The influence of chemical composition of a soil upon the mass attenua-
tion coefficient was also reported by Reginato and Van Bavel (72). They
calculated theoretical values of g for 662 KeV photons for nine repre-
sentative soils of the United States. Reginato (69), calculated mass at-
tenuation coefficients of 60 KeV photons for the same soils. Coppola
and Reiniger (10) extensively discussed the dependence of k upon
chemical composition of soils and calculated attenuation coefficients for
four soils over a range from 10 to 3000 KeV.


 B. Experimental Values
 Experimental determinations of mass attenuation coefficients for soil
have provided values with magnitudes similar to theoretical values.
Proper determination of mass attenuation coefficients requires a small
standard deviation, good collimation of the radiation beam, a small dis-
criminator window for the spectrometer, high intensity of the incident
gamma beam, correction of counts for dead time, and use of optimum
thickness of the soil sample. These factors have been discussed in other
sections.
 In subsection B of section II the theoretical optimum thickness of
the soil sample was discussed. However, if very great accuracy and pre-
cision is needed in the measurement of mass attenuation coefficients,
especially when the dual-energy gamma beam method is used, addi-
tional considerations of the Compton effect is necessary. For example, if
the sample thickness is large and the photon energy low, the Compton
scattering influence increases and may become important to the ratio of
radiation intensities (I/Io).
 Gopal and Sanjeevaiah (38) reported that an increase in the experi-
 mental value of g is due to the multiple scattering of gamma photons
 within the soil sample, which increases the counts. They studied the
 sample thickness influence upon f measurements and their investigations
 indicate that multiple scattering of photons could be minimized if the
 product of the mass attenuation coefficient times the sample thickness is
 less than one mean free path ([x<l), especially for low energies.
 In addition to consideration of the cited factors, the accuracy of a
 measurement for A depends upon the nature of the absorber. Determina-
 tions of A for homogeneous and pure materials such as water is con-
 siderably simpler than for heterogeneous materials such as soil. Conner
 et al. (9) determined mass attenuation coefficients for several pure
 metals using equipment similar to that used by soil physicists, and they
 obtained a very low standard deviation. For example, they found for
 aluminum the value for the mass attenuation coefficient plus or minus
 the value of the standard deviation to be 0.1827 0.0008 cm2/g at
 88.09 KeV energy. For soils, a standard deviation value of 0.4% of the
 21