@article{zhang_tang_jia_zhang_wang_li_gardner_2019, title={Application of the Monte Carlo Library Least-Squares (MCLLS) approach for chromium quantitative analysis in aqueous solution}, volume={150}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2019.02.018}, abstractNote={In the present work, a new in-situ prompt gamma-ray neutron activation analysis (PGNAA) setup was developed for the quantitative measurement of chromium (Cr) in aqueous solutions which consists of a 4" × 4″ inch Bismuth Germanate detector and a 300 mCi 241Americium-beryllium neutron source. A series of standard samples were prepared by dissolving Cr compounds in deionized water of analytical pure grade and measured using the in situ PGNAA setup. Quantitative spectrum analysis was conducted using Monte Carlo Library Least-Squares approach (MCLLS). Simulates of elemental library spectra were in silico modeled using the CEARCPG code, which was developed by Prof. Robin Gardner research group in North Carolina State University. The fitted spectra presented were in excellent agreement with the total experimental spectrum, and the correlation coefficients were all nearly 1. After applying the MCLLS approach, the minimum detectable concentration of Cr was 301.5 mg/L, better than that obtained with other setups, and the relative deviation of the Cr quantitative analysis accuracy was less than 4.09%.}, journal={APPLIED RADIATION AND ISOTOPES}, author={Zhang, Yan and Tang, Bin and Jia, WenBao and Zhang, Xiongjie and Wang, Renbo and Li, Fusheng and Gardner, Robin}, year={2019}, month={Aug}, pages={39–42} }
 @article{zhang_zhang_gardner_yan_wu_tian_chen_ti_2018, title={A method for determining density based on gamma ray and fast neutron detection using a Cs2LiYCl6 detector in neutron-gamma density logging}, volume={142}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2018.09.011}, abstractNote={With the increasing demand for radioisotope-free operations, pulsed neutron-gamma density (NGD) has become increasingly important for logging-while-drilling (LWD) development. However, current NGD tools, adopting the multiple-detector array design, are not conducive to the simplification of instrument design and measurement system. To break obstacles, based on the fast neutron-gamma coupled theory, a new density measurement method was proposed. Further, combined with the neutron-gamma simultaneous detection characteristics of the Cs2LiYCl6 (CLYC) detector, an NGD measurement system consisting of a D-T source and one CLYC detector was used. Results show that the new method is capable of determining formation density using a single CLYC detector, which can not only avoid complex instrument systems but also improve density sensitivity. Moreover, the applicability of the new density method was well verified by Monte Carlo simulation. Additionally, the method was successfully applied in a simulated well, and density results are in good agreement with the benchmarked formations. The research provides theoretical guidance for NGD instrument design.}, journal={APPLIED RADIATION AND ISOTOPES}, author={Zhang, Quanying and Zhang, Feng and Gardner, Robin P. and Yan, Huizhong and Wu, Guoli and Tian, Lili and Chen, Qian and Ti, Yongzhou}, year={2018}, month={Dec}, pages={77–84} }
 @article{chang_meric_sudac_nad_obhodas_hou_zhang_gardner_2019, title={Implementation of the Monte Carlo Library Least-Squares (MCLLS) approach for quantification of the chlorine impurity in an on-line crude oil monitoring system}, volume={155}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2018.05.012}, abstractNote={Prompt gamma-ray neutron activation analysis (PGNAA) has been widely used for elemental analysis of bulk samples as it provides an on-line, rapid and non-destructive means of performing such analysis. The Monte Carlo Library Least-Squares (MCLLS) approach is one of the methods for quantitative analysis providing elemental weight fractions via an assumption that the total prompt gamma-ray spectrum is a linear combination of the contributions from the individual prompt gamma-ray spectra of the sample constituents. For the generation of prompt gamma-ray libraries of each constituent, a specific purpose Monte Carlo code system called Center for Engineering Applications of Radioisotopes Coincidence Prompt Gamma-Ray (CEARCPG) is utilized. In this work, the focus was on detecting and measuring chlorine impurity in crude oil samples, i.e. trace amounts of chlorine. A preliminary study investigating the feasibility of PGNAA method in conjunction with the MCLLS approach for measuring trace amounts of chlorine in oil samples was performed. For this purpose, an extended version of the MCLLS approach, the so-called MCLLSX approach, was proposed and applied for the quantitative analysis. The results presented in this paper prove the feasibility of the proposed approach.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Chang, Hao Ping and Meric, Ilker and Sudac, Davorin and Nad, Karlo and Obhodas, Jasmina and Hou, Guojing and Zhang, Yan and Gardner, Robin P.}, year={2019}, month={Feb}, pages={197–201} }
 @article{zhang_zhang_gardner_liu_zhang_qiu_chen_tian_wang_2018, title={Quantitative monitoring of CO2 sequestration using thermal neutron detection technique in heavy oil reservoirs}, volume={79}, ISSN={["1878-0148"]}, DOI={10.1016/j.ijggc.2018.10.003}, abstractNote={CO2 enhanced oil recovery (CO2-EOR) project is of significance for CO2 sequestration and heavy oil recovery. Quantitative monitoring of CO2 saturation (SCO2) is essential to recognizing and understanding the migration and distribution of CO2 injected into the geological formations. In this paper, based on the difference in the neutron moderation ability of CO2, water and heavy oil, thermal neutron detection technique is applied in heavy oil reservoirs to monitor CO2 sequestration. By Monte Carlo simulation, the responses of thermal neutron count ratio versus different porosities and CO2 saturation were studied. Then, a mathematical model of CO2 saturation versus thermal neutron count ratio and formation porosity was established to quantitatively calculate CO2 saturation. Besides, the effects of formation pressure and temperature, heavy oil density, lithology, and other factors on the method were studied. Results show that variations of formation pressure, formation temperature, and density of heavy oil have little impact on the CO2 saturation measurement. However, the change of formation lithology results in larger CO2 saturation errors and needs corrections. In addition, the method has a low discrimination between CO2 and CH4 gas, and the results are easily affected by the CH4 content. Finally, a simulated case demonstrates the application of the method. For the heavy-oil sandstone with different porosities, the method shows a perfect performance: the SCO2 errors are less than 1% for the high and low gas saturated formation. This research provides an effective strategy to monitor CO2 storage and residual oil saturation in CO2-EOR reservoirs.}, journal={INTERNATIONAL JOURNAL OF GREENHOUSE GAS CONTROL}, author={Zhang, Feng and Zhang, Quanying and Gardner, Robin P. and Liu, Juntao and Zhang, Xiaoyang and Qiu, Fei and Chen, Qian and Tian, Lili and Wang, Yang}, year={2018}, month={Dec}, pages={154–164} }
 @article{feinberg_gardner_2019, title={Uncertainty quantification for Gamma-ray spectroscopy using the Library-Least Squares technique}, volume={155}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2018.08.015}, abstractNote={This work explores aspects of uncertainty quantification for inverse gamma-ray problems that use the Library Least Squares method by analyzing a simple example. Two uncertainty quantification techniques (first-order propagation and forward Monte-Carlo) are used in this work. The effects of resolution are explored by comparing results found with NaI and HPGe detectors. These results are confounded with the effects of total counts and binning. A conclusion is established that in some, very-specific cases, there is no reduction in uncertainty from using better-resolution detectors. The limitations of this conclusion and of the uncertainty quantification techniques used are discussed at length.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Feinberg, Aaron and Gardner, Robin P.}, year={2019}, month={Feb}, pages={191–196} }
 @article{zhang_jia_gardner_shan_zhang_hou_chang_2018, title={A distance correction method for improving the accuracy of particle coal online X-ray fluorescence analysis - Part 1: Theoretical dependence of XRF intensity on the distance}, volume={147}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2017.07.005}, abstractNote={During online X-ray fluorescence (XRF) measurement, the distance from XRF spectrometer to the sample surface always changes due to the rough surface textures of pulverized coal, resulting in changes in the X-ray fluorescent intensity and inaccuracy of online XRF measurement. To solve the impact of the sample's rough surface textures on the measurement accuracy, the theory and validity details of the dependence of XRF intensity on the distance as well as the comparison of the formula derivation and experimental verification were elaborated in the present research. A typical XRF calculation model has been built for theoretical derivation. The expression of the relationship between the XRF intensity and the distance from XRF spectrometer to sample surface was derived, where the variation of the distance influenced five physical phenomena: (1) absorption of X-rays by the air, (2) irradiated surface area of the samples, (3) changes in exit angle, (4) changes in solid angle and (5) absorption of X-ray fluorescence by the air. The intensities of the Fe Kα spectral lines in iron were calculated and agreed with the experiments very well, indicating that the expression of the dependence was accurate. The dependence of X-ray fluorescence intensity on the distance is the theoretical basis of the distance correction method to improve the accuracy of online XRF analysis techniques in industrial processes.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Zhang, Yan and Jia, Wen Bao and Gardner, Robin and Shan, Qing and Zhang, Xin Lei and Hou, Guojing and Chang, Hao Ping}, year={2018}, month={Jun}, pages={118–121} }
 @article{zhang_jia_gardner_shan_zhang_hou_chang_2017, title={A distance correction method for improving the accuracy of particle coal online X-ray fluorescence analysis - Part 2: Method and experimental investigation}, volume={141}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2017.07.004}, abstractNote={The distance from X-Ray Fluorescence (XRF) spectrometer to sample surface always changes with the different coal's particle sizes, resulting in the inaccuracy of online XRF measurement. To improve the accuracy of particle coal online XRF analysis, a distance correction method was established elaborated by iteration, which was based on the relationship between the XRF intensity and the distance. In order to verify the effectiveness of this method, five different particle size coal samples with same components have been measured by the online XRF analyzer directly above the conveyor belt, in the meanwhile, the distances between XRF spectrometer and samples' surface were obtained by a laser rangefinder. The results showed that the average distances are decreased with decreasing the particle size. By comparing the results of before and after applying the distance correction method, we demonstrated that the measurement accuracy of online XRF analysis for particle coal can be significantly increased. The distance correction method can be used for the development of online XRF analysis techniques applicable for real-time industrial processes.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Zhang, Yan and Jia, Wen Bao and Gardner, Robin and Shan, Qing and Zhang, Xin Lei and Hou, Guojing and Chang, Hao Ping}, year={2017}, month={Dec}, pages={235–238} }
 @article{chang_meric_sudac_nad_obhodas_gardner_2017, title={Development of a method for on-line determination of chlorine impurity in crude oil by using fast neutrons}, volume={209}, ISSN={["1873-7153"]}, DOI={10.1016/j.fuel.2017.06.123}, abstractNote={An important challenge in the petroleum industry is to reduce the salt content to acceptable levels in crude oil. Continuous monitoring of crude oil salinity is thus an important aspect. However, development of safe and reliable on-line and non-intrusive monitoring systems is still a challenging task. In this work, a method for non-intrusive, on-line monitoring of the chlorine impurity in crude oil based on prompt gamma-ray neutron activation analysis (PGNAA) in conjunction with a portable pulsed neutron generator producing 14 MeV neutrons in deuterium-tritium (DT) nuclear reactions is proposed and tested. Simulations with Monte Carlo N-Particle version 6 (MCNP6) transport code combined with Center for Engineering Applications of Radioisotopes Detector Response Function (CEARDRF) and a specific purpose Monte Carlo code called Center for Engineering Applications of Radioisotopes Coincidence Prompt Gamma-Ray (CEARCPG) were used for the design optimization of the experiments. Preliminary results indicate that a minimum detection limit for chlorine impurity of about 71 mg/L is achievable for an integration time of 30 min.}, journal={FUEL}, author={Chang, Hao Ping and Meric, Ilker and Sudac, Davorin and Nad, Karlo and Obhodas, Jasmina and Gardner, Robin P.}, year={2017}, month={Dec}, pages={643–649} }
 @article{zhang_jia_gardner_shan_hei_2018, title={Study on the PGNAA. measurement of heavy metals in aqueous solution by the Monte Carlo Library Least-Squares (MCLLS) approach}, volume={132}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2017.10.037}, abstractNote={In the present work, a prompt gamma neutron activation analysis (PGNAA) setup, which consists of a 300mCi 241Americium-Beryllium (Am-Be) neutron source and a 4 × 4-in. Bismuth germanium oxide (BGO) detector, was developed for heavy metal detection in aqueous solutions. A series of standard samples with analytical purity were prepared by dissolving heavy metals in deionized water. Quantitative spectrum analysis was performed by the Monte Carlo-Least-Squares (MCLLS) approach to measure the standard samples. The detector response functions of 4 × 4-in. BGO detector were generated by using the CEARDRF code. The element libraries were simulated in silico by the CEARCPG code, which was developed by Dr. Gardner. The simulation results presented were in very good agreement with the experimental results. The correlation coefficients were very close to 1 when the fitted spectrum was compared with the experimental spectrum. By applying the MCLLS approach, the relative deviation of the measurement accuracy was less than 2.27% for Ni, Mn, and Cu and up to 69.33% for Pb.}, journal={APPLIED RADIATION AND ISOTOPES}, author={Zhang, Yan and Jia, WenBao and Gardner, Robin and Shan, Qing and Hei, Daqian}, year={2018}, month={Feb}, pages={13–17} }
 @article{nelson_azmy_gardner_mattingly_smith_worrall_dewji_2017, title={Validation and uncertainty quantification of detector response functions for a 1″×2″ NaI collimated detector intended for inverse radioisotope source mapping applications}, volume={410}, ISSN={0168-583X}, url={http://dx.doi.org/10.1016/J.NIMB.2017.07.015}, DOI={10.1016/J.NIMB.2017.07.015}, abstractNote={Detector response functions (DRFs) are often used for inverse analysis. We compute the DRF of a sodium iodide (NaI) nuclear material holdup field detector using the code named g03 developed by the Center for Engineering Applications of Radioisotopes (CEAR) at NC State University. Three measurement campaigns were performed in order to validate the DRF's constructed by g03: on-axis detection of calibration sources, off-axis measurements of a highly enriched uranium (HEU) disk, and on-axis measurements of the HEU disk with steel plates inserted between the source and the detector to provide attenuation. Furthermore, this work quantifies the uncertainty of the Monte Carlo simulations used in and with g03, as well as the uncertainties associated with each semi-empirical model employed in the full DRF representation. Overall, for the calibration source measurements, the response computed by the DRF for the prediction of the full-energy peak region of responses was good, i.e. within two standard deviations of the experimental response. In contrast, the DRF tended to overestimate the Compton continuum by about 45–65% due to inadequate tuning of the electron range multiplier fit variable that empirically represents physics associated with electron transport that is not modeled explicitly in g03. For the HEU disk measurements, computed DRF responses tended to significantly underestimate (more than 20%) the secondary full-energy peaks (any peak of lower energy than the highest-energy peak computed) due to scattering in the detector collimator and aluminum can, which is not included in the g03 model. We ran a sufficiently large number of histories to ensure for all of the Monte Carlo simulations that the statistical uncertainties were lower than their experimental counterpart's Poisson uncertainties. The uncertainties associated with least-squares fits to the experimental data tended to have parameter relative standard deviations lower than the peak channel relative standard deviation in most cases and good reduced chi-square values. The highest sources of uncertainty were identified as the energy calibration polynomial factor (due to limited source availability and NaI resolution) and the Ba-133 peak fit (only a very weak source was available), which were 20% and 10%, respectively.}, journal={Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms}, publisher={Elsevier BV}, author={Nelson, N. and Azmy, Y. and Gardner, R.P. and Mattingly, J. and Smith, R. and Worrall, L.G. and Dewji, S.}, year={2017}, month={Nov}, pages={1–15} }
 @article{liu_zhang_gardner_hou_zhang_zhang_li_li_hu_2016, title={A method to improve the sensitivity of neutron porosity measurement based on D-T source}, volume={33}, ISSN={["2212-3865"]}, DOI={10.1016/j.jngse.2016.06.028}, abstractNote={Compensated porosity logging tool utilizing deuterium-tritium (D-T) source shows a lower sensitivity to the variation of formation porosity compared with that adopting Am-Be source. In order to improve the sensitivity, the factors of an infinite homogeneous formation influencing slowing-down length and the near to far counts ratio are analyzed. Then Monte Carlo simulation method is used to build well logging models to study the responses of a neutron porosity logging tool to hydrogen index and formation density. It shows that in addition to hydrogen index, the variation of the density also has a great impact on slowing-down length and the ratio which reduces the response sensitivity to porosity. A new model depicts the relationship between the count ratio and porosity is proposed. When the model is used to process the measured ratio, the ratio shows improved dynamic range and sensitivity to porosity compared with the values without processing.}, journal={JOURNAL OF NATURAL GAS SCIENCE AND ENGINEERING}, author={Liu, Juntao and Zhang, Feng and Gardner, Robin P. and Hou, Guojing and Zhang, Quanying and Zhang, Yan and Li, Xianghui and Li, Hu and Hu, Cong}, year={2016}, month={Jul}, pages={879–884} }
 @article{liu_zhang_gardner_hou_zhang_li_2015, title={A method to evaluate hydraulic fracture using proppant detection}, volume={105}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2015.08.003}, abstractNote={Accurate determination of the proppant placement and propped fracture height are important for evaluating and optimizing stimulation strategies. A technology using non-radioactive proppant and a pulsed neutron gamma energy spectra logging tool to determine the placement and height of propped fractures is proposed. Gd2O3 was incorporated into ceramic proppant and a Monte Carlo method was utilized to build the logging tools and formation models. Characteristic responses of the recorded information of different logging tools to fracture widths, proppant concentrations and influencing factors were studied. The results show that Gd capture gamma rays can be used to evaluate propped fractures and it has higher sensitivity to the change of fracture width and traceable proppant content compared with the exiting non-radioactive proppant evaluation techniques and only an after-fracture measurement is needed for the new method; The changes in gas saturation and borehole size have a great impact on determining propped fractures when compensated neutron and pulsed neutron capture tool are used. A field example is presented to validate the application of the new technique.}, journal={APPLIED RADIATION AND ISOTOPES}, author={Liu, Juntao and Zhang, Feng and Gardner, Robin P. and Hou, Guojing and Zhang, Quanying and Li, Hu}, year={2015}, month={Nov}, pages={139–143} }
 @article{hou_gardner_2015, title={A new G-M counter hybrid dead-time correction model}, volume={116}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2015.05.014}, abstractNote={A new G–M counter dead-time model based on the hybrid model is proposed and studied. The new model contains the idealized paralyzable and non-paralyzable dead-time models, but also allows the non-paralyzable dead-time to vary with the true counting rate. To validate this model, the decaying source experiment with a very pure 56Mn source was used. The calculated true counting rates obtained by fitting the experimental data to the model were compared to the data obtained by table lookup. In general, the fitting was in good agreement (within 4.8% up to 5.35×106counts/minute) for a G–M counter with a paralyzable dead-time component of 85.9 μs and a non-paralyzable dead-time component ranging from 438 μs to 24.3 ms.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Hou, Guojing and Gardner, Robin P.}, year={2015}, month={Nov}, pages={125–129} }
 @article{miller_holmes_gardner_2015, title={An analytical approach for treating background in spectral analysis measurements}, volume={116}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2015.01.018}, abstractNote={A method of determining the spectral shape of background radiation present in experimental spectra via a mathematical approach is presented. Elements of interest will be subtracted from an experimental spectrum using the linear correlation coefficient across a characteristic peak to determine their contribution. Once all elements of interest are removed, the remainder of the experimental spectrum should represent an approximation of the background. This approximation can then be used in conjunction with library least-squares to determine the background and elemental contributions to the unknown spectrum.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Miller, Ian and Holmes, Thomas W. and Gardner, Robin P.}, year={2015}, month={Nov}, pages={87–91} }
 @article{holmes_gardner_2015, title={Spectral analysis of shielded gamma ray sources using precalculated library data}, volume={116}, ISSN={0969-806X}, url={http://dx.doi.org/10.1016/J.RADPHYSCHEM.2015.04.017}, DOI={10.1016/J.RADPHYSCHEM.2015.04.017}, abstractNote={In this work, an approach has been developed for determining the intensity of a shielded source by first determining the thicknesses of three different shielding materials from a passively collected gamma-ray spectrum by making comparisons with predetermined shielded spectra. These evaluations are dependent on the accuracy and validity of the predetermined library spectra which were created by changing the thicknesses of the three chosen materials lead, aluminum and wood that are used to simulate any actual shielding. Each of the spectra produced was generated using MCNP5 with a sufficiently large number of histories to ensure a low relative error at each channel. The materials were held in the same respective order from source to detector, where each material consisted of three individual thicknesses and a null condition. This then produced two separate data sets of 27 total shielding material situations and subsequent predetermined libraries that were created for each radionuclide source used. The technique used to calculate the thicknesses of the materials implements a Levenberg–Marquardt nonlinear search that employs a tri-linear interpolation with the respective predetermined libraries within each channel for the supplied input unknown spectrum. Given that the nonlinear parameters require an initial guess for the calculations, the approach demonstrates first that when the correct values are input, the correct thicknesses are found. It then demonstrates that when multiple trials of random values are input for each of the nonlinear parameters, the average of the calculated solutions that successfully converges also produced the correct thicknesses. Under situations with sufficient information known about the detection situation at hand, the method was shown to behave in a manner that produces reasonable results and can serve as a good preliminary solution. This technique has the capability to be used in a variety of full spectrum inverse analysis problems including homeland security issues.}, journal={Radiation Physics and Chemistry}, publisher={Elsevier BV}, author={Holmes, Thomas Wesley and Gardner, Robin P.}, year={2015}, month={Nov}, pages={355–358} }
 @article{wang_lee_gardner_2014, title={A DUAL SYSTEM FOR MONITORING THE POSITIONS OF MULTIPLE RADIOACTIVE TRACER PEBBLES IN SCALED PEBBLE BED REACTORS}, volume={185}, ISSN={["1943-7471"]}, DOI={10.13182/nt13-13}, abstractNote={A dual measurement system for monitoring the simultaneous positions of multiple radioactive tracer pebbles in scaled pebble bed reactors (PBRs) has been developed and benchmarked to the prototype stage. The first system (the collimated system) is an updated version of a previously developed system that is now a completely automatic system that uses three collimated directionally variable NaI detectors that are programed to continuously search for a maximum counting rate from a single radioactive pebble. This system can be used by itself when a single radioactive tracer pebble is of interest and the pebble is relatively slow moving. In the present case, its primary use is to provide an independent measurement of the position of a stationary tracer pebble that is used to provide a point for calibration of the second system. The second system (the uncollimated system) is a modified version of a multiple uncollimated NaI detector system commonly called CARPT. The modified version involves those changes necessary to allow for use of the entire gamma-ray spectra for the inverse problem instead of only the gamma-ray full energy peaks. This allows one to use multiple radioisotopes each in a different tracer pebble so that up to ten individual tracer pebbles can be followed simultaneously with the best possible accuracy. The inverse problem is treated with the Monte Carlo library least-squares approach in which Monte Carlo–generated library spectra for each radioisotope are made available for a complete range of reference positions within the scaled PBR. Then, any unknown total gamma-ray spectra can be analyzed in an iterative fashion with the radioisotope library spectra to yield the position of all the radioisotope tracer pebbles. The scaled PBR used was a 30-cm-high and 30-cm-diam circular cylindrical section on the top and a cone with a 25-deg angle on the bottom. The pebbles are 1.2-cm glass marbles. Results have been obtained with both single tracer radioisotope marbles and multiple tracer radioisotope marbles, simultaneously.}, number={3}, journal={NUCLEAR TECHNOLOGY}, author={Wang, Zhijian and Lee, Kyoung O. and Gardner, Robin P.}, year={2014}, month={Mar}, pages={259–269} }
 @article{gardner_lee_2015, title={Personal reflections on the highlights and changes in radiation and radioisotope measurement applications}, volume={116}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2014.11.019}, abstractNote={This paper describes the recent changes that the authors have perceived in the use of radiation and radioisotope measurement applications. The first change is that due to the increased use of Monte Carlo simulation which has occurred from a normal evolutionary process. This is due in large part to the increased accuracy that is being obtained by the use of detector response functions (DRFs) and the simultaneous increased computational efficiency that has become available with these DRFs, the availability of a greatly improved weight windows variance reduction method, and the availability of inexpensive computer clusters. This first change is a happy one. The other change that is occurring is in response to recent terrorist activities. That change is the replacement or major change in the use of long-lived radioisotopes in radioisotope measurement and other radioisotope source applications. In general this can be done by improving the security of these radioisotope sources or by replacing them altogether by using machine sources of radiation. In either case one would like to preclude altogether or at least minimize the possibility of terrorists being able to obtain radioisotopes and use them for clandestine purposes.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Gardner, Robin P. and Lee, Kyoung O.}, year={2015}, month={Nov}, pages={28–31} }
 @article{meric_johansen_mattingly_gardner_2014, title={On the ill-conditioning of the multiphase flow measurement by prompt gamma-ray neutron activation analysis}, volume={95}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2012.12.047}, abstractNote={Abstract Prompt gamma-ray neutron activation analysis (PGNAA) in conjunction with the so-called Monte Carlo library least-square (MCLLS) approach for the quantitative analysis is currently considered for rapid, non-intrusive and online measurements of multiphase oil/gas/seawater flow. The results of this work indicate that the current method would be feasible for measurements of multiphase flow provided that the ill-conditioning in the MCLLS approach could be treated appropriately.}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Meric, Ilker and Johansen, Geir A. and Mattingly, J. and Gardner, R. P.}, year={2014}, month={Feb}, pages={401–404} }
 @article{lee_gardner_2013, title={Prediction of Pebble Motion in Pebble-Bed Reactors Using Monte Carlo Molecular Dynamics Simulation}, volume={174}, ISSN={["1943-748X"]}, DOI={10.13182/nse12-23}, abstractNote={Abstract Pebble motions in pebble-bed reactors (PBRs) have been investigated by generating pebble motion histories with Monte Carlo molecular dynamics simulations. This extension of molecular dynamics to PBR-sized pebble motion is accomplished by splitting the simulation into two parts. The first part simulates the dropping of pebbles into the PBR with a closed exit that allows one to obtain the correct initial placement of all pebbles within the pebble bed. The second part simulates what happens when the PBR exit is opened and normal pebble flow begins. Using this combined approach the pebble piling up and subsequent discharge are predicted. Simulations have been conducted with this approach by monitoring the mass flow rate, the pebble piling up, and the subsequent discharge for a range of pertinent parameters using the Hertz-Mindlin force for pebble interactions. The simulation output data include the force, velocity, and position of the pebbles as a function of time. Note that arching or locked flow, a very important phenomenon, is predicted by this approach under certain operating conditions. Using this approach, PBR results (including arching) for a range of the parameters of interest are reported and are discussed herein.}, number={3}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Lee, Kyoung O. and Gardner, Robin P.}, year={2013}, month={Jul}, pages={264–285} }
 @article{meric_johansen_holstad_calderon_gardner_2012, title={Enhancement of the intrinsic gamma-ray stopping efficiency of Geiger-Muller counters}, volume={696}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2012.08.086}, abstractNote={Geiger–Müller counters are frequently employed in many industrial radiation gauges. However, it is of practical interest to further extend the usage areas of these detectors to include applications such as dual modality densitometry and industrial gamma-ray tomography. This is mainly because these counters exhibit a high degree of robustness in demanding environments, their relatively low cost and the fact that they require simple read-out electronics. The main drawback of these counters is their relatively poor intrinsic gamma-ray stopping efficiency which is of the order of 1.0% for a wide range of primary photon energies. Therefore, identification of a method whereby the poor stopping efficiency in these detectors could be improved is of great interest. For this purpose Monte Carlo analyses were carried out, at photon emission energy of 59.5 keV, to investigate the feasibility of introducing high density and high atomic number insulating disks inside a cylindrical Geiger–Müller counter. Calculations were performed using a specific purpose Monte Carlo code that was validated against experimental stopping efficiency data at 59.5 keV. The preliminary simulation results show that a threefold increase in the efficiency of the counter could be expected as well as the fact that the simulation of secondary electron transport in the fill gas becomes critical. The results reported in this work also show that the maximum achievable low energy gamma-ray detection efficiency in Geiger–Müller counters is still significantly less than the efficiency of semiconductor and scintillation detectors at the corresponding photon energies. Therefore, the development of efficient Geiger–Müller counters will remain a challenge.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Meric, Ilker and Johansen, Geir A. and Holstad, Marie B. and Calderon, Adan F. and Gardner, Robin P.}, year={2012}, month={Dec}, pages={46–54} }
 @article{lee_holmes_calderon_gardner_2012, title={Molecular Dynamics simulation for PBR pebble tracking simulation via a random walk approach using Monte Carlo simulation}, volume={70}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.11.043}, abstractNote={Using a Monte Carlo (MC) simulation, random walks were used for pebble tracking in a two-dimensional geometry in the presence of a biased gravity field. We investigated the effect of viscosity damping in the presence of random Gaussian fluctuations. The particle tracks were generated by Molecular Dynamics (MD) simulation for a Pebble Bed Reactor. The MD simulations were conducted in the interaction of noncohesive Hertz–Mindlin theory where the random walk MC simulation has a correlation with the MD simulation. This treatment can easily be extended to include the generation of transient gamma-ray spectra from a single pebble that contains a radioactive tracer. Then the inverse analysis thereof could be made to determine the uncertainty of the realistic measurement of transient positions of that pebble by any given radiation detection system designed for that purpose.}, number={5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Lee, Kyoung O. and Holmes, Thomas W. and Calderon, Adan F. and Gardner, Robin P.}, year={2012}, month={May}, pages={827–830} }
 @article{meric_johansen_holstad_mattingly_gardner_2012, title={On the treatment of ill-conditioned cases in the Monte Carlo library least-squares approach for inverse radiation analyzers}, volume={23}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/23/5/055603}, abstractNote={Prompt gamma-ray neutron activation analysis (PGNAA) has been and still is one of the major methods of choice for the elemental analysis of various bulk samples. This is mostly due to the fact that PGNAA offers a rapid, non-destructive and on-line means of sample interrogation. The quantitative analysis of the prompt gamma-ray data could, on the other hand, be performed either through the single peak analysis or the so-called Monte Carlo library least-squares (MCLLS) approach, of which the latter has been shown to be more sensitive and more accurate than the former. The MCLLS approach is based on the assumption that the total prompt gamma-ray spectrum of any sample is a linear combination of the contributions from the individual constituents or libraries. This assumption leads to, through the minimization of the chi-square value, a set of linear equations which has to be solved to obtain the library multipliers, a process that involves the inversion of the covariance matrix. The least-squares solution may be extremely uncertain due to the ill-conditioning of the covariance matrix. The covariance matrix will become ill-conditioned whenever, in the subsequent calculations, two or more libraries are highly correlated. The ill-conditioning will also be unavoidable whenever the sample contains trace amounts of certain elements or elements with significantly low thermal neutron capture cross-sections. In this work, a new iterative approach, which can handle the ill-conditioning of the covariance matrix, is proposed and applied to a hydrocarbon multiphase flow problem in which the parameters of interest are the separate amounts of the oil, gas, water and salt phases. The results of the proposed method are also compared with the results obtained through the implementation of a well-known regularization method, the truncated singular value decomposition. Final calculations indicate that the proposed approach would be able to treat ill-conditioned cases appropriately.}, number={5}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Meric, Ilker and Johansen, Geir A. and Holstad, Marie B. and Mattingly, John and Gardner, Robin P.}, year={2012}, month={May} }
 @article{meric_johansen_holstad_lee_calderon_wang_gardner_2011, title={A single scatter electron Monte Carlo approach for simulating gamma-ray stopping efficiencies of Geiger-Müller counters}, volume={654}, ISSN={0168-9002}, url={http://dx.doi.org/10.1016/J.NIMA.2011.06.065}, DOI={10.1016/J.NIMA.2011.06.065}, abstractNote={Abstract In spite of their relatively poor gamma-ray stopping efficiencies, the Geiger-Muller (GM) counter is still preferred in many radioisotope gauges for industrial measurements. This is because these detectors exhibit a high degree of robustness in harsh environments, are relatively insensitive to temperature changes in the environment, and are inexpensive compared to other types of radiation detectors. These properties could make the use of GM counters very feasible in a number of industrial applications, such as gamma-ray tomography and gamma-ray density gauges, provided that their gamma-ray stopping efficiencies can be improved. The Monte Carlo (MC) method is a powerful computational physics tool that is utilized very often in the design of radiation detectors and radioisotope gauges. In this work a MC model for GM counters that is benchmarked with experiments at the primary photon energy of 59.5 keV is proposed. This is a specific purpose MC simulation code that, as opposed to publicly available general purpose MC codes, employs single scatter (or microscopic) electron transport and is currently under development. In this paper, the MC code is described in detail and the results of the specific purpose MC code are benchmarked with experiments and two general purpose MC codes, MCNP5 and PENELOPE. It was observed that the specific purpose MC code improved the reduced chi-square value when compared to MCNP5 and PENELOPE.}, number={1}, journal={Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment}, publisher={Elsevier BV}, author={Meric, Ilker and Johansen, Geir A. and Holstad, Marie B. and Lee, Kyoung O. and Calderon, Adan F. and Wang, Jiaxin and Gardner, Robin P.}, year={2011}, month={Oct}, pages={279–287} }
 @article{yu_sun_wang_gardner_2011, title={Accuracy and borehole influences in pulsed neutron gamma density logging while drilling}, volume={69}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.04.023}, abstractNote={A new pulsed neutron gamma density (NGD) logging has been developed to replace radioactive chemical sources in oil logging tools. The present paper describes studies of near and far density measurement accuracy of NGD logging at two spacings and the borehole influences using Monte-Carlo simulation. The results show that the accuracy of near density is not as good as far density. It is difficult to correct this for borehole effects by using conventional methods because both near and far density measurement is significantly sensitive to standoffs and mud properties.}, number={9}, journal={APPLIED RADIATION AND ISOTOPES}, author={Yu, Huawei and Sun, Jianmeng and Wang, Jiaxin and Gardner, Robin P.}, year={2011}, month={Sep}, pages={1313–1317} }
 @article{wang_wang_peeples_yu_gardner_2012, title={Development of a simple detector response function generation program: The CEARDRFs code}, volume={70}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.11.003}, abstractNote={A simple Monte Carlo program named CEARDRFs has been developed to generate very accurate detector response functions (DRFs) for scintillation detectors. It utilizes relatively rigorous gamma-ray transport with simple electron transport, and accounts for two phenomena that have rarely been treated: scintillator non-linearity and the variable flat continuum part of the DRF. It has been proven that these physics and treatments work well for 3×3″ and 6×6″ cylindrical NaI detector in CEAR's previous work. Now this approach has been expanded to cover more scintillation detectors with various common shapes and sizes. Benchmark experiments of 2×2″ cylindrical BGO detector and 2×4×16″ rectangular NaI detector have been carried out at CEAR with various radiactive sources. The simulation results of CEARDRFs have also been compared with MCNP5 calculations. The benchmark and comparison show that CEARDRFs can generate very accurate DRFs (more accurate than MCNP5) at a very fast speed (hundred times faster than MCNP5). The use of this program can significantly increase the accuracy of applications relying on detector spectroscopy like prompt gamma-ray neutron activation analysis, X-ray fluorescence analysis, oil well logging and homeland security.}, number={7}, journal={APPLIED RADIATION AND ISOTOPES}, author={Wang, Jiaxin and Wang, Zhijian and Peeples, Johanna and Yu, Huawei and Gardner, Robin P.}, year={2012}, month={Jul}, pages={1166–1174} }
 @article{li_gardner_2012, title={Implementation of the elemental library stratified sampling technique on the GUI-based Monte Carlo library least squares (MCLLS) approach for EDXRF analysis}, volume={70}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.09.012}, abstractNote={The Monte Carlo-based Energy Dispersive X-ray Fluorescence (MCXRF) elemental analyzer approach can be used to accurately calculate elemental weight fractions in rock formations or metal alloys, by least-squares regression of the measured X-ray spectrum with computer-generated elemental library spectra. The analog sampling scheme in a Monte Carlo simulation cannot generate satisfactory results for trace elements due to their low sampling probability. A stratified sampling variance-reduction technique is proposed and implemented to enable each element in the measured sample to have equal opportunity of being sampled with adjusted weights. Essentially, all computed elemental libraries have good statistical precision and improve the accuracy of the elemental analysis results.}, number={7}, journal={APPLIED RADIATION AND ISOTOPES}, author={Li, Fusheng and Gardner, Robin P.}, year={2012}, month={Jul}, pages={1243–1249} }
 @article{li_smith_hehlen_mckigney_gardner_2012, title={Light yield measurement method for milled nanosize inorganic crystals}, volume={70}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.09.021}, abstractNote={Composite scintillators consisting of nanosize inorganic crystals embedded in an organic matrix have been actively pursued in recent years. One method of producing nanosize crystals is through wet milling; however, since milling is known to introduce defects, the light yield of the milled crystals must be characterized. In this work, a new method of characterizing the light yield of milled inorganic crystals will be explored and discussed; this method will take into account explicitly the concentration of the inorganic crystals and the difference in stopping power between the crystals and the solvent.}, number={7}, journal={APPLIED RADIATION AND ISOTOPES}, author={Li, Andy and Smith, N. and Hehlen, M. P. and McKigney, E. A. and Gardner, R.}, year={2012}, month={Jul}, pages={1219–1222} }
 @article{meric_johansen_holstad_gardner_2011, title={Monte Carlo modelling of gamma-ray stopping efficiencies of Geiger-Muller counters}, volume={636}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2011.01.083}, abstractNote={A detailed study of the intrinsic stopping efficiency mechanisms of low energy gamma-rays (59.5 keV) in a Geiger–Müller counter is presented. MCNP5 and PENELOPE 2006 simulations have been carried out and the simulation results have been benchmarked with experimentally obtained stopping efficiencies. The results show that the inner wall interactions are critical for the gamma-ray stopping efficiencies.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Meric, I. and Johansen, G. A. and Holstad, M. B. and Gardner, R. P.}, year={2011}, month={Apr}, pages={61–66} }
 @article{peeples_gardner_2012, title={Monte Carlo simulation of the nonlinear full peak energy responses for gamma-ray scintillation detectors}, volume={70}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2011.12.006}, abstractNote={A Monte Carlo code has been developed, which predicts the nonlinear full peak energy responses of scintillation detectors to incident gamma-rays. It is illustrated here for the popular scintillation detectors, NaI and BGO. The full energy response can be determined by treating the detector as effectively infinite and assuming that all photons and electrons are fully absorbed within the detector. This assumption means that no geometrical direction or position tracking is required, only the selection of sequential photon interactions based on the appropriate energy-dependent interaction cross-sections. The full energy pulse-height response is determined by the sum of the pulse-height responses from all secondary electrons. Results from infinite NaI and BGO detectors indicate that even though the maximum difference in electron scintillation efficiency is about the same for the two scintillation detectors, the overall effect on the extent of the difference in pulse height is much less for BGO than NaI. This result is due to the larger density and effective atomic number of BGO, which causes significantly fewer Compton scattering events. Compton scattering interactions reduce the incident photon energy without absorption and therefore give more responses at reduced energy where the electron scintillation efficiency is most different.}, number={7}, journal={APPLIED RADIATION AND ISOTOPES}, author={Peeples, Johanna L. and Gardner, Robin P.}, year={2012}, month={Jul}, pages={1058–1062} }
 @article{meric_johansen_holstad_wang_gardner_2011, title={Produced water characterization by prompt gamma-ray neutron activation analysis}, volume={22}, DOI={10.1088/0957-0233/22/12/125701}, abstractNote={Multiphase gas/oil/water measurement has been and still is a challenging task within the petroleum industry. The requirements of measurement efficiency and accuracy have been continuously increasing at the same time as the trend is to use subsea installations. As a response, the design of the SOFA (Subsea Online Fluid Analyser) concept was initiated by Christian Michelsen Research in cooperation with the University of Bergen. The final design of the analyser is yet to be completed. The ultimate goal of this design is the development of a permanently installed subsea metering station which is capable of characterizing all of the components that can be found in multiphase flow. One of the key challenges in this design is the detailed characterization of produced water samples which includes the identification of salt ions in the sample and their weight percentages. In this work, use of prompt gamma-ray neutron activation analysis (PGNAA) in conjunction with the so-called Monte Carlo library least-squares (MCLLS) approach is considered for characterization of the produced water component. A relatively simple produced water sample was prepared and the prompt gamma-ray spectrum of the sample was recorded using a large 6" × 6" NaI(Tl) scintillation detector. A specific purpose Monte Carlo code named CEARCPG was used to generate the pertinent elemental libraries that are required in the MCLLS approach. The results of this feasibility study have shown that the PGNAA in conjunction with the MCLLS approach would be feasible for determining the amounts of salt ions that can be found in produced water samples.}, number={12}, journal={Measurement Science & Technology}, author={Meric, I. and Johansen, G. A. and Holstad, M. B. and Wang, J. X. and Gardner, R. P.}, year={2011} }
 @article{gardner_ai_peeples_wang_lee_peeples_calderon_2011, title={Use of an iterative convolution approach for qualitative and quantitative peak analysis in low resolution gamma-ray spectra}, volume={652}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2010.12.224}, abstractNote={In many applications, low resolution gamma-ray spectrometers, such as sodium iodide scintillation detectors, are widely used primarily due to their relatively low cost and high detection efficiency. There is widespread interest in improved methods for analyzing spectral data acquired with such devices, using inverse analysis. Peak means and peak areas in gamma- and X-ray spectra are needed for both qualitative and quantitative analysis. This paper introduces the PEAKSI code package that was developed at the Center for Engineering Applications of Radioisotopes (CEAR). The basic approach described here is to use accurate forward models and iterative convolution instead of direct deconvolution. Rather than smoothing and differentiation a combination of linear regression and non-linear searching is used to minimize the reduced chi-square, since this approach retains the capability of establishing uncertainties in the estimated peak parameters. The PEAKSI package uses a Levenberg–Marquardt (LM) non-linear search method combined with multiple linear regression (MLR) to minimize the reduced chi-square value for fitting single or multiple overlapping peaks to determine peak parameters, including peak means, peak standard deviations or full width at half maximum (FWHM), net peak counts, and background counts of peaks in experimental gamma-ray spectra. This approach maintains the natural error structure so that parameter uncertainties can be estimated. The plan is to release this code to the public in the near future.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Gardner, Robin P. and Ai, Xianyun and Peeples, Cody R. and Wang, Jiaxin and Lee, Kyoung and Peeples, Johanna L. and Calderon, Adan}, year={2011}, month={Oct}, pages={544–549} }
 @article{gardner_li_2011, title={Use of the CEARXRF GUI-Based Monte Carlo-Library Least-Squares (MCLLS) Code for the Micro-Focused EDXRF analyzer}, volume={40}, ISSN={["0049-8246"]}, DOI={10.1002/xrs.1365}, abstractNote={The CEARXRF GUI‐Based Monte Carlo–Library Least‐Squares (MCLLS) Code is demonstrated with results from a micro‐focused EDXRF analyzer, which can be used to calculate elemental weight fractions in metal alloys or rock samples accurately by library least‐squares regression of the measured X‐ray spectrum with computer‐generated elemental library spectra. An elemental stratified sampling variance reduction technique has been implemented in the CEARXRF5 code, which equalizes the statistical precision of the elemental libraries within the measured sample independent of the relative elemental amounts that are present. Also, an improved Si(Li) detector response function (DRF) has been obtained for micro‐focused X‐ray fluorescence (XRF) analyzers, and the DRF parameters are obtained based on regression from pure elemental experimental spectra. It is demonstrated that the resulting MCLLS approach can greatly improve the accuracy of elemental XRF analysis results. Copyright © 2011 John Wiley & Sons, Ltd.}, number={6}, journal={X-RAY SPECTROMETRY}, author={Gardner, Robin P. and Li, Fusheng}, year={2011}, pages={405–410} }
 @article{holmes_calderon_peeples_gardner_2011, title={A proposed benchmark problem for cargo nuclear threat monitoring}, volume={652}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2010.10.070}, abstractNote={There is currently a great deal of technical and political effort focused on reducing the risk of potential attacks on the United States involving radiological dispersal devices or nuclear weapons. This paper proposes a benchmark problem for gamma-ray and X-ray cargo monitoring with results calculated using MCNP5, v1.51. The primary goal is to provide a benchmark problem that will allow researchers in this area to evaluate Monte Carlo models for both speed and accuracy in both forward and inverse calculational codes and approaches for nuclear security applications. A previous benchmark problem was developed by one of the authors (RPG) for two similar oil well logging problems (Gardner and Verghese, 1991, [1]). One of those benchmarks has recently been used by at least two researchers in the nuclear threat area to evaluate the speed and accuracy of Monte Carlo codes combined with variance reduction techniques. This apparent need has prompted us to design this benchmark problem specifically for the nuclear threat researcher. This benchmark consists of conceptual design and preliminary calculational results using gamma‐ray interactions on a system containing three thicknesses of three different shielding materials. A point source is placed inside the three materials lead, aluminum, and plywood. The first two materials are in right circular cylindrical form while the third is a cube. The entire system rests on a sufficiently thick lead base so as to reduce undesired scattering events. The configuration was arranged in such a manner that as gamma-ray moves from the source outward it first passes through the lead circular cylinder, then the aluminum circular cylinder, and finally the wooden cube before reaching the detector. A 2 in.×4 in.×16 in. box style NaI (Tl) detector was placed 1 m from the point source located in the center with the 4 in.×16 in. side facing the system. The two sources used in the benchmark are 137Cs and 235U.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Holmes, Thomas Wesley and Calderon, Adan and Peeples, Cody R. and Gardner, Robin P.}, year={2011}, month={Oct}, pages={52–57} }
 @article{wang_calderon_peeples_ai_gardner_2011, title={Monte Carlo investigation and optimization of coincidence prompt gamma-ray neutron activation analysis}, volume={652}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2010.08.011}, abstractNote={Normal Prompt Gamma-Ray Neutron Activation Analysis (PGNAA) suffers from a large inherent noise or background. The coincidence PGNAA approach is being investigated for eliminating almost all of the interfering backgrounds and thereby significantly improving the signal-to-noise ratio (SNR). This can be done since almost all of the prompt gamma rays from elements of interest are emitted in coincidence except hydrogen. However, it has been found previously that while the use of two normal NaI detectors greatly reduces the background, the signal is also greatly reduced so that very little improvement in standard deviation is obtained. With the help of MCNP5, the general-purpose Monte Carlo N-Particle code, and CEARCPG, the specific purpose Monte Carlo code for Coincidence PGNAA, further optimization of the proposed coincidence system is being accomplished. The idea pursued here is the use of a large area plastic scintillation detector as the trigger for coincidence events together with a normal large NaI detector. In this approach the detection solid angle is increased greatly, which directly increases the probability of coincidence detection. The 2D-coincidence spectrum obtained can then be projected to the axis representing the NaI detector to overcome the drawback of low energy resolution and photopeak intensity of the plastic scintillation detector and utilize the overall higher coincidence counting rate. To reach the best coincidence detection, the placement of detectors, sample, and the moderator of the neutron source have been optimized through Monte Carlo simulation.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Wang, Jiaxin and Calderon, Adan and Peeples, Cody R. and Ai, Xianyun and Gardner, Robin P.}, year={2011}, month={Oct}, pages={572–577} }
 @article{peeples_mickael_gardner_2010, title={On replacing Am-Be neutron sources in compensated porosity logging tools}, volume={68}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2009.11.042}, abstractNote={Authors explored the direct replacement of Am-Be neutron sources in neutron porosity logging tools through Monte Carlo simulations using MCNP5. (252)Cf and electronic accelerator neutron sources based on the Deuterium-Tritium fusion reaction were considered. Between the sources, a tradeoff was noted between sensitivity to the presence of hydrogen and uncertainty due to counting statistics. It was concluded that both replacement sources as well as accelerator sources based on the Deuterium-Deuterium fusion reaction warrant further consideration as porosity log sources.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Peeples, Cody R. and Mickael, Medhat and Gardner, Robin P.}, year={2010}, pages={926–931} }
 @article{gardner_sood_2010, title={On the future of Monte Carlo simulation for nuclear logs}, volume={68}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2009.10.021}, abstractNote={The oil and oil well logging companies have long been innovators in many technologies that are used in oil well logging. This includes the development and use of radiation detection equipment in the harsh environment of oil wells. It also certainly includes the use of Monte Carlo simulation in the study, optimum design, and calculation of corrections and correction factors for the inverse logging problem. This latter area is the subject of this paper. Past milestones in this area are discussed along with those of the present and future work. The perspective is from the viewpoint of the authors.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, Robin P. and Sood, Avneet}, year={2010}, pages={932–935} }
 @article{li_gardner_2010, title={Semi-empirical modeling of gamma-ray density logs with the possibility of obtaining more information}, volume={68}, ISSN={["0969-8043"]}, DOI={10.1016/j.apradiso.2009.10.047}, abstractNote={A semi-empirical model based on the forward scatter principle has been devised for describing the gamma-ray density log for oil well logging. This approach is significantly different from the existing primarily empirical model and approach that is presently used for this purpose. It offers the possibility of providing a more fundamental and accurate basis for log interpretation since the natural parameters used in this approach are also significantly different and more fundamental than the ones presently in use.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Li, Fusheng and Gardner, Robin P.}, year={2010}, pages={936–940} }
 @article{gardner_xu_2009, title={Status of the Monte Carlo library least-squares (MCLLS) approach for non-linear radiation analyzer problems}, volume={78}, ISSN={["0969-806X"]}, DOI={10.1016/j.radphyschem.2009.04.023}, abstractNote={The Center for Engineering Applications of Radioisotopes (CEAR) has been working for over a decade on the Monte Carlo library least-squares (MCLLS) approach for treating non-linear radiation analyzer problems including: (1) prompt gamma-ray neutron activation analysis (PGNAA) for bulk analysis, (2) energy-dispersive X-ray fluorescence (EDXRF) analyzers, and (3) carbon/oxygen tool analysis in oil well logging. This approach essentially consists of using Monte Carlo simulation to generate the libraries of all the elements to be analyzed plus any other required background libraries. These libraries are then used in the linear library least-squares (LLS) approach with unknown sample spectra to analyze for all elements in the sample. Iterations of this are used until the LLS values agree with the composition used to generate the libraries. The current status of the methods (and topics) necessary to implement the MCLLS approach is reported. This includes: (1) the Monte Carlo codes such as CEARXRF, CEARCPG, and CEARCO for forward generation of the necessary elemental library spectra for the LLS calculation for X-ray fluorescence, neutron capture prompt gamma-ray analyzers, and carbon/oxygen tools; (2) the correction of spectral pulse pile-up (PPU) distortion by Monte Carlo simulation with the code CEARIPPU; (3) generation of detector response functions (DRF) for detectors with linear and non-linear responses for Monte Carlo simulation of pulse-height spectra; and (4) the use of the differential operator (DO) technique to make the necessary iterations for non-linear responses practical. In addition to commonly analyzed single spectra, coincidence spectra or even two-dimensional (2-D) coincidence spectra can also be used in the MCLLS approach and may provide more accurate results.}, number={10}, journal={RADIATION PHYSICS AND CHEMISTRY}, author={Gardner, Robin P. and Xu, Libai}, year={2009}, month={Oct}, pages={843–851} }
 @article{wang_li_gardner_2008, title={On the use of prompt gamma-ray neutron activation analysis for determining phase amounts in multiphase flow}, volume={19}, ISSN={["1361-6501"]}, DOI={10.1088/0957-0233/19/9/094005}, abstractNote={Prompt gamma-ray neutron activation analysis (PGNAA) is considered for the measurement of the in situ multiphase flow amounts of oil, gas, water and salt in a deep sea oil well. PGNAA has the advantages for this application that: (1) useful characteristic prompt gamma rays are produced by neutron interactions with almost all elements, (2) it is a rapid non-destructive measurement method, (3) a large sample volume is measured and (4) it can be used under the relatively extreme conditions present for undersea oil recovery. Feasibility calculations have been made with the previously developed Monte Carlo–library least-squares (MCLLS) measurement approach used with the specific purpose Monte Carlo code named CEARCPG that was previously developed at CEAR for PGNAA bulk material analysis. A slight modification of the MCLLS measurement approach previously developed for the nonlinear PGNAA and energy dispersive x-ray analysis (EDXRF) measurement applications is used for the present application. This modification allows the use of the very accurate forward Monte Carlo calculation of the PGNAA response and consists of using first the three components oil plus gas, water and salt as library spectra rather than the normal use of individual elemental libraries. Then the gamma-ray transmission density gauge response from the Cs-137 source is used to obtain the amount of gas. This approach allows one to determine the four parameters of primary interest directly. The arrangement considered is the use of a Cf-252 neutron source and a Cs-137 gamma-ray source with a large NaI detector placed on the opposite side of a right circular cylindrical sample holder for an assumed homogeneous mixture of oil, gas and seawater. A background that was previously obtained experimentally in bulk analysis applications was added in various amounts to the response here to make the calculations more reasonable. More experimental results for benchmarking will be taken in the future. Results indicate that this approach would be accurate and is feasible.}, number={9}, journal={MEASUREMENT SCIENCE AND TECHNOLOGY}, author={Wang, Jiaxin and Li, Fusheng and Gardner, Robin P.}, year={2008}, month={Sep} }
 @article{han_gardner_metwally_2006, title={CEARCPG: A Monte Carlo simulation code for normal and coincidence prompt-gamma-ray neutron activation analysis}, volume={155}, DOI={10.13182/nse07-a2652}, abstractNote={Abstract A new Monte Carlo code named CEARCPG has been developed to generate both the normal and coincidence library spectra for the prompt-gamma-ray neutron activation analysis (PGNAA) inverse analysis problem. A new algorithm for sampling the neutron-induced prompt gamma rays has been developed and implemented within the CEARCPG code, making it possible to calculate the coincidence spectrum by the Monte Carlo method. Compared to the previous code CEARPGA II, which was for normal PGNAA, several improvements have been made, including implementation of the coincidence spectrum simulation and elimination of the “big weight” problem by implementing a new algorithm to generate prompt gamma rays. Several experiments have been carried out to benchmark the new CEARCPG code, and simulation results are also compared with MCNP5 calculations.}, number={1}, journal={Nuclear Science and Engineering}, author={Han, X. G. and Gardner, R. P. and Metwally, W. A.}, year={2006}, pages={143–153} }
 @article{kulisek_hartwell_mcilwain_gardner_2007, title={Design and preliminary Monte Carlo calculations of an active Compton-suppressed LaBr3(Ce) detector system for TRU assay in remote-handled wastes}, volume={580}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2007.05.060}, abstractNote={Recent studies indicate LaBr3(Ce) scintillating detectors have desirable attributes, such as room temperature operability and excellent energy resolution, which may make them viable alternatives as primary detectors (PD) in a Compton suppression spectrometer (CSS) used for remote-handled transuranic (RH-TRU) waste assay. A CSS with a LaBr3(Ce) PD has been designed and its expected performance evaluated using Monte Carlo analysis. These results indicate that this detector will have a relatively high Compton-suppression capability, with greater suppression ability for large angle-scattered photons in the PD.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Kulisek, J. A. and Hartwell, J. K. and McIlwain, M. E. and Gardner, R. P.}, year={2007}, month={Sep}, pages={226–229} }
 @misc{gardner_2007, title={Gamma ray detectors having improved signal-to-noise ratio and related systems and methods for analyzing bulk materials}, volume={7,214,942}, number={2007 May 7}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Gardner, R. P.}, year={2007} }
 @misc{garnder_2007, title={Gamma ray detectors having improved signal-to-noise ratio and related systems and methods for analyzing materials in an oil well}, volume={7,309,857}, number={2007 Dec. 18}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Garnder, R. P.}, year={2007} }
 @article{lee_jae_gardner_2007, title={Non-poisson counting statistics of a hybrid G-M counter dead time model}, volume={263}, ISSN={["0168-583X"]}, DOI={10.1016/j.nimb.2007.04.041}, abstractNote={The counting statistics of a G–M counter with a considerable dead time event rate deviates from Poisson statistics. Important characteristics such as observed counting rates as a function true counting rates, variances and interval distributions were analyzed for three dead time models, non-paralyzable, paralyzable and hybrid, with the help of GMSIM, a Monte Carlo dead time effect simulator. The simulation results showed good agreements with the models in observed counting rates and variances. It was found through GMSIM simulations that the interval distribution for the hybrid model showed three distinctive regions, a complete cutoff region for the duration of the total dead time, a degraded exponential and an enhanced exponential regions. By measuring the cutoff and the duration of degraded exponential from the pulse interval distribution, it is possible to evaluate the two dead times in the hybrid model.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Lee, Sang Hoon and Jae, Moosung and Gardner, Robin P.}, year={2007}, month={Oct}, pages={46–49} }
 @article{han_gardner_2007, title={The Monte Carlo code CEARCPG for coincidence prompt gamma-ray neutron activation analysis}, volume={263}, ISSN={["0168-583X"]}, DOI={10.1016/j.nimb.2007.04.238}, abstractNote={Prompt gamma-ray neutron activation analysis (PGNAA) is widely used to determine the elemental composition of bulk samples. The detection sensitivities of PGNAA are often restricted by the inherent poor signal-to-noise ratio (SNR). There are many sources of noise (background) including the natural background, neutron activation of the detector, gamma-rays associated with the neutron source and prompt gamma-rays from the structural materials of the analyzer. Results of the prompt gamma-ray coincidence technique show that it could greatly improve the SNR by removing almost all of the background interferences. The first specific Monte Carlo code (CEARCPG) for coincidence PGNAA has been developed at the Center for Engineering Application of Radioisotopes (CEAR) to explore the capabilities of this technique. Benchmark bulk sample experiments have been performed with coal, sulfur, and mercury samples and indicate that the code is accurate and will be very useful in the design of coincidence PGNAA devices.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Han, Xiaogang and Gardner, Robin P.}, year={2007}, month={Oct}, pages={320–325} }
 @article{metwally_gardner_sood_2007, title={Using gamma-gamma coincidence measurements to validate Monte Carlo generated detector response functions}, volume={263}, ISSN={["0168-583X"]}, DOI={10.1016/j.nimb.2007.04.137}, abstractNote={Monte Carlo simulation of gamma-ray transport for the purpose of performing elemental analysis of bulk samples requires the tracking of gamma rays in the sample and also in the detector(s) used. Detector response functions (DRF’s) are an efficient and accurate variance reduction technique that greatly decreases the simulation time by substituting the tracking of gamma rays inside the detector by predefined single energy gamma-ray spectra. These spectra correspond to the average response of the detector for incident gamma rays. DRF’s are generated by Monte Carlo methods and are benchmarked with experimental data. In this work, prompt gamma–gamma coincidence measurements are presented as a way to validate DRF’s for high-energy gamma rays.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Metwally, W. A. and Gardner, R. P. and Sood, A.}, year={2007}, month={Oct}, pages={50–53} }
 @article{gardner_metwally_han_2005, title={A new NaI detector arrangement for efficient detection of high energy gamma-rays}, volume={264}, ISSN={["1588-2780"]}, DOI={10.1007/s10967-005-0685-x}, number={1}, journal={JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY}, author={Gardner, RP and Metwally, WA and Han, XG}, year={2005}, month={Apr}, pages={133–137} }
 @article{guo_gardner_mayo_2005, title={A study of the real-time deconvolution of digitized waveforms with pulse pile up for digital radiation spectroscopy}, volume={544}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2004.12.036}, abstractNote={Two new real-time approaches have been developed and compared to the least-squares fit approach for the deconvolution of experimental waveforms with pile-up pulses. The single pulse shape chosen is typical for scintillators such as LSO and NaI(Tl). Simulated waveforms with pulse pile up were also generated and deconvolved to compare these three different approaches under cases where the single pulse component has a constant shape and the digitization error dominates. The effects of temporal separation and amplitude ratio between pile-up component pulses were also investigated and statistical tests were applied to quantify the consistency of deconvolution results for each case. Monte Carlo simulation demonstrated that applications of these pile-up deconvolution techniques to radiation spectroscopy are effective in extending the counting-rate range while preserving energy resolution for scintillation detectors.}, number={3}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Guo, WJ and Gardner, RP and Mayo, CW}, year={2005}, month={Jun}, pages={668–678} }
 @article{zhang_gardner_2005, title={CEARPGA II: A Monte Carlo simulation code for prompt-gamma-ray neutron activation analysis}, volume={151}, ISSN={["1943-748X"]}, DOI={10.13182/NSE05-A2556}, abstractNote={Abstract A Monte Carlo simulation code, CEARPGA II, has been developed to generate the complete set of library spectra that are required for the application of the Monte Carlo–Library Least-Squares approach in prompt-gamma-ray neutron activation analysis. Compared to the previous version, the CEARPGA I code, several important improvements have been made including eliminating the “big weight” problem by implementing the Analog Linear Interpolation technique, generating the appropriate detector response functions using improved simulation models that account for NaI detector nonlinearity and flat continua, generating the neutron activation backgrounds by directly sampling detector-activated gamma-ray energies, generating the natural background libraries by interpolating the energy-score tables, and tracking the annihilation gamma rays from the pair production interaction that occurs outside the detector. The coal sample spectrum calculated with the CEARPGA II code is benchmarked against those calculated from the CEARPGA I code, the MCNP code, and experimentally measured data.}, number={3}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Zhang, WC and Gardner, RP}, year={2005}, month={Nov}, pages={361–373} }
 @article{metwally_mayo_han_gardner_2005, title={Coincidence counting for PGNAA applications: Is it the optimum method?}, volume={265}, ISSN={["1588-2780"]}, DOI={10.1007/s10967-005-0826-2}, number={2}, journal={JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY}, author={Metwally, WA and Mayo, CW and Han, X and Gardner, RP}, year={2005}, month={Aug}, pages={309–314} }
 @article{gardner_guo_2005, title={Development of a Monte Carlo - Library Least-Squares code package for the EDXRF inverse problem}, volume={20}, ISSN={["1945-7413"]}, DOI={10.1154/1.1913722}, abstractNote={The Monte Carlo—Library Least-Squares (MCLLS) approach has now been developed, implemented, and tested for solving the inverse problem of EDXRF sample analysis. It consists of a linear library least-squares code and a comprehensive Monte Carlo code named CEARXRF that is capable of calculating the unknown sample spectrum, all the elemental library spectra in the sample, and the differential operators for each library spectrum with respect to each element. Two codes with graphical user interface have been designed to implement the MCLLS approach and benchmark results are presented for the two stainless steel samples; SS304 and SS316. The results are accurate, the system is easy to use, and all indications are that this approach will be very useful for the EDXRF practitioner.}, number={2}, journal={POWDER DIFFRACTION}, author={Gardner, RP and Guo, W}, year={2005}, month={Jun}, pages={146–152} }
 @article{gardner_zhang_metwally_2005, title={Status of software for PGNAA bulk analysis by the Monte Carlo - Library Least-Squares (MCLLS) approach}, volume={264}, ISSN={["0236-5731"]}, DOI={10.1007/s10967-005-0697-6}, number={1}, journal={JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY}, author={Gardner, RP and Zhang, W and Metwally, WA}, year={2005}, month={Apr}, pages={221–228} }
 @article{metwally_gardner_2004, title={Stabilization of prompt gamma-ray neutron activation analysis (PGNAA) spectra from NaI detectors}, volume={525}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2004.02.011}, abstractNote={Abstract NaI detectors are still used frequently in industrial Prompt Gamma-Ray Neutron Activation Analysis applications such as in bulk material analysis. They have the advantages of being efficient for high-energy gamma rays, being relatively rugged, and being able to be used without cooling. When using NaI detectors, and consequently photomultiplier tubes, the quality of the data can drastically deteriorate through gain and zero shifts that result in spectral smearing due to temperature and/or counting rate changes. A new offline approach is presented to stabilize the NaI spectral drift. The approach is not sensitive to the cause of the drift and takes into account the NaI and ADC non-linearities. Peak resolution is improved substantially when this approach is used in the presence of spectral drift.}, number={3}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Metwally, WA and Gardner, RP}, year={2004}, month={Jun}, pages={518–521} }
 @article{guo_lee_gardner_2004, title={The Monte Carlo approach MCPUT for correcting pile-up distorted pulse-height spectra}, volume={531}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2004.05.089}, abstractNote={Pulse pile-up distortion is a common problem for radiation spectroscopy measurements involving high counting rates. The Monte Carlo pile-up to true approach (MCPUT) is proposed and benchmarked in this article for correcting pile-up distorted pulse-height spectra to true spectra. In previous work, a Monte Carlo approach was used for predicting the pile-up distorted pulse-height spectra for high counting-rate measurements (“the forward calculation”). The present work improves the previous simulation by employing a better ADC dead-time model. Based on this improved “forward calculation”, the MCPUT approach introduces an iterative procedure for correcting pile-up distortions. Experiments with an Fe-55 source and a Si(Li) detector are used for benchmarking purposes. The MCPUT corrected spectrum for the high counting-rate measurement shows excellent agreement with the measured true spectrum at low counting rate with reduced chi-square as the quantitative measure. The approach is also efficient, as accurate calculations are possible in a few minutes.}, number={3}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Guo, WJ and Lee, SH and Gardner, RP}, year={2004}, month={Oct}, pages={520–529} }
 @article{metwally_gardner_mayo_2004, title={Two-dimensional diagonal summing of coincidence spectra for bulk PGNAA applications}, volume={525}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2004.02.012}, abstractNote={Abstract In the past 10 years, new electronic devices have been developed that allow fast coincidence measurements to be performed that are capable of simultaneously recording the individual spectra as well as the coincidence spectra of multiple detectors. Utilizing these devices with computer software allows multiparameter data acquisition which adds much more flexibility in data analysis. One of the capabilities that is enabled is that of obtaining two-dimensional spectra. In this work, the use of this equipment and the two-dimensional spectra obtained with it are used to allow two-dimensional diagonal summing. The main advantages of this approach are improved peak resolution and very low background (Compton continuum). Possible uses of the two-dimensional diagonal summing are identifying coincidence schemes, performing elemental analysis, and identifying trace elements in bulk samples. The spectra obtained are very promising for these applications.}, number={3}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Metwally, WA and Gardner, RP and Mayo, CW}, year={2004}, month={Jun}, pages={511–517} }
 @article{gardner_sood_2004, title={A Monte Carlo simulation approach for generating NaI detector response functions (DRFs) that accounts for non-linearity and variable flat continua}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01539-8}, abstractNote={A new approach to generating detector response functions (DRFs) for NaI detectors is developed and demonstrated. It consists of using Monte Carlo simulation with a specific purpose code (named g03) that utilizes relatively rigorous gamma-ray transport with very simple electron transport. The approach accounts for two phenomena that have not been previously treated in this way: NaI non-linearity and the variable flat continua part of the DRF. The first is accounted for by utilizing the known non-linear relationship between NaI scintillation efficiency and deposited electron energy within the Monte Carlo code. The second is accounted for by using a semi-empirical relationship between electron range and incident gamma-ray energy within the Monte Carlo code. The resulting DRFs for 3″ × 3″ and 6″ × 6″ NaI detectors are benchmarked with the extensive existing data of Heath [AEC Research and Development Report, Physics, TID-4500] for the 3″ × 3″ detectors and with new data for the 6″ × 6″ detectors. Results indicate that the approach is accurate and the use of this approach should increase the accuracy of prompt gamma-ray neutron activation analysis significantly.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Gardner, RP and Sood, A}, year={2004}, month={Jan}, pages={87–99} }
 @article{sood_gardner_2004, title={A new Monte Carlo assisted approach to detector response functions}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01540-4}, abstractNote={The physical mechanisms that describe the components of NaI, Ge and Si(Li) detector responses have been investigated using Monte Carlo simulation. The mechanisms described focus on the shape of the Compton edge, the magnitude of the flat continuum, and the shape of the exponential tails features. These features are not accurately predicted by previous Monte Carlo simulations. Probable interaction mechanisms for each detector response component are given based on this Monte Carlo simulation.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Sood, A and Gardner, RP}, year={2004}, month={Jan}, pages={100–104} }
 @article{gardner_metwally_shehata_2004, title={A semi-empirical model for a Sr-90 beta-particle transmission thickness gauge for aluminum alloys}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01582-9}, abstractNote={A semi-empirical model is derived and tested for a 90Sr beta-particle transmission thickness gauge for aluminum alloys. Monte Carlo simulation is also used to verify the forward scatter part of the model. The model accurately accounts for thickness as well as forward scatter, gamma-ray background, and the measurement interference of sample density and composition. The model parameters are obtained and the model is benchmarked with industrial experimental data from a 90Sr(90Y) gauge.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Gardner, RP and Metwally, WA and Shehata, A}, year={2004}, month={Jan}, pages={357–363} }
 @article{dunn_fernández_gardner_tartari_2004, title={Editorial}, volume={213}, ISSN={0168-583X}, url={http://dx.doi.org/10.1016/S0168-583X(03)01525-8}, DOI={10.1016/S0168-583X(03)01525-8}, journal={Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms}, publisher={Elsevier BV}, author={Dunn, William and Fernández, Jorge E. and Gardner, Robin P. and Tartari, Agostino}, year={2004}, month={Jan}, pages={vii-viii} }
 @article{metwally_gardner_mayo_2004, title={Elemental PGNAA analysis using gamma-gamma coincidence counting with the library least-squares approach}, volume={213}, ISSN={["1872-9584"]}, DOI={10.1016/S0168-583X(03)01660-4}, abstractNote={Abstract An accurate method for determining elemental analysis using gamma–gamma coincidence counting is presented. To demonstrate the feasibility of this method for PGNAA, a system of three radioisotopes (Na-24, Co-60 and Cs-134) that emit coincident gamma rays was used. Two HPGe detectors were connected to a system that allowed both singles and coincidences to be collected simultaneously. A known mixture of the three radioisotopes was used and data was deliberately collected at relatively high counting rates to determine the effect of pulse pile-up distortion. The results obtained, with the library least-squares analysis, of both the normal and coincidence counting are presented and compared to the known amounts. The coincidence results are shown to give much better accuracy. It appears that in addition to the expected advantage of reduced background, the coincidence approach is considerably more resistant to pulse pile-up distortion.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Metwally, WA and Gardner, RP and Mayo, CW}, year={2004}, month={Jan}, pages={394–399} }
 @article{sun_gardner_2004, title={Optimization of the steady neutron source technique for absorption cross section measurement by using an Sb-124-Be neutron source}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01527-1}, abstractNote={An improved experimental approach has been developed to determine thermal neutron absorption cross sections. It uses an 124Sb–Be neutron source which has an average neutron energy of only about 12 keV. It can be moderated in either a water tank or a paraffin filled box and can be used for aqueous or powder samples. This new design is first optimized by MCNP simulation and then benchmarked and calibrated with experiments to verify the simulations and realize the predicted improved measurement sensitivity and reproducibility. The 124Sb–Be source device is from 1.35 to 1.71 times more sensitive than the previous method based on the use of a 252Cf source.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Sun, J and Gardner, RP}, year={2004}, month={Jan}, pages={22–28} }
 @article{guo_gardner_metwally_2004, title={Preliminary studies on K and L coincidence spectroscopy for optimizing the in vivo XRF measurement of lead in bone}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01674-4}, abstractNote={Previous studies treated the optimal combined K and L XRF system which consists of a Cd-109 point source, a low energy Ge detector (LEGe) and a Si(Li) detector. The Monte-Carlo library least-squares (MCLLS) approach and differential operator approach were treated separately. In this work, an approach for combining the MCLLS approach with the differential operator approach (MCDOLLS) is presented and an optimal configuration for coincidence spectroscopy of K and L X-rays is proposed based on preliminary experimental data.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Guo, WJ and Gardner, RP and Metwally, WA}, year={2004}, month={Jan}, pages={574–578} }
 @article{zhang_gardner_2004, title={The analog linear interpolation approach for Monte Carlo simulation of PGNAA: The CEARPGA code}, volume={213}, ISSN={["0168-583X"]}, DOI={10.1016/S0168-583X(03)01544-1}, abstractNote={The analog linear interpolation approach (ALI) has been developed and implemented to eliminate the big weight problem in the Monte Carlo simulation code CEARPGA. The CEARPGA code was previously developed to generate elemental library spectra for using the Monte Carlo – library least-squares (MCLLS) approach in prompt gamma-ray neutron activation analysis (PGNAA). In addition, some other improvements to this code have been introduced, including (1) adopting the latest photon cross-section data, (2) using an improved detector response function, (3) adding the neutron activation backgrounds, (4) generating the individual natural background libraries, (5) adding the tracking of annihilation photons from pair production interactions outside of the detector and (6) adopting a general geometry package. The simulated result from the new CEARPGA code is compared with those calculated from the previous CEARPGA code and the MCNP code and experimental data. The new CEARPGA code is found to give the best result.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION B-BEAM INTERACTIONS WITH MATERIALS AND ATOMS}, author={Zhang, WC and Gardner, RP}, year={2004}, month={Jan}, pages={116–123} }
 @article{guo_gardner_todd_2004, title={Using the Monte Carlo - Library Least-Squares (MCLLS) approach for the in vivo XRF measurement of lead in bone}, volume={516}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2003.09.030}, abstractNote={The Monte Carlo – Library Least-Squares (MCLLS) method has been developed by the Center for Engineering Applications of Radioisotopes for various XRF applications of multi-elemental composition analysis and implemented with the CEARXRF code. In the present work, it is successfully applied to the in vivo XRF measurement of lead in bone and benchmarked by the measurement of a plaster of Paris phantom of known lead concentration. It is implicitly assumed that if the approach works for this sample that closely approximates the real problem of interest, it will also work for the real in vivo case when the proper description of the real case is used. Traditional techniques for XRF analysis are reviewed briefly and the full advantages of the MCLLS method are discussed. Simulation results are presented that are in good agreement with experimental results. The applicability of the MCLLS method to the lead in bone measurement is supported by the good fitting results obtained with simulated Monte Carlo elemental library spectra and close agreement between simulated and experimental spectra from a calcium-rich matrix-based calibration standard in a test geometrical configuration.}, number={2-3}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Guo, WJ and Gardner, RP and Todd, AC}, year={2004}, month={Jan}, pages={586–593} }
 @article{lee_gardner_todd_2001, title={Preliminary studies on combining the K and L XRF methods for in vivo bone lead measurement}, volume={54}, DOI={10.1016/s0969-8043(00)00350-x}, abstractNote={Lead is a toxic material that invokes irreversible neurological problems. Once ingested, lead accumulates in the bones. To study detailed lead poisoning effects it is essential to have an in vivo bone lead measurement tool with a small minimum detectable concentration (MDC). Both K- and L-based XRF methods for the tibia bone have been suggested and developed in the past and are presently in use. In this work a combined K and L XRF method for the tibia bone is proposed. The proposed system consists of a 109Cd point source and Ge and Si(Li) detectors for optimum detection of the K and L X-rays, respectively. Experimental and Monte Carlo simulated results are given here for a prototype combined K and L XRF system. This system promises to yield a better MDC and the possibility of obtaining information on the near-surface bone lead content as well as the average lead content throughout the bone.}, number={6}, journal={Applied Radiation and Isotopes}, author={Lee, S. H. and Gardner, R. P. and Todd, A. C.}, year={2001}, pages={893–904} }
 @article{gardner_mayo_el-sayyed_metwally_zheng_poezart_2000, title={A feasibility study of a coincidence counting approach for PGNAA applications}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00206-2}, abstractNote={Prompt gamma-ray nutron activation analysis (PGNAA) has an inherently low signal-to-noise (S/N) ratio primarily because of the large background (noise) associated with it. Most elements emit a significant fraction of their prompt gamma rays in coincidence with one or more other prompt gamma rays. This paper reports on initial efforts to use coincidence counting in PGNAA to significantly reduce the several sources of background and thereby increase the S/N ratio. An added benefit is the elimination of the often dominant hydrogen prompt gamma-ray spectrum which emits only a single prompt gamma ray with an energy of 2.223 MeV. Preliminary results are given for both in situ bulk analysis applications with a 252Cf neutron source and for nuclear reactor thermal neutron beam applications for small laboratory samples.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Mayo, CW and El-Sayyed, ES and Metwally, WA and Zheng, Y and Poezart, M}, year={2000}, pages={515–526} }
 @article{lee_gardner_2000, title={A new G-M counter dead time model}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00261-X}, abstractNote={A hybrid G–M counter dead time model was derived by combining the idealized paralyzable and non-paralyzable models. The new model involves two parameters, which are the paralyzable and non-paralyzable dead times. The dead times used in the model are very closely related to the physical dead time of the G–M tube and its resolving time. To check the validity of the model, the decaying source method with 56Mn was used. The corrected counting rates by the new G–M dead time model were compared with the observed counting rates obtained from the measurement and gave very good agreement within 5% up to 7 × 104 counts/s for a G–M tube with a dead time of about 300 μs.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Lee, SH and Gardner, RP}, year={2000}, pages={731–737} }
 @article{gardner_liu_2000, title={Monte Carlo simulation for IRRMA}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00233-5}, abstractNote={Monte Carlo simulation is fast becoming a standard approach for many radiation applications that were previously treated almost entirely by experimental techniques. This is certainly true for Industrial Radiation and Radioisotope Measurement Applications--IRRMA. The reasons for this include: (1) the increased cost and inadequacy of experimentation for design and interpretation purposes; (2) the availability of low cost, large memory, and fast personal computers; and (3) the general availability of general purpose Monte Carlo codes that are increasingly user-friendly, efficient, and accurate. This paper discusses the history and present status of Monte Carlo simulation for IRRMA including the general purpose (GP) and specific purpose (SP) Monte Carlo codes and future needs--primarily from the experience of the authors.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Liu, LY}, year={2000}, pages={837–855} }
 @article{gardner_sayyed_zheng_hayden_mayo_2000, title={NaI detector neutron activation spectra for PGNAA applications}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00198-6}, abstractNote={When NaI detectors are used in prompt gamma-ray neutron activation analysis devices, they are activated by neutrons that penetrate the detector. While thermal neutron filters like boron or lithium can be used to reduce this activation, it can never be completely eliminated by this approach since high energy neutrons can penetrate the detector and thermalize inside it. This activation results in the emission of prompt gamma rays from both the I and Na and the production of the radioisotopes 128I and 24Na that subsequently decay and emit their characteristic beta particles and gamma rays. The resulting three spectra represent a background for this measurement. An experimental method for obtaining these three spectra is described and results are reported for 2" x 2", 5" x 5", 6" x 6", and 1" x 6" NaI detectors using the thermal neutron beam of the NCSU PULSTAR nuclear reactor. In addition, Monte Carlo simulation programs have been developed and used for simulating these spectra. Good results have been obtained by the Monte Carlo method for the two radioisotope spectra, and it is anticipated that good results will also be obtained for the prompt gamma-ray spectrum when the I and Na coincidence schemes are known.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Sayyed, E and Zheng, YS and Hayden, S and Mayo, CW}, year={2000}, pages={483–497} }
 @article{earnhart_prettyman_lestone_gardner_2000, title={Simulation of Compton camera imaging with a specific purpose Monte Carlo code}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00203-7}, abstractNote={A specific purpose Monte Carlo model that treats the physical processes of image formation in Compton cameras is presented. Features of the model include: detailed Compton scattering physics, including incoherent scattering functions and Doppler broadening; multiple scattering; and peak detector response functions. Calculation efficiency is enhanced by making every history result in a partial success through the extensive use of pathway sampling variance reduction techniques. The code was benchmarked to data obtained from a prototype camera using CdZnTe and silicon room-temperature detector technology. Reasons for observed discrepancies between the simulation model and experiments are explored. Implicit assumptions, related to our implementation of pathway sampling, appear to be the greatest source of error.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Earnhart, J and Prettyman, T and Lestone, J and Gardner, R}, year={2000}, pages={673–680} }
 @article{sood_gardner_gray_2000, title={Steady neutron source measurement method for Sigma(a) and Sigma(s) in geological samples}, volume={53}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(00)00235-9}, abstractNote={An improved experimental method, using a steady neutron source in a moderating medium, has been developed to determine thermal neutron absorption and scattering cross sections for bulk geological media using discrete samples. The system design has been optimized and experimental results have been benchmarked using Monte Carlo simulation. Studies have been performed to improve the measurement sensitivity and reproducibility over previous designs. A semi-empirical model has been developed for determining both absorption and scattering cross sections of the sample.}, number={4-5}, journal={APPLIED RADIATION AND ISOTOPES}, author={Sood, A and Gardner, RP and Gray, TK}, year={2000}, pages={603–616} }
 @article{gardner_liu_1999, title={Monte Carlo simulation of neutron porosity oil well logging tools: Combining the geometry-independent fine-mesh importance map and one-dimensional diffusion model approaches}, volume={133}, ISSN={["0029-5639"]}, DOI={10.13182/NSE99-A2074}, abstractNote={The generation of first estimate geometry-independent fine-mesh three-dimensional importance maps with simple one-dimensional diffusion models is demonstrated for the Monte Carlo simulation of the neutron porosity oil well logging tool response benchmark problem. By combining the approach of using simple one-dimensional steady-state diffusion models for calculating neutron adjoint flux with the geometry-independent fine-mesh-based Monte Carlo importance approach previously developed, an automated and efficient variance reduction method is obtained for this specific problem. A surprising result is that the converged figures of merit after iteration are consistently larger when the initial importance map is based on the one-dimensional diffusion model rather than that obtained from an analog Monte Carlo simulation.}, number={1}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Gardner, RP and Liu, LY}, year={1999}, month={Sep}, pages={80–91} }
 @article{gardner_mayo_1999, title={NaI detector nonlinearity for PGNAA applications}, volume={51}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(98)00183-3}, abstractNote={The nonlinearity of a 6″×6″ NaI detector used for PGNAA has been measured. The prompt γ-rays from pure element samples mixed with graphite were used for this determination in the range from 1.712 to 10.829 MeV and γ-rays from radioisotope sources were used in the range from 0.356 to 1.333 MeV. A surprising result is that the pulse height per unit absorbed energy is slightly higher from single escape as opposed to full energy peaks. One explanation for this is that the light production efficiency for positrons may be different than that for electrons. By interpolation a value for the coincidence spectral full energy peak exhibited by NaI detectors that are neutron activated is obtained. The value reported is 6.885 MeV which is slightly higher than the cross section weighted value of 6.834 MeV. (The reported SN values for I and Na are 6.826 and 6.959 MeV, respectively.)}, number={2}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Mayo, CW}, year={1999}, month={Aug}, pages={189–195} }
 @article{gardner_guo_sood_mayo_gehrke_dobbs_1998, title={Monte Carlo aided treatments of the nonlinear inverse PGNAA measurement problem for various continuous on-line applications}, volume={233}, ISSN={["0236-5731"]}, DOI={10.1007/BF02389655}, number={1-2}, journal={JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY}, author={Gardner, RP and Guo, P and Sood, A and Mayo, CW and Gehrke, RJ and Dobbs, CL}, year={1998}, month={Jul}, pages={105–107} }
 @inbook{gardner_dobbs_1998, title={Nuclear radiation gages and analyzers}, booktitle={Sensing for materials characterization, processing, and manufacturing}, publisher={Columbus, OH: American Society for Nondestructive Testing, Inc.}, author={Gardner, R. P. and Dobbs, C. L.}, editor={G. Birnbaum and Auld, B A.Editors}, year={1998}, pages={153–182} }
 @article{liu_gardner_1997, title={A geometry-independent fine-mesh-based Monte Carlo importance generator}, volume={125}, ISSN={["1943-748X"]}, DOI={10.13182/NSE97-A24265}, abstractNote={A new importance map approach for Monte Carlo simulation that can be used in an adaptive fashion has been identified and developed. It is based on using a mesh-based system of weight windows that are independent of any physical geometric cells. It consists of an importance map generator and a splitting and Russian roulette algorithm for a mesh-based weight windows game that is used in an iterative fashion to obtain increasingly efficient results. The general purpose Monte Carlo code MCNP is modified to incorporate this new mesh-based importance map generator and matching weight window technique for variance reduction. Two nuclear well logging problems- one for neutrons and the other for gamma rays- are used to test the new importance map generator. Results show that the new generator is able to produce four to six times larger figures of merit than MCNP's physical geometry cell-based importance map generator. More importantly, the superior user friendliness of this new mesh-based generator makes variance reduction easy to accomplish.}, number={2}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Liu, LY and Gardner, RP}, year={1997}, month={Feb}, pages={188–195} }
 @article{titus_clapp_zhu_gardner_1997, title={A preliminary investigation of a beta-particle transmission gauge for seam quality determination}, volume={67}, ISSN={["0040-5175"]}, DOI={10.1177/004051759706700107}, abstractNote={ Automation of assembly operations throughout the textile industry requires the development and implementation of new quality monitoring systems. A beta-particle transmission gauge using a 90Sr source is investigated as a potential on-line seam inspection technique. The fundamentals of beta-particle transmission radio gauges are discussed in the context of their applicability to textile materials, and preliminary results are obtained with a laboratory prototype device. Unknown sample thicknesses for a variety of woven and knit fabrics are determined from the counting rate intensities of radiation transmitted through the fabric and the experimentally derived areal density. Single-fold knit hems and felled denim seams are evaluated by the gauge system. Results indicate that the number of layers present within the hem or seam can be rapidly and accurately determined to assess seam quality. }, number={1}, journal={TEXTILE RESEARCH JOURNAL}, author={Titus, KJ and Clapp, TG and Zhu, Z and Gardner, RP}, year={1997}, month={Jan}, pages={23–34} }
 @article{gardner_guo_ao_dobbs_1997, title={Black box radiation gauges and analyzers: Dream or reality?}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00120-6}, abstractNote={This paper describes our attempts to make radiation gauges and analyzers operate in more of a ‘block box’ manner — being more independent of or at least more insensitive to changes in such things as sample composition and position. Our overall approach relies heavily on Monte Carlo simulation and that is discussed first in relation to radiation gauge and analyzer design and usage. Then the principles and applications of radiation gauges and analyzers including the Measurement Chi-Square and Monte Carlo Library Least-Squares, respectively, are treated. Finally future work in this area is discussed.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Guo, P and Ao, Q and Dobbs, CL}, year={1997}, pages={1273–1288} }
 @article{ao_lee_gardner_1997, title={Development of the specific purpose Monte Carlo code CEARXRF for the design and use of in vivo X-ray fluorescence analysis systems for lead in bone}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00136-X}, abstractNote={X-ray fluorescence (XRF) systems have been increasingly used for in vivo toxic trace-element analysis in the human body, such as lead in the tibia. Monte Carlo simulation can provide an efficient and flexible method for designing and using in vivo XRF systems. The Monte Carlo code CEARXRF has been developed specifically to simulate the complete pulse height spectrum of energy-dispersive XRF systems. This code is capable of tracking photons in a general geometry and modelling all of the physics of photon interactions in the energy range 1-150 keV for elements Z = 1-94, including primary and higher degree excitations of K and L XRF, the Doppler broadening of Compton-scattered photon energies, and the polarization effects in low-energy photon scatterings. The scattering background for minimum detectable concentration (MDC) analysis may be simulated more accurately by taking into account Doppler broadening in the distribution of the Compton-scattered photon energy due to electron-binding effects. The use of polarized excitation photons has been shown to be important in producing a low scattering background and good measurement sensitivity. The code has two very unique and important features: (1) complete composition and density correlated sampling that is extremely useful for studying measurement sensitivity to small changes in sample composition and density; and (2) Monte Carlo library spectra calculation for the determination of elemental amounts by the Monte Carlo-Library Least-Squares (MCLLS) method. The capability of CEARXRF to aid the design and optimization of in vivo XRF analysis has been verified by modelling hypothesized lead K and L XRF measurement systems.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Ao, Q and Lee, SH and Gardner, RP}, year={1997}, pages={1403–1412} }
 @article{gardner_guo_wang_sood_lee_dobbs_1997, title={Feasibility of neutron activation methods for measurement of sodium and aluminum in green liquor}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00131-0}, abstractNote={The feasibility of neutron activation methods including prompt gamma-ray neutron activation analysis (PGNAA), neutron activation analysis (NAA), and continuous neutron activation analysis (CNAA) has been investigated for both the off-line and on-line continuous measurement of sodium and aluminum in green liquor. It is found that in the measurement of sodium and aluminium dissolved in green liquor at the 11% and 5.5% levels, respectively, one must also know the chlorine content to determine the average neutron flux for any method involving neutron activation. Chlorine may be present in green liquor in amounts up to 0.5% which would significantly (as much as 10%) affect the average thermal neutron flux in a sample. It was found that PGNAA is quite sensitive to chlorine and somewhat to sodium, but very insensitive to aluminum. However, NAA and CNAA are quite sensitive to aluminum, then sodium, but very insensitive to chlorine. The final proposed approaches for discrete samples and continuous samples are to use NAA and CNAA, respectively, with time lags incorporated between activation and detection to reduce the effect of the otherwise overly intense aluminum spectrum on the less intense sodium spectrum and use of a 3He detector to monitor the average neutron flux (a function primarily of chlorine content) in the samples. Extensive experimental and benchmarked Monte Carlo simulations are used to evaluate the feasibility of this approach. It appears that the approach is viable and will allow accurate (1% relative standard deviation for both sodium and aluminum) discrete sample or on-line continuous analysis of both sodium and aluminum in green liquor in reasonable times (10–30 min) with a relatively small (15 μg) 252Cf source. Prototype devices are planned for future development.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Guo, P and Wang, YY and Sood, A and Lee, SH and Dobbs, CL}, year={1997}, pages={1355–1372} }
 @article{gardner_liu_1997, title={On extending the accurate and useful counting rate range of GM counter detector systems}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00161-9}, abstractNote={A study is described to extend the accurate and useful counting rate range of the GM (Geiger-Muller) counter detector system. The idealized simple one-parameter paralyzable and non-paralyzable models were first investigated with the decaying source method and were found to be inadequate for all the systems tested. The use of a slightly more complex two-parameter model was investigated next and, in spite of initial positive indications, the resulting model yielded inaccuracies larger than ± 10% for over one-third of the counting rate range. However, use of the decaying source method to provide a wide range of accurate useful counting rates was found to be reproducible to within ± 1%. Previous formulas for variance found in the literature were studied by Monte Carlo simulation and were found to be valid when the respective appropriate assumptions of the idealized models were applicable. Actual experimental variances were found to be fairly well predicted by a modified Kosten (1943) model that has a variable dead time. This model has the advantage that it always gives conservative estimates. At present the counting rate range of GM counter detector systems can be extended to the ± 1% accuracy level only at the cost of having to use the decaying source method, which involves the use of a pure short-lived radioisotope such as 56Mn. Use at this accuracy level without having to use the decaying source method must await further modeling or other experimental developments.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Liu, LY}, year={1997}, pages={1605–1615} }
 @article{ao_lee_gardner_1997, title={Optimization of in vivo X-ray fluorescence analysis methods for bone lead by simulation with the Monte Carlo code CEARXRF}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00137-1}, abstractNote={In the design of X-ray fluorescence (XRF) systems for in vivo measurements of lead in human bone, the most important considerations are the minimum detectable concentration (MDC), and accuracy and precision. Possible design optimizations can be investigated much more easily and economically by Monte Carlo simulation than by experiment. The specific purpose Monte Carlo code CEARXRF has been used in the present study for: (1) improving the MDC of a hypothesized in vivo 109Cd source-based KXRF system and a 109Cd source or X-ray tube source-based LXRF system by investigating the effects of source polarization and source-bone-detector geometry modification on reducing the scattering background, and (2) investigating the effects of sample variables, such as overlying skin thickness on the MDC and the lead XRF intensity precision. In addition, the feasibility of the Monte Carlo-Library Least-Squares (MCLLS) approach has been investigated in a preliminary fashion for 109Cd-based KXRF spectroscopy analysis.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Ao, Q and Lee, SH and Gardner, RP}, year={1997}, pages={1413–1423} }
 @article{gardner_sood_wang_liu_guo_gehrke_1997, title={Single peak versus library least-squares analysis methods for the PGNAA analysis of vitrified waste}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00127-9}, abstractNote={A feasibility study (Gardner et al., 1995) for the neutron capture prompt gamma-ray analysis (PGNAA) of simulated vitrified nuclear waste containing SiO2/Al2O3 and SiO2/Fe2O3 mixtures has been carried out. Analyses for elements Al, Si, and Fe were made by both the single peak area and the library least-squares methods using measured and simulated data. It is shown that the library least-squares method is about 2.5 times better than the single peak analysis method.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Gardner, RP and Sood, A and Wang, YY and Liu, L and Guo, P and Gehrke, RJ}, year={1997}, pages={1331–1335} }
 @article{guo_ao_gardner_1997, title={Spectral gamma-ray log interpretation algorithms using the Monte Carlo multiply scattered components approach}, volume={48}, ISSN={["0969-8043"]}, DOI={10.1016/S0969-8043(97)00132-2}, abstractNote={Abstract New algorithms for spectral gamma-ray log interpretation have been developed on the basis of a novel approach originally developed for natural gamma-ray log interpretation using the lithology-independent Monte Carlo multiply scattered components (MCMSC) approach. With separation of density and composition effects, the new algorithms have fewer parameters and have been designed for both natural gamma-ray and gamma-gamma litho-density logs.}, number={10-12}, journal={APPLIED RADIATION AND ISOTOPES}, author={Guo, P and Ao, Q and Gardner, RP}, year={1997}, pages={1373–1383} }