Page 161 - Radiochemistry and nuclear chemistry

146 Radiochemistry and Nuclear Chemistry

tightly botmd electrons to be emitted. These discontinuities coincide with the K, L, etc.,
edges observed in X-ray absorption.
Gamma-rays of higher energy, rather than interacting with the field of the whole atom as
in the photoelectric effect, interact with the field of one electron directly. This mode of
interaction is called the Compton effect after its discoverer, A. H. Compton. In the
Compton effect an electron is ejected from an atom while the ),-ray is deflected with a
lower energy. The energy of the scattered ),-ray, Ev', is expressed by the equation

Ev' = E, v - E e (6.24)

where E e is the kinetic energy of the Compton electron. The probability for Compton
scattering increases with target Z and decreases with E. t. Since the Compton interaction
occurs only with the most weakly bound electrons and high energy )'-rays, the binding
energy of the electron is negligible compared to Ev. The Compton electrons and scattered
),-rays have angles and energies which can be calculated from the relationships between the
conservation of energy and momentum, correcting for the relativistic mass of the electrons
at these kinetic energies. The scattered ),-ray may still have sufficient energy to interact
further by the Compton effect, the photoelectric effect or pair production. Again, emission
of X-rays and Auger electrons usually accompanies Compton interaction and extensive
secondary ionization follows. Since the Compton electron can have a spread of values, the
scattered ),-rays exhibit a broad spectrum. The Compton electrons, as in the case of
photoelectrons, are eventually stopped by the processes described for B-particles.
Figure 6.17 shows the division of energy between the scattered Compton ), and the
Compton electron as a function of ),-ray energy. Only the energy of the electron is
deposited in the absorber as the scattered )'-ray has a high probability of escape. Thus
Compton electrons contribute to the (energy) absorption coefficient #a while the Compton
), contributes to the total attenuation coefficient # through the scattering coefficient/t s in
(6.9).
The fourth mode of interaction for ),-rays with an absorber involves conversion in the
Coulomb field of the nucleus of a )'-ray into an electron and a positron (Fig. 6.18). This
process is termed pair production since a pair of electrons, one positive and one negative,
is produced. The process can be considered as the inverse phenomenon of positron
annihilation. Since the rest mass of an electron corresponds to 0.51 MeV, the )'-ray must
have a minimum value of 1.02 MeV to interact by pair production. As the energy of the
)'-ray is increased beyond this value, the probability of pair production increases (see Fig.
6.17, where/.tpair is denoted x). The excess energy (above the 1.02 MeV) appears as the
kinetic energy of the electron pair.

E,v= 1.02+Ec- +Ec+ (6.25)

The pair of electrons are absorbed as described in w The annihilation of positrons
produce 0.51 MeV )"s, which are absorbed by the processes described previously.
Figure 6.19 summarizes the domains of interaction of the main )'-ray absorption processes
as a function of )'-ray energy and absorber Z-value.