Improving the MCLLS Method Applied to the In Vivo XRF Measurement of Lead in Bone by Using the Differential Operator Approach (MCDOLLS) and X-ray Coincidence Spectroscopy

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Title: Improving the MCLLS Method Applied to the In Vivo XRF Measurement of Lead in Bone by Using the Differential Operator Approach (MCDOLLS) and X-ray Coincidence Spectroscopy
Author: Guo, Weijun
Advisors: Matthias Stallmann, Committee Member
Dmitriy Y. Anistratov, Committee Member
Robin P. Gardner, Committee Chair
Charles W. Mayo, Committee Member
Abstract: Lead is a toxic chemical element with irreversible neurological effects on human being, which accumulates in human bones after ingestion or inhalation. To make in vivo measurement of the lead concentration in human bones, the Monte Carlo ¨C Library Least-Squares (MCLLS) method has been applied with an energy-dispersive X-ray fluorescence (EDXRF) Germanium spectrometer. The quantitative result is accurate compared to the certified lead concentration of the bone sample for measurement. To quantitatively study the matrix effect for EDXRF measurement and the measurement sensitivity, the Monte Carlo - Differential Operator method has been implemented to simulate differential responses of sample and elemental library spectra to variations of elemental compositions. The MCLLS method requires initial guesses for elemental compositions of the sample, which causes possibly several iterations of the Monte Carlo simulation code. To improve the efficiency of the MCLLS method, a combined method (MCDOLLS) has been implemented by using Taylor series expansion to re-adjust those elemental library spectra instead of running the simulation code one more time. Simulation cases of MCDOLLS show promising results for the in vivo lead in bone measurement. This method is generally applicable to other EDXRF applications with further investigation and benchmark. Source photons back-scattered from the sample are the dominant background for in vivo lead in bone measurement that prevents the improvement of measurement sensitivity. X-ray coincidence spectroscopy has been investigated in this thesis to minimize the detection of events that are not correlated in time, in other word, to relatively enhance the measurement sensitivity of K and L X rays of lead, which are in true coincidence theoretically. Coincidence experiments show good results to support the theory and Monte Carlo simulation results have been benchmarked with experimental data. For setting the electronics precisely for coincidence experiments, a complete procedure is also documented. To apply the X-ray coincidence spectroscopy to trace level lead in bone measurement, a customized spectrometer has been proposed by combining the high efficiency and low cost feature of big X-ray NaI(Tl) detectors with the fine resolution of low energy Ge detectors. This coincidence spectrometer has been simulated with the benchmarked Monte Carlo code and preliminary results are promising.
Date: 2003-11-16
Degree: PhD
Discipline: Nuclear Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/3165


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