5. Calibration

5.1 Outline

Calibration of the MCA consists of determining the relationship between channel number and energy. This will be affected by various settings on the amplifier and the MCA, the temperature history of the detector, and other things beyond the control of the user. As a result it should be repeated before every experiment.

The IDL MCA program has built-in calibration procedures, which should be used initially. These procedures separate the energy calibration from the 2q calibration. Collect a spectrum containing a low energy line and a high energy line, and as many additional lines as possible. Run the energy calibration procedure (see below), to get the energy calibration terms. Save the file, which will put that calibration into the detector. Then collect another spectrum, using any known diffraction standard, and run the 2q calibration procedure. Again save this file. All future data sets will have these calibration parameters automatically included.

There are, however, a few limitations:

1. Energy and 2q calibration are separate.

2. Each is based on one data set, so, for example, you cannot use two separate sources to get your energy calibration.

3. The energy equation is limited to linear or quadratic.

If you need a more accurate calibration, or one which does not have the limitations listed above, you can perform another calibration later, using PLOT85. That procedure is described below.

• 1. For each detector, collect spectra containing as many fluorescence or gamma lines as possible; save them into separate files.
• 2. Do the same for diffraction data. Save this data. You may use more than one file.
• 3. Load the PLOT85 program and, using the GPLS option, locate all the peaks visible. The program will use the EDF parameters included in the data files, which, presumably, have been saved from the IDL MCA program.
• 4. Edit or create the STANDARD.EDF file with the peak locations of each peak found.
• 5. Using PLOT85, do the calibration procedure, which will write a new DEFAULT.EDF file.

5.3 Creating the DEFAULT.EDF file

Summary:

• 1. Collect spectra for radioactive and/or x-ray fluorescence lines (usually ⁵⁷Co), and one or more diffraction spectra (usually Al₂O₃). Save these as files using the IDL MCA program.

• 2. Using PLOT85, identify the various lines and determine their channel numbers, using GPLS. Save the Peaks files

• 4. Use the calibration option in PLOT85

Details:

For the more accurate calibration to be used for your real data, you must first set up the appropriate file names. Several different sources may be used for the energy calibration. Possible gamma lines are:

Co57 gamma1 14.413 KeV

Co57 gamma2 122.0614 KeV

Pb Ka1 74.956 KeV

Pb Ka2 72.794 KeV

• 1. Collect and save this data using the IDL MCA program. The filename will be automatically incremented to filen.MED. Make a plot of the spectrum using PLOT85 (in another window).

• 2. Using PLOT85, plot a each of the data files and determine channel numbers for specific peaks. You should use the GPLS option or you can enlarge the region around each peak to get an accurate center. Save the PKS file. If your data file was named QTZ01.MED, and you are doing this for detector 1, then your peaks file will be QTZ01001.PK1. Print it by loading the .pks file into Notepad or WordPad.

• 3. If your first data set was a fluorescence or a gamma source, you will have to collect another data set for diffraction, such as Al₂O₃.
• 4. Use the calibration option. The program calculates the energy for each line using the Bragg equation. A cubic equation of the form E=A₀+A₁C+A₂C² +A₃C³ is used to determine the energy (E) - channel number (C) relation. The four coefficients A₀, A₁, A₂ and A₃ are determine using a least squares routine. The value of 2q is then incremented by 0.1° and the process repeated. The value for which 2q is a minimum is then stored. The process is then repeated for increment steps of 0.01°, 0.001°, and 0.0001°. The value of 2q which results in the minimum standard error is accepted. This procedure must be repeated for each detector.
• Because PLOT85 selects 2q based on minimizing errors, one line with a large error can result in spurious results. Therefore, it is necessary to ascertain that there are no "bad" lines.

   

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LABEL ENERGY OR D DET1 DET2 DET3 DET4

'Co57 G1', 14.413, 207.99, 198.24, 215.79, 448.77

'Co57 G2', 122.0614, 1795.50, 1720.16, 1861.60, 1784.59

'Co57 G3', 136.4743, 2008.24, 1924.21,-2082.00, 1964.00

'FeKa', 6.4, 89.99, 85.45, 93.61, 196.00

'PdKa', 21.123, 306.78, 293.02, -318.52, 659.82

'PdKb', 23.859, 346.91, 330.31, -358.77, 744.00

'Al2O3 (0-1-2) ' 3.4790, -450.90, 446.11, -461.53, -451.23

'Al2O3 (1-0-4) ' 2.5520, -615.36, 610.67, -631.76, -611.02

'Al2O3 (1-1-0) ' 2.379 , -660.33, 655.24, -677.83, -659.54

'Al2O3 (1-1-3) ' 2.085 , -753.87, 748.66, -774.10, -752.62

'Al2O3 (0-2-4) ' 1.740, -904.46, 898.80, -928.97, -902.22

'Al2O3 (1-1-6) ' 1.601, -984.14, 977.23,-1009.88, -980.52

'Al2O3 (1-2-2) ' 1.514, -1075.88,-1116.45,-1075.88,-1075.88

'Al2O3 (2-1-4) ' 1.404, -1158.11, 1116.45,-1158.11,-1158.11

'Al2O3 (3-0-0) ' 1.374, -1184.38, 1141.21,-1178.21,-1184.38

'Al2O3 (1-2-5) ' 1.337, -1413.72,-1342.35,-1413.72,-1413.72

'Al2O3 (1-0-10)' 1.239, -1269.62,-1269.62,-1269.62,-1269.62

'Al2O3 (2-2-6) ' 1.0426, -1506.98,-1506.98,-1506.98,-1506.98

'Al2O3 (2-0-10)' 1.099, -1479.75,-1479.75,-1479.75,-1479.75

'AL2O3 (2-1-10)' 0.9976, -1574.54,-1574.54,-1574.54,-1574.54

'NACL (1-1-1)', 3.25658, 481.74, -473.98, 493.10, -473.98

'NaCl (2-0-0)', 2.82028, 556.50, -548.01, 570.50, -548.01

'NaCl (2-2-0)', 1.99424, 788.56, -777.86, 808.31, -777.86

'NaCl (2-2-2)', 1.62829, 966.29, -954.24, 990.93, -954.24

'NaCl (4-2-2)', 1.26127, -1253.16,-1235.49, 1281.73,-1235.49

Table IX STANDARD.EDF file