TAMANHO DO COLIMADOR E ESPESSURA DA AMOSTRA EM MEDIDAS DO COEFICIENTE DE ATENUAÇÃO DE RAIOS GAMA DO SOLO

Abstract

The gamma-ray attenuation technique is based on the interaction of radiation with matter. In soil physics the technique is mainly used to determine the soil bulk density, porosity, and soil moisture. The mass attenuation coefficient ) is defined as the probability of interaction per unit of volume of a photon with a given energy with the matter. An accurate measurement of is important for evaluation of representative soil physical parameters. Usually,is considered constant during evaluations of soil physical properties, even for different sample sizes or nuclear system geometry. The main objective of this study was to analyze the effect of sample size and collimator diameter in evaluations of for a heterogeneous porous medium (soil). The radioactive sources of 241Am (59,4keV) and 137Cs (661,6 keV) were used. These sources are chosen because they are the most commonly used for soil physical properties measurements. Sample thickness and solid angle variations were obtained using acrylic boxes with sizes from 2 to 15 cm and four circular collimators (1-4 mm). Two different soil textures (sandy-loam and silty-clay) were also used, besides the different sample sizes and collimator diameters. The results obtained show that the influence of collimator size and sample thickness in the variations of is less than 5% (relative deviation considering each value for each thickness and the average value calculated for all thicknesses) for both soils and radioactive sources. However, for the 241Am only specific sample thicknesses (2-12 cm) can be used when collimators with small diameters (1 and 2 mm) are used. This occurs due to problems in the photopeak evaluation after determined soil thickness. In order to compare the obtained experimental results of with theoretical ones, theoretical values were generated via XCOM for water and soils. A good agreement was found for water (3,8% - 241Am and 6,3% - 137Cs, relative deviation considering the theoretical value as reference), which represents a good gamma-ray system operation. However, for the soil samples the same was not observed. A possible explanation for this result is the qualitative soil chemical composition evaluation used to generate the via XCOM. To conclude, the results of this work show that even small changes in the value with the collimator size and sample thickness can cause important changes in measurements of soil physical properties such its bulk density.