Page 168 - Radiochemistry and nuclear chemistry

152 Radiochemistry and Nuclear Chemistry

emission of a K-x-ray decreases with increasing Z of the sample; e.g. for 5 MeV protons
the reaction cross section is ca. 10 -25 m 2 for Z = 10 to 20 and about 10 -28 m 2 (1 barn)
for Z = 50; cf. Table 6.1. It can be shown that the reaction probability has a maximum
when the incident particle has a velocity equal to the Bohr orbital velocity of the electron.
Though the sensitivity decreases with Z elements up to Pb can be determined. The
technique has primarily been used to determine trace elements in environmental and
biological samples, see Figure 6.22(a).

6.8.3. ESCA (Electron spectrometry for chemical analysis)

High resolution 0-spectroscopy can be used to determine chemical properties. A sample
is irradiated with mono-energetic photons of Ehp, leading to the emission from the sample
surface of photoelectrons. The relevant equation is

Ehp = Ebe(X,Y) + E e (6.28)
where E e is the kinetic energy of the emitted electrons, which can be determined very
accurately (presently to about 0.01 eV) in the magnetic spectrograph; photoelectron
spectroscopy. This sensitivity is much greater than chemical binding energies, Ebe(X,Y),
where X refers to an atom in compound Y. The probability for ejection of photoelectrons
increases with decreasing photon energy (Fig. 6.16 and 6.19) and therefore low energy
X-rays are used as a source.
Although it is the outermost (or most weakly bound) electrons which form the valency
orbitals of a compound, this does not leave the inner orbitals unaffected. An outer electron
(which we may refer to as eL) of an atom X 1 which takes part in bond formation with
another atom X 2 decreases its potential, which makes the inner electrons (which we may
call eK) more strongly bound to X 1. Thus Ebe(eK) increases by an amount depending on
Ebe(eL). Although this is a somewhat simplified picture, it leads to the practical
consequence that the binding energy of e K, which may be in the 100 - 1000 eV range,
depends on the chemical bond even if its orbital is not involved directly in the bond
formation. Figure 6.23 shows the ESCA spectrum of trifluoroacetate. Since the largest
chemical shift, relative to elementary carbon, is obtained for the most electronegative
atoms, the peaks refer to the carbon atoms in the same order as shown in the structure.

6.8.4. XFS (X-ray fluorescence analysis)

If a sample containing atoms of a particular element (e.g. Ag) is irradiated with photons
of energy high enough to excite an inner electron orbital (e.g. the Kt~ orbital in Ag at 22.1
keV), X-rays are emitted in the de-excitation. If the photon source is an X-ray tube with
a target made of some element (Ag in our example), the probability is very high that the
K s X-ray emitted from the source would be absorbed by the sample atoms and re-emitted
(fluorescence). (This is an "electron shell resonance absorption" corresponding to the
MSssbauer effect, although the width of the X-ray line is so large that recoil effects can be
neglected.) The spectrum of the scattered X-radiation (or, more correctly, photon radiation