Browsing by Author "Robin P. Gardner, Committee Chair"

Now showing 1 - 7 of 7

The Analog Linear Interpolation Approach for Monte Carlo Simulation of Prompt Gamma-Ray Neutron Activation Analysis
(2003-04-15) Zhang, Wenchao; Richard M. Lindstrom, Committee Member; Charles W. Mayo, Committee Member; Robin P. Gardner, Committee Chair; Griff Bilbro, Committee Member
The Monte Carlo code (CEARPGA I) was developed to generate the elemental library spectra required for implementing the Monte Carlo Library Least-Squares algorithm for prompt gamma-ray neutron activation analysis (PGNAA). The existing big weight problem in which a few histories yield very large weights with very large variance has been investigated thoroughly. It has been found that the expected value splitting technique, a powerful variance reduction technique used in the code is the primary cause of this problem. Two Monte Carlo simulation approaches have been investigated to eliminate the big weight problem while still maintaining high efficiency. They are 1) score importance map with batch tracking and 2) analog linear interpolation. Both approaches were implemented separately in the corresponding Monte Carlo codes on the basis of CEARPGA I code and demonstrated to be feasible for solving the big weight problem. The analog linear interpolation approach was selected to incorporate into the new CEARPGA Monte Carlo code (CEARPGA II). A comparison of the results obtained by both CEAEPGA I and CEARPGA II codes as well as experimentally measured data shows that the big weight problem has been successfully eliminated, the accuracy of the simulation has improved greatly, and the simulated results agree very well with the measured data. In addition, some other important improvements have been made in the CEARPGA II to make it more accurate, efficient and powerful, including: 1) tracking pair production gamma rays, 2) incorporating neutron activation background spectra, 3) generating natural background spectra, 4) taking into account the non-linearity of NaI detectors, and 5) adopting a general geometry package.
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
(2003-11-16) Guo, Weijun; Matthias Stallmann, Committee Member; Dmitriy Y. Anistratov, Committee Member; Robin P. Gardner, Committee Chair; Charles W. Mayo, Committee Member
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.
Investigation of Prompt Gamma-Ray Neutron Activation Analysis for Determining the Phase Amounts in Multiphase Flow
(2008-06-05) Mutiso, Athanas M; Sharon Lubkin, Committee Member; Robin P. Gardner, Committee Chair; Mohamed A. Bourham, Committee Member
Monte Carlo Application for the use of Detector Response Function on Scintillation Detector Spectra
(2009-08-07) Speaker, Daniel P; Robin P. Gardner, Committee Chair; Hany S. Abdel-Khalik , Committee Member; Robert White, Committee Member
The DRF is the pulse height distribution for an incident radiation, and is also a PDF which has the properties of always being greater than or equal to zero and also integrates to unity. The application of the DRF on a simulated spectrum results in the benchmarking of the simulation results with experimental results. The results are the nice Gaussian shapes that are caused by the statistical fluctuations in the energy and collection efficiency of the detector. To find the perfect simulation of the DRF is impossible due to the fact that the detector might have imperfections, where electrons can essentially become trapped and not be collected. One must rely on empirical models of nonlinearity and simulation data to do this. This is what CEARÃ¢â‚¬â„¢s DRF code g03 does. The time consuming task of a code like g03 is the time it takes to simulate the Monte Carlo simulation, in particular the electron transport of it. G03 couples rigorous gamma ray transport with very simple electron transport. By this methodology the non-linearity and the variable flat continua part of the DRF is accounted for. There are some problems and upgrades that needed to be addressed, for instance the difference in the valley region between the Photopeak and Compton Edge and parts of the Compton Continuum. This Monte Carlo simulation also simulates the detector as a bare crystal. It was found that this could account for as much of a reduction of as much 5 percent of the incident energy. And also distort the response function in the lower energy range of the function. For this MCNP was employed to simulate the difference between the bare and covered crystal. The MCNP simulation also included a surface current tally for electrons and photons on the interface between the can and the crystal, and also the interface between the side of the crystal and the can. From the results of the simulation of the can and no can simulation for the pulse height spectra are different. It here when it was determined to add a patch to make the simulation of the detector response function more accurate. This causes a sizeable difference in valley region, which can be explained as many different photopeaks in the valley region, due to Compton scatters in the can. Also one can distinguish between the plots and conclude that the side of the can contributes to the continuum due to the backwards continuum which starts around 0.2 MeV. The way that this will be added is different for the place where in the can contributes and type. For the electrons in the front and the side, the spectrum will be run through a program that will calculate the energy deposited and this will be added directly to the spectrum. The photons on the side will be run though MCNP with an f8 tally which will be in turn added to the spectrum. The photons from the front, and perhaps the most significant, will be added by g03 having a spectrum of incident photons on the crystal instead of the way it is done now with a monoenergetic energy. Then a patch will be added to make the code more accurate.
Monte Carlo Simulation of Energy-Dispersive X-Ray Fluorescence and Applications
(2009-03-13) Li, Fusheng; Dmitriy Y. Anistratov, Committee Member; Hao Zhang, Committee Member; Hany S. Abdel-Khalik, Committee Member; Robin P. Gardner, Committee Chair
Four key components with regards to Monte Carlo Library Least Squares (MCLLS) have been developed by the author. These include: a comprehensive and accurate Monte Carlo simulation code â€“ CEARXRF5 with Differential Operators (DO) and coincidence sampling, Detector Response Function (DRF), an integrated Monte Carlo â€“ Library Least-Squares (MCLLS) Graphical User Interface (GUI) visualization System (MCLLSPro) and a new reproducible and flexible benchmark experiment setup. All these developments or upgrades enable the MCLLS approach to be a useful and powerful tool for a tremendous variety of elemental analysis applications. CEARXRF, a comprehensive and accurate Monte Carlo code for simulating the total and individual library spectral responses of all elements, has been recently upgraded to version 5 by the author. The new version has several key improvements: input file format fully compatible with MCNP5, a new efficient general geometry tracking code, versatile source definitions, various variance reduction techniques (e.g. weight window mesh and splitting, stratifying sampling, etc.), a new cross section data storage and accessing method which improves the simulation speed by a factor of four and new cross section data, upgraded differential operators (DO) calculation capability, and also an updated coincidence sampling scheme which including K-L and L-L coincidence X-Rays, while keeping all the capabilities of the previous version. The new Differential Operators method is powerful for measurement sensitivity study and system optimization. For our Monte Carlo EDXRF elemental analysis system, it becomes an important technique for quantifying the matrix effect in near real time when combined with the MCLLS approach. An integrated visualization GUI system has been developed by the author to perform elemental analysis using iterated Library Least-Squares method for various samples when an initial guess is provided. This software was built on the Borland C++ Builder platform and has a user-friendly interface to accomplish all qualitative and quantitative tasks easily. That is to say, the software enables users to run the forward Monte Carlo simulation (if necessary) or use previously calculated Monte Carlo library spectra to obtain the sample elemental composition estimation within a minute. The GUI software is easy to use with user-friendly features and has the capability to accomplish all related tasks in a visualization environment. It can be a powerful tool for EDXRF analysts. A reproducible experiment setup has been built and experiments have been performed to benchmark the system. Two types of Standard Reference Materials (SRM), stainless steel samples from National Institute of Standards and Technology (NIST) and aluminum alloy samples from Alcoa Inc., with certified elemental compositions, are tested with this reproducible prototype system using a 109Cd radioisotope source (20mCi) and a liquid nitrogen cooled Si(Li) detector. The results show excellent agreement between the calculated sample compositions and their reference values and the approach is very fast. The funding of this work is provided by the Center for Engineering Application of Radioisotopes (CEAR) at North Carolina State University (NCSU).
Optimization of the Neutron Absorption Steady Source Measurement Method --by using an 124Sb-Be Source
(2002-12-16) Sun, Jing; Wushow Chou, Committee Member; Robin P. Gardner, Committee Chair; Charles W. Mayo, Committee Co-Chair
In the mineral exploration and oil well logging industries where logging tools are used based on neutron moderation and absorption principles, one needs to have an accurate knowledge of the neutron absorption cross-sections of the sample of interest for calibration purposes. This is difficult to ascertain by ordinary chemical analysis since some elements that are present in trace amounts (like boron, samarium, and gadolinium) have very large neutron absorption cross-sections. In oil well logging applications, chlorine appears in larger than trace amounts and often dominates the absorption cross-sections. For this reason a sensitive measurement is obtained by using the response to a source of thermal neutrons when a small sample is placed within a good moderating medium. Tittle and Crawford (1983) described and demonstrated such an approach. This method has evolved into one that uses a 252Cf source of neutrons with a 3He proportional counter. Attempts to optimize the sensitivity of this technique by Monte Carlo simulation with both MCNP and a specific purpose Monte Carlo code (Mickael, 1988) involving different detector positions relative to the sample, variable source-to-sample distances, and different moderators (Bussian, Jetter, and Supernaw, 1991, Talavarjula, 1995, and Sood, Gardner, and Gray, 2000) have had only limited success. The present approach involves the use of a 124Sb-Be neutron source with an average neutron energy of about 12 Kev compared to the 3 to 4 MeV average of the 252Cf source. Monte Carlo calculations with MCNP and subsequent benchmark experiment indicate a much improved sensitivity and accuracy with the 124Sb-Be source.
Prompt Gamma-ray Imaging for Small Animals
(2006-11-02) Xu, Libai; Avneet Sood, Committee Member; Dmitriy Y. Anistratov, Committee Member; Robin P. Gardner, Committee Chair; Wesley E. Snyder, Committee Member
A new imaging modality called prompt gamma-ray imaging (PGI) has been identified and investigated primarily by Monte Carlo simulation. Currently it is suggested for use on small animals. This new technique could greatly enhance and extend the present capabilities of PET and SPECT imaging from ingested radioisotopes to the imaging of selected non-radioactive elements, such as Gd, Cd, Hg, and B, and has the great potential to be used in Neutron Cancer Therapy to monitor neutron distribution and neutron-capture agent distribution. This approach consists of irradiating small animals in the thermal neutron beam of a nuclear reactor to produce prompt gamma rays from the elements in the sample by the radiative capture (n, γ) reaction. These prompt gamma rays are emitted in energies that are characteristic of each element and they are also produced in characteristic coincident chains. After measuring these prompt gamma rays by surrounding spectrometry array, the distribution of each element of interest in the sample is reconstructed from the mapping of each detected signature gamma ray by either electronic collimations or mechanical collimations. In addition, the transmitted neutrons from the beam can be simultaneously used for very sensitive anatomical imaging, which provides the registration for the elemental distributions obtained from PGI. The primary approach is to use Monte Carlo simulation methods either with the specific purpose code CEARCPG, developed at NC State University or with the general purpose codes GEANT4 or MCNP5, to predict results and investigate the feasibility of this new imaging idea. Benchmark experiments have been conducted to test the capability of the code to simulate prompt gamma rays, which are produced by following the nuclear structures of each irradiated isotope, and coincidence counting techniques, which are considered the most important improvement in neutron-related gamma-ray detection applications to reduce gamma background and improve system signal-to-noise ratios. With coincidence prompt gamma rays available, two major imaging techniques, electronic collimations and mechanic collimations, are implemented in the simulation to illustrate the feasibility of imaging elemental distribution by this new technique. The expectation maximization algorithm is employed in electronic collimation to reconstruct images. The common SPECT imaging algorithms are used in mechanical collimation to get an image. Several critical topics concerning practical applications have already been discussed, such as the radiation dose to the mouse and the detection efficiency of high-energy gamma rays.