Calibrate SGM

This option lets you calibrate the spherical grating monochromator. This section describes the specific procedure of how to do it from SM_GUI. Refer to the beamline manual^5.3 for background information and exact tabulated absorption peak positions.

Note that you need access to the current SGM calibration file on to local file system (mounted from x1a.nsls.bnl.gov, which is the case when you do the calibration from the microscope control client computers)^5.4. The SGM is driven by a stepping motor, and the x-ray wavelength (not energy!) changes linearly with motor position (to a very good approximation). Therefore, the calibration requires two parameters (see Eq. 10.2). In daily operation, however, only the zero_order_steps might change (steps_per_angstrom should stay fine), and you can calibrate only this one parameter in SM_GUI. A full calibration has to be done outside the GUI.

The regular procedure is as follows. Ask the beamline staff for help in special cases like iron edge etc.

1. Record a point spectrum over the appropriate energy range (280 to 310eV on outboard, 520 to 540eV on inboard, in steps of 0.1eV each). On inboard the chamber must be evacuated and backfilled with Helium. Set this spectrum to $I_0$ by clicking the button in the GUI window. You might want to save the scan and write down the file name.

2. Add the calibration gas by opening the CO$_2$ flow on outboard, or letting some air (Oxygen) leak into the chamber on inboard (use the valve on the outboard-downstream side of the chamber). Record another point spectrum over the same energy range. Again, save the file and remember the scan number for future reference. Check the "Show $I/I_0$" box. Identify the calibration peak (e.g., the 292.74eV CO$_2$ absorption peak, see the beamline manual for a table of calibration peaks).

3. Fit the peak by dragging the left mouse button over it (see Fig. 5.19). Don't include any part of a neighboring peak! A red line will be displayed at the center of the fitted Gaussian, and the peak position will be shown below the plot as well as in the display information area. (You can also just click on the peak rather than fitting).

4. If the calibration is off (the fitted peak position deviates from the tabulated value), click the "Calibrate SGM" button in the menu. A window will pop up (see Fig. 5.19). The fitted "wrong" value will already be entered in the upper field. Into the lower field, enter the correct, tabulated value. If you click "Accept", you'll be asked for a confirmation to overwrite the SGM calibration file. A backup copy of the file will be made automatically.

   Note: If you do the calibration from a computer other than the microscope control client computer, you'll have to specify the SGM calibration file. Better know what you are doing!

   Figure 5.19: Monochromator calibration