JUST A PHASE? BETTER NEUTRON ANALYSIS WITH LOWER RADIATION.

Researchers from NIST, the University of Melbourne, and the University of Missouri at Columbia have demonstrated a new form of neutron radiography that produces greatly enhanced images of fine structural details in test samples with relatively low levels of radiation. Conventional radiography, using either x rays or neutrons, works by detecting the absorption of the radiation as it passes through different materials. For example, the radiograph shows the "shadows" cast by tumors (x rays) or cracks in jet engine turbine blades (neutrons). But if two materials have very similar absorption characteristics or the feature under inspection is very small, radiographs often show very little detail.

In recent years, researchers have explored a more subtle effect called phase-sensitive imaging. As radiation passes through a specimen, the phase of the wave can shift at the boundary between materials of two different densities, just as light diffracts when passing through a glass of water. Because the phase-shift effect is often much stronger than the absorption effect, images based on the phase differences of the emerging waves can show much greater detail--particularly of boundaries or edges--with relatively smaller doses of radiation.

Phase-contrast x-ray images have been studied for several years, producing dramatic images of tissue specimens that clearly show fine details. The experiments at NIST are the first to demonstrate the same phenomenon--and achieve equally dramatic image enhancement--using neutron beams, which have broad industrial applications in materials research.

Preliminary details of this research were published in Nature, Brief Communications, Vol. 408, November 2000. For more information, contact Muhammad Arif, [phone], [e-mail].