during the period 1960-1962. These workers determined bulk density
and water content in soil columns under conditions of steady-state water
flow. Davidson, Biggar and Nielsen (17) used a 200 mCi 137Cs source
to determine soil water content under conditions of transient water
flow. In each of these investigations, '37Cs was selected as the radiation
source because of the relatively high 660 KeV primary energy of the
photons. Lead collimators were used to provide a beam of monoener-
getic photons.
 The radioisotope 241Am was proposed by King (50) in 1962 as a
gamma radiation source for use in determining bulk density of soil.
Earlier in 1955, Miller (62) proposed the use of 241Am for measuring
concentrations of uranium and plutonium salts in aqueous solutions. The
low 60 keV primary energy for radiation from the 241Am radioisotope
and the long half-life of 460 years are advantageous for measurements
of water content and density of soils. Probably because of its limited
availability at the time, 241Am did not receive wide attention until about
1966. Gardner and collaborators (32, 33), Soane (77), and Groene-
velt, DeSwart, and Cisler (40), Vauchaud et al. (86), Stroosnijder (82),
and Cisler et al. (8) used 241Am as a radiation source. Today, the 241Am
gamma ray is commonly used by many researchers to determine water
content and density of soils and of low density materials such as wood
(27).
 For stable soils, bulk density may be assumed to remain constant dur-
 ing water flow, and thus changes in radiation intensity may be attributed
 to changes in volumetric water content. However, for soils which swell
 upon wetting and shrink upon drying or moist soils undergoing cycles of
 freezing and thawing, changes in intensity of a single beam of mono-
 energetic radiation may result from changes in both water content and
 density. Because of this limitation, either a single beam of dual-energy
 photons or two mono-energetic beams must be used to provide simul-
 taneous measurement of both bulk density and water content. The pos-
 sibility for using radiation from two gamma ray sources each with dif-
 ferent primary energy spectra was first considered theoretically by
 Durante et al. (22) in 1957 and later by Gardner and Calissendorf (34)
 in 1967. Soane (77, 78) in 1967, Nofziger and Swartzendruber (64) in
 1976 and Gardner and Fischer (32) in 1966 successfully tested this
 method to simultaneously determine water content and bulk density of
 soil columns.

 III. Theory of Radiation Attenuation Methods in Soils
 A. Interaction of Gamma Rays with Matter
 When monoenergetic gamma radiation passes through a homogeneous
 material gamma photons interact with electrons, nuclei and electrical
 3