Proton Induced Xray Emission

In the preceding sections we have discussed the formation of electron vacancies in inner shells (n = 1, 2, ... ) by incident x-rays. Characteris-

TABLE F.4

X-Ray Energies in keV for Characteristic X-rays* Emitted from Elements in Common Pigments

X-ray Energies keV

Element

Symbol

Z

Ka

La

Ma

Aluminum

Al

13

1.49

1.56

_

-

Silicon

Si

14

1.74

1.83

-

-

Phosphorus

P

15

2.01

2.14

-

-

Sulfur

S

16

2.31

2.46

-

-

Titanium

Ti

22

4.51

4.93

0.45

-

Chromium

Cr

24

5.41

5.95

0.57

-

Iron

Fe

26

6.40

7.04

0.70

-

Cobalt

Co

27

6.93

7.65

0.78

-

Copper

Cu

29

8.04

8.92

0.93

-

Zinc

Zn

30

8.63

9.58

1.01

-

Arsenic

As

33

10.53

11.73

1.28

-

Selenium

Se

34

11.21

12.66

1.38

-

Cadmium

Cd

48

23.11

26.10

3.13

0.39

Tin

Sn

50

25.20

28.50

3.44

0.46

Antimony

Sb

51

26.28

29.73

3.60

0.50

Mercury

Hg

80

70.18

80.25

10.0

2.20

Lead

Pb

82

74.25

84.93

10.533

2.34

*In x-ray notation, the capital letter (K, L, M, ... ) indicates the shell containing the vacancy. The subscript a indicates the most intense line. The electron transitions are: Ka (l3 to k), Kp (m5 to k), La (m5 to l3), Ma (n7 to m5).

*In x-ray notation, the capital letter (K, L, M, ... ) indicates the shell containing the vacancy. The subscript a indicates the most intense line. The electron transitions are: Ka (l3 to k), Kp (m5 to k), La (m5 to l3), Ma (n7 to m5).

X-Ray Fluorescence (XRF)

Fig. F.8. X-ray fluorescence (XRF) in which x-rays incident on a painting generate inner shell vacancies in an atom of a pigment particle. Characteristic x-rays are emitted from the atom, and their energies are measured in the x-ray detector, which allows identification of the atom.

Energy

Fig. F.9. Comparison of ion-, electron-, and photon-induced x-ray emission.

tic x-rays are emitted in the subsequent electron transistion to fill the vacancy. This process is called x-ray fluorescence (XRF).

The apparatus for generating x-rays is simple, and x-ray fluorescence is used for in situ examination of works of art (Figure F.8). The disadvantage is that the x-ray beam strikes a large area, millimeters to a centimeter in diameter. This large beam area limits the system in analysis of sample areas and paint cross sections. The advantages of XRF are that

Fig. F.10A,B. Comparison of (A) x-ray and (B) electron beam generated x-ray emission. X-ray fluorescence (XRF) analysis is carried out in air but has a larger beam spot, typically millimeters (mm) in diameter, whereas the electron beam has a smaller size (less than one micron) but the analysis is carried out in a vacuum chamber, which requires removal of the sample from the painting.

Fig. F.10A,B. Comparison of (A) x-ray and (B) electron beam generated x-ray emission. X-ray fluorescence (XRF) analysis is carried out in air but has a larger beam spot, typically millimeters (mm) in diameter, whereas the electron beam has a smaller size (less than one micron) but the analysis is carried out in a vacuum chamber, which requires removal of the sample from the painting.

Chi Kung Breathing Full Breath

Fig. F.11. Schematic diagram of the setup for proton-induced x-ray emission (PIXE). The million electron volt (MeV) proton beam is produced in an accelerator, and the protons travel in vacuum to the thin window at the end of the beam line. The protons have sufficient energy (usually 3.5 to 5 MeV) to pass through the window and to travel in air to the painting. The emitted x-rays are detected in a system similar to that used in XRF and electron microprobe analysis.

the analysis is carried out in air and that portable units (x-ray generator and detector) are used to analyze pigments without removing samples from the painting.

The x-ray emission process is independent of the manner in which the vacancies are formed. Energetic electrons and ions can also be used

5 10

1 1 1 1 1

External PIXE :

_

White Paint with Ti I

I ArK

-

1

TiK :

I (1 '

■ i * . 1 •

iii \ i i i i i i 'i V/i Vii i i

External PIXE White Paint with Pb

5 10

X-Ray Energy (keV)

Fig. F.12a,b. External proton beam analysis (PIXE) is shown of two white paints (a) titanium (Ti) white and (b) lead (Pb) white. The spectra are displayed as the yield of x-rays (the number of x-rays incident on the detector for a fixed analysis time) in square root of the number of detected x-rays (counts) versus x-ray energy. The argon (Ar) signals are generated by the proton's passage through air, which contains argon. Titanium K-shell (TiK) is shown in (a) and lead K-shell (PbL) in (b).

to create vacancies as long as the incident electron energy £electron is greater than the binding energy of electron in the shell. Figure F.9 is a summary of the three methods of pigment analysis by x-ray emission.

Electron-induced x-ray emission is called electron microprobe analysis (EMA) because the incident electron beam can be focused to a one-micron diameter spot size (a microprobe). In comparison with x-ray fluorescence (Figure 10a), the striking drawback to the electron microprobe technique is that the pigment sample must be mounted in vacuum (Figure 10b) because of the small penetration distance of electrons in air. A paint fragment must be removed from the sample, placed in a potting compound, and polished so that a cross section of the pigment layers can be examined in the vacuum chamber of the electron microscope. The penetration of 20-keV electrons is only 40 microns, so that top-surface irradiation by 20-keV electrons would produce x-rays only in the outermost layers.

Protons of MeV energies produced by a particle accelerator can also produce electron vacancies (Figure F.11). The process is called proton-induced x-ray emission (PIXE). Figure F.12 shows PIXE spectra of titanium white (Figure 12a) and lead white (Figure 12b). Comparison of the two spectra show that both have peaks from pigments containing zinc (zinc white) as well as argon (Ar) due to the passage of the proton beam through air before striking the painting (argon is a noble gas present in air at 0.93%). The presence of the titanium peaks in Figure 12a and lead peaks in Figure 12b serve to identify the two white paints.

Range of Protons in Air

Energy (MeV)

Energy (MeV)

Fig. F.13. The range of protons in air. A 4-million electron volt (MeV) proton can travel nearly 25 cm (10 inches) in air before it loses all its energy in collisions with air molecules along its path.

The range of 5-MeV protons in air is many centimeters. Consequently, the protons can be taken out into the air (external beam analysis), as shown in Figure F.13. The range of 5-MeV protons in paint layers is somewhat greater than 200 microns. Thus, nearly the full thickness of a multilayer painting may be analyzed.

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    Can proton induced xray emission be used for cobalt analysis?
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