Magazine article Science News

Looking into Metal Specimens

Magazine article Science News

Looking into Metal Specimens

Article excerpt

Looking into metal specimens

In magnetic resonance imaging, certain atomic nuclei in samples held within a magnetic field absorb, then emit, radio waves at characteristic frequencies. Such data can be used to construct images showing the location of particular atoms in the sample. These images have important medical applications because researchers can use the technique to examine animal and human tissue without having to remove a specimen.

Now, Stephen J. Norton of the National Bureau of Standards (NBS) in Gaithersburg, Md., has proposed a new imaging technique based on a somewhat different nuclear effect. Unlike magnetic resonance imaging, this technique can be used for studying the internal structure of metal alloys, composites and other materials.

Norton's technique is based on a nuclear phenomenon known as the Mossbauer effect. Nuclei of atoms such as iron, when embedded in a solid lattice, can emit and absorb gamma rays without suffering any recoil. As a result, the absorption or emission frequency of a typical gamma-ray signal is confined to a narrow range, allowing precise measurements of nuclear phenomena or a remarkably sensitive means of marking the presence of certain elements in a sample.

In a conventional Mossbauer experiment, a moving radioactive source emits gamma rays, which are then absorbed by the sample. A detector counts the number of gamma rays that manage to pass through the sample. Such measurements, however, provide only bulk averages. …

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