soil water, this value is lower and can be determined for each soil. This
method needs a high energy and high activity gamma source due to the
low cross-section of the reaction and the low deuterium content in nat-
ural waters. This method was first used by Haskell and Hawkins (45)
in a study of deuterium as a tracer, with a 24Na (2.75 MeV) source.
Corey et al. (11, 12) used a 1.4 Curies 208T1 source in secular equilib-
rium (4.3 Curies of 22sTh).


 X. Summary and Conclusions
 During the transmission of collimated monoenergetic gamma radia-
tion through a homogeneous material such as water the gamma photons
undergo scattering and absorption (primarily due to the Compton ef-
fect). Thus the intensity of the radiation decreases as the path length
for the transmission increases. The Lambert-Beer equation states that
the fraction of gamma rays attenuated is directly proportional to the
thickness of the material, x. The proportionality constant is the linear
attenuation coefficient. Dividing this value by the density provides the
well-known mass attenuation coefficient which is controlled by the
chemical composition of the material and by the primary energy of the
gamma photons. For a given material, an effective mass attenuation co-
efficient can be calculated by summing the individual products of at-
tenuation coefficients and contents for each chemical element. High
contents of elements like iron tend to give higher values for the effective
attenuation coefficient for the material. Also, for a given material, the
mass attenuation coefficient tends to be highest for low energy gamma
photons. For example, mass attenuation coefficients for water tend to be
higher for 60 KeV photons than for 662 KeV photons.
 For a heterogeneous material, such as soil, which is comprised of air,
 water, and solid components the Lambert-Beer equation can be used to
 obtain an attenuation equation which attributes changes in radiation in-
 tensity to each of the air, water, and solids components. The attenuating
 influence of soil air is infinitesimally small compared to water and solids
 and can thus be ignored. Thus the attenuation equation can be rear-
 ranged to provide an expression for water content if the density is known,
 or for bulk density if the water content is known.
 Attenuation of the incident intensity of a collimated beam of mono-
 energetic gamma rays transmitted through a non-swelling soil provides
 a convenient method to determine the dry bulk density and water con-
 tent of the soil. This technique is well developed for use under tem-
 perature-controlled laboratory conditions where precise, rapid, and non-
 destructive determinations are needed for packed columns or undisturbed
 cores of soil. Attenuation of the intensity of gamma radiation is not
 specific with respect to water or soil solids since an increase in the overall