B. System Geometry and Collimation of the Radiation
 Collimation is especially important in the single-energy gamma ray
 attenuation method for determining water content or bulk density in
 soil columns and particularly in simultaneous measurements with dual-
 energy photons. The collimator must provide either a single monoener-
 getic or two parallel monoenergetic beams or a single dual-energy beam,
 as thin as possible, but with maximum intensity. This ideal collimator is
 impossible to obtain due to the practical geometry limitations but many
 authors have obtained good collimation with materials such as lead.
 For water content or bulk density measurements by a monoenergetic
 gamma beam, configurations for the geometry and collimation of the
 beam have been discussed by Gurr (43) and Davidson et al. (17). Fer-
 guson (25) used radiation collimation only on the detector side. In order
 to meet specific objectives, many researchers have also optimized colli-
 mation by adjustment of the length, position, size, and shape of colli-
 mators.
 For the simultaneous measurements of water content and bulk density
with dual-energy photons, several important configurations have been
used. Gardner and Calissendorf (34) and other investigators, separately
passed two gamma beams having different energies through the same
collimator and used a single detector. Ferraz (26) used two parallel
beams which penetrated the sample in two locations separated by dis-
tances of 10 mm and detected by one or two scintillator detectors.
Stroosnijder and DeSwart (83) used two perpendicular beams which
penetrated the sample on different sides and was detected by two separate
detectors. Bridge and Collis-George (5) used a fixed geometry with only
one collimator in conjunction with a mechanical mechanism to alter-
nately change the sources.
 Under field conditions, the relative position of the source and detector
was investigated by Van Bavel et al. (87 and 89), Reginato et al. (70
and 71), and Ryhiner and Pankow (74). De Vries (21) used collima-
tors in the field and Reginato (69) used simultaneous measurements
with two sources in the field.
 Collimators are generally constructed of lead, because of its high
density, and sometimes of tungsten as well as other heavy metals. Sev-
eral collimator geometries have been used. The collimator cross section
and its length is determined by resolution and beam intensity. Some sci-
entists have used a circular cross section with different sizes. Two such
cases are Mansell et al. (58) and Ferraz (26), who used 78.5 and 4.45
mm2 cross sections, respectively. Others such as Gardner et al. (35) and
Nofziger and Swartzendruber (63) used rectangular slits with 6.75 and
18 mm2 cross sections, respectively.
 For best results, collimation of the radiation is required on both source
and detector sides. Collimating slits or holes should be parallel, of the