collimators (on both the radiation source and detector) for gamma ra-
diation.


 C. Detection of Gamma Radiation Intensity
 In the early work of Vomocil (90) a 60Co source and a Geiger Muller
detector were used for field measurements of soil bulk density. Van
Bavel et al. (89) later introduced the use of a sodium iodide (Nal)
scintillator detector, optically coupled to a photomultiplier tube and
connected to a single channel pulse analyzer. This kind of equipment is
currently used by soil physicists. The single channel gamma ray spec-
trometer with a sodium iodide detector has been described by Crouthanel
(15) or Davisson and Evans (18).
 The NaI(TI) detectors used for measuring intensity of narrow colli-
mated beams of medium energy such as 662 KeV (137Cs) require the
use of a thick Nal crystal (5.08 or 7.62 cm) to provide proper energy
discrimination. Ferraz (26) used a 7.62-cm thick scintillator with a
diameter of 7.62 cm for simultaneous detection of 60 KeV photons from
241Am and 662 KeV photons from 137Cs. Resolutions of 16% and 10%,
respectively, were obtained for 60 and 662 KeV photons. Mansell et al.
(58) used a 5.08-cm thick crystal with a diameter of 5.08 cm for the
same objective, and the resolution of this detector may be seen in Fig-
ure 4.
 Although used extensively in gamma spectroscopy analysis, the high
resolution (less than 1%) germanium Ge(Li) detector has not been
used in the majority of soil science laboratories, primarily because of its
low efficiency (0.2% for 662 KeV gamma photons), which is about 100
times less than for the Nal(TI). The Ge(Li) detector also needs per-
manent cooling with liquid nitrogen which results in many operational
difficulties in the laboratory and it is much more expensive than NaI(TI)
detectors. Corey et al. (13) examined advantages and disadvantages for
the Ge(Li) and NaI(TI) detectors with single channel and multichannel
analyzers of 400 and 4096 channels. For simultaneous measurements of
1'8Cs and 241Am peaks, they recommended the NaI(TI) detector with
a 400 channel pulse analyzer. Herkelrath and Miller (47) reported that
plastic scintillator detectors have the advantage of a low dead time (about
30 nano-seconds) but the disadvantage of very low efficiency. Plastic
scintillators operate best for strong (2 Ci) radioisotope sources. Kirkham
and Corey (51) have reviewed the types of radiation detectors used by
soil scientists.
 Measurements of intensity of a beam of medium energy gamma radia-
tion such as 137Cs is not difficult; however, for low energy radiations or
the use of two monoenergetic beams of radiation, simultaneous deter-
minations of bulk density and water content require special techniques.
 16