Scanning Electron Microscopy and X-Ray Microanalysis

227 16

use this value for all your data acquisition. Before each day’s
measurements ensure that the detector is calibrated consis-
tently by following the same protocol to check and, if neces-
sary, recalibrate your detector.
On a modern pulse processor, calibration is usually per-
formed using the EDS vendor’s software. The software will
prompt you to collect a spectrum from an established mate-
rial. The software will examine the spectrum and extract the
positions of various characteristic X-ray features. The soft-
ware will then perform an internal adjustment to center these
features in the correct channels. Most modern pulse proces-
sors 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 elec-
tronic 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 resolu-

tion (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 per-

fectly 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 com-

plete EDS Quality Control (QC) program, the validation will

be discussed in a later section.

16.3.7 Other Items

z 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.

3,000,000

2,500,000

f(x) = 35876.1202787172x

R^2 = 0.9999923818

2,000,000

1,500,000

1,000,000

Counts

500,000

36,050

36,000

35,950

35,900

35,850

35,800

Co

unts / (nA.s)35,750

35,700

35,650

35,600

35,550

0 0.2 0.4

Probe current (nA)

0.6 0.8 1 1.2 1.4

Det 2

0

0102030

Probe dose (nA.s)

40 50 60 70 80 90

Linear (Det 2)

Det 2

. 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)
16. 3 · Practical Aspects of Ensuring EDS Performance for a Quality Measurement Environment