. Fig. 16.21  The spectrum image was collected on a 128 by 128 pixel grid representing 4 × 4 mm. After data rotation to view the x-y plane, the first energy slice, 1 out of 1024, is shown. It appears black because it contains no counts

16

. Fig. 16.22  Extracting the Cu L-family lines from the spectrum planes 89 to 97

probe current, due to the loss of useful electrons during the dead-time.

It is very important for accurate quantitative analysis that the measured X-ray intensity is linear with respect to the effective probe dose. Twice the effective probe dose should produce twice the measured intensity. Not only is this

important because it is often hard to replicate the identical probe current but it is also important because different materials measured with the same probe current will produce different dead times.

There really is no excuse for a modern X-ray detector not to be linear. However, it is worth checking because the test can expose other potential problems like a non-linear or offset probe current meter.

zz Check 5: Count Rate Linearity with Effective Probe Current

Equipment: 55Faraday cup

55Picoammeter

55Flat, polished copper sample Procedure:

\1.\ Mount the Faraday cup and the polished copper sample on the stage at the same nominal working distance.

\2.\ Image the sample at the optimal working distance and a beam energy selected in the 15–25 keV range.

3.\ Start a factor of 10 or more below the optimal probe current. Measure and record the probe current using the Faraday cup and the picoammeter.

4.\ Collect a 60 live-time second spectrum from the copper sample.

5.\ Increase the probe current through a sequence of approximately 10–20 steps from the initial probe current to approximately 2 times the optimal probe current. Collect a 60 live-time second spectrum at each probe current and measure and record the probe current using the Faraday cup. A plot of such a measurement sequence is shown in .  Fig. 16.23 (upper).

6.\ Integrate the total number of counts in the range of channels representing the Cu K-L2,3 characteristic peak. (You don’t need to background correct the integral.) Plot the measured intensity divided by the probe current against the measured probe current. The result should be a horizontal line, as shown in .  Fig. 16.23 (lower).

1.\ If the line is not horizontal, the problem may be in the detector or in the probe current measurement.

2.\ The probe current measurement could be non-linear meaning the plot of the measured probe current to true probe current is not a straight line.

3.\ Alternatively, the probe current may have a zero offset. The zero offset can be measured by blanking the beam and recording the measured current at zero true current. Subsequent probe current measurement can be offset by this value.

16.3.6  Energy Calibration Linearity

Consistent energy calibration is critically important for reproducible quantitative analysis. Pick a nominal channel width (5-eV/channel will work fine in almost all cases) and

. Fig. 16.23  The number of X-rays recorded in the spectrum should scale linearly with the dose (the product of live-time and probe current)

use this value for all your data acquisition. Before each day’s measurements ensure that the detector is calibrated consistently by following the same protocol to check and, if necessary, recalibrate your detector.

On a modern pulse processor, calibration is usually performed using the EDS vendor’s software. The software will prompt you to collect a spectrum from an established material. The software will examine the spectrum and extract the positions of various characteristic X-ray features. The software will then perform an internal adjustment to center these features in the correct channels. Most modern pulse processors perform a continuous zero offset calibration using a “zero strobe pulse” the pulse processor adds to the signal stream for diagnostic purposes. Thus the only parameter they usually adjust when performing a calibration is an electronic gain. Usually, this involves identifying a single high energy characteristic line (like the Mn K-L2,3 or the Cu K-L2,3) and adjusting the gain until this feature is centered on the appropriate channel. The calibration is thus a two-point cali- bration—either a low energy characteristic line or the zero strobe at low energy and a second characteristic line at high

energy. Two points are sufficient to unambiguously calibrate a linear function. The calibration (peak position) and resolution (peak shape) should be constant with input count rate (or dead-time), as shown in .  Fig. 16.24.

To a very high degree, modern EDS detectors are linear. However if you look carefully in the mid-range of energies, you many notice the KLM markers may be misaligned by a channel or two. This is evidence that your detector is not perfectly linear but this need not represent a true performance problem.

Since energy calibration is so critical but is also one of many parameters that should be measured as part of a complete EDS Quality Control (QC) program, the validation will be discussed in a later section.

16.3.7\ Other Items

zz Light Transparency and IR Cameras

Most (but not all) modern EDS detectors have a vacuum tight window that is opaque to infrared and visible light.