Gamma-Ray Attenuation for Soil-Water-Content Measurements Using 241Am

For several years, soil-water content has been measured by the attenuation of gamma radiation, both on laboratory samples and in the field. Usually, 137Cs has been used as the radioactive source emitting the gamma rays. This paper describes the use of 241Am as the source for such measurements, gives some advantages for using 241Am instead of 137Cs, and describes necessary precautions for the design of counting equipment. Both isotopes have the desired property of emitting nearly monoenergetic photons. The spectrum for 137Cs shows a major peak at about 662 keV while for 241Am, the peak occurs at 60 keV. Thus, the less energetic 241Am source requires substantially less shielding for operator safety than does 137Cs. The apparatus can be made smaller, lighter, and less costly; especially if it is desired to traverse the sample with a moving source and detector. The mass absorption coefficient for water is about 0.086 with 137Cs and about 0.20 with 241Am. The optimum thickness for best precision in gamma-ray attenuation techniques is between 1 and 3 half-thicknesses, with preference towards the upper value. Since, for water content measurements, we are primarily interested in the differences in over-all densities between saturated and dry soil, the effective half-thickness for water as it exists in the pores of the soil is an important value. For similar soil conditions, the optimum thickness of water in the soil sample is about 2.3 times greater for 137Cs than for 241Am. Thus, if the optimum thickness for 137Cs is 14 cm, the best thickness for 241Am is only 6 cm. For the smaller samples there is greater sensitivity using 241Am. Some precautions must be taken to ensure good counting efficiency for the less energetic 241Am source. The system used in this study included a 1-mm-thick Nal(Tl) crystal coupled to an EMI 9536B photomultiplier tube. The signal from the tube was passed through a special pre amplification section with gain of 20 before being fed to the preamplifier section of the RIDL scaler and timer. Data are presented showing the comparison of soil-water content measured volumetrically with the content determined by attenuation of gamma rays from 241Am. The sample was contained in a small cell where the moisture content was controlled by controlling the capillary pressure. The water was extracted through a saturated porous material and measured in a calibrated horizontal tube. (author)