sample to be aligned we are looking to perform the following steps:

  1. Stage leveling

  2. Theta calibration

  3. Stage height calibration

  4. x-y calibration

  5. xspress3 calibration

  6. Sample alignment

[image of stage, top down, axes labeled]

[image of stage in 3d, beam path context]

Stage leveling

Stage leveling is performed by adjusting the pico motors and reading the stage height with the laser range finder. The level_s_stage plan attempts to do both the x and y axes, but to level only one level_stage_single can be used.

In [1]: %mov py 0

In [2]: # detector, pico, stage motor, pos over pico, pos to reference
RE(level_stage_single(lrf, vx, px, -50, 50))

In [3]: %mov px 43

In [4]: RE(level_stage_single(lrf, vy, py, 75, -75))

Theta Calibration

With the stage leveled, we set the theta motor offset such that theta reads 0. Rough calibration is usually done with a simple bubble level and later refined using the beam. This is done by iterating through:

  • Scan beam stop w.r.t. theta

  • Move to theta where signal is maximized

  • Scan beam stop w.r.t. stage height (pz)

  • Move to theta where derivative is maximized

Stage height calibration

This should be done in coordination with theta calibration.

Stage x-y Alignment

Centering the beam on the stage is obviously important, and can be accomplished in a variety of ways. The simplest is often to move theta to some angle (1-2 degrees), find the center of a sample (copper dot), then center the laser on the dot. The stage can then be moved such that the laser is in the center of the stage, and px/py zeroed. Alternately one can use phosphor tape to find the beam.

It is important to remember the offsets found during this alignment, as we will want to return to this as a ‘home’ setting.

Xspress3 MCA calibration

If this has not yet been done, move to a piece of copper foil and calibrate the Xspress3. See Calibrating the Xspress3 section for detailed instructions.