Spectrum Calibration

Operating note 1: accurate calibration is crucial to accurate Quant results. 4pi tech support has found that 95% of the problems with Quant results can be traced to bad calibration.

Operating note 2: Proper autocalibration requires a preliminary detector setup procedure to be performed!

To begin the calibration procedure, activate the EDX Spectra Toolbar from the Mode menu...

... and select Calibrate Spectra from the Options menu. The following window appears:

The spectrum calibration window is similar to the standard spectrum window in Revolution: tools are available for zooming in and out of the channels, sliding channels left and right, and scaling the spectrum. The realtime and livetime counters are also available. Not shown: any spectrum that has already been acquired into any catalog can be cut and pasted into this Calibrate window, thus circumventing the additional acquire step that is outlined next!

To begin the calibration procedure, select from the Sample menu according to the type of sample in the microscope...

... and select the Acquire Spectra button:

KLM reference markers are automatically placed in the spectrum according to the sample selected. A spectrum will acquire for the amount of time entered in the EDX Spectra Toolbar dwell field (see top of this page). As the calibration is being performed, the spectrum will update in real time. The goal of course is to align the spectrum lines with the KLM reference markers. If extensive adjustments are required, the clr button should be used to restart the spectrum as needed.

For this procedure to work well, the calibration sample loaded into the microscope should have a minimal number of well-known x-ray lines in both a lower and a higher energy range. A common and effective choice is any sample that contains both Aluminum and Copper. Al-Cu will be assumed for rest of this discussion.

Manually Calibrating the EDX Spectrum

The manual calibration process is to iterate the adjustment of the gain and offset controls of the Scanning Interface Unit (SIU) until the spectrum comes into alignment (for systems without an SIU, the gain and offset controls of the pulse processor itself must be used).

The general rule of thumb when performing a calibration is that the offset control is used to adjust low-energy peaks while the gain control is used to adjust high-energy peaks. Note: condensed spectrum snapshots are shown to conserve space.

Initially, it is quite likely that the element peaks will appear at the wrong energies:

The first step is to change the offset control of the SIU or pulse processor until the low-energy (copper) peak is adjusted (upward in this example) to the correct energy:

Use the mag control to zoom in on the spectrum to perform a precise adjustment of the lowest energy. It is not unusual for lines at a higher energy (here, Al) to still be out of adjustment:

Next, adjust the gain control of the SIU or pulse processor to make the high-energy (copper) peak line up to the markers:

By iterating back and forth between the low-energy and high-energy peaks, the SIU or pulse processor offset and gain controls can be used to dial in the correct calibration. Once the high and low energy peaks are calibrated, the intermediate peak(s) will be also:

The most precise calibrations are performed with the lowest low-energy and highest high-energy peaks available. The choice of calibration sample is dependent on the user's needs and available samples. For users of SEII 2.2 (and later) boards, even closer calibrations can be performed using the manual software gain and offset controls; however, the preferred method is to perform a software Autocalibration step, detailed below.

Autocalibrating the EDX Spectrum

Remember: Autocalibration must always be performed after the manual calibration is completed (see above). Do not skip the manual step - a relatively good manual calibration is required for the autocalibration to work properly.

Operating Note: As a general rule, the manual calibration shown above is acceptable for the autocal step but is not sufficient for accurate Quant results.

As the above manual adjustment is being made, the measured peak values are continually updated in comparison with the known values of peak energy for the sample(s) selected (see blue highlights in diagram below). The most precise calibration will show a close match between the actual and measured peaks. Due to the statistics of the x-ray process, there will always be a fluctuation in the measured value, so a perfect match is not realistic. The equivalent Manganese FWHM is continuously calculated to display detector performance (see red highlight in diagram below). The end result for a Copper/Aluminum sample should be very comparable to the following example:

In actuality, the autocalibration is performed for the entire time the window is open. When a satisfactory result has been achieved (as determined by the closeness of the actual and measured line energies), hit the OK button to record the calibration preferences. All spectra acquired after this point will have the calculated software correction applied.