error. The observed counting rate, R, in the detection system can be
corrected for this coincidence loss with the quadratic equation given by
Chase and Rabinowitz (6) for the true count rate I

 1= -f- V 1- 4TR [11]
 27 L I
where r = pulse resolving time (microseconds/photon). A commonly
accepted (29) approximation of this equation
 I = R/(1 rR) [12]
is valid only when used with low count rates. A plot of R versus I re-
veals that at high count rates equation [12] overcompensates for coinci-
dence counting loss. For example if R = 0.25/r where 7 is the pulse
resolving time (with r = 2 microseconds per pulse, R = 7.5 x 106
cpm), equation [12] yields only I = 0.33/r whereas equation [11] yields
I = 0.50/r.


 V. Mass Attenuation Coefficients for Gamma Photons
 in Water and Soils
 A. Theoretical Values
 The mass attenuation coefficient, x, may be defined as the probability
per unit volume of a given absorber for collision with a photon of a
specific energy. This probability is controlled by the nature of the ab-
sorber and by the energy of the photon. For a given photon energy, the
mass attenuation coefficient for a heterogeneous material such as soil is
directly related to the chemical composition of that material. Because of
this, a theoretical mass attenuation coefficient for a given soil can be
calculated by summing the products of the mass attenuation coefficients
and the contents for the respective chemical elements by the equation


 S= E (MiAi) = M + 2+ Mn [13]

where n is the number of different elements in the absorber, Mj is the
percentage of the ith chemical element in the soil and p is the mass
attenuation coefficient for the ith element.
 In order to illustrate the validity of equation [13], /. values for several
geologic materials, soils and other gamma absorbers were determined
experimentally by Ferraz (26) using a gamma ray attenuation apparatus
with a 60 KeV 241Am source. The absorber materials were then an-
alyzed for chemical composition. Using the chemical analyses and mass
attenuation coefficients for the elements present (Table 2), theoretical
 18