Number 76, April 17, 1992 by Phillip F. Schewe and Ben Stein|
NON-OPTICAL DIFFRACTION PATTERNS in solids have been imaged. It has been known for almost a decade that phonons, packets of acoustical or thermal energy in solids, can interfere to form diffraction patterns, but these patterns had not clearly been
seen prior to a new experiment performed by Richard Weaver (217-333-3656) and co-workers at the University of Illinois (Matt R. Hauser et al., Physical Review Letters, 27 April 1992). In their experiment, an ultrasonic wave source produces long-wavelength phonons which can yield much larger, easy-to-discern patterns than
earlier methods, which employed laser pulses that engendered much smaller-wavelength phonons. Since the arrangement of atoms in solids is anisotropic, certain preferred directions for phonon travel exist. As a consequence, even though phonons may
enter the material from many different directions, the material's for more, geometry will cause phonons to converge, interfere, and ultimately form diffraction patterns which can be picked up by an external acoustical detector. Such patterns may eventually help workers detect defects in materials and help them to determine the structures of unknown materials non-invasively. Unlike previous methods of imaging phonons, the ultrasonic imaging method can be performed at ambient temperatures with all types of materials, including non-crystalline solids.
DETECTORS FOR THE LARGE HADRON COLLIDER (LHC) will be selected from among four competing proposals. Like the SSC, the LHC would collide protons with protons at multi-TeV energies; to study these collisions, detectors capable of
tracking hundreds of particles will be needed. Although the LHC has not yet been fully approved by the governing body of CERN, the detector competition is keen. The leaders of the four contenders are Peter Norton of the Rutherford-Appleton lab in
the UK (the "Ascot" detector), Peter Jenni of CERN (the "Eagle" detector), Michael Della Negra of CERN ("CMS"), and Sam Ting of MIT (the L3+1 detector, a rebuilt version of the detector Tingis using in LEP).
TITAN'S ATMOSPHERE may have come from a comet. Kevin Zahnle and his colleagues at the NASA Ames Research Center suggest that in the early stages of the solar system comets from beyond Uranus may have struck Saturn's moon Titan, which had enough gravity to retain the material. Smaller moons, or those farther in, such as Jupiter's moon Ganymede and Callisto, may not have been able to capture or keep the comet debris. (New Scientist, 11 April 1992.)
WATCHING BEER BUBBLES RISE allows one to calculate the radius of the carbon dioxide molecule. In response to an article in the October 1991 issue Physics Today ("Through a Beer Glass Darkly," by Neil Shafer and Richard Zare) T.G.M. van de Ven
and S.S. Dukhin of McGill University use the measured properties of CO2 bubbles in beer to compute a radius for them of about 3 angstroms. (Letter in the April 1992 Physics Today.)