P = In [(l + x,]. [5]1

 Thus for a collimated beam of gamma photons with a specific primary
 energy transmitted through an undisturbed core or packed column of a
 given soil, the value of 0 or p can be calculated from the ratio of mea-
 sured intensities of radiation that has passed through the container filled
 with soil and through the empty container. Prior to measurements of
 the required radiation intensities, the thickness of the bulk soil (x), the
 mass attenuation coefficient for water (t,) and the specific photon en-
 ergy, and the mass attenuation coefficient for water ( ,,) and the spe-
 cific photon energy must be known as input parameters to either equa-
 tion [4] or [5]. Once the magnitude of these parameters is known, the
 magnitude of the water content (0) or bulk density (p) will vary accord-
 ing to changes in intensities (1) of the attenuated radiation. For example,
 if the water content (0) is being measured at given times under transient
 water flow conditions in a soil column (example: redistribution of soil
 water following infiltration) measurements of the radiation intensity (I)
 must be made at the required times.




 I I

 // 1 . .
 0 *10 *
 S* ..*1... .. Xc
 1. I Y Y .




Figure 1. Schematic photon path in a sample of soil. The thickness of the
soil sample is represented as x. The equivalent thicknesses of soil, air, and
water in the sample are represented as x,, x,, and x,c. The radiation source is
designated as S and the detector as D. The wall thickness of the container
is given as xc. The distances between the soil column and the source and
between the column and the detector are given as x,,1 and x,2, respectively.


 C. Dual-Energy Gamma Attenuation Method
 If gamma radiation is transmitted through a soil column either as a
single collimated beam of dual-energy photons or as two separate beams
of monoenergetic photons with different energies, mathematical equa-

 6