In the selection of a radiation source, several factors should be con-
sidered. First, the radiation spectrum must show a well distinguished
primary energy peak in a region free of interfering radiations, because
the commonly used solid NaI(Tl) scintillator detector has high effi-
ciency but limited resolution. Secondly, the radioisotope half-life must
be of the same or greater order than the duration of the programmed
experiments to minimize or eliminate corrections for decay. Also, the
cost for the system assemblage must be considered.




E
U



 R A
 .002





 C0C
 2 .001



 E D




 5 10
 Soil Sample Thickness, x(cm)

 Figure 3. Error in water content determination as a function of soil column
 thickness. Curve A corresponds to a mineral soil having large values for the
 mass attenuation coefficient (,= 0.4260 cm2/g for 60 KeV photons, p= 1.2
 g/cm3, and 0=0.2 cm3/cm3) due to a high content of iron. Curve B is for
 the same soil but with a higher bulk density (p= 1.4 g/cm3). Curve C is for
 an organic soil (,,=0.2552 cm2/g for 60 KeV photons, p=1.2 g/cm3, and
 0=0.20 cm3/cm3). Curve D represents any soil for high energy gamma
 radiation (662 KeV photons). This data was obtained by Ferraz (26).