CONTENTS
 I. Introduction ........ ............................. 1
 II. Historical Review of the Development and Use of
 Gamma Radiation Methods ......................... 2
 S III. Theory of Radiation Attenuation Methods in Soils ....... 3
 A. Interaction of Gamma Rays with Matter ........... 3
 B. Attenuation of Monoenergetic Gamma Radiation ..... 5
 M: C. Dual-Energy Gamma Attenuation Method .......... 6
 D. Experimental Factors Associated with Gamma
.a Attenuation M ethods ........................... 7
 E. Optimum Thickness of the Soil Column ............ 8
 IV. System Parameters ................................ 10
 A. Gamma Radiation Sources ....................... 10
 B. System Geometry and Collimation of the Radiation .... 14
 C. Detection of Gamma Radiation Intensity ............ 16
 V. Mass Attenuation Coefficients for Gamma Photons in
 W ater and Soils ................................. 18
 A. Theoretical Values ............................. 18
 0 B. Experimental Values ........................... 21
 VI. Single-Energy Method for Separate Determinations of
 Water Content and Bulk Density of Soil ............... 23
 SA. General Considerations ........................ 23
 SB. Example of Water Content Determination ........... 24
 S; C. Examples of Density Determination ................ 26
 VII. Dual-Energy Method for Simultaneous Determinations of
 Soil Water Content and Bulk Density .................. 27
 A. General Considerations .......................... 27
 B. Accuracy and Precision ......................... 28
 1. Sensitivity of the Method for Determinations for
 Soil W ater Content ........................... 29
 2. Sensitivity of the Method for Determination of
 Soil Bulk Density ............................ 31
 C. Examples of Simultaneous Determinations of 0 and p .. 32
 VIII. An Uncollimated Radiation Attenuation Method for In Situ
 Determination of Soil Water Content .................. 34
 A. Description of the Method ..................... 34
 B. Example of an In Situ Method ................... 35