@article{crozier_hawari_2023, title={Ab Initio Evaluation of Uranium Carbide S(alpha, beta) and Thermal Neutron Cross Sections}, volume={284}, ISBN={["*****************"]}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/202328417005}, abstractNote={Uranium Carbide (UC) is a nuclear fuel material which offers better neutron economy and lower fuel-cycle costs compared to conventional mixed-oxide fuels. UC’s lattice binding and dynamical properties impact thermal neutron scattering and low temperature epithermal resonance absorption. The Thermal Scattering Law (TSL) describes the scattering system available energy and momentum transfer states. There is no TSL evaluation for UC in the ENDF/B-VIII.0 database; herein,ab-initiolattice dynamics (AILD) techniques are invoked to calculate the phonon spectrum for UC using spin-orbit-coupling density functional theory (DFT). The TSLs, inelastic and elastic thermal scattering cross sections for Uranium238 and Carbon12 in UC, respectively, are calculated inFLASSHfor use in higher fidelity reactor design calculations.}, journal={15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022}, author={Crozier, Jonathan and Hawari, Ayman}, year={2023} }
 @article{lee_fleming_hawari_2023, title={Benchmark of Neutron Thermalization in Graphite Using a Pulsed Slowing-Down-Time Experiment}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2022.2162789}, abstractNote={Abstract A benchmark has been developed using a pulsed slowing-down-time experiment to isolate the thermalization process in graphite. The experiment was conducted at the Oak Ridge Electron Linear Accelerator facility at Oak Ridge National Laboratory, and it measured the time spectrum of neutrons leaking from a graphite pile during slowing down and thermalization within graphite. Simulations of the benchmark experiment were performed using the MCNP6.1 Monte Carlo code and the ENDF/B-VII.1 and ENDF/B-VIII.0 cross-section databases. The benchmark provides a time spectrum (i.e., time-dependent counts in a detector) that allows for validation of the graphite thermal scattering libraries (TSLs). The impact on the simulations using a suite of graphite TSLs was compared with the experimental results. Given the density of nuclear graphite, the TSL corresponding to graphite with 30% porosity, as implemented in ENDF/B-VIII.0, was found to most accurately represent the measured time spectrum corresponding to the thermal energy range with an average deviation of ±1.7%.}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Lee, Eunji and Fleming, N. Colby and Hawari, Ayman I.}, year={2023}, month={Feb} }
 @article{vaglio-gaudard_destouches_hawari_avramova_ivanov_valentine_blaise_hudelot_2023, title={Challenge for the validation of high-fidelity multi-physics LWR modeling and simulation: Development of new experiments in research reactors}, volume={11}, ISSN={["2296-598X"]}, DOI={10.3389/fenrg.2023.1110979}, abstractNote={Current approaches to validate multi-physics coupling mainly rely upon experimental data from the operation of the current reactor fleet. These data allow global experimental validation based on Light Water Reactor (LWR) macroscopic physical parameters of interest. However, they are insufficient for validating detailed coupling at the assembly and pin level. The use of well-controlled experimental data provided by research reactors is essential to implement a rigorous and consistent step-wise validation process of high-fidelity multi-physics coupling. That is why experimental data, such as the core power evolution in a transient-state coming from the SPERT-III experimental program and the CABRI research reactor, are analyzed as a first step towards this objective for the simulation of LWR transients initiated by reactivity insertion. The analysis of the state-of-the-art shows no existing experimental benchmark available worldwide for LWRs to consistently and rigorously validate advanced reactor physics/thermal-hydraulics/fuel performance coupling at the pin- or sub-channel scale. In this context, a discussion is therefore initiated in this paper on the perspective of developing new experiments dedicated to high-fidelity multi-physics tools, focusing on a first application: the validation of reactivity feedback effects. Very few existing light-water experimental reactors containing UO2fuel could today have the capacity to host these experiments. The development of a new validation experiment could only be achievable by considering a two-stage process for the experiment realization: a first stage involving a distributed network of sensors in the reactor core using instrumentation commonly used in research reactors, and a second stage implementing an instrumented fuel pin and innovative experimental techniques, in the longer term. Even if the OECD/NEA activities in the Expert Group on Multi-Physics Experimental Data, Benchmarks and Validation (EGMPEBV) (currently merged in the Expert Group on Multi-Physics of Reactor Systems – EGMUP) have started to pave the way for the development of such a high-fidelity multi-physics experiment, most of the work is still ahead of us.}, journal={FRONTIERS IN ENERGY RESEARCH}, author={Vaglio-Gaudard, Claire and Destouches, Christophe and Hawari, Ayman and Avramova, Maria and Ivanov, Kostadin and Valentine, Timothy and Blaise, Patrick and Hudelot, Jean-Pascal}, year={2023}, month={Jan} }
 @article{ahmed_fleming_hawari_2023, title={Effects of Hydrogen Bonding on Nuclear Data Development of Liquid Anhydrous HF}, volume={284}, ISBN={["*****************"]}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/202328417003}, abstractNote={Anhydrous Hydrogen Fluoride (HF) at high temperatures and pressures is used to process and manufacture nuclear fuel. As HF is often used directly with uranium, correct neutron thermal scattering cross sections are crucial to criticality safety applications. Classical molecular dynamics (CMD) simulation of the flexible HF system was used to create the thermal scattering law (TSL) and cross sections. The initial 2-site model is used in LAMMPS, and it can not capture the H-bond. To correctly represent the H-bond effects, a second, 3-site model was constructed in GROMACS. The 3-site model handled H-bonds by connecting a massless charge to the molecule. Key model parameters were compared to experimental data to verify the approach and models. To get the normalized VACF, the model was compared using hydrogen and fluorine bond length, density, potential energy, and diffusion coefficient. The phonon DOSs for both models were derived from the normalized VACF. DOSs were used to estimate the TSL (S(α,β)) and neutron thermal scattering cross sections for hydrogen in HF. The TSLs were evaluated using the FLASSH code with the Schofield diffusion model. It was observed that the representation of the hydrogen bonding changes the TSL's diffusional contributions. This is represented in the low energy scattering cross section, where intermolecular binding effects shift the cross section.}, journal={15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022}, author={Ahmed, Tanvir and Fleming, N. Colby and Hawari, Ayman I.}, year={2023} }
 @article{laramee_hawari_2023, title={Evaluation of Thermal Neutron Scattering Law and Cross Sections for Calcium Hydride}, volume={284}, ISBN={["*****************"]}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/202328417014}, abstractNote={Presented here are the calculated thermal scattering law (TSL) and thermal neutron scattering cross sections for Calcium Hydride, hereafter referred to by its chemical symbol CaH2. The only other such data prior to this evaluation are thermal neutron scattering libraries in the JEFF database, which do not fully capture the scattering physics of the CaH2 system. The data in this evaluation are calculated from first principles; Density Functional Theory (DFT) is used to calculate the phonon density of states (DOS), which is the primary input required to derive the TSL. The TSL and cross sections have been evaluated for the three non-equivalent atom cites in the CaH2: Ca, H1, and H2. Each evaluation has been submitted to the NNDC for consideration in the next ENDF/B database release.}, journal={15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022}, author={Laramee, Briana K. and Hawari, Ayman I.}, year={2023} }
 @article{fleming_manring_laramee_crozier_lee_hawari_2023, title={FLASSH 1.0: Thermal Scattering Law Evaluation and Cross Section Generation}, volume={284}, ISBN={["*****************"]}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/202328417007}, abstractNote={The Full Law Analysis Scattering System Hub (FLASSH) is an advanced code which evaluates the thermal scattering law (i.e. TSL, S(α,β)) for thermal scattering cross sections and resonance Doppler broadening. The ability to accurately capture these two key cross section features is dependent on accurate, high fidelity TSL evaluations. FLASSH 1.0 provides advanced physics capabilities resulting in an improved, generalized TSL to most accurately represent the lattice dynamics within any material. This improved TSL will allow for consistent analysis in both the thermal and epithermal energy ranges. The features for TSL analysis are packaged within the FLASSH GUI for easy user interface along with data output in many file formats including ENDF File 7 and ACE files.}, journal={15TH INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY, ND2022}, author={Fleming, N. Colby and Manring, Cole A. and Laramee, Benjamin K. and Crozier, Jonathan P. W. and Lee, Eunji and Hawari, Ayman I.}, year={2023} }
 @article{fleming_manring_laramee_crozier_lee_hawari_2023, title={FLASSH 1.0: Thermal Scattering Law Evaluation and Cross-Section Generation for Reactor Physics Applications}, ISSN={["1943-748X"]}, DOI={10.1080/00295639.2023.2194195}, abstractNote={Abstract The Full Law Analysis Scattering System Hub (FLASSH) is a modern, advanced code that evaluates the thermal scattering law (TSL) along with accompanying cross sections. FLASSH features generalized methods that accommodate any material structure. Historical approximations including incoherent and cubic approximations have been removed. Instead, the latest release of FLASSH features advanced physics options including distinct effect corrections (one-phonon contributions) and noncubic formulations. Noncubic elastic and inelastic contributions are necessary to accurately evaluate one-phonon contributions. Both noncubic and one-phonon calculations require high-density sampling of the various scattering directions. Optimization and parallelization of these routines were therefore necessary to produce results in a reasonable computational time frame. With these notable improvements to the generalized TSL, FLASSH 1.0 can meet benchmark requirements by permitting realistic comparisons with experiments for both TSLs and the resulting integrated cross sections. Within FLASSH, these high-fidelity TSLs can be applied also to the resonance region to evaluate accurate, material structure-dependent Doppler broadening that captures the observed experimental behavior. Additional features including a graphical user interface (GUI), plotting diagnostics, and formatted output options including ACE files allow users to complete a TSL evaluation with minimal input and maximum flexibility. The user GUI creates input files for FLASSH, reducing user error and also providing built-in error checks. Autofill options and suggested input values help make TSL evaluation accessible to novice users. The FLASSH code is compiled to run on both Windows and Linux platforms with automatic parallelization.}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Fleming, N. Colby and Manring, Cole A. and Laramee, Briana K. and Crozier, Jonathan P. W. and Lee, Eunji and Hawari, Ayman I.}, year={2023}, month={Apr} }
 @article{saxena_hawari_2023, title={High-Resolution Gamma-Ray Spectrometry of Pebble Bed Reactor Fuel Using Adaptive Digital Pulse Processing}, ISSN={["1943-7471"]}, DOI={10.1080/00295450.2022.2148839}, abstractNote={Abstract In this work, an investigation was performed to assess the feasibility of passive gamma-ray spectrometry using adaptive digital pulse processing for online interrogation of pebble bed reactor (PBR) fuel. This work incorporates the physics of the radiation emission phenomenon with advanced pulse processing techniques to develop a high-resolution gamma-spectrometry system capable of handling ultrahigh count rates in various applications of nuclear science and technology. Computational modeling was used to simulate the irradiation of PBR fuel and to design the adaptive digital pulse processing–based gamma-ray spectrometry system. Monte Carlo simulations were performed to study the gamma-ray spectra of the PBR fuel and to perform coupled photon-electron transport analysis to calculate the pulse-height spectrum of PBR fuel. A Monte Carlo computer routine was used to predict the effect of pulse pileup at high-count-rate conditions. This code utilizes the random interval distribution function based on Poisson statistics to simulate the pileup behavior. Combined with pileup logic, a recursive trapezoid filter with adaptive shaping parameters was implemented to simulate the pileup behavior of a digital gamma-ray spectrometry system. The adaptive shaping algorithm selects the rise time of the trapezoid shaping filter based on the separation between the input pulses for each incoming signal. The simulation results using the proposed adaptive digital pulse processing demonstrated that with the improved energy resolution, the burnup information can be more accurately determined on a pebble-by-pebble basis as compared to fixed shaping, and tasks related to in-core fuel management, safeguards, and waste management become feasible to perform efficiently and accurately.}, journal={NUCLEAR TECHNOLOGY}, author={Saxena, Shefali and Hawari, Ayman I.}, year={2023}, month={Feb} }
 @article{wall_parish_dixon_hawari_liu_breon_2023, title={Irradiation of ultrasonic sensors and adhesive couplants for application in light water reactor primary loop piping and components}, volume={414}, ISSN={["1872-759X"]}, DOI={10.1016/j.nucengdes.2023.112594}, abstractNote={The Electric Power Research Institute (EPRI) Nuclear Sector and US Department of Energy Light Water Reactor Sustainability Program are committed to engaging in research and development endeavors to address materials aging issues specific to long term operation of light water power reactors. To this effect, EPRI launched an industry initiative to develop nondestructive evaluation systems for online monitoring of existing cracks in light water reactor primary coolant loop piping and components. One of the goals of this initiative is to develop a sensor system (or systems) that can determine nondestructively if cracks are growing or arrested and, in the case of the former, to characterize their growth rates. A missing component of this initiative is an experimental assessment of how sensors and adhesive couplants will perform in service when exposed to chronic energetic neutron radiation, particularly at the primary coolant loop hot and cold leg dissimilar metal welds, which join the primary loop piping to the reactor pressure vessel and reside in the vicinity of the reactor core. The objective of this experimental study was to determine how ultrasonic transducers and adhesive couplants perform when exposed to irradiation in a test reactor to simulate and accelerate in-service exposure. To achieve this objective, the signal stability of piezoelectric transducers and performance of adhesive couplants as a function of accumulated fast neutron fluence were characterized by collecting ultrasonic data in-situ during irradiation. Of particular interest were the ultrasonic signal quality and time decay of the amplitude of acoustic reflections as a function of fast neutron fluence. The results of the study showed that, of the 8 transducer/substrate sample assemblies tested, only 3 generated usable ultrasonic signals through the conclusion of the irradiation campaign. It was found that high temperature epoxy tends to ultrasonically couple the sensors to the substrates better than three types of refractory ceramic cements studied, as is supported by post irradiation examination. The results obtained through this experimental study will be utilized in the achievement of the overall goal of development of a sensor system to perform online monitoring of primary loop components. This manuscript has been authored in part by UT-Battelle, LLC, under contract DE AC05-00OR22725 with the US Department of Energy (DOE). The publisher acknowledges the US government license to provide public access under the DOE Public Access Plan (https://energy.gov/downloads/doe-public-access-plan [energy.gov]).}, journal={NUCLEAR ENGINEERING AND DESIGN}, author={Wall, James J. and Parish, Chad M. and Dixon, J. Travis and Hawari, Ayman I. and Liu, Ming and Breon, Luke}, year={2023}, month={Dec} }
 @article{wormald_hawari_2022, title={Modeling fission spikes in nuclear fuel using a multigroup model of electronic energy transport}, volume={566}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2022.153797}, abstractNote={In fission based nuclear reactors the fuel is subject to an intense neutron environment that drives the fission chain reaction. Due to this process fission fragments are created with energies reaching 1 MeV/amu that lose energy primarily through inelastic interactions with the electronic structure producing electronic excitations . Subsequently, these excitations thermalize through electron-phonon interactions resulting in the formation of a high temperature thermal spike and associated pressure spike. This process promotes atomic mobility that is expected to evolve lattice defects, including the annealing of latent ion tracks. In this work, a multigroup model for electron energy transport is developed and applied to molecular dynamics simulations in the LAMMPS code to examine fission energy deposition and fission effects in nuclear fuel. This technique utilizes MCNP Monte Carlo electron transport calculations to determine the initial injection of fission energy. To provide a more predictive approach than semi-empirical two-temperature models, the electron-phonon interactions are defined to include multiphonon energy transfer as a function of atomic and electron temperature, and are evaluated from electronic structure calculations using the VASP density functional theory code and PHONON lattice dynamics code. Application of this model to fission energy deposition in uranium dioxide predicts ion track formation and fission enhanced atomic mobility behavior within reasonable agreement of experimental trends. Furthermore, simulations of fission fragment interactions with latent ion tracks demonstrate an annealing effect due to this enhanced mobility.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Wormald, J. L. and Hawari, A. I.}, year={2022}, month={Aug} }
 @article{burba_feightner_liu_hawari_2022, title={The Effect of Fluorinated Solvents on the Physicochemical Properties, Ionic Association, and Free Volume of a Prototypical Solvate Ionic Liquid}, ISSN={["1439-7641"]}, DOI={10.1002/cphc.202100548}, abstractNote={Abstract}, journal={CHEMPHYSCHEM}, author={Burba, Christopher M. and Feightner, Kylie and Liu, Ming and Hawari, Ayman}, year={2022}, month={Jan} }
 @article{datta_hawari_2021, title={Characterization and Implementation of A Dynamic Neutron Imaging System at the PULSTAR Reactor}, ISSN={["1939-8115"]}, DOI={10.1007/s11265-021-01694-8}, journal={JOURNAL OF SIGNAL PROCESSING SYSTEMS FOR SIGNAL IMAGE AND VIDEO TECHNOLOGY}, author={Datta, Arka and Hawari, Ayman I}, year={2021}, month={Sep} }
 @article{benson_harp_xie_yao_tolman_wright_king_hawari_cai_2021, title={Out-of-pile and postirradiated examination of lanthanide and lanthanide-palladium interactions for metallic fuel}, volume={544}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2020.152727}, abstractNote={Palladium is being investigated as a fuel additive to bind with and potentially immobilize lanthanide fission products. A primary cause of fuel-cladding chemical interaction (FCCI) is the lanthanide fission products migrating to the fuel periphery and interacting with the cladding. This interaction will lead to wastage of the cladding and eventually to a cladding breach. Palladium has previously been identified as a promising additive used to prevent or decrease FCCI by reacting with the lanthanide fission products. In the current study, an alloy cast from the four highest abundant lanthanides found in irradiated metallic fuel, Nd, Ce, Pr, and La, with and without Pd, has been characterized using neutron diffraction, scanning electron microscopy, and electron probe microanalysis. In the lanthanide-Pd intermetallic compounds, all of the constituent compounds, i.e. Nd-Pd, Ce-Pd, La-Pd and Pr-Pd are known. There is very good agreement, both structurally and compositionally, between the out-of-pile lanthanide alloy and lanthanide fission products characterized in irradiated fuels. In both cases, the lanthanide elements form a solid solution in a hexagonal crystal structure. The out-of-pile lanthanide alloy follows Vegard's Law, with the measured and calculated (weighted average of constituents) lattice parameters being within 1% for both the a and c parameters. Pd bonds with the lanthanides (Ln) forming the phases LnPd and Ln7Pd3. The results indicate the properties of lanthanide compounds in irradiated metallic fuel can be reliably simulated in out-of-pile experiments.}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Benson, Michael T. and Harp, Jason M. and Xie, Yi and Yao, Tiankai and Tolman, Kevin R. and Wright, Karen E. and King, James A. and Hawari, Ayman I. and Cai, Qingsheng}, year={2021}, month={Feb} }
 @article{wormald_hawari_zerkle_2020, title={Impact of magnetic structure and thermal effects on vibrational excitations and neutron scattering in uranium mononitride}, volume={143}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2020.107447}, abstractNote={Uranium mononitride (UN) is a nuclear fuel material of interest in the design of advanced reactors. Phonon spectra and dispersion relations of UN in its anti-ferromagnetic and paramagnetic structures were calculated using ab initio lattice dynamics. Subsequently, the dynamic structure factors of uranium and nitrogen in UN were generated in the incoherent approximation using the phonon expansion method for inclusion in the US National ENDF/B-VIII.0 database of neutron thermal scattering law evaluations. Phonons from spin polarized density functional theory simulations demonstrate good agreement with inelastic neutron scattering measurements; however, phonons from non-spin polarized simulations deviate from experiment, indicating a previously unexplored impact of magnetism on the vibrational characteristics. An oscillatory behavior due to multi-phonon scattering, previously observed in neutron scattering experiments at low temperatures, was captured in the dynamic structure factor. Moreover, calculated dynamic structure factors at 296 K and 1200 K demonstrate that the oscillatory behavior is present at elevated temperatures.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Wormald, J. L. and Hawari, A. and Zerkle, M. L.}, year={2020}, month={Aug} }
 @article{yang_phan_liu_hawari_kim_2020, title={Material defect study of thallium lead iodide (TlPbI3) crystals for radiation detector applications}, volume={954}, ISSN={0168-9002}, url={http://dx.doi.org/10.1016/J.NIMA.2018.10.194}, DOI={10.1016/j.nima.2018.10.194}, abstractNote={TlPbI3 is a promising semiconductor material for fabricating room-temperature radiation detectors, which have wide applications in national security, medical imaging, astrophysics research, industrial process monitoring and environmental survey. TlPbI3 has a large energy bandgap at 2.3 eV, a high density (6.04 g/cm3) and high concentrations of the high atomic number elements Tl and Pb. Such physical properties offer great potential to use TlPbI3 to detect gamma-ray at room temperature with high detection efficiency. In this work, we used the positron annihilation lifetime spectroscopy (PALS) measurement and infrared transmission microscopy to study the material defects in bulk TlPbI3 crystals. These crystals were grown with Bridgman method. For the PALS measurements, we used the positron experimental setup at North Carolina State University’s PULSTAR reactor facility. A 15 μCi Na-22 positron source sealed with 7.6μm thick Kapton films was sandwiched between two identical pieces of TlPbI3 samples. Two cylindrical plastic scintillators (1 inch diameter by 1 inch long) combined with Hamamatsu H3378-50 photomultiplier tubes (PMT) were used to detect the 1.27 MeV gamma-rays in coincidence with the 511 keV annihilation gamma-rays as the start and the stop signals, respectively. A LeCroy Wavepro 7300A digital oscilloscope was used to digitize the raw PMT pulses and acquire the PALS spectra. The dominating positron lifetime in TlPbI3 is 393 ps and its intensity is more than 92%. This component is typically attributed to some vacancy type (or more likely, vacancy cluster) positron trapping sites. The first component of ∼ 140 ps could be related to mono-vacancies or positrons annihilate in a delocalized lattice state. Compared with MAPbI3, the higher average lifetime, τav, and the higher intermediate lifetime (τ2) in TlPbI3 indicate the presence of more anion-type vacancies and imply an increase in ionic conductivity. Using infrared transmission microscopy, we also observed the formation of large volume TlPbI3 single crystal even in the transition portion between the conical seeding pocket and the normal growth chunk.}, journal={Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment}, publisher={Elsevier BV}, author={Yang, G. and Phan, Q.V. and Liu, M. and Hawari, A. and Kim, H.}, year={2020}, month={Feb}, pages={161516} }
 @article{hawari_2020, title={On a measurement approach to support evaluation of thermal scattering law data}, volume={135}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2019.106940}, abstractNote={Inelastic thermal neutron scattering in materials that act as neutron moderators, reflectors, and filters results in shaping the neutron spectrum at low energies. This phenomenon is described using differential scattering cross sections calculated from three components including the bound atom (i.e., nuclear) scattering cross section of the neutron, the ratio of the outgoing and incoming neutron energy, and the thermal scattering law (TSL), i.e., S(α,β), where α and β represent dimensionless momentum and energy exchange variables, respectively. To date, no TSL libraries are generated using measured data. However, valuable information may be derived from measurements and “targeted” experiments that can validate TSL data and the related inelastic scattering cross sections. As a demonstration, a suite of coordinated measurements and experiments is described that was designed and used to support the evaluation of the TSL for “nuclear” graphite. This experimental suite includes neutron powder diffraction (for structure analysis), positron annihilation (for nano porosity assessment), inelastic neutron scattering measurements using a chopper spectrometer, transmission experiments using neutrons with energy below the Bragg cutoff thereby accessing the total (inelastic) cross section, and a slowing-down-time experiment to observe the developing neutron spectrum in the material. This experimental suite was key to understanding the difference in TSL between “nuclear” and “ideal” graphite and for the inclusion of “nuclear” graphite in the ENDF/B-VIII.0 nuclear data library release.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Hawari, Ayman I.}, year={2020}, month={Jan} }
 @article{fleming_bernard_brown_chadwick_saint jean_dupont_ge_harada_hawari_herman_et al._2020, title={Overview of the OECD-NEA Working Party on International Nuclear Data Evaluation Cooperation (WPEC)}, volume={239}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/202023915002}, abstractNote={The OECD Nuclear Energy Agency (NEA) Working Party on International Nuclear Data Evaluation Cooperation (WPEC) was established in 1989 to facilitate collaboration in nuclear data activities. Over its thirty year history, different Subgroups have been created to address topics in nearly every aspect of nuclear data, including: experimental measurements, evaluation, validation, model development, quality assurance of databases and the development of software tools.}, journal={ND 2019: INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY}, author={Fleming, M. and Bernard, D. and Brown, D. and Chadwick, M. and Saint Jean, C. and Dupont, E. and Ge, Z. and Harada, H. and Hawari, A. and Herman, M. and et al.}, year={2020} }
 @article{wu_yang_han_liu_hawari_du_peng_foster_chen_koschan_et al._2020, title={Role of Lithium Codoping in Enhancing the Scintillation Yield of Aluminate Garnets}, volume={13}, ISSN={["2331-7019"]}, DOI={10.1103/PhysRevApplied.13.064060}, abstractNote={The aim of this work is to clarify the scintillation-yield enhancement in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ scintillators obtained by $\mathrm{Li}$ codoping via integrated study of the valence state of activators, the preferential site occupancy of $\mathrm{Li}$ codopants, and defect structures from experimental and theoretical insights. With $\mathrm{Li}$ codoping, the light yield and energy resolution of $10\ifmmode\times\else\texttimes\fi{}10\ifmmode\times\else\texttimes\fi{}10\phantom{\rule{0.1em}{0ex}}{\mathrm{mm}}^{3}$ $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ samples are improved from 15 600 to 24 800 photons/MeV, and 5.3 to 4.3% at 662 keV, respectively. The optical absorption spectra indicate that $\mathrm{Li}$ codoping does not induce conversion of stable ${\mathrm{Pr}}^{3+}$ to ${\mathrm{Pr}}^{4+}$ in $\mathrm{Lu}\mathrm{YAG}:\mathrm{Pr}$ single crystals. Based on the formation energies of substitutional and interstitial $\mathrm{Li}$ sites using density-functional-theory (DFT) calculations and the ${}^{7}\mathrm{Li}$ nuclear magnetic resonance results, it is shown that the $\mathrm{Li}$ ions prefer to dominantly occupy the fourfold coordinated interstitial sites and fourfold coordinated $\mathrm{Al}$ sites. The systematic analysis of thermoluminescence glow curves, positron annihilation lifetime spectroscopies, and defect formation energies derived from DFT calculations reveals that the concentration of isolated $\mathrm{Lu}$ and $\mathrm{Al}$ vacancies as dominant acceptor defects is reduced by $\mathrm{Li}$ codoping, whilst the shallow ${\mathrm{Li}}_{i}$ interstitial defects and the deep ${V}_{O}$ oxygen vacancies are introduced simultaneously. We propose that the lowering of hole trapping at defects resulting from $\mathrm{Li}$ codoping contributes to the scintillation-yield enhancement.}, number={6}, journal={PHYSICAL REVIEW APPLIED}, author={Wu, Yuntao and Yang, Ge and Han, Dan and Liu, Ming and Hawari, Ayman and Du, Mao-Hua and Peng, Jing and Foster, Camera and Chen, Shiyou and Koschan, Merry and et al.}, year={2020}, month={Jun} }
 @article{ahmed_alarcon_aleksandrova_baeßler_barron-palos_bartoszek_beck_behzadipour_berkutov_bessuille_et al._2019, title={A new cryogenic apparatus to search for the neutron electric dipole moment}, volume={14}, ISSN={1748-0221}, url={http://dx.doi.org/10.1088/1748-0221/14/11/P11017}, DOI={10.1088/1748-0221/14/11/P11017}, abstractNote={A cryogenic apparatus is described that enables a new experiment, nEDM@SNS, with a major improvement in sensitivity compared to the existing limit in the search for a neutron Electric Dipole Moment (EDM). This apparatus uses superfluid 4He to produce a high density of Ultra-Cold Neutrons (UCN) which are contained in a suitably coated pair of measurement cells. The experiment, to be operated at the Spallation Neutron Source at Oak Ridge National Laboratory, uses polarized 3He from an Atomic Beam Source injected into the superfluid 4He and transported to the measurement cells where it serves as a co-magnetometer. The superfluid 4He is also used as an insulating medium allowing significantly higher electric fields, compared to previous experiments, to be maintained across the measurement cells. These features provide an ultimate statistical uncertainty for the EDM of 2−3× 10−28 e-cm, with anticipated systematic uncertainties below this level.}, number={11}, journal={Journal of Instrumentation}, publisher={IOP Publishing}, author={Ahmed, M.W. and Alarcon, R. and Aleksandrova, A. and Baeßler, S. and Barron-Palos, L. and Bartoszek, L.M. and Beck, D.H. and Behzadipour, M. and Berkutov, I. and Bessuille, J. and et al.}, year={2019}, month={Nov}, pages={P11017–P11017} }
 @article{manring_hawaii_2019, title={Assessment of thermal neutron scattering in a heavy paraffinic molecular material}, volume={128}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2018.12.042}, abstractNote={Base oil lubricants are prevalent in many mechanical systems and can have a wide range of chemical compositions. Their carbon numbers generally span from C20–40, and they are often classified by their predominant molecular species (e.g., paraffinic, naphthenic). These heavy, viscous fluids are of interest when used in or around nuclear fuel cycle facilities, where they may influence the local neutronic environment. Furthermore, variations in molecular structure impact the oil’s dynamic properties and render the modeling process a challenging endeavor. In this work, a model for a specific paraffinic oil was developed for obtaining the vibrational/translational density of states (DOS) for hydrogen and carbon, the primary and secondary scattering species, respectively. The DOS was subsequently used to generate the thermal scattering law (TSL). The molecular ensemble, constructed using the MedeA material design platform, was benchmarked using available static and dynamic properties (i.e., density, viscosity, viscosity index, diffusivity). All simulations were performed using the LAMMPS code, and the COMPASS force field, a semi-empirical molecular potential, was selected as a suitable representation of the system dynamics. Additionally, a modified version of the nuclear data processing code, NJOY, was employed to more accurately treat a viscous fluid. Through this investigation, the viscous behavior characteristic of a heavy paraffinic oil was captured and was found to minimally impact the TSL when compared to the solid approximation.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Manring, Cole A. and Hawaii, Ayman I.}, year={2019}, month={Jun}, pages={140–147} }
 @article{sorrell_hawari_2019, title={TREAT M2 experiment modeling for transient benchmark analysis}, volume={128}, ISSN={["0306-4549"]}, DOI={10.1016/j.anucene.2019.01.026}, abstractNote={The Transient Reactor Test Facility (TREAT) is a graphite moderated, air-cooled reactor historically operated for experimental fuel transient testing. This facility has returned to operation, and as part of that effort, the ability to model and predict the conditions for upcoming experiments is paramount to the development of the next generation of transient experiments and fuel testing. In order to design future experiments, methods are being developed to predictively reproduce data from experiments performed at TREAT prior to its shutdown in 1994. In this work, the M2 and M3 experiments, representing a set of transient tests, were selected to explore as TREAT transient benchmarks. In TREAT, transients are controlled largely by control/transient rod movement and temperature feedback that is attributed to the core’s graphite-fuel matrix. To capture these effects, a methodology for modeling these mechanisms using multi-physics coupled Monte Carlo simulations is developed. This includes performing full-core transient simulations using the Serpent Monte Carlo code with feedback based on temperature estimates derived from the OpenFOAM computational fluid dynamics code. Using this methodology, the developed model for the TREAT reactor was able to reproduce the pre-transient power maneuver from low power up to 27 MW and to initiate the power transient for the M2 2580 experiment. The results demonstrate the utility of this approach for dynamic modeling of the reactor. Additional investigation of the impact of key phenomena such as neutron thermalization and temperature response in TREAT’s graphitic core and the surrounding reflector was also performed under steady state conditions using the developed models.}, journal={ANNALS OF NUCLEAR ENERGY}, author={Sorrell, Nina C. and Hawari, Ayman I.}, year={2019}, month={Jun}, pages={398–405} }
 @article{saxena_hawari_2018, title={Digital pulse deconvolution with adaptive shaping for real-time high-resolution high-throughput gamma spectroscopy}, ISSN={0168-9002}, url={http://dx.doi.org/10.1016/J.NIMA.2018.09.123}, DOI={10.1016/J.NIMA.2018.09.123}, abstractNote={A high-resolution high-throughput real-time adaptive digital pulse processing system is developed for high count rate gamma-ray spectroscopy applications. The adaptive digital pulse processing algorithms are implemented on a reconfigurable FPGA and include a pulse deconvolver, adaptive shaping filter, timing filter, baseline restorer, and pile-up rejecter. Digital pulse deconvolution is implemented to reconstruct the original detector signal from the preamplifier signal, which reduces the resolution deterioration due to pulse pile-up. The deconvoluted signal is shaped with a trapezoid filter and the shaping parameter is selected adaptively based on the time separation between successive input pulses. Experimental measurements are performed with a 137Cs source under varying count-rate conditions and using germanium detectors equipped with resistive feedback and transistor reset preamplifiers (TRP). The results demonstrate that when using a TRP, adaptive digital signal processing allows handling 106 counts/s. In addition, the implementation of a deconvolution approach limits resolution deterioration for throughput rates that are 4 to 10 times better than achievable in typical digital and analog gamma-ray spectroscopy systems.}, journal={Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment}, publisher={Elsevier BV}, author={Saxena, Shefali and Hawari, Ayman I.}, year={2018}, month={Oct} }
 @article{brown_chadwick_capote_kahler_trkov_herman_sonzogni_danon_carlson_dunn_et al._2018, title={ENDF/B-VIII.0: The 8 th  Major Release of the Nuclear Reaction Data Library with CIELO-project Cross Sections, New Standards and Thermal Scattering Data}, volume={148}, ISSN={0090-3752}, url={http://dx.doi.org/10.1016/J.NDS.2018.02.001}, DOI={10.1016/J.NDS.2018.02.001}, abstractNote={We describe the new ENDF/B-VIII.0 evaluated nuclear reaction data library. ENDF/B-VIII.0 fully incorporates the new IAEA standards, includes improved thermal neutron scattering data and uses new evaluated data from the CIELO project for neutron reactions on 1H, 16O, 56Fe, 235U, 238U and 239Pu described in companion papers in the present issue of Nuclear Data Sheets. The evaluations benefit from recent experimental data obtained in the U.S. and Europe, and improvements in theory and simulation. Notable advances include updated evaluated data for light nuclei, structural materials, actinides, fission energy release, prompt fission neutron and γ-ray spectra, thermal neutron scattering data, and charged-particle reactions. Integral validation testing is shown for a wide range of criticality, reaction rate, and neutron transmission benchmarks. In general, integral validation performance of the library is improved relative to the previous ENDF/B-VII.1 library.}, journal={Nuclear Data Sheets}, publisher={Elsevier BV}, author={Brown, D.A. and Chadwick, M.B. and Capote, R. and Kahler, A.C. and Trkov, A. and Herman, M.W. and Sonzogni, A.A. and Danon, Y. and Carlson, A.D. and Dunn, M. and et al.}, year={2018}, month={Feb}, pages={1–142} }
 @article{brown_chadwick_capote_kahler_trkov_herman_sonzogni_danon_carlson_dunn_et al._2018, title={ENDF/B-VIII.0: The 8th major release of the nuclear reaction data library with CIELO-project cross Sections, new standards and thermal scattering data}, volume={148}, journal={Nuclear Data Sheets}, author={Brown, D. A. and Chadwick, M. B. and Capote, R. and Kahler, A. C. and Trkov, A. and Herman, M. W. and Sonzogni, A. A. and Danon, Y. and Carlson, A. D. and Dunn, M. and et al.}, year={2018}, pages={1–142} }
 @article{datta_hawari_2018, title={Investigation of a model based reconstruction technique for neutron tomography at the PULSTAR reactor}, ISSN={0168-9002}, url={http://dx.doi.org/10.1016/J.NIMA.2018.09.035}, DOI={10.1016/J.NIMA.2018.09.035}, abstractNote={A model based neutron computed tomography (CT) reconstruction technique is developed and implemented to improve the spatial resolution of the reconstructed images. The spatial resolution of a scintillator and a charge-coupled device (CCD) based digital neutron imaging system is quantified and used as the system response function in the iterative reconstruction process. The response function is included in the forward projection model to provide better estimation of the actual projection, thus improving the spatial resolution of the reconstructed images. Experiments were performed at the neutron imaging facility of the PULSTAR research reactor to corroborate the performance of the developed algorithm with a 50μm and a 250μm 6LiF:ZnS thick scintillator. The reconstruction results of the model based technique have shown an improvement of nearly 40% in achievable spatial resolution as compared to the standard filtered back projection (FBP) technique.}, journal={Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment}, publisher={Elsevier BV}, author={Datta, Arka and Hawari, Ayman I.}, year={2018}, month={Sep} }
 @article{zhou_liu_williams_griffin_cress_rivas_rudy_polcawich_glaser_bassiri-gharb_et al._2018, title={Radiation-induced changes of vacancy-type defects in ferroelectric capacitors as revealed by Doppler broadening positron annihilation spectroscopy}, volume={124}, ISSN={["1089-7550"]}, DOI={10.1063/1.5045189}, abstractNote={Thin film ferroelectric capacitors of composition Pb(Zr0.52Ti0.48)O3 were exposed to Fe3+ radiation (1011 to 1013 ions/cm2), and the change in the defect structure was investigated by Doppler broadening positron annihilation spectroscopy and other characterization techniques. As the radiation fluence increases, a systematic drop of the S parameter of the positron annihilation photopeak is observed and attributed to an increase in the Zr- and Ti-site related vacancies relative to the Pb-sites. The results demonstrate that the radiation has a more significant influence on the Zr- and Ti-sites relative to the Pb-sites. It is also observed that the S parameter of the Mn-doped samples is higher than the undoped counterparts. Coupled with ferroelectricity measurements and X-ray diffraction, the results suggest that the Mn dopant modifies the initial structure of the material and leads to a different functional response.}, number={24}, journal={JOURNAL OF APPLIED PHYSICS}, author={Zhou, Hanhan and Liu, Ming and Williams, Samuel C. and Griffin, Lee A. and Cress, Cory D. and Rivas, Manuel and Rudy, Ryan Q. and Polcawich, Ronald G. and Glaser, Evan R. and Bassiri-Gharb, Nazanin and et al.}, year={2018}, month={Dec} }
 @article{anghel_bailey_bison_blau_broussard_clayton_cude-woods_daum_hawari_hild_et al._2018, title={Solid deuterium surface degradation at ultracold neutron sources}, volume={54}, ISSN={1434-6001 1434-601X}, url={http://dx.doi.org/10.1140/epja/i2018-12594-2}, DOI={10.1140/epja/i2018-12594-2}, abstractNote={Solid deuterium (sD_2) is used as an efficient converter to produce ultracold neutrons (UCN). It is known that the sD_2 must be sufficiently cold, of high purity and mostly in its ortho-state in order to guarantee long lifetimes of UCN in the solid from which they are extracted into vacuum. Also the UCN transparency of the bulk sD_2 material must be high because crystal inhomogeneities limit the mean free path for elastic scattering and reduce the extraction efficiency. Observations at the UCN sources at Paul Scherrer Institute and at Los Alamos National Laboratory consistently show a decrease of the UCN yield with time of operation after initial preparation or later treatment (`conditioning') of the sD_2. We show that, in addition to the quality of the bulk sD_2, the quality of its surface is essential. Our observations and simulations support the view that the surface is deteriorating due to a build-up of D_2 frost-layers under pulsed operation which leads to strong albedo reflections of UCN and subsequent loss. We report results of UCN yield measurements, temperature and pressure behavior of deuterium during source operation and conditioning, and UCN transport simulations. This, together with optical observations of sD_2 frost formation on initially transparent sD_2 in offline studies with pulsed heat input at the North Carolina State University UCN source results in a consistent description of the UCN yield decrease.}, number={9}, journal={The European Physical Journal A}, publisher={Springer Nature}, author={Anghel, A. and Bailey, T. L. and Bison, G. and Blau, B. and Broussard, L. J. and Clayton, S. M. and Cude-Woods, C. and Daum, M. and Hawari, A. and Hild, N. and et al.}, year={2018}, month={Sep} }
 @article{torstensen_liu_jin_deng_hawari_syverud_spontak_gregersen_2018, title={Swelling and Free-Volume Characteristics of TEMPO-Oxidized Cellulose Nanofibril Films}, volume={19}, ISSN={["1526-4602"]}, DOI={10.1021/acs.biomac.7b01814}, abstractNote={Cellulose nanofibrils (CNFs) are becoming increasingly ubiquitous in diverse technologies requiring sustainable nanoscale species to form or modify films. The objective of the present study is to investigate the swelling behavior and accompanying free volume of self-standing TEMPO-oxidized (TO) CNF films in the presence of water vapor. For this purpose, we have performed time-resolved swelling experiments on films, prepared according to different experimental protocols, at 90% relative humidity (RH) and ambient temperature. Corresponding free-volume characteristics are elucidated by positron annihilation lifetime spectroscopy (PALS) conducted at ambient temperature and several RH levels. Increasing the drying temperature of the films (from ambient to 50 °C) is observed to promote an increase in film density, which serves to reduce bulk swelling. These elevated drying temperatures likewise cause the free-volume pore size measured by PALS to decrease, while the corresponding total free-volume fraction remains nearly constant. Similarly, dispersion of TO-CNF into aqueous suspensions by ultrasonication prior to film formation increases both the total free-volume fraction and pore size but reduces the size of individual nanofibrils with little net change in bulk swelling. The swelling and concurrent free-volume measurements reported here generally reveal an increase in the free volume of TO-CNF films with increasing RH.}, number={3}, journal={BIOMACROMOLECULES}, author={Torstensen, Jonathan O. and Liu, Ming and Jin, Soo-Ah and Deng, Liyuan and Hawari, Ayman I. and Syverud, Kristin and Spontak, Richard J. and Gregersen, Oyvind W.}, year={2018}, month={Mar}, pages={1016–1025} }
 @article{datta_hawari_2017, title={Geant4 Analysis of a Thermal Neutron Real-Time Imaging System}, volume={64}, ISSN={["1558-1578"]}, DOI={10.1109/tns.2017.2708031}, abstractNote={Thermal neutron imaging is a technique for nondestructive testing providing complementary information to X-ray imaging for a wide range of applications in science and engineering. Advancement of electronic imaging systems makes it possible to obtain neutron radiographs in real time. This method requires a scintillator to convert neutrons to optical photons and a charge-coupled device (CCD) camera to detect those photons. Alongside, a well collimated beam which reduces geometrical blurriness, the use of a thin scintillator can improve the spatial resolution significantly. A representative scintillator that has been applied widely for thermal neutron imaging is 6LiF:ZnS (Ag). In this paper, a multiphysics simulation approach for designing thermal neutron imaging system is investigated. The Geant4 code is used to investigate the performance of a thermal neutron imaging system starting with a neutron source and including the production of charged particles and optical photons in the scintillator and their transport for image formation in the detector. The simulation geometry includes the neutron beam collimator and sapphire filter. The 6LiF:ZnS (Ag) scintillator is modeled along with a pixelated detector for image recording. The spatial resolution of the system was obtained as the thickness of the scintillator screen was varied between 50 and  $400~\mu \text{m}$ . The results of the simulation were compared to experimental results, including measurements performed using the PULSTAR nuclear reactor imaging beam, showing good agreement. Using the established model, further examination showed that the resolution contribution of the scintillator screen is correlated with its thickness and the range of the neutron absorption reaction products (i.e., the alpha and triton particles). Consequently, thinner screens exhibit improved spatial resolution. However, this will compromise detection efficiency due to the reduced probability of neutron absorption.}, number={7}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Datta, Arka and Hawari, Ayman I.}, year={2017}, month={Jul}, pages={1652–1658} }
 @article{wormald_hawari_2017, title={Generation of phonon density of states and thermal scattering law using ab initio molecular dynamics}, volume={101}, ISSN={0149-1970}, url={http://dx.doi.org/10.1016/J.PNUCENE.2017.02.011}, DOI={10.1016/J.PNUCENE.2017.02.011}, abstractNote={The operation and safety of thermal nuclear reactors is dependent on the ability to accurately predict the thermal neutron spectrum; a distribution which is correlated to the inelastic thermal neutron scattering cross-section of the neutron moderator. The inelastic thermal neutron scattering law, S(α,β), of a moderator is a fundamental property of the material describing the permitted vibrational excitations of the atoms, i.e. phonons, and may be calculated using atomistic methods. The current state-of-the-art ab initio lattice dynamics (AILD) methods have been used to calculate phonon density of states (DOS) at 0 K for use in the generation of S(α,β) under reactor conditions. Modern computational power, however, has made accessible ab initio molecular dynamics (AIMD) methods. The AIMD technique captures temperature effects and permits the calculation of the phonon DOS from first principles. This is in contrast to AILD, where temperature effects are lacking, and classical molecular dynamics methods that use semi-empirical force fields. The aim of this work is to demonstrate the use of AIMD in the generation of the phonon DOS for beryllium as an exemplar moderating material. The phonon DOS was computed from AIMD simulations at 300 K and compared to AILD. Subsequently, S(α,β) and the inelastic scattering cross-section were generated using the NJOY package for both methods. The predicted phonon DOS and inelastic scattering cross-section for the AIMD method were found to be consistent with those predicted using AILD.}, journal={Progress in Nuclear Energy}, publisher={Elsevier BV}, author={Wormald, J.L. and Hawari, A.I.}, year={2017}, month={Nov}, pages={461–467} }
 @article{saxena_hawari_2017, title={Investigation of FPGA-Based Real-Time Adaptive Digital Pulse Shaping for High-Count-Rate Applications}, volume={64}, ISSN={["1558-1578"]}, DOI={10.1109/tns.2017.2692219}, abstractNote={Digital signal processing techniques have been widely used in radiation spectrometry to provide improved stability and performance with compact physical size over the traditional analog signal processing. In this paper, field-programmable gate array (FPGA)-based adaptive digital pulse shaping techniques are investigated for real-time signal processing. National Instruments (NI) NI 5761 14-bit, 250-MS/s adaptor module is used for digitizing high-purity germanium (HPGe) detector’s preamplifier pulses. Digital pulse processing algorithms are implemented on the NI PXIe-7975R reconfigurable FPGA (Kintex-7) using the LabVIEW FPGA module. Based on the time separation between successive input pulses, the adaptive shaping algorithm selects the optimum shaping parameters (rise time and flattop time of trapezoid-shaping filter) for each incoming signal. A digital Sallen–Key low-pass filter is implemented to enhance signal-to-noise ratio and reduce baseline drifting in trapezoid shaping. A recursive trapezoid-shaping filter algorithm is employed for pole-zero compensation of exponentially decayed (with two-decay constants) preamplifier pulses of an HPGe detector. It allows extraction of pulse height information at the beginning of each pulse, thereby reducing the pulse pileup and increasing throughput. The algorithms for RC–CR2 timing filter, baseline restoration, pile-up rejection, and pulse height determination are digitally implemented for radiation spectroscopy. Traditionally, at high-count-rate conditions, a shorter shaping time is preferred to achieve high throughput, which deteriorates energy resolution. In this paper, experimental results are presented for varying count-rate and pulse shaping conditions. Using adaptive shaping, increased throughput is accepted while preserving the energy resolution observed using the longer shaping times.}, number={7}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Saxena, Shefali and Hawari, Ayman I.}, year={2017}, month={Jul}, pages={1733–1738} }
 @article{zhu_hawari_2017, title={Thermal neutron scattering cross section of liquid FLiBe}, volume={101}, ISSN={0149-1970}, url={http://dx.doi.org/10.1016/J.PNUCENE.2017.03.028}, DOI={10.1016/J.PNUCENE.2017.03.028}, abstractNote={The molten salt material Li2BeF4 (FLiBe) has been widely proposed as a moderator and coolant material in nuclear applications. Its usage and impact on neutron thermalization in the system requires accurate generation of FLiBe thermal neutron scattering libraries. In this work, liquid FLiBe is modeled using the classical molecular dynamics code LAMMPS. Experimental limits of liquid FLiBe properties are determined from reported databases. Predicted properties of FLiBe from molecular dynamics simulations including density, viscosity, and atomic species diffusivities are computed and compared to experimental data. To initiate the calculation of the thermal neutron scattering law, the velocity autocorrelation functions are calculated from molecular dynamics trajectories and subjected to Fourier transformation to obtain the density of states of possible excitations. Bound modes and diffusive modes of the density of states are separated to calculate the corresponding scattering law Sbound(α,β) and ,Sdiff(α,β) respectively. Thermal neutron scattering cross section libraries of F, Li, Be in FLiBe are generated at temperatures of 873 K, 923 K and 973 K in ENDF/B VII.1 format.}, journal={Progress in Nuclear Energy}, publisher={Elsevier BV}, author={Zhu, Y. and Hawari, A.I.}, year={2017}, month={Nov}, pages={468–475} }
 @article{wormald_hawari_2017, title={Thermal neutron scattering law calculations using ab initio molecular dynamics}, volume={146}, ISSN={["2100-014X"]}, DOI={10.1051/epjconf/201714613002}, abstractNote={In recent years, methods for the calculation of the thermal scattering law (i.e. S(α,β), where α and β are dimensionless momentum and energy transfer variables, respectively) were developed based on ab initio lattice dynamics (AILD) and/or classical molecular dynamics (CMD). While these methods are now mature and efficient, further advancement in the application of such atomistic techniques is possible using ab initio molecular dynamics (AIMD) methods. In this case, temperature effects are inherently included in the calculation, e.g. phonon density of states (DOS), while using ab initio force fields that eliminate the need for parameterized semi-empirical force fields. In this work, AIMD simulations were performed to predict the phonon spectra as a function of temperature for beryllium and graphite, which are representative nuclear reactor moderator and reflector materials. Subsequently, the calculated phonon spectra were utilized to predict S(α,β) using the LEAPR module of the NJOY code. The AIMD models of beryllium and graphite were 5 × 5 × 5 crystal unit cells (250 atoms and 500 atoms respectively). Electronic structure calculations for the prediction of Hellman-Feynman forces were performed using density functional theory with a GGA exchange correlation functional and corresponding core electron pseudopotentials. AIMD simulations of 1000–10,000 time-steps were performed with the canonical ensemble (NVT thermostat) for several temperatures between 300 K and 900 K. The phonon DOS were calculated as the power spectrum of the AIMD predicted velocity autocorrelation functions. The resulting AIMD phonon DOS and corresponding inelastic thermal neutron scattering cross sections at 300 K, where anharmonic effects are expected to be small, were found to be in reasonable agreement with the results generated using traditional AILD. This illustrated the validity of the AIMD approach. However, since the impact of the temperature on the phonon DOS (e.g. broadening of spectral peaks) was observed in AIMD analysis, this technique may be envisioned as the approach for deriving the needed atomistic data for thermal scattering law calculations under realistic temperature and structural conditions for a given material.}, journal={ND 2016: INTERNATIONAL CONFERENCE ON NUCLEAR DATA FOR SCIENCE AND TECHNOLOGY}, author={Wormald, Jonathan and Hawari, Ayman I.}, year={2017} }
 @article{holmes_hawari_zerkle_2016, title={A Phonon-Based Covariance Methodology for ENDF S(alpha, beta) and Thermal Neutron Inelastic Scattering Cross Sections}, volume={184}, ISSN={["1943-748X"]}, DOI={10.13182/nse15-89}, abstractNote={Abstract The S(α, β) double-differential thermal neutron scattering law tabulated in Evaluated Nuclear Data File (ENDF) File 7 is, by convention, produced theoretically through fundamental scattering physics models. Currently, no published ENDF evaluations contain covariance data for S(α, β) or associated scattering cross sections. Furthermore, no accepted methodology exists for quantifying or representing these covariances. Thermal scattering cross sections depend on the interatomic structure and dynamics of the material. For many solids, the influence of these properties on inelastic scattering cross sections can be adequately described through the phonon energy spectrum. The phonon spectrum can be viewed as a probability density function and is commonly the fundamental input for calculating S(α, β). Probable variation in the shape of the phonon spectrum may be established that characterizes uncertainties in the physics models and methodology employed in its production. Through Monte Carlo sampling of perturbations from the reference phonon spectrum, an S(α, β) covariance matrix may be generated. With appropriate sensitivity information, the S(α, β) covariance matrix can be propagated to generate covariance data for differential and integral cross sections. In this work, hexagonal graphite is used as an example material for demonstrating the proposed procedures for analyzing, calculating, and processing uncertainty information for theoretically generated thermal neutron inelastic scattering data.}, number={1}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Holmes, Jesse C. and Hawari, Ayman I. and Zerkle, Michael L.}, year={2016}, month={Sep}, pages={84–113} }
 @misc{phillips_snow_babul_banerjee_baxter_berezhiani_bergevin_bhattacharya_brooijmans_castellanos_et al._2016, title={Neutron-antineutron oscillations: Theoretical status and experimental prospects}, volume={612}, ISSN={["1873-6270"]}, DOI={10.1016/j.physrep.2015.11.001}, abstractNote={The observation of neutrons turning into antineutrons would constitute a discovery of fundamental importance for particle physics and cosmology. Observing the n–n̄ transition would show that baryon number (B) is violated by two units and that matter containing neutrons is unstable. It would provide a clue to how the matter in our universe might have evolved from the B=0 early universe. If seen at rates observable in foreseeable next-generation experiments, it might well help us understand the observed baryon asymmetry of the universe. A demonstration of the violation of B–L by 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics. Slow neutrons have kinetic energies of a few meV. By exploiting new slow neutron sources and optics technology developed for materials research, an optimized search for oscillations using free neutrons from a slow neutron moderator could improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutron beam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutron annihilation detector. Antineutron annihilation in a target downstream of a free neutron beam is such a spectacular experimental signature that an essentially background-free search is possible. An authentic positive signal can be extinguished by a very small change in the ambient magnetic field in such an experiment. It is also possible to improve the sensitivity of neutron oscillation searches in nuclei using large underground detectors built mainly to search for proton decay and detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutron oscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.}, journal={PHYSICS REPORTS-REVIEW SECTION OF PHYSICS LETTERS}, author={Phillips, D. G., II and Snow, W. M. and Babul, K. and Banerjee, S. and Baxter, D. V. and Berezhiani, Z. and Bergevin, M. and Bhattacharya, S. and Brooijmans, G. and Castellanos, L. and et al.}, year={2016}, month={Feb}, pages={1–45} }
 @article{wormald_hawari_2015, title={Examination of the impact of electron-phonon coupling on fission enhanced diffusion in uranium dioxide using classical molecular dynamics}, volume={30}, ISSN={["2044-5326"]}, DOI={10.1557/jmr.2014.405}, abstractNote={Abstract}, number={9}, journal={JOURNAL OF MATERIALS RESEARCH}, author={Wormald, Jonathan L. and Hawari, Ayman I.}, year={2015}, month={May}, pages={1485–1494} }
 @inbook{wormald_hawari_2014, title={Exploring Fission Enhanced Diffusion of Uranium in Uranium Dioxide Using Classical Molecular Dynamics Simulations}, ISBN={9783319485935 9783319482378}, url={http://dx.doi.org/10.1007/978-3-319-48237-8_21}, DOI={10.1007/978-3-319-48237-8_21}, booktitle={TMS 2014: 143rd Annual Meeting & Exhibition}, publisher={Springer International Publishing}, author={Wormald, J. L. and Hawari, A. I.}, year={2014}, pages={155–162} }
 @inproceedings{wormald_hawari_2014, title={Exploring fission enhanced diffusion of uranium in uranium dioxide using classical molecular dynamics simulations}, DOI={10.1002/9781118889879.ch21}, abstractNote={This chapter contains sections titled: Introduction Methodology Results and Discussion Conclusion}, booktitle={TMS 2014 Supplemental Proceedings}, author={Wormald, J. L. and Hawari, A. I.}, year={2014}, pages={155–162} }
 @article{holmes_hawari_2014, title={Generation of an S(alpha, beta) Covariance Matrix by Monte Carlo Sampling of the Phonon Frequency Spectrum}, volume={118}, ISSN={["1095-9904"]}, DOI={10.1016/j.nds.2014.04.089}, abstractNote={Formats and procedures are currently established for representing covariances in the ENDF library for many reaction types. However, no standard exists for thermal neutron inelastic scattering cross section covariance data. These cross sections depend on the material's dynamic structure factor, or S ( α , β ) . The structure factor is a function of the phonon density of states (DOS). Published ENDF thermal neutron scattering libraries are commonly produced by modeling codes, such as NJOY/LEAPR, which utilize the DOS as the fundamental input and directly output the S ( α , β ) matrix. To calculate covariances for the computed S ( α , β ) data, information about uncertainties in the DOS is required. The DOS may be viewed as a probability distribution function of available atomic vibrational energy states in a solid. In this work, density functional theory and lattice dynamics in the harmonic approximation were used to simulate the structure of silicon dioxide ( α -quartz) to produce the DOS. A range for the variation in the partial DOS for silicon in α -quartz was established based on limits of variation in the crystal lattice parameters. Uncertainty in an experimentally derived DOS may also be incorporated with the same methodology. A description of possible variation in the DOS allowed Monte Carlo generation of a set of perturbed DOS spectra which were sampled to produce the S ( α , β ) covariance matrix for scattering with silicon in α -quartz. With appropriate sensitivity matrices, it is shown that the S ( α , β ) covariance matrix can be propagated to generate covariance matrices for integrated cross sections, secondary energy distributions, and coupled energy-angle distributions.}, journal={NUCLEAR DATA SHEETS}, author={Holmes, J. C. and Hawari, A. I.}, year={2014}, month={Apr}, pages={392–395} }
 @article{hawari_gillete_2014, title={Inelastic Thermal Neutron Scattering Cross Sections for Reactor-grade Graphite}, volume={118}, ISSN={["1095-9904"]}, DOI={10.1016/j.nds.2014.04.030}, abstractNote={Current calculations of the inelastic thermal neutron scattering cross sections of graphite are based on representing the material using ideal single crystal models. However, the density of reactor-grade graphite is usually in the range of 1.5 g/cm3 to approximately 1.8 g/cm3, while ideal graphite is characterized by a density of nearly 2.25 g/cm3. This difference in density is manifested as a significant fraction of porosity in the structure of reactor-grade graphite. To account for the porosity effect on the cross sections, classical molecular dynamics (MD) techniques were employed to simulate graphite structures with porosity concentrations of 10% and 30%, which are taken to be representative of reactor-grade graphite. The phonon density of states for the porous systems were generated as the power spectrum of the MD velocity autocorrelation functions. The analysis revealed that for porous graphite the phonon density of states exhibit a rise in the lower frequency region that is relevant to neutron thermalization. Using the generated phonon density of states, the inelastic thermal neutron scattering cross sections were calculated using the NJOY code system. While marked discrepancies exist between measurements and calculations based on ideal graphite models, favorable agreement is found between the calculations based on the porous graphite models and measured data.}, journal={NUCLEAR DATA SHEETS}, author={Hawari, A. I. and Gillete, V. H.}, year={2014}, month={Apr}, pages={176–178} }
 @article{hawari_2014, title={Modern Techniques for Inelastic Thermal Neutron Scattering Analysis}, volume={118}, ISSN={["1095-9904"]}, DOI={10.1016/j.nds.2014.04.029}, abstractNote={A predictive approach based on ab initio quantum mechanics and/or classical molecular dynamics simulations has been formulated to calculate the scattering law, S(κ⇀,ω), and the thermal neutron scattering cross sections of materials. In principle, these atomistic methods make it possible to generate the inelastic thermal neutron scattering cross sections of any material and to accurately reflect the physical conditions of the medium (i.e, temperature, pressure, etc.). In addition, the generated cross sections are free from assumptions such as the incoherent approximation of scattering theory and, in the case of solids, crystalline perfection. As a result, new and improved thermal neutron scattering data libraries have been generated for a variety of materials. Among these are materials used for reactor moderators and reflectors such as reactor-grade graphite and beryllium (including the coherent inelastic scattering component), silicon carbide, cold neutron media such as solid methane, and neutron beam filters such as sapphire and bismuth. Consequently, it is anticipated that the above approach will play a major role in providing the nuclear science and engineering community with its needs of thermal neutron scattering data especially when considering new materials where experimental information may be scarce or nonexistent.}, journal={NUCLEAR DATA SHEETS}, author={Hawari, A. I.}, year={2014}, month={Apr}, pages={172–175} }
 @article{korobkina_medlin_wehring_hawari_huffman_young_beaumont_palmquist_2014, title={Ultracold neutron source at the PULSTAR reactor: Engineering design and cryogenic testing}, volume={767}, ISSN={1872-9576}, DOI={10.1016/j.nima.2014.08.016}, abstractNote={Construction is completed and commissioning is in progress for an ultracold neutron (UCN) source at the PULSTAR reactor on the campus of North Carolina State University. The source utilizes two stages of neutron moderation, one in heavy water at room temperature and the other in solid methane at ~40K, followed by a converter stage, solid deuterium at 5 K, that allows a single down scattering of cold neutrons to provide UCN. The UCN source rolls into the thermal column enclosure of the PULSTAR reactor, where neutrons will be delivered from a bare face of the reactor core by streaming through a graphite-lined assembly. The source infrastructure, i.e., graphite-lined assembly, heavy-water system, gas handling system, and helium liquefier cooling system, has been tested and all systems operate as predicted. The research program being considered for the PULSTAR UCN source includes the physics of UCN production, fundamental particle physics, and material surface studies of nanolayers containing hydrogen. In the present paper we report details of the engineering and cryogenic design of the facility as well as results of critical commissioning tests without neutrons.}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Korobkina, E. and Medlin, G. and Wehring, B. and Hawari, A. I. and Huffman, P. R. and Young, A. R. and Beaumont, B. and Palmquist, G.}, year={2014}, month={Dec}, pages={169–175} }
 @inproceedings{liu_moxom_hawari_gidley_2013, title={The intense slow positron beam facility at the PULSTAR reactor and applications in nano-materials study}, volume={1525}, url={http://dx.doi.org/10.1063/1.4802370}, DOI={10.1063/1.4802370}, abstractNote={An intense slow positron beam has been established at the PULSTAR nuclear research reactor of North Carolina State University. The slow positrons are generated by pair production in a tungsten moderator from gammarays produced in the reactor core and by neutron capture reactions in cadmium. The moderated positrons are electrostatically extracted and magnetically guided out of the region near the core. Subsequently, the positrons are used in two spectrometers that are capable of performing positron annihilation lifetime spectroscopy (PALS) and positron Doppler broadening spectroscopy (DBS) to probe the defect and free volume properties of materials. One of the spectrometers (e+-PALS) utilizes an rf buncher to produce a pulsed beam and has a timing resolution of 277 ps. The second spectrometer (Ps-PALS) uses a secondary electron timing technique and is dedicated to positronium lifetime measurements with an approximately 1 ns timing resolution. PALS measurements have been conducted in the e+-PALS spectrometer on a series of nano-materials including organic photovoltaic thin films, membranes for filtration, and polymeric fibers. These studies have resulted in understanding some critical issues related to the development of the examined nano-materials.}, publisher={AIP}, author={Liu, Ming and Moxom, Jeremy and Hawari, Ayman I. and Gidley, David W.}, year={2013}, pages={455–459} }
 @article{xiao_mishra_hawari_bingham_bilheux_tobin_2011, title={Coded source neutron imaging at the PULSTAR reactor}, volume={652}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2010.10.049}, abstractNote={A neutron imaging facility is located on beam-tube #5 of the 1-MW PULSTAR reactor at North Carolina State University. An investigation of high resolution imaging using the coded source imaging technique has been initiated at the facility. Coded imaging uses a mosaic of pinholes to encode an aperture, thus generating an encoded image of the object at the detector. To reconstruct the image data received by the detector, the corresponding decoding patterns are used. The optimized design of coded mask is critical for the performance of this technique and will depend on the characteristics of the imaging beam. In this work, a 34×38 uniformly redundant array (URA) coded aperture system is studied for application at the PULSTAR reactor neutron imaging facility. The URA pattern was fabricated on a 500 μm gadolinium sheet. Simulations and experiments with a pinhole object have been conducted using the Gd URA and the optimized beam line.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Xiao, Ziyu and Mishra, Kaushal K. and Hawari, Ayman I. and Bingham, Philip R. and Bilheux, Hassina Z. and Tobin, Kenneth W.}, year={2011}, month={Oct}, pages={606–609} }
 @article{hawari_gidley_moxom_hathaway_mukherjee_2011, title={Operation and testing of the PULSTAR reactor intense slow positron beam and PALS spectrometers}, volume={262}, ISBN={["*****************"]}, ISSN={["1742-6588"]}, DOI={10.1088/1742-6596/262/1/012024}, abstractNote={An intense slow positron beam has been established at the 1-MW PULSTAR nuclear reactor. The beam is operational generating mono-energetic positrons with an energy of 1-keV. The maximum measured intensity slightly exceeds 109 e+/s. The beam is operated routinely with an intensity of approximately 5×108 e+/s. The positrons are generated through gamma-ray pair production interactions in two back-to-back banks of tungsten converter/moderators. The gamma-rays are produced in the PULSTAR core and by thermal neutron capture in a cadmium shroud that surrounds the tungsten. The primary utilization of the PULSTAR positron beam is the characterization of nanoscale structure in materials. Consequently, the beam has been equipped with two state-of-the-art PALS spectrometers. The first spectrometer is dedicated to measurements in materials such as metals and semiconductors. This spectrometer is based on pulsing and bunching of the primary beam and is currently operating with a timing resolution of approximately 390 picoseconds. The second spectrometer is dedicated to measurements in materials where positronium formation is promoted. The timing resolution of this spectrometer is designed to be ~ 0.5 nanosecond with an on-sample spot size of 1–2 mm. For both spectrometers, the energy of the positrons can be varied to allow depth profiling with on-sample intensity exceeding 106 e+/s.}, journal={12TH INTERNATIONAL WORKSHOP ON SLOW POSITRON BEAM TECHNIQUES (SLOPOS12)}, author={Hawari, Ayman I. and Gidley, David W. and Moxom, Jeremy and Hathaway, Alfred G. and Mukherjee, Saurabh}, year={2011} }
 @article{mishra_hawari_2011, title={Phase contrast neutron imaging at the PULSTAR reactor}, volume={652}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2010.09.113}, abstractNote={Abstract Non-interferometric phase contrast effects have been shown to enhance material edges in neutron images. The achieved contrast enhancement in the image depends upon the neutron coherent scattering lengths of the materials present in the object and the degree of spatial coherence of the neutron beam. Spatial coherence of the beam is achieved using design-based spatial filters, a large L / d ratio (∼10,000) and low average neutron energy. Physically, a large L / d ratio is realized by a pinhole neutron source thereby significantly reducing the neutron beam intensity at the image plane. Thus, performance of such imaging exercises at low/medium intensity neutron sources is associated with additional design considerations that are not needed at high intensity neutron sources, where it has been demonstrated. In the present work, phase contrast neutron imaging was conducted using a suitably designed collimator at the 1-MWth PULSTAR reactor located at North Carolina State University (NCSU). Results of the imaging exercises that depict phase contrast edge enhancement are being presented along with the collimator design. Digital image plate detectors were used to capture images with a range of exposure times between 45 and 120 min.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Mishra, Kaushal K. and Hawari, Ayman I.}, year={2011}, month={Oct}, pages={615–619} }
 @article{gould_hawari_sharapov_2010, title={Reanalysis of Recent Neutron Diffusion and Transmission Measurements in Nuclear Graphite}, volume={165}, ISSN={["0029-5639"]}, DOI={10.13182/nse09-48}, abstractNote={Abstract We revisit the determination by Bowman et al. of unusual neutron transport characteristics for a newly fabricated form of graphite [Nucl. Sci. Eng., 159, 182 (2008); Nucl. Sci. Eng., 161, 68 (2009)]. From MCNP modeling and consideration of data from other experiments, we determine revised values for the neutron transport parameters of this graphite. Our reanalysis gives a coherent scattering cross section σcoh ˜ 4 b at 50 meV, a small-angle neutron scattering cross section σsans ˜ 11 to 13 b at 1 meV, and an effective capture cross section σa = 5.8 ± 0.5 mb. Scaled to a graphite reference density of 1.60 g/cm3, we find a diffusion coefficient D̄ = 0.94 ± 0.03 cm and a diffusion length L = 47.7 ± 3.7 cm. Apart from the somewhat larger values of σa and D̄, these are not untypical parameters for graphite. Based on our investigation, the recent experiments and analysis of Bowman et al. do not give evidence for different transport properties for this newly fabricated graphite.}, number={2}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Gould, C. R. and Hawari, A. I. and Sharapov, E. I.}, year={2010}, month={Jun}, pages={200–209} }
 @article{dijulio_hawari_2009, title={Examination of reactor grade graphite using neutron powder diffraction}, volume={392}, ISSN={["1873-4820"]}, DOI={10.1016/j.jnucmat.2009.03.014}, abstractNote={Graphite is of principal interest in Generation IV nuclear reactor concepts. In particular, graphite will be the moderator for the Very High Temperature Reactor. In support of experimental and computational investigations that aim at understanding the behavior of reactor grade graphite under operating conditions, neutron powder diffraction experiments have been performed at the North Carolina State University PULSTAR reactor. The collected diffraction patterns exhibit intense broadening of several of the reflections, characteristic of turbostratic stacking. In order to quantify this disorder structurally, a model combined with a Rietveld-like refinement approach was implemented, which includes several refinable parameters that aim at describing this type of structure. Stacking parameters representing the probabilities of a random and registered shift between stacking packages were defined. The results indicate that the studied reactor grade graphite specimens contain a small fraction of layer disorder. The inferred interlayer spacing for the specimens is slightly larger than the theoretical value for graphite of 0.335 nm and the lattice constant is slightly less than 0.246 nm. The developed methodology is found to be successful in fitting the neutron diffraction patterns of reactor grade graphite.}, number={2}, journal={JOURNAL OF NUCLEAR MATERIALS}, author={Dijulio, D. D. and Hawari, A. I.}, year={2009}, month={Jul}, pages={225–229} }
 @article{pattie_anaya_back_boissevain_bowles_broussard_carr_clark_currie_du_et al._2009, title={First measurement of the neutron beta asymmetry with ultracold neutrons}, volume={102}, number={1}, journal={Physical Review Letters}, author={Pattie, R. W. and Anaya, J. and Back, H. O. and Boissevain, J. G. and Bowles, T. J. and Broussard, L. J. and Carr, R. and Clark, D. J. and Currie, S. and Du, S. and et al.}, year={2009} }
 @article{mishra_hawari_2009, title={Investigating Phase Contrast Neutron Imaging for Mixed Phase-Amplitude Objects}, volume={56}, ISSN={["1558-1578"]}, DOI={10.1109/TNS.2009.2016962}, abstractNote={Phase contrast imaging is an imaging modality that has been extensively applied in X-ray imaging and was demonstrated using neutrons over the past few years. In this case, contrast in the image, especially at edges, is enhanced due to phase shifts that take place as the neutron wave passes through regions in the sample that differ in the coherent scattering length density. Usually, a pure phase object approximation is used to formulate the problem, whereas realistic samples represent mixed phase-amplitude objects. In this work, a formulation for mixed phase-amplitude objects with moderate neutron attenuation coefficients and its effect on the neutron image is presented. A computational simulation technique has been devised to study this effect on different types of samples. Using simulations, it is observed that the pure phase object approximation results in over enhancement of edges for a phase-amplitude object, with significant variation (in the case of neutron imaging) depending upon the edge forming material characteristics. The total contrast for the mixed phase-amplitude object is less than the sum of the individual attenuation and phase contrast components. The difference depends on the scalar product of the gradient of the coherent scattering length density and the attenuation coefficient. The presented formulation can aid in predicting and optimizing the performance characteristics of neutron phase contrast imaging experiments.}, number={3}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Mishra, Kaushal K. and Hawari, Ayman I.}, year={2009}, month={Jun}, pages={1629–1636} }
 @inproceedings{mishra_hawari_2008, title={Development of neutron phase contrast Imaging at the NC State University PULSTAR Reactor}, booktitle={Neutron Radiography}, author={Mishra, K. K. and Hawari, A. I.}, year={2008}, pages={123–133} }
 @article{dijulio_hawari_berliner_2007, title={A Gd-based gaseous electron multiplier detector for neutron scattering applications}, volume={579}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2007.04.015}, abstractNote={The optimum configuration of a Gaseous Electron Multiplier (GEM) neutron detector using a CsI–Gd–Kapton–Gd–CsI neutron converter is investigated using Monte-Carlo simulations. Neutrons absorbed in the converter produce secondary electrons that are emitted from the CsI layers. The detector can be assembled from multiple modules where each module consists of the neutron converter, several cascaded GEMs and anode pickup plates on both sides. Position sensitive anodes and localization electronics are then used to detect, timestamp and record the resulting signal. For a single GEM module, the performance can be assessed by estimating the secondary electron (SE) leakage from the converter sandwich. The simulations show that the optimum for a single module, double-sided detector would have a neutron converter composed of 0.1 μm CsI on top of 3 μm Gd plated on each side of a 7.5 μm thick Kapton foil. This detector would have a SE yield of approximately 0.6 SE/neutron and neutron absorption of 60%. Significant enhancements of the SE yield can be obtained for detectors composed of multiple modules with thinner Gd converters. The multi-module design allows for enhanced SE leakage from each converter while maintaining the high neutron absorption efficiency of thicker converters and dividing the detector count rate among multiple sets of decoding electronics.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={DiJulio, D. D. and Hawari, A. I. and Berliner, R.}, year={2007}, month={Aug}, pages={71–74} }
 @article{korobkna_wehring_hawari_young_huffman_golub_xu_palmquist_2007, title={An ultracold neutron source at the NC state university PULSTAR reactor}, volume={579}, ISSN={0168-9002}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-34547682155&partnerID=MN8TOARS}, DOI={10.1016/j.nima.2007.04.116}, abstractNote={Research and development is being completed for an ultracold neutron (UCN) source to be installed at the PULSTAR reactor on the campus of North Carolina State University (NCSU). The objective is to establish a university-based UCN facility with sufficient UCN intensity to allow world-class fundamental and applied research with UCN. To maximize the UCN yield, a solid ortho-D2 converter will be implemented coupled to two moderators, D2O at room temperature, to thermalize reactor neutrons, and solid CH4, to moderate the thermal neutrons to cold-neutron energies. The source assembly will be located in a tank of D2O in the space previously occupied by the thermal column of the PULSTAR reactor. Neutrons leaving a bare face of the reactor core enter the D2O tank through a 45×45 cm cross-sectional area void between the reactor core and the D2O tank. Liquid He will cool the disk-shaped UCN converter to below 5 K. Independently, He gas will cool the cup-shaped CH4 cold-neutron moderator to an optimum temperature between 20 and 40 K. The UCN will be transported from the converter to experiments by a guide with an inside diameter of 16 cm. Research areas being considered for the PULSTAR UCN source include time-reversal violation in neutron beta decay, neutron lifetime determination, support measurements for a neutron electric-dipole-moment search, and nanoscience applications.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Korobkna, E. and Wehring, B. W. and Hawari, A. I. and Young, A. R. and Huffman, P. R. and Golub, R. and Xu, Y. and Palmquist, G.}, year={2007}, month={Aug}, pages={530–533} }
 @article{chen_hawari_2007, title={Analysis of the impact of random summing on passive assay of pebble bed reactor fuel using gamma-ray spectrometry}, volume={579}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2007.04.064}, abstractNote={Pebble bed reactors (PBR) are characterized by multi-pass fuel systems in which spherical fuel pebbles are circulated through the core until they reach a proposed burnup limit. The fuel is assayed on-line to ensure that the burnup limit is not breached. However, random summing effects can impact the response of the burnup measurement system and result in distortions that degrade the accuracy of the assay results. Monte Carlo analysis was performed to estimate the magnitude and effect of random summing on the absolute and relative indicators that have been identified as usable in on-line assay. For a throughput rate of 105 counts/s and trapezoidal pulse shaping of the signals, the results show that absolute indicators suffer from severe distortions due to this effect. Relative indicators are found to be resistant to random summing with the deviation in the ratio of peak areas remaining less than 5–15% depending on pulse width.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Chen, J. and Hawari, A. I.}, year={2007}, month={Aug}, pages={297–300} }
 @article{hathaway_skalsey_frieze_vallery_gidley_hawari_xu_2007, title={Implementation of a prototype slow positron beam at the NC State University PULSTAR reactor}, volume={579}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2007.03.036}, abstractNote={Abstract A prototype beam system was designed and tested to verify the potential of utilizing the North Carolina State University PULSTAR reactor in generating slow positrons. An annealed tungsten converter/moderator was placed at the end of beam tube ♯6 near the PULSTAR core and surrounded by 0.5-mm-thick cadmium cover. Fission and cadmium capture γ-rays were used to drive pair production reactions in the tungsten. The positrons emitted from tungsten were electrostatically focused and guided using a magnetic solenoid until they reached the exit of the biological shield. An aluminum plate that was placed near the exit served as an annihilation target for the positrons. A coincidence counting system was set up to measure the true coincidence rate of the 511 keV annihilation photons that are produced once the slow positrons strike the aluminum target. The data demonstrated the production and control of slow positrons at a rate greater than 10 7 e + /s.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Hathaway, A. G. and Skalsey, M. and Frieze, W. E. and Vallery, R. S. and Gidley, D. W. and Hawari, A. I. and Xu, J.}, year={2007}, month={Aug}, pages={538–541} }
 @article{hawari_al-qasir_ougouag_2007, title={Investigation of the impact of simple carbon interstitial formations on thermal neutron scattering in graphite}, volume={155}, ISSN={["1943-748X"]}, DOI={10.13182/NSE07-A2676}, abstractNote={Abstract In both the prismatic and pebble bed designs of very high temperature reactors, the graphite moderator is expected to reach exposure levels of 1021 to 1022 n/cm2 over the lifetime of the reactor. This exposure results in damage to the graphite structure. Studies of the thermal properties of irradiated graphite show changes in the thermal conductivity and (to a lesser extent) the heat capacity at fluences <1021 n/cm2. In graphite, these properties depend on the behavior of atomic vibrations (phonons) in the solid. Therefore, it can be expected that alterations in the phonon behavior that would produce changes in these properties would have an impact on the thermal neutron scattering behavior of that material. In this work, an atomistic ab initio investigation is performed to explore the potential impact of simple carbon interstitial formations on the inelastic thermal neutron scattering behavior of graphite. Using the VASP/PHONON code system, graphite supercells were modeled with and without either a single carbon interstitial or a di-interstitial (C2) molecule between the graphite planes. This resulted in the production of the phonon frequency spectra for these structures. From the phonon data, the inelastic thermal neutron scattering cross sections were generated, using the NJOY code system, at temperatures of 300 and 1200 K. A comparison of the generated cross sections shows that accounting for the interstitials in the calculations affects the cross sections mainly in the energy range from 0.01 to 0.1 eV.}, number={3}, journal={NUCLEAR SCIENCE AND ENGINEERING}, author={Hawari, Ayman I. and Al-Qasir, Iyad I. and Ougouag, Abderrafi M.}, year={2007}, month={Mar}, pages={449–462} }
 @article{hehr_hawari_gillette_2007, title={Molecular dynamics simulations of graphite at high temperatures}, volume={160}, ISSN={["1943-7471"]}, DOI={10.13182/NT07-A3897}, abstractNote={Graphite, a key structural and moderator material in the proposed Generation IV roadmap, is expected to experience irradiation at temperatures up to 1800 K. In this study, a molecular dynamics (MD) code is developed for the purpose of performing atomistic simulations of high-temperature graphite. The MD computations are benchmarked against thermal expansion and mean-squared displacement data, and modifications to the potential energy function are devised as needed to fit experimental measurements. Graphite-specific alterations include a plane-by-plane center-of-mass velocity correction, anisotropy in the potential energy cutoff function, and temperature-dependent parameterization of the interatomic potential. The refined MD model is then employed to investigate the threshold displacement energy at temperatures of 300 and 1800 K. It was found that the threshold displacement energy depends strongly on the knock-on direction, yet the angle-averaged threshold energy exhibits relatively little variation with temperature.}, number={2}, journal={NUCLEAR TECHNOLOGY}, author={Hehr, Brian D. and Hawari, Ayman I. and Gillette, Victor H.}, year={2007}, month={Nov}, pages={251–256} }
 @article{moxom_hathaway_bodnaruk_hawari_xu_2007, title={Performance analysis of the intense slow-positron beam at the NC State University PULSTAR reactor}, volume={579}, ISSN={["0168-9002"]}, DOI={10.1016/j.nima.2007.04.117}, abstractNote={An intense positron beam, for application in nanophase characterization, is now under construction at the 1 MW PULSTAR nuclear reactor at North Carolina State University (NCSU). A tungsten converter/moderator is used, allowing positrons to be emitted from the surface with energies of a few electron volts. These slow positrons will be extracted from the moderator and formed into a beam by electrostatic lenses and then injected into a solenoidal magnetic field for transport to one of three experimental stations, via a beam switch. To optimize the performance of the beam and to predict the slow-positron intensity, a series of simulations were performed. A specialized Monte-Carlo routine was integrated into the charged-particle transport calculations to allow accounting for the probabilities of positron re-emission and backscattering from multiple-bank moderator/converter configurations. The results indicate that either a two-bank or a four-bank tungsten moderator/converter system is preferred for the final beam design. The predicted slow-positron beam intensities range from nearly 7×108 to 9×108e+/s for the two-bank and the four-bank systems, respectively.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Moxom, J. and Hathaway, A. G. and Bodnaruk, E. W. and Hawari, A. I. and Xu, J.}, year={2007}, month={Aug}, pages={534–537} }
 @article{harp_hawari_bourham_2007, title={Simulation of gamma-ray spectrometry of failed TRISO fuel}, volume={579}, ISSN={["1872-9576"]}, DOI={10.1016/j.nima.2007.04.065}, abstractNote={Very High Temperature Reactors (VHTR) utilize the TRISO microsphere as the fundamental fuel unit in the core. To understand better its behavior under in-core conditions, the fuel is irradiated in a controlled environment that resembles anticipated operating conditions. In this work, simulated γ-ray spectra are developed for the fission product gases that are released upon fuel failure. The simulations are based on Monte Carlo calculations of spectra assuming the use of high-purity germaniun (HPGe) and high-pressure xenon (HPXe) detectors. The γ-ray source terms for the simulations are derived from ORIGEN 2.2 fuel depletion calculations. In addition, measured detector resolution information are included in the Monte Carlo simulations to produce realistically broadened spectra. The effect of fission product release processes are incorporated in the simulation by reconstructing the Monte Carlo photon source terms using a specific gas-release model. Consequently, birth and release γ-ray spectra are constructed for the Kr and Xe gaseous fission products.}, number={1}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Harp, J. M. and Hawari, A. I. and Bourham, M. A.}, year={2007}, month={Aug}, pages={301–304} }
 @article{su_zhao_chen_hawari_2006, title={Assessment of on-line burnup monitoring of pebble bed reactor fuel by passive neutron counting}, volume={48}, ISSN={["0149-1970"]}, DOI={10.1016/j.pnucene.2006.06.013}, abstractNote={Due to the multi-pass fuel circulation design for pebble bed reactors, an on-line measurement system is needed to accurately assess whether a given pebble has reached its End-of-Life burnup limit and thereby provide an on-line, automated go/no-go decision on fuel disposition on a pebble-by-pebble basis. Gamma spectrometry has been used for burnup determination of pebble bed reactor fuels. In this work a preliminary investigation was conducted to assess the feasibility of using passive neutron counting techniques to analyze fuel pebbles in real time to provide the speed, accuracy, and burnup range required for burnup determination. Specifically, numerical simulations were performed to study the correlation between passive neutron emission rate of an irradiated pebble and its burnup level, the detectability of passive neutron emission from an irradiated fuel pebble by commonly used neutron detectors, and these detectors' abilities to discriminate gamma interference. The overall conclusion is that there is an acceptable correlation between burnup and passive neutron emission rate of an irradiated pebble at high-burnup levels; and if the neutron detection system is well designed, passive neutron counting can be used to provide an on-line, go/no-go decision on fuel disposition on a pebble-by-pebble basis for pebble bed reactors.}, number={7}, journal={PROGRESS IN NUCLEAR ENERGY}, author={Su, Bingjing and Zhao, Zhongxiong and Chen, Jianwei and Hawari, Ayman I.}, year={2006}, pages={686–702} }
 @article{mishra_hawari_gillette_2006, title={Design and performance of a thermal neutron imaging facility at the North Carolina State University PULSTAR reactor}, volume={53}, ISSN={["1558-1578"]}, DOI={10.1109/tns.2006.884323}, abstractNote={A thermal neutron imaging facility has been set up at the North Carolina State University PULSTAR reactor. The PULSTAR is an open pool light water moderated 1 MWth research reactor with six beam tubes. The present facility is set up on beam tube # 5 of the reactor. The facility is intended to have radiographic and tomographic capabilities. The design of the neutron collimator was performed using MCNP5. The collimator includes a 4-in bismuth filter followed by a 6-in single-crystal sapphire filter. Thermal neutron scattering cross-section libraries for sapphire and bismuth were generated and used in the MCNP simulation of the system. Based on the current design, the L/D of the facility ranges between 100 and 150. The neutron flux at the image plane can be varied from 1.8times106 to 7times106 n/cm2middots with a Cd-ratio of ~450. The resolution of the system for different imaging media was also estimated and found to be ~33 mum for conventional radiography film and ~110 mum for digital image plates. Initial measurements, using ASTM standards, show that the imaging facility achieves a beam quality classification of IA}, number={6}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Mishra, Kaushal K. and Hawari, Ayman I. and Gillette, Victor H.}, year={2006}, month={Dec}, pages={3904–3911} }
 @article{hawari_chen_2005, title={Computational investigation of on-line interrogation of pebble bed reactor fuel}, volume={52}, ISSN={["0018-9499"]}, DOI={10.1109/TNS.2005.856760}, abstractNote={Pebble bed reactors are characterized by multipass fuel systems in which spherical fuel pebbles are circulated through the core until they reach a proposed burnup limit (80000-100000 MWD/MTU). For such reactors, the fuel is assayed on-line to ensure that the burnup limit is not breached. We considered assaying the fuel using an HPGe detector to perform passive gamma-ray spectrometry of fission products. Since neither fresh nor irradiated fuel is readily available, computer simulations were utilized to identify the radionuclides that can be used as burnup indicators, and to visualize the gamma-ray spectra at various levels of burnup. Specifically, we used the ORIGEN-MONTEBURNS-MCNP code system. This allowed the establishment of the burnup dependent one-group gas reactor cross-sections for the radionuclides of interest. Subsequently, ORIGEN was used to simulate in-core pebble depletion to establish the irradiated pebble isotopics. Finally, the codes MCNP and SYNTH were used to simulate the response of the HPGe gamma-ray spectrometer. The results show that absolute and relative indicators can be used on-line to determine unambiguously the enrichment and burnup on a pebble-by-pebble basis. The activity of Cs-137 or the activity ratio of Co-60/Cs-134 can be combined with the activity ratio of Np-239/I-132 to yield the enrichment and burnup information. To use the relative indicators, a relative efficiency calibration of the gamma-ray spectrometer can be performed using the La-140 gamma lines that are emitted by the irradiated pebble. I-132, Cs-134, Cs-137, La-140, and Np-239 are produced upon the irradiation of the fuel. Co-60 is produced by doping the fuel with a small amount (/spl sim/100 ppm) of Co-59. Using this approach, the uncertainty in burnup determination due to factors such as power history variation, detector efficiency calibration, and counting statistics is expected to remain in the range of /spl plusmn/5% to /spl plusmn/10%.}, number={5}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Hawari, AI and Chen, JW}, year={2005}, month={Oct}, pages={1659–1664} }
 @article{hawari_chen_su_2005, title={Efficiency self-calibration of the HPGe detector for on-line assay of pebble bed reactor fuel}, volume={264}, ISSN={["0236-5731"]}, DOI={10.1007/s10967-005-0704-y}, number={1}, journal={JOURNAL OF RADIOANALYTICAL AND NUCLEAR CHEMISTRY}, author={Hawari, AI and Chen, J and Su, B}, year={2005}, month={Apr}, pages={265–270} }
 @article{chen_hawari_zhao_su_2003, title={Gamma-ray spectrometry analysis of pebble bed reactor fuel using Monte Carlo simulations}, volume={505}, ISSN={["0168-9002"]}, DOI={10.1016/S0168-9002(03)01105-7}, abstractNote={Abstract Monte Carlo simulations were used to study the gamma-ray spectra of pebble bed reactor fuel at various levels of burnup. A fuel depletion calculation was performed using the ORIGEN2.1 code, which yielded the gamma-ray source term that was introduced into the input of an MCNP4C simulation. The simulation assumed the use of a 100% efficient high-purity coaxial germanium (HPGe) detector, a pebble placed at a distance of 100 cm from the detector, and accounted for Gaussian broadening of the gamma-ray peaks. Previously, it was shown that 137 Cs, 60 Co (introduced as a dopant), and 134 Cs are the relevant burnup indicators. The results show that the 662 keV line of 137 Cs lies in close proximity to the intense 658 keV of 197 Nb, which results in spectral interference between the lines. However, the 1333 keV line of 60 Co, and selected 134 Cs lines (e.g., at 605 keV) are free from spectral interference, which enhances the possibility of their utilization as relative burnup indicators.}, number={1-2}, journal={NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT}, author={Chen, JW and Hawari, AI and Zhao, ZX and Su, BJ}, year={2003}, month={Jun}, pages={393–396} }
 @article{hawari_chen_su_zhao_2002, title={Assessment of on-line burnup monitoring of pebble bed reactor fuel using passive gamma-ray spectrometry}, volume={49}, ISSN={["0018-9499"]}, DOI={10.1109/TNS.2002.1039646}, abstractNote={An investigation was performed to assess the feasibility of passive gamma-ray spectrometry assay as an approach for on-line burnup determination for the Modular Pebble Bed Reactor (MPBR). In addition to its inherently safe design, a unique feature of this reactor is its multipass fuel cycle in which graphite fuel pebbles are randomly loaded and continuously circulated through the core until they reach their prescribed end-of-life burnup limit (/spl sim/80 000 MWD/MTU). Unlike the situation with conventional light water reactors, depending solely on computational methods to perform in-core fuel management will be highly inaccurate. As a result, an on-line measurement approach becomes the only accurate method to assess whether a particular pebble has reached its end-of-life burnup limit. The results of this investigation indicate that the fission products Cs-137 and Eu-154 have the potential to provide accurate and power-history-resistant signatures that can be directly correlated with burnup. Furthermore, depending on the fuel manufacturing process, artificially introduced dopants (e.g., Co) can provide gamma-ray lines that are usable for burnup monitoring. In fact, it was found that the relative activity of Co-60 to Cs-134 could form a burnup indicator that is resistant to power-history variations. In this case, the use of a relative indicator has several advantages, among them the elimination of the need for absolute knowledge of the detector full-energy peak efficiency curve and the establishment of a system quality-assurance figure of merit based on the peak area ratio of the Co lines.}, number={3}, journal={IEEE TRANSACTIONS ON NUCLEAR SCIENCE}, author={Hawari, AI and Chen, JW and Su, BJ and Zhao, ZX}, year={2002}, month={Jun}, pages={1249–1253} }