In the previous sections (Sections 5.4.1—5.4.5), the different interactions (namely, coherent and incoherent scattering), photoelectric effect, and pair formation of the gamma radiation have been discussed. As seen, the cross sections, or the absorption coefficients of all these interactions depends on the energy of gamma radiation and the atomic number of the absorber. The cross sections versus energy or atomic number functions are significantly different for the different processes. The total absorption of the gamma radiation is the sum of the different interactions, expressed by the cross sections:

P pRayleigh + ^Thomson + ^photoelectric + pCompton + Ppair (5:92)

The absorption law (Eq. (5.3)) for the gamma radiation can be expressed as:

/ = /0 e-px (5.93)

Equation (5.93) can be transformed to mass absorption coefficients, as is done in the case of beta radiation (see Section 5.3.4).

Gamma energy (keV)

The scheme of the total absorption of gamma radiation as a function of the gamma energy is shown in Figure 5.25.

As seen in Figure 5.25, the mass absorption coefficients of the individual inter­actions show the range of gamma energy that is characteristic of the given interac­tion. The mass absorption coefficient of the total absorption (д) as a function of gamma energy shows a minimum: the mass absorption coefficient decreases until the gamma energy exceeds 1.02 MeV; it is the start of the pair formation.

In Figure 5.26, the mass absorption coefficient for different gamma energies as a function of the atomic number of the absorbers is shown.