Solid-state physicists have used this spectroscopy technique for over 20 years to study the properties of many metals and alloys. Scientists have also discovered that measurement of positron annihilation is an excellent tool for studying the structure of microscopic defects on and inside metals and other materials. With the increasing use of exceedingly small electronic devices and thin films in the semiconductor and polymer industries has come the challenge of detecting proportionately small material defects. A void as small as several atoms can render a microdevice defective. A void means no atoms and hence fewer electrons, so the positrons live longer than they would in a defect-free material.
Defects affect much of a material's behavior.
They evolve from changes in temperature and pressure or over long periods of time, affecting the ultimate strength of the whole system. Livermore researchers have studied many problems caused by defects, including defect species in high-temperature superconductors, defects active during oxygen transport in uranium corrosion, and radiation damage in metals. Optical microscopy, neutron scattering, transmission electron microscopy, scanning tunneling microscopy, atomic force microscopy, and x-ray scattering are methods that have been developed to find vacancies, voids, and other defects in materials. But positron annihilation lifetime spectroscopy is the best of the lot for finding the smallest defects and a range of defect concentrations at virtually any depth in metals, semiconductors, and molecular or organic compounds. As part of the Department of Energy's science-based Stockpile Stewardship Program, researchers at Livermore's Positron Facility, under the leadership of physicist Richard Howell, are enhancing current capabilities as well as developing new ones to detect changes in weapons materials.