2

2.3 Controls

The tab in the MCA Display allows set up the following functions:

2.2.1 Presets

Presets allows you to change the time for data collection. Setting this parameter to zero will allow the MCA to collect data indefinitely. When performing an ROI scan (such as maximizing peak intensity by moving certain LVP motors), the preset time has to be set at a reasonable value (typically 5 or 10 sec). See Scan for details.

2.2.2 Calibrate Energy

The standard way of calibrating energy for the Ge solid state detector is to use two radio-active sources (Co⁵⁷ and Cd¹⁰⁹), both are located in the Chemistry Lab in Drawer 1. Obtain the key to the drawer from the key box mounted on the wall (right hand side of the lab entrance) and sign off the log book. Mount the sources over the opening on the lead housing over the Ge solid state detector head using scotch tape. Stack them one over the other.

Note: Do NOT remove the lead housing over the detector head and NEVER place the sources directly on the Be window, as doing so may puncture the Be window and destroy the vacuum in the detector.

Collect fluorescence lines on the Ge detector. The peaks commonly used are:

Peak	Peak name used in MCA	Energy (keV)	Typical FWHM (keV)
Silver Ka	Ag ka*	22.104	0.33
Silver Kb1	Ag kb1	24.942	0.25
Lead Ka2	Pb ka2**	72.794	0.41
Lead Ka2	Pb ka1**	74.956	0.41
Cd¹⁰⁹ g	Cd109 g1	88.040	0.42
Co⁵⁷ g2	Co57 g2	122.061	0.49
Co⁵⁷ g3	Co57 g3	136.474	0.52

Notes:

* - This is weighted energy position of AgKa1 and AgKa2.

** - Pb peaks are present due to lead shielding on the detector.

Define regions of interest (ROIs) of all the lines to be used for energy calibration (see Overview), and label each peak using the peak name precisely given in column 2 in the table above. MCA will find the energy values for these peaks. After sufficient data collection time, open the following table by click on Controls -> Calibrate Energy:

For Calibration type: choose Linear. We haven’t found any evidence for an non-linear behavior.

Choose Compute Calibration to start energy calibration. After it’s done, errors in keV will appear in the last column in the ROI list and the resultant calibration coefficients will be shown in the “Calibration coefficients:” panel. Two coefficients, Offset and Slope will appear in the appropriate fields. The Quadratic term is zero by choice.

You may wish to plot calibration error or plot FWHM to examine the actual fitting results. Click OK when you are done. MCA will take the new calibration and write it to every data file. All the displays opened after this point will also be based on the new calibration.

For files collected before this calibration, the energy calibration information will be incorrect. You may want to edit the data files by changing the coefficients in the files.

If for some reason you want to go back to previous energy calibration coefficients, you’ll have to load those values into MCA manually. To do so, go to MCA Electronics and follow the instructions there.

2.2.3 Calibrate 2-Theta

The standard way to calibrate two-theta is collect diffraction data on a standard material, which can be a NIST standard (such as Al2O3, LiB6, etc.). This must be done independently from the cell assembly and some time the centering may be slightly different from the cell. So in most cases, the pressure standard in your high pressure cell is used for two-theta calibration.

After collecting the diffraction data, open the JCPDS file for the material you are using from the Displays tab on the MCA Display. Load ROIs (see Overview) for the peaks you are interested. You may need to modify the ROIs slightly, as some of the ROIs may be too wide for you, especially if there is some peak overlap because of other materials present in the cell. Then click on the Calibrate 2-Theta button to open the following window: