Scanning Electron Microscopy and X-Ray Microanalysis

46

4

0510 15 20 25 30

1.4e-20

1.2e-20

1.0e-20

8.0e-21

6.0e-21

4.0e-21

2.0e-21

0.0

Beam energy (keV)

K-shell ionization cross section of silicon

lonization cross section (cm

2 )

. Fig. 4.6 Ionization cross section
for the silicon K-shell calculated with
Eq. 4.4

0102030405060708090 100

Atomic number (Z)

Critical ionization energy of the elements

30

25

20

15

10

5

0

Critical ionization energy (keV

)

K-shell

L 3 shell

M 5 shell

. Fig. 4.7 Critical ionization
energy for the K-, L-, and M-shells

energy loss. The X-ray production in a thin foil of thick-

ness t can be estimated from the cross section by calculat-

ing the effective density of atom targets within the foil:

neXIphotons Qeionizations atom cm^2

Xio

///

/

−−=

×

   ()







ω -raysnnizationatoms /mole

molesg gcm

cm

0

3

[][]

()[] 

[]

×

××

×

Ν

1/A //ρ

t ==×QNI0×ωρ××tA/

(4.7)

where A is the atomic weight and N 0 is Avogadro’s number.

When several elements are mixed at the atomic level in a

thin specimen, the relative production of X-rays from differ-

ent elements depends on the cross section and fluorescence

yield, as given in Eq. 4.7, and also on the partitioning of the

X-ray production among the various possible members of

the X-ray families, as plotted in. Fig. 4.5a–c. The relative

production for the most intense transition in each X-ray fam-

ily is plotted in. Fig. 4.8 for E 0 = 30 keV. . Figure 4.8 reveals

Chapter 4 · X-Rays