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Title: Use of Californium-252 in laboratory testing for moisture and density of soils
Authors: Lewis, Jack T.
Keywords: Californium-252
Soil moisture
Nuclear methods
Soil testing
Radioactive isotopes
Soil density
Soil tests
Publisher: Soils and Pavements Laboratory (U.S.)
Engineer Research and Development Center (U.S.)
Description: Miscellaneous Paper
Abstract: The U.S. Army Engineer Waterways Experiment Station is currently investigating the use of the radioisotope californium-252 for detailed soil investigations. This report, on Phase I of the investigation, is concerned primarily with determination of moisture and, to a limited extent, density. A major portion of the study was devoted to quantifying moisture and density along thin, discrete soil layers in unopened 2. 875-in.-diam steel-encased soil cores. Both gaging and film imagery methods were evaluated for their applicability in the detailed soil studies. Through shielding devices, gamma and thermal neutron beams 1/4 in. wide by 2 in. high were used in direct radiation transmission for gaging. By using the proper detector for gamma rays and for thermal neutrons, radiation beams passed through a core were recorded in counts per second (CPS). The CPS is proportional to bulk density and weight of water, respectively. The data are plotted simultaneously on an X-Y plotter, thus producing a graphic log of the radiation characteristics of the sample investigated. Calibration curves prepared from samples of known moisture and density permit the interpretation of water and density from any position in a scanned tube of unknown moisture and density. Several factors may contribute to errors; principal ones are variations in tube diameter, impurities in the steel, adsorbed water, and water of crystallization in clays. Correction factors can be applied. The film imagery method uses a collimated beam of thermal neutrons for transmission through a sample and registration on radiographic film. A conversion screen converts neutrons into low-energy X-rays which, in turn, expose the film and produce an image of the distribution of water in a sample. Quantification of moisture is then accomplished from calibration and scanning of film density. The exposure times, using a 5.0-mg californium-252 source, for a 2.875-in.-diam tube required as much as 16 hr for samples with up to 25% moisture. Images of samples with more than 25% moisture showed only slight variation in film density with an increase in moisture. Thin soil slabs (3/8 in. thick) cut from undisturbed soils did produce good neutron radiographs within practical time limits. Such radiographs from 3/8-in. thick soil slabs have provided useful data with regard to moisture distributions.
Rights: Approved for public release; distribution is unlimited.
Appears in Collections:Miscellaneous Paper

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