SLAC Today is
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In this issue:
Symmetry Explains It in 60 Seconds: Particle Event
Science Today: Lensless Nanoscale Imaging
SLAC at the LHC: The ATLAS Trigger
Thursday - September 4, 2008 |
Symmetry Explains It in 60 Seconds: Particle EventA particle event is a particle collision or interaction that is observed by some type of particle detector. Collected by the hundreds, thousands, or millions, particle events are the raw material that scientists use to explore the subatomic world. To capture these precious events, particle physicists build "cameras" that record signals such as the tracks of particles emerging from a collision. The interesting features of particle events often occur on submillimeter scales, and the cameras act as extremely powerful microscopes. The cameras have a wide variety of shapes and sizes. At particle laboratories around the world, detectors as large as houses take snapshots of the bursts of matter and energy that emerge when particles ram into each other. In other places, large arrays of detectors—sometimes covering thousands of square kilometers—record cosmic-ray showers created when protons originating from outer space smash into air molecules high above the ground. Each experiment archives its particle events and assigns a unique number to each event. Physicists refer to these event numbers when discussing unusual events that might hint at particles or phenomena never seen before. See the original article in symmetry magazine. |
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Lensless Nanoscale ImagingA team of researchers working at the Stanford Synchrotron Research Laboratory Beamline 13-3 have devised an imaging technique that combines methods from traditional X-ray crystallography and X-ray holography, circumventing one of the major technical hurdles associated with capturing detailed images of non-crystal, or "nonperiodic" materials. The results were published in the August 15 edition of Physical Review Letters. X-ray diffraction has been widely used to determine the structure of large molecules like proteins, but samples must first be grown into a crystal form, with molecules arranged in a periodic pattern. The regular ordering of the molecules in the sample makes it much easier to recover information about the phase of diffracted X-rays, which then enables researchers to recreate the structure of the molecules within the sample. The desire to image all kinds of natural and artificial nanostructures or materials that exhibit nanoscale ordering has led to the development of X-ray imaging techniques that do not rely on any form of sample periodicity. One such method is real space X-ray microscopy, using special X-ray lenses. Other "lensless" methods use coherent X-ray scattering, wherein the reciprocal space "speckle" pattern needs to be inverted into real space to determine structure. This step is impeded by a well-known "phase problem." The SSRL researchers found a new way to solve the phase problem by capturing two scattering patterns of a sample—in this case, microscopic polystyrene beads on a specially prepared thin film—using coherent X-rays of two different energies. A "resonant" beam was used to obtain a diffraction pattern that highlighted the carbon atoms in the sample, while a "non-resonant" beam captured a baseline diffraction pattern as a reference. The researchers then combined the two images to recover the phase information and recreate a two-dimensional image of the sample. The ability to capture detailed images of non-periodic structures holds great promise for imaging all kinds of nanostructures, which in most cases are not periodic. For more details, see the full scientific highlight. |
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