fields to become absorbed and scattered. These interactions result in an
attenuation of the intensity, I, of the incident beam of photons. The
fraction (dl/I) of gamma rays attenuated is directly proportional to
the thickness of the material, x, where the proportionality constant, k,
is referred to as the linear attenuation coefficient (6)
 dl/ = k dx. [1]
 Many interaction processes (19, 48, 65) are known to occur between
gamma photons and matter, but only a few of these are important to the
measurement of volumetric water content and bulk density of soil.
Compton effect, which includes scalteillng and absorption, is the main
attenuation process, but photo-electric, p:lir-produc(ioii and Rayleigh
effects also may become important, depending upon the energy level.
 The linear attenuation coefficient termed k (units of 1/cm) for a
given radiation energy and for a specific homogeneous (example: liquid
water) material is the sum of several individual attenuation coefficients,
which represent all of the individually occurring attenuation processes:
 k = ke + k + kp +k ..... [2]
where the subscripts C, E, P, and R represent the Compton, photo-
electric, pair-production and Rayleigh effects, respectively. Each of these
coefficients taken separately represents the statistical prob:ibilit\ for oc-
currence of each of the individual interactions. The sum of all thc<e co-
efficients represents the total probability\ for attenuation of gamma rays
with a specific energy during transmission though a specific material
with a given chemical composition.
 Gamma-ray attenuation coefficients may also be expressed as a mass
attenuation coefficient, A, which is simply the linear coefficient (k)
divided by the densitl, (p). The coefficient termed A has units of cm2/g
and for a given radiation energy, depends upon the chemical properties
of the absorber material. The mass attenuation coefficient is generally
independent of the physical state of the material. For most purposes, the
mass attenuation coefficient, u, can be used more conveniently than the
linear coefficient (k).
 For a given absorber material, the fractional contributions of each of
the various absorption and scattering mechanisms to the total magnitude
of the mass attenuation coefficient (t) is greatly influenced by the pri-
mary vnerg\ level of the gamma photons. For example, during inter-
actions between 60 KeV photons and water the Compton Effect (ko/p)
coniributc' about 89% (81% of Compton scattering and 8% of Comp-
ton absorption) of total attenuation (L) and photo-electric (kW/p) and
Rayleigh (kg/p) effects with only 7% and 4%, respectively. For the
medium energy) region, such as 662 KeV of 137Cs, the Compton effect
(ko/p) accounts for about 100% of total attenuation (62% due to
 4