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Number 384, July 28, 1998 by Phillip F. Schewe and Ben Stein
MEASURING GRAVITY GRADIENTS WITH ATOM INTERFEROMETRY. Splitting an atom into two wave packets and then recombining them will produce an interference pattern that depends on any forces that are present, including gravity. By putting two such atom interfermometers together, Mark Kasevich (203-432-3826, mark.kasevich@yale.edu) and his Yale colleagues have made a highly sensitive gradiometer that will soon be able to detect changes in gravity as small as 10^-10 g over a distance of 1 meter, as good as any technique now available, and likely to get even better after more development. Because the test objects whose acceleration is being measured are individual atoms rather than bulk objects and because the readout is based on stable laser beams, systematic uncertainties are eliminated or greatly reduced. Also, the device is immune to vibrations incurred on moving platforms, which should make it valuable for pending gradiometer applications such as covert navigation, underground structure detection, oil exploration, and geodesy, as well as for fundamental studies of general relativity. (Snadden et al., Physical Review Letters, 3 August 1998.)
A WATER SOFTENER FOR RADIOACTIVE WASTE has been designed at Sandia, providing a potential new method for removing highly radioactive cesium-137 and strontium- 90 isotopes from nuclear waste sites. Separating these isotopes from lower-level waste would make the cleanup process cheaper and easier. At last week's meeting of the American Crystallographic Association (ACA) in Virginia, Abraham Clearfield of Texas A&M (409- 845-2936) described the fabrication of titanium silicate crystals containing specially tailored, 8-angstrom-wide tunnels into which Cs-137 ions can fit exactly. The researchers had previously filled each tunnel with a smaller, narrower, sodium ion. By passing liquid solutions of nuclear waste through a bead of such crystals, a perfectly fitting cesium ion could enter each tunnel, displacing the more loosely held sodium ions. (After absorbing the isotopes, the beads could then be encased and buried directly.) A similar "ion-exchange process" takes place in water softeners, which employ a carbon-based substance (containing sodium ions) to remove calcium and magnesium ions that ordinarily promote the formation of unwanted soap scum in dishwashing and clothes washing.
ULTRASMALL ANGLE X-RAY SCATTERING has been used to obtain precise information on deformities in a metal crystal in the act of being stretched. How metal acquires defects when it undergoes molding or stamping is no small matter; auto makers must make one die after another, by trial and error, until the pressed part has just the right shape. Physicists from NIST, wanting to make this art of mechanical deformation more like a science, have set an aluminum sample in an x-ray beam at Brookhaven's National Synchrotron Light Source. The x rays scatter from defects in the sample. Ironically, the more widely scattered x rays provide information at the angstrom scale, whereas knowledge of the defect structures (up to 10 microns in size) must be extracted with great difficulty from the pattern of x rays scattering through very small angles. NIST's new high-efficiency, low-angle (as small as 6 arcseconds) measurements yield the best information yet of what the defect structures look like. Unexpectedly, the defects are not entirely disordered; a fraction of them lie at regular intervals. The researchers (Gabrielle Long, gabrielle.long@nist.gov and Lyle Levine, lel@argo.nist.gov) reported at the ACA meeting that their next step will be to bring these experiments to the x-ray source at Argonne, where intensities will be 100 times greater.
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