0 10




o
 5-


 O
 0 .5 \

 (0

 S.1 ^^ -WATER

 .05F SOILL- '
 .05

 U)
 O 60 662

 10 100 1000

 Primary Gamma Photon Energy
 E(KeV)
Figure 2. Mass attenuation coefficients for water and a representative soil as
functions of the primary energy of gamma photons (Ferraz, 26).


 IV. System Parameters

 A. Gamma Radiation Sources
 Most gamma-ray attenuation determinations of 0 by soil scientists
have been made with 137Cs (662 KeV) sources; however, 241Am (60
KeV) is frequently used as a gamma ray source. Preference for these
radioisotopes may be attributed to several inherent advantages over other
sources.