Scanning Electron Microscopy and X-Ray Microanalysis

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22

Au, while at E 0 = 2.5 keV, the range for Na collapses to 0.12 μm

(120 nm) in a C matrix to 0.02 μm (20 nm) in Au.

22.2.2 Reduced Matrix Absorption

Correction

When the range of X-ray production is reduced by lowering

the beam energy, the generated X-rays undergo lower absorp-

tion because of the reduced path length to the surface. This

can be a strong effect, because X-ray absorption follows an

exponential relationship:

I/Ie 0 =−xp ()μρ/ ρs

(22.6)

where I 0 is the original intensity and I is the intensity

remaining after passing through a distance s (cm) of a mate-

rial of density ρ (g/cm^3 ) and of mass absorption coefficient,

μ/ρ (cm^2 /g). For strongly absorbed photons, which is

typically the case for low energy photons, the matrix correc-

tion for absorption diminishes rapidly (i.e., approaches

unity) as the beam energy is reduced, as shown in. Fig. 22.10

for O K-L2,3 and Cu L 3 -M4,5 in Cu 2 O (measured relative to

MgO and Cu).

22.2.3 Accurate Analysis of Low Atomic

Number Elements at Low Beam

Energy

Low atomic number elements with Z ≤ 10 have characteristic

X-ray energies below 1  keV, and these low energy photons

suffer especially high absorption. By minimizing absorption

through operation at low beam energy, Li and Be can be

detected; and B, C, N, and F, can be quantitatively analyzed

with accuracy such that the analyzed value is generally

within ± 5 % relative to the true value, as presented in

. Table 22.1 (borides),. Table 22.2 (carbides),. Table 22.3

0

1.1

1.0

0.9

0.8

Absorption correc

tion, A

0.7

0.6

0.5

246

O K-L2,3

Cu L 3 -M4,5

8

Beam energy (keV)

X-ray absorption in Cu 2 O

10 12 14 16 18 20

. Fig. 22.10 Absorption correc-
tion factor for O K-L2,3 (relative to
MgO) and Cu L 3 -M4,5 (relative to
Cu) as a function of beam energy
. Table 22.1 Analysis of metal borides at E 0 = 5 keV (5 replicates); atomic concentrations

Compound Metal, Cav Relative

accuracy,%

σrel, % Boron, Cav Relative

accuracy,%

σrel, %

CrB 2 0.3482 4.5 0.32 0.6518 −2.2 0.17

CrB 0.5149 3.0 0.17 0.4851 −3.0 0.19

Cr 2 B 0.6769 1.5 0.65 0.3231 −3.1 1.4

TiB 2 0.3373 1.2 3.6 0.6627 −0.6 1.8

Chapter 22 · Low Beam Energy X-Ray Microanalysis