@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{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{mishra_ghosh_2009, title={Estimation of SCRAM Rate Trends in Nuclear Power Plants Using Hierarchical Bayes Models}, volume={38}, ISSN={["1532-415X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-70249097019&partnerID=MN8TOARS}, DOI={10.1080/03610920902947196}, abstractNote={Nuclear reactors are equipped with reactor scram systems to ensure rapid shutdown of the system in the event of leaks, failure of power conversion systems, or other operational abnormalities. The U.S. Nuclear Regulatory Commission (NRC) collects data on scram rates from various nuclear power plants over time to estimate the trend of proper functioning of the plants which in turn is used to regulate them. The annual scram data obtained from 66 commercial nuclear power plants indicate an increase in the number of plants having no scrams from 1.5% in 1986 to 33% in 1993. To analyze correlated count data with excess zeros (e.g., no scrams), a zero-inflated model that accounts for both temporal and plant-to-plant variation is being developed in this article. A wide class of possibly non-nested models was fitted using Markov Chain Monte Carlo (MCMC) methods and compared using a predictive criterion. Out-of-sample tests were also performed to study the performance of the models in predicting the scram rates of the plants. For the NRC data on scram rates, the stochastic time trend models that account for zero-inflation were found to provide a much better fit compared to the deterministic trend models.}, number={16-17}, journal={COMMUNICATIONS IN STATISTICS-THEORY AND METHODS}, author={Mishra, Kaushal K. and Ghosh, Sujit K.}, year={2009}, pages={2856–2871} }
 @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{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} }