photon energies termed Ia and I,, i.e., simnultaneousl. and alternately. For
simultaneous measurements of I, and Ic, the complex energy spectrum
with two prominent peaks (see Fig. 4) can be electronically separated
with a two-channel Lanjl',,er and two scalers to record the count-rates.
For this kind of measurement, special attention must be given to mini-
mizing and correcting the high energy peak interference in the low
energy peak counts. These interference have been documented by sev-
eral inmestig.atos. Gardner et al. (36), Corey et al. (13), Mansell et al.
(58) and Nofziger and Swartzendruber (63),
 Another way to obtain m tenItti.es 1 and 1, is to obtain the attenuated
 beam intensity in the 662 KeV channel (I,) alternately with that in the
 60 KeV channel (I ), in order to prevent Cesium interference in the
 Americium counts. Ferraz (2-') used a geometry with two parallel beams,
 where the Cesium radiation is shielded when the Americium peak is
 being counted. Bridge and Collis-George (5) used a mechanical device
 in order to change the position of the sources in the collimator. Stroos-
 nijder and De Swart (83) used a cross-beam device with two inde-
 pendent counting systems to obtain the same objectii eV. When the cross-
 beam device is used, the soil column must be rotated during measure-
 ment of radiation intensity. If the incident intensities for both photon
 energie, are high, sufficiently high counts in the attenuated beam may
 be obtained in only 10 to 20 seconds to permit measurements of 0 in
 soil during transient water flow.


 B. Accuracy and Precision
 Error in water content determinations by the dual-energy method
appear to be restricted to standard Je-\iations of appro\imntel. 0.010
cm '/cm8 (58) due to practical limitations upon measurement precision
of attenuation coefficients for soil and water. Calculated values for bulk
den'irt (p) and water content (0) should have an accuracy or standard
deviation approaching the limit imposed by the random nature of the
sources (36). In order to obtain p and 0 within a useful degree of ac-
curac\ (< 1%) in simultaneous measurements (see Fig. 9 and Ap-
pendix D), the mass absorption coefficients for the soil and water at
both gamma energies must be known with a standard deviation of better
than 10-4 cm2/g, if the total counts for low and high energy) photons
through air are about 4 X 106. The counting time must be kept short
in order to measure rapidly changing water contents and/or soil bulk
densities. Thus the incident gamma intensities must be high and the
processing of high counting rates becomes more complicated.
 Another important consideration concerns the water mass attenutation
coefficient value for low energy In most soils, the water which moves
in the column carries electrol\i e in solution, and the mass attenuation
 28