mediate Zabsorbers at medium energies (~1 MeV). At higher energies
(>10 MeV), pair production predominates in all Zabsorbers.
Photodisintegration
When the g-ray photon energy is very high (>10 MeV), the photon may
interact with the nucleus of the absorber atom and transfer sufficient energy
to the nucleus such that one or more nucleons may be emitted. This process
is called the photodisintegration reaction, or photonuclear reactionand pro-
duces new nuclides. The (g, n) reactions on targets such as^12 C and^14 N have
been used to produce^11 C and^13 N radionuclides but now are rarely used to
produce radionuclides.
Attenuation of g-Radiations
Linear and Mass Attenuation Coefficients
g-ray and x-ray photons are either attenuated or transmitted as they travel
through an absorber. Attenuation results from absorption by the photo-
electric effect, Compton scattering, and pair production at higher energies.
Depending on the photon energy and the density and thickness of the
absorber, some of the photons may pass through the absorber without any
interaction leading to the transmission of the photons (Fig. 6.7). Attenua-
tion of g-radiations is an important factor in radiation protection.
As shown in Figure 6.7, if a photon beam of initial intensity I 0 passes
through an absorber of thickness x, then the transmitted beam Itis given
by the exponential equation
64 6. Interaction of Radiation with Matter
Fig. 6.6. Relative contributions of the photoelectric effect, Compton scattering, and
pair production as a function of photon energy in absorbers of different atomic
numbers. (Adapted with permission from Hendee WR.Medical Radiation Physics.
1st ed. Chicago: Year Book Medical Publishers, Inc; 1970:141.)