Scanning Electron Microscopy and X-Ray Microanalysis

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16

measurements are made on the basis of the same “live-time”

so as to achieve constant dose for quantitative measurements.

The level of activity of the EDS is reported to the user as a

percentage “dead-time”:

Deadtime()%=()ICRO− CR /ICR×100%

(16.5)

Dead-time increases as the beam current increases. The

dead time correction circuit can correct the measurement

time over the full dead-time range to 80 % or higher. (Note

that as a component of a quality measurement system, the

dead-time correction function should be periodically

checked by systematically changing the beam current and

comparing the measured X-ray intensity with predicted.)

However, as the dead-time increases and the arrival rate of

X-rays at the EDS increases, coincidence events become

progressively more prominent. This effect is illustrated in

. Fig. 16.10 for a sequence of spectra from a glass with six

SiK-L+

Ca

K-L,

n

ot Cr

K-L

Al

K+

Ca

K-L,

not

V K-L

2M

gK

2A

lK

2S

iK

SiK+

O K

2O

K not Na

K

Count

s

Coincidence

peaks

Coincidence

peaks

K412 (mass frac)

O0.428

Mg 0.11 7

Al 0.0491

Si 0.21 2

Ca 0.10 9

Fe 0.0774

Energy (keV)

20DT

16DT

12DT

10DT

7DT

14 000

12 000

10 000

8 000

6 000

4 000

2 000

0

0246810

K412

E 0 = 20 keV

7% deadtime

Counts

K412 (mass frac)

O 0.428

Mg 0.117

Al 0.0491

Si 0.212

Ca 0.109

Fe 0.0774

Photon energy (keV)

140 000

120 000

100 000

80 000

60 000

40 000

20 000

0

0246810

. Fig. 16.10 Development of coincidence peaks as a function of dead-
time. NIST Standard Reference Material SRM 470 (Mineral Glasses) K412.
(upper) SDD-EDS spectrum at 7 % dead-time showing the characteristic

peaks for O, Mg, Al, Si, Ca, and Fe. (lower) SDD-EDS spectra recorded over

arrange of dead-times showing in-growth of coincidence peaks. Note

elemental misidentifications that are possible

Chapter 16 · Energy Dispersive X-ray Spectrometry: Physical Principles and User-Selected Parameters