A third consideration is that both the total and specific activities of
 the radiation source are important considerations. Because of errors due
 to the random nature of radioactive disintegration, a large number of
 photons must reach the detector. The actual number of photons counted
 is a function of source activity and specific activity, geometry, collima-
 tion, and electronic discrimination. Usually a source with 100 to 200
 mCi is used for a collimated beam in laboratories and 3 to 7 mCi for in
 situ 0 determinations in the field using uncollimated gamma radiation.
 Certain investigators have used stronger sources in the laboratory, such
 as Finnemore and Schaaf (28) with 250 mCi, Mansell et al. (58) and
 Gardner and Calissendorff (34) 300 mCi and Nofziger and Swartzen-
 druber (63) 280 and 389 mCi; Gardner, Campbell and Calissendorf
 (36) 500 mCi. Herkelrath and Miller (47) used a 1300 mCi 138Cs
 source and recommended 2000 mCi for use with plastic scintillator
 detectors having low deadtime.
 The thickness of the soil sample, its density and the gamma radiation
 energy determines the optimum experimental conditions. The product
 of density and thickness determines the best gamma energy to be used.
 Thus the primary energy of the gamma radiation is a fourth considera-
 tion when selecting a radiation source. For most soils with columns 4
 to 8 cm thick, the 60 KeV 241Am is recommended. For columns with
 thickness between 10 to 25 cm or for dense materials, the 662 KeV
 137Cs is normally used and sometimes in special cases, 1170 and 1310
 KeV of 6oCo or 1250 KeV of 22Na. For very thick samples of low-density
 materials, X-ray sources can also be used.
 Detailed discussion about sources of gamma transmission methods
has been given by Christensen (7). King (50) reported the advantages
for 241Am source in comparison with others and especially with 137Cs.
Gardner and Calissendorf (34) and Ferraz (26) discussed the possi-
bility for using sources other than 241Am and 837Cs in the dual-energy
gamma beam method for simultaneous measurements of bulk density
and water content. Careful examination of a radioisotope table verifies
that only a few radioisotopes can be used (Table 1).
 For the dual-energy gamma energy method, a collimated beam with
two very well distinguished energies is required. These energies must
provide adequate mass attenuation coefficients for soil and water, in
order to give acceptable resolution. Figure 2 shows variations in the
mass attenuation coefficients, for water and for a typical soil, as a func-
tion of photon energy. Note that these two mass attenuation coefficients
become greatly different only for low energies, that is below 70 KeV.
However, 203Hg, 170Tm and 109Cd, with energies 78, 84, and 88 KeV,
respectively, are just above this point. The 210Pb isotope with a half-life
of 20 years, has a peak at 47 KeV that could be an ideal source, but its
spectrum is complex and this peak represents only 4% of the entire
 12