MIT Department of Nuclear Science and Engineering
Professor of Nuclear Engineering
B.S. 1956 National Taiwan University; M.S. 1958 National Tsing-Hua University; M.S. 1962 (Nuclear Science) University of Michigan; PhD 1964 (physics) McMaster University
Neutron and X-ray scattering and spectroscopy; Applications of laser light scattering to complex fluids and biological problems.
We have developed a new technique for measuring high-resolution Doppler frequency shifts in scattered laser light from slowly moving particles. This photon correlation spectroscopy is a completely digital technique in the time domain whereby the intensity correlation function of scattered light can be simultaneously measured at 256 values of the delay time by using a delay coincidence method. The accessible range for the delay time in this instrument is from 1 µsec to 100 sec, which covers the useful range of slow fluctuation phenomena. We have applied the method to the study of slow fluctuations of the concentration in a binary liquid mixture near the critical point with a great deal of success. We have also applied this technique to measurements of isotropic random motions of bacteria in liquid media, and also to directed biased motions when a chemotactic agent is present. More recently, the critical slowing of concentration fluctuations in micellar solutions and microemulsions have been studied, and the critical exponents have been determined. Our present thrust in this project is the study of the percolation and kinetic glass transitions in dense copolymer micellar solutions when the volume fraction of the micelles is increased to above 50 percent.
We have developed a new method for determining the inter-micellar structure factor in a strongly interacting ionic micellar system using SANS technique. The method has been applied to a series of alkali dodecyl-sulfate micelles in both dilute and concentrated solutions. We were able to extract both the aggregation number of the micelle and its renormalized surface charge at all concentrations with good accuracy. A contrast variation method, which takes advantage of the large difference between scattering length densities of hydrocarbons constituting the micelles and heavy water solvent, has also been used to study the internal structure of small micelles.
We have studied the recently found critical phenomena in a three-component microemulsion, AOT/ n-decane/water system. The main interest is in determining the nature of the critical point and its associated order parameter fluctuation. Recently, the structure of the dense phase has also been studied. We discovered interesting percolation and glass transitions as functions of temperature and volume fraction.
When a small amount of salt is added to the AOT/decane/water system, the water-in-oil microemulsion transforms into a bicontinuous structure characterized by a morphology of interpenetrating domains of oil and water. We succeeded in determining the 3-D structure and the average interfacial curvatures of such bicontinuous microemulsions by combination of computer simulation and SANS experiment.
When the world’s most powerful Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is completed in 2006, SANS and cold and hot neutron spectroscopy will see a new level of activity. As the result, we expect that our studies of interfacial phenomena, which are the main focus of the above project, will gain unprecedented impetus.
Professor of Nuclear Engineering and Materials Science and Engineering
B.S. 1958, M.S. 1959, PhD 1962 (Nuclear Engineering) University of Michigan
Theory and atomistic simulations in transport and collisional phenomena; Multiscale materials modeling.
An area of interdisciplinary research focused on modeling complex physical structures is emerging that exploits theory and simulation across scales, from the atomistic to the macroscopic. Goal of such research is to develop molecular models that integrate the many-body physics with robust simulations and selected measurements to give insights toward the discovery and optimization of technological materials. Our program has broadened from studies of strength and deformation of metals and ceramics to include problems of thermal and charge transport in functional nanostructures. Following are problems illustrative of our progress and directions of future efforts.
The notion of shear localization is fundamental to our understanding of the mechanics of non-uniform deformation in condensed matter. Through electronic structure calculations and atomistic simulations we examine the intrinsic shearability of metallic and ceramic crystals, the nucleation and propagation of dislocation and deformation twin, the emission of a dislocation loop from a crack tip, and hydrolytic attack on quartz. A characteristic feature of these studies is the sampling of reaction pathway to determine the saddle-point configurations, thereby connecting the underlying deformation mechanisms ultimately with the redistribution of charge densities in the solids.
Conductance and switching in a single molecule are current issues in the development of molecular electronics. We propose a time-dependent density functional theory treatment of charge transport in which electrons are injected from metallic contacts as wave packets and propagated in the manner of a scattering problem. The role of solitons (domain walls) as the fundamental charge carrier in p-conjugated molecules is emphasized, along with the use of a new localized basis set to visualize and interpret the behavior of molecule-specific orbitals in terms of bonded atoms.
We find by high-level quantum chemistry calculations that a neutral polypyrrole chain should be nonplanar, with a helical or bent conformation depending on the chain length. Upon oxidation a dihedral torsion-polaron coupling acts to constrain the chain to be planar and its conformation to straighten out. This behavior is a form of charge localization in low-dimensional structures; it suggests an actuation mechanism that is intrinsic to single p-conjugated polymer chains. Together with our studies of shear deformation and charge conductance, we find that localization phenomena play a key role in controlling functional behavior in nanoscience and technology.
Nanoindentation. Molecular dynamics snapshots superposed to show the emission of a prismatic dislocation loop beneath a nano-indenter (upper left) at critical loading. Metallic thin-film sample occupies the region between the top and bottom plates; only the defective atoms are shown and color coding denotes local coordination.
Molecular actuation. Hartree-Fock calculations show the transformation of a straight polyacetylene chain in the neutral state to a bent conformation upon insertion of negative charges (color coding). Resulting strain provides an intrinsic mechanism for molecular actuation.
Water-quartz reaction. Molecular orbital calculations showing a siloxane (Si-O-Si) bond being hydrolyzed to form a silanol group (SiOH) when a water molecule reacts with a strained quartz nanorod. Si and O are denoted as large and small atoms respectively, and color coding shows charge transfer.
David Cory Professor of Nuclear Engineering
BA ‘81, PhD 1987 (Chemistry) Case Western Reserve University
Our research is concerned with Quantum Information Processing (QIP), a new and rapidly expanding field with the potential to make spectacular scientific and technological advances beyond the reach of classical technologies. Our objective is to develop the engineering necessary to coherently control large quantum systems. Once we can create and manipulate large multi-particle states, these can be used for secure communication, quantum computation, and more efficient simulation of many physical systems. The applications range from simulating properties of nano-particles to following complex fluid dynamics.
We have focused on using spin (nuclear, electron, and that of free neutrons) as test-beds of quantum control schemes. The studies include understanding and translating the ideas of quantum information processing into the language of the physical Hamiltonians that control spin dynamics, then designing and building the experiments that allow us to test these ideas. In a still-emerging field such as QIP, theory and experiments are developed hand in hand. At MIT we have some of the most complex and precise implementations of quantum information processors available anywhere, and there is a large community of researchers from all engineering and science disciplines working on this.
Richard Lanza Senior Research Scientist
AB ‘59 Princeton University; MSc ‘61 University of Pennsylvania; PhD ‘66 (Physics) University of Pennsylvania.
Radiation imaging; radiation detectors; nondestructive testing; radiological and industrial applications of radiation; development of new radiation sources.
Our projects are part of several long-term programs sponsored by a number of agencies including the Federal Aviation Administration, Office of National Drug Control Policy, and the Department of Defense. Recent events have created problems related to the detection of contraband materials, explosives and special nuclear materials. Our approach to these problems has been through the development of techniques for non-intrusively determining the elemental composition of materials, rather than simply measuring densities and shapes.
We have developed a new approach for detecting materials, based on Neutron Resonance Radiography (NRR). This technique is capable of good spatial resolution (~3mm), penetration of heavy objects, as well as the determination of elemental composition. Element-specific resonances in total neutron attenuation cross-sections, which are in the 1 to 8 MeV range, are exploited to enhance the contrast for imaging elements such as carbon, nitrogen, oxygen, and others. This contrast enhancement mechanism is then used to produce elementally-resolved images which can be used to determine the composition of objects under inspection. We are developing a system based on the use of compact accelerator neutron sources which could be scaled to accommodate a range of objects, from baggage to small cargo containers.
In cooperation with Lawrence Livermore National Laboratory, we have been working on the use of large area coded aperture imagers to detect radioactive materials at distances of up to 100 meters. Coded apertures are a particularly powerful technique for imaging, especially with point-like sources typical of radioactive source and fissile materials in a background.
Based on our work in security applications we have shown how coded apertures can be used to provide high-resolution (~100 μm) nuclear medicine images, retaining higher sensitivity as compared to more tradition collimator techniques. Our applications are currently for molecular and small animal imaging. Molecular imaging, specifically designed for imaging of small animal models, provides researchers with detailed information about cellular physiology and function, and holds great promise for early detection and treatment of numerous diseases, and for facilitating the goal of personalized medicine. We are developing a novel gamma microscope capable of imaging low energy radioisotopes with a spatial resolution of ~10 μm. We have also demonstrated three-dimensional reconstruction techniques, which make use of the properties of coded apertures to produce three-dimensional images from a small numbers of views.
Neutron radiography has typically relied on the absorption of neutrons to provide image contrast. An alternative approach makes use of the interference between neutrons that have been scattered and those transmitted without scatter. Scattered neutrons differ in phase from unscattered neutrons and thus are sensitive to small changes in the objects under investigation. At thermal energies, phase shifts of 2π can occur after passing through only a few 10’s of μm for many engineering materials. The contrast enhancement of phase contrast images can be as large as a factor of 104 depending on the materials. We produce such phase coherent beams by using a pinhole source and imaging with a variety of electronic detectors. The use of conventional neutron radiography with these detectors makes it possible to image very small quantities of hydrogen and water. Some applications have been in the study of corrosion and in investigations of the performance of fuel cdls.
