Compton scattering and 38% due to Compton absorption), because the
occurrence of other processes are negligible. For gamma photons with
energy levels greater than 1 MeV, the pair-production (kp/p) effect can
occur but only becomes important for energy levels greater than 3 or
4 MeV.
 Knowledge of these interaction processes which result in absorption
and scattering of photons during movement through specific absorber
material provides the basis for using gamma-ray attenuation as an ex-
cellent method for studying selected physical properties of matter.


 B. Attenuation of Monoenergetic Gamma Radiation
 After a collimated beam of single-energy gamma photons has passed
through a selected absorber material, measurements of the attenuated
radiation permit calculation of the density of that material. As a colli-
mated beam of mono-energetic photons with intensity Io (number of
photons per cm2 per sec) is transmitted through a homogeneous sub-
stance having density p (g/cm3) and thickness x (cm), the radiation
becomes attenuated in accordance with the well-known Lambert-Beer
equation used in physical chemistry:
 I = I. exp[-/zpx] [3]

 where I is the resulting intensity (number of photons per cm2 per sec)
 and A is the mass attenuation coefficient (cm2/g). Equation [3] is the
 integral form of equation [1].
 When gamma radiation is transmitted through a column of hetero-
 geneous material such as soil, gamma ray attenuation occurs not only
 by the soil' but also by other absorbers along the path of the photons.
 In a sample of moist soil, the intensity of a collimated beam undergoes
 attenuation by soil, water, and air components as well as the walls of
 the container and by the air separating the soil column from both the
 radiation source and the detector (see Figure 1). Since attenuation of
 gamma radiation by air is very, very small compared to that by the soil
 and soil water components, the influence of the air within the soil and
 the air surrounding the soil column is usually ignored in determinations
 of soil water content and bulk density. Equuaions [Aiv] and [Av] in
 Appendix A can be used to calculate the water content (0) of the soil,
 provided the soil density is known and remains constant with time,

 0 = 1 I(n -+ xp, [4]

 or to calculate the soil bulk density (p), provided the water content is
 known and remains constant with time,
 5