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Nondestructive Elemental Analysis in Three Dimensions A nuclear microprobe based on a new type of focusing lens has been constructed at Sandia National Labo ratories in New Mexico, and Sandia scientists have been able to perform nondestructive three-dimensional ele mental analyses of solid materials with the instrument. For most MeV ion beam analytical experiments, the beam spot is a few square millimeters or larger. But the recent development of strong focusing lenses for high-energy ion beams has made it possible to obtain beam spot sizes of micrometer dimensions. The signals generated when these ion beams strike sample surfaces contain depth information, making it possible to construct three-dimensional con centration profiles of elements close to the surface. Other methods used for three-di mensional surface analysis, such as secondary ion mass spectrometry, damage the sample by sputtering away parts of the surface. Nuclear mi croprobe analysis does introduce some radiation damage, but the sample re mains basically intact. According to Sandia scientist Bar ney L. Doyle, the nondestructive na ture of the nuclear microprobe tech nique is a significant advantage: "When the amount of sample is ex-
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A wide variety of ion-induced signals can be detected in the target chamber of Sandia's nuclear microprobe. Backscattered or forward recoiled ions are detected and energy-analyzed by an an nular-surface barrier detector. The mi croprobe also includes a Si(Li) detector for PIXE analysis
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tremely limited—small grains of lunar material, for example—it is wise not to exhaust it." Nondestructive analy sis also enables the investigator to per form repetitive analytical procedures on the same sample or to place the sample back into service, assuming it is a useful device such as a microchip. Two analytical techniques are used for three-dimensional profiling— Rutherford backscattering (RBS) and elastic recoil detection (ERD). A com bination of these two techniques makes the nuclear microprobe capable of detecting every element in the peri odic table. The instrument is also out fitted for proton-induced X-ray emis sion (PIXE) analysis. In RBS, lightweight accelerated particles, such as hydrogen or helium ions, collide with heavy atoms in the sample and rebound to a detector, where their energy is measured. In ERD, on the other hand, heavy accel erated atoms, such as silicon, strike less massive atoms in the material, such as hydrogen, and knock them out of the sample. The energies of the de tected particles in both cases provide information about the depth at which collisions took place. The PIXE tech nique, in which surface atoms emit characteristic X-rays in response to proton bombardment, is used primarly for trace elemental analysis. "One of the biggest problems in three-dimensional microbeam analy sis," said Doyle, "is displaying the data in a way that's easy to assimilate, because with RBS you really have five degrees of freedom: the x, y, and ζ di mensions, concentration, and atomic number. Eventually you have to dis play this on a two-dimensional matrix. That's one area we're working on." According to a recent paper written by Doyle and his colleague Norman D. Wing, "The ability to measure nondestructively the three-dimensional con centration of elements ranging from Η to U is extremely useful and has pro vided insight into many material problems which would have been diffi cult, if not impossible, to study with other techniques." Stuart A. Borman
