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A Review Of Fundamental Nuclear Physics Applies To Gamma Ray
The fundamental nuclear physics principles required for a thorough understanding of gamma spectral logging are discussed in terms which should be especially helpful to persons with limited experience inn nuclear techniques. After a brief discussion of radioactivity, a description of the various nuclear particles is give which leads to the discussion of artificial and accelerator produced neutron sources. The interaction of neutrons in materials by elastic and inelastic scattering and capture is treated with particular emphasis on those interactions pertinent to well logging. A brief description of cross sections is included. Neutron detection techniques utilizing B10(n,oc)Li7 and Li6(n,0c)H3 capture reactions and (n,p) recoil type reactions are presented. In addition to the conventional Geiger counter, detection of gamma rays by a scintillation counter is discussed in detail. The three processes of gamma ray interaction in matter (photoelectric, Compton and pair production) are covered since a thorough understanding of these processes is required to unscramble the complex spectra recorded in well logging. A brief section on processing data obtained from various counters presents the concepts of integral and differential type measurements. A tabulation of thermal neutron cross sections with most intense gamma rays resulting and the inelastic scattering cross section with its resulting gamma rays is given for the common earth materials. Data is presented to show the false interpretation that can be made by examination of gamma ray spectra without prior knowledge of lithology, porosity and presence of casing in a well. However, with a knowledge of these factors, a gamma spectral log may be used in certain cases to give elemental analysis to a reasonable degree of accuracy.
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