Table 1. A list of radioisotopes used as sources of gamma radiation for
 gamma-ray attenuation methods.

 Half-Life Main-Peaks
Radioisotopes (years) (%) (KeV)
Americium 241Am 458 86 60
Cadmium 109Cd 1.24 100 88
Cerium 144Ce 0.78 11 134
Cesium 134Cs 2.50 23 570
 98 605
 99 796
Cesium 137Cs 30 85 662
Lead 210Pb 20 4 47
Cobalt 60Co 5.30 100 1173
 100 1332
Iridium 192Ir 0.20 29 296
 20 308
 81 317
 49 468
Mercury 203Hg 0.13 38 78
 77 279
Sodium 22Na 2.60 180 511
 100 1275
Thulium 170Tm 0.37 3 84



spectrum. Among all of the low energy sources, only the 241Am has the
desirable characteristics of a long half-life, clean spectrum, low cost and
relatively large values of mass attenuation coefficients which are signifi-
cantly different between values for soil and for water. Since 241Am is
probably the best radioisotope to be used as a low energy source, the
obvious question is what is the best second source with higher energy
for the dual energy method? Calculations using the sensitivity equations
show that 110 KeV energy source would be ideal, but unfortunately
such an ideal radioisotope with this energy and practical conditions is
not available. Among the available gamma radiation sources (Table 1),
theoretically the best would be 144Ce, which has a peak at 134 KeV, but
the peak represents only a portion of the spectrum and has a short half-
life. The next best choice is '37Cs which is suited for practical conditions.
Today, most of the investigations with the dual-energy method for simul-
taneous measurements of bulk density and water content in soils are
made with paired 24'Am and 137Cs gamma-ray sources.