How to perform flux/charge calibration in GeoPIXE

You will need to perform a flux/charge calibration using foil data at every major beam energy you use during your run.



1. Import spectra from your calibration foil using the Spectrum Display window.
  • In the Flux PV window, fill in the IC scaler channel, preamp sensitivity, preamp scale units, and dwell time as usual, but set the Q/IC conversion = 1.
2. Delete all XYT using the Spectrum Select window. Save the foil E-spectra.

3. Calibrate the energy of the spectra.
  • In the Spectrum Display window, go to Display > Get all energy cals
  • Load a set of energy-calibrated spectra from a sample with similar counts at the same energy as your foil (possibly from a pair of foil spectra- see tutorial on generating energy calibrations from foil data). Save calibrated E-spectra.
  • Add spectra using Process > Add (Remap cals). Save the sum spectrum.
4. Open the x-ray spectrum fit window. Load a recent fit setup for a calibration foil (.pcm file) to set most of the fit parameters to a reasonable first guess. Make sure the element selection is correct for your foil. Check that you are fitting a reasonable energy window (no cuts active in crucial regions of spectrum). Also check the filter file. For a foil 1.5mm from the detector, the filter should be 5mm air + 6.5 um Mylar.

5. In the x-ray spectrum fit window, load an appropriate or set up a new yields file (.yields). Make sure the beam energy, target layer formula, and areal density (mass per area, mg/cm2- leave density set at 0) are correct for your foil experiment. The areal density of the reference foils can found in the CHESS Maia binder. Let's call this number C_foil. Use this yields file in your fit.

6. When satisfied with your fit, open Window > Fit results. Look at the most recent fit and write down the concentration for your foil element in ng/cm2. You might need to change the units next to the "Conc." drop-down menu. If you're not sure which is the most recent fit, clear the existing fit data and re-do the fit to repopulate the results window. Let's call this number C_fit.

7. Look at the x-ray spectrum fit window, and write down the number that shows up in the Q box. Let's call this number Q.

8. Now you are ready to do the following calculation: (C_fit/C_foil)*Q

  • Example (same as video above): Say I am using a copper foil, and the fit gives me 0.0000114 ng/cm2 of copper. I know the concentration of the copper foil is 15.9 ug/cm2. The existing Q in my fit window is 28125600. I then calculate 0.0000114/15900*28125600 = 0.0201655.
  • Note: The foil concentrations are given in ug/cm2, but the GeoPIXE concentrations are in ng/cm2. Make sure your units match!
9. Replace the Q value in the x-ray spectrum fit window with your calculated value, and re-run the fit. Now check the concentration in the fit results window- your foil element should show up with the proper concentration. In the copper foil example, I put 0.0201655 in for Q, which gave me a new copper concentration of 15905 ng/cm2, close to the 15.9 ug/cm2 that I expect.

10. In the x-ray spectrum fit window, press the "?" by the Q button. A window will pop up- the number in the upper right, "Conv- change to" should be on the order of 1E-9 to 1E-10. This is your flux/charge conversion factor. Write this number down and use it for the flux/Q conversion factor in your data processing for your entire run at this beam energy. You will definitely need this number when importing your raw data as spectra (in the Flux PV window), and when processing your raw data to build images in the Sort EVT window.

-- LouisaSmieska - 10 May 2016
Topic revision: r4 - 21 Aug 2018, ArthurWoll
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