MRI: Development of an Intense Positron Annihilation Spectrometry System for Nanophase Characterization

North Carolina State University, Raleigh NC

Investigators

Abstract

North Carolina State University (NCSU), Oak Ridge National Laboratory (ORNL), and the University of Michigan (UM) have formed a collaboration to develop an intense positron (antimatter electrons) facility at the PULSTAR reactor on the campus of NCSU. Positrons are the antiparticles of electrons and when injected into normal matter they "annihilate" with electrons-that is they both disappear with all of their combined mass being converted into a pulse of electromagnetic radiation according to Einstein's famous formula E=mc2. Positron annihilation is emerging as an ideal probe of advanced materials being developed as part of the nation's Nanotechnology Initiative. Many new applications of nanotechnology involve understanding and manipulating the atomic-scale microstructure of voids within a material. However, fundamental understanding of the role of voids in enhancing or degrading materials properties is often hampered by the inadequacy of traditional techniques (such as electron microscopes) to characterize such voids at the true nanometer scale. Positrons in matter seek out and directly probe such open structures. In the new facility called (NC)3, the North Carolina National Center for Nanophase Characterization, positrons produced near the core of the NCSU nuclear reactor will be piped out to several target stations where students, faculty, and industrial collaborators from around the world can monitor the annihilation process in newly developed materials. By vastly enhancing the access of researchers to an intense, focused beam of antimatter positrons next-generation materials research will benefit from a new way to "see" the structure inside them. The center's research impact is highly leveraged through many applications in science and engineering with broad societal impacts including miniaturized and faster computing, higher strength and longer lasting materials, cleaner energy production, and corrosion- and radiation-resistant coatings. North Carolina State University (NCSU), Oak Ridge National Laboratory (ORNL), and The University of Michigan (UM) have formed a collaboration to develop an intense positron beam facility and associated spectrometers at the PULSTAR reactor on the campus of NCSU. Positrons are emerging as an ideal probe of matter on the nanoscale. The positively charged positron, with its affinity for open structures in matter, has been gaining an important role as a probe of nanostructure and in understanding the role of engineered nanovoids on macroscopic properties. This is mainly due to the increased ability to tailor the macroscopic properties of a material by fine-tuning and engineering its microstructure. The intense positron beam will be created in a converter by pair-production using fission gamma rays produced in the reactor core and by (n,?) reactions in a cadmium cladding surrounding the converter assembly. The positron beam will drive two complementary positron/positronium spectrometers: First, a "next generation" positronium PALS spectrometer (Ps-PALS), with a 1-mm diameter beam, that is capable of performing high accuracy lifetime studies on nanoporus thin films and patterned microelectronic devices, and second, a time-bunched positron PALS spectrometer (e+-PALS) that will be directed for studying annihilation in metals and semiconductors. Ultimately, our goal is to establish (NC)3, the North Carolina National Center for Nanophase Characterization. The core apparatus of (NC)3 is the intense positron beam-line at the PULSTAR reactor, complemented by existing PULSTAR and NCSU systems for neutron scattering, and electron microscopy. The positron beam facility will have applicability in fields such as materials science and engineering, bioscience/biomedical engineering, chemical engineering, electrical engineering, environmental science and engineering, physics and chemistry to name a few. Its main technical focus will be on three aspects of nanophase materials: (a) engineered nanoporosity for improved material properties, (b) the role of nanovoid/defect coalescence in the degradation of materials properties, and (c) interface effects between phases. The development of this facility on a university campus in the United States represents a major research and educational opportunity. The positron facility will also be integrated into the PULSTAR on-line educational network.