@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{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{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{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{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} }