@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{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{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{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_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} }