Public Release: 

Physics news update 609

Neutron holography, positronium, lithium superconductivity

American Institute of Physics

NEUTRON HOLOGRAPHY with atomic-scale resolution has been performed, for the first time, with an "inside-detector" approach. Holography generally includes a source of illuminating waves, an object to be imaged, and a detector or film in which waves direct from the source interfere with waves scattered from parts of the object. The interference pattern, stored in the detector medium, is later read out (and a 3D image of the object viewed) by sending waves into the detector. Holograms with visible light are common enough: they adorn most credit cards. Holograms using electrons (considered in their "wave" manifestation, not as particles) provide sharp pictures, but because the electrons cannot penetrate far into a solid sample, the imaging process is usually restricted to surface regions. Holograms using x rays go can penetrate much farther, but their limitation consists of the fact that the penetration depth improves as the square of the atomic number.

Therefore x-holography is not very good for materials with light elements. Holograms with neutrons are different; rather than scattering from the electrons in the atoms of the sample, neutrons scatter only from nuclei, which are 100,000 times smaller than the atoms in which they reside. This is important when it comes time to reconstruct an image of the interior of a crystal lattice. In an experiment carried out with a beam of neutrons from a reactor at the Institute Laue-Langevin in Grenoble, a group of scientists has produced, for the first time, an atomic-scale map of a crystal, in particular a sample of lead atoms, using a technique in which the "detector," a trace amount of atoms (cadmium-113) whose nuclei readily absorb neutrons, are embedded inside the sample itself. The holographic process unfolds as follows: neutron waves can strike a Cd nucleus directly (reference beam) or by first scattering from a Pb nucleus. In either case, the absorption of a neutron stimulates a Cd nucleus to emit a high energy photon observable in a nearby detector. The overall interference pattern for these two processes (absorbing scattered or direct neutron waves) is monitored as the profile of the sample to the beam is stepped through various angles.

The result: a crisp picture of a unit cell of 12 lead atoms (see figure at http://www.aip.org/mgr/png/2002/165.htm). This process should be great for spotting foreign atoms in a solid (dopants if the atoms are desired, impurties if they're not). Since the neutron has a magnetic moment, n-holography might also to contribute to an understanding of the magnetic nature of the sample atoms, in addition to imaging their whereabouts. (Cser et al., Physical Review Letters, 21 October 2002; contact Laszlo Cser, Central Research Institute for Physics, Budapest, cser@sunserv.kfhi.hu, 36-1-392-2222 extension 1526.)

FIRST DETAILED POSITRONIUM SCATTERING EXPERIMENT. The lightest atom made of an electron and a positively charged mate is not hydrogen but positronium (abbreviated Ps), a bound electron-positron pair. The lifetime for these no-nucleus atoms is hardly more than about 100 nanoseconds but, if things are expedited, this is long enough for doing an experiment. (The brief lifespan comes not from the intrinsic instability of the Ps "atom" but from the fact that the constituents will, left to themselves, annihilate each other.) In recent years physicists have been able to gather Ps beams, made by sending a beam of positrons through a neutralizing gas, and have measured the total cross section (likelihood of scattering) for Ps scattering from various targets. Now a team of scientists at University College London reports the first experiment in which a specific type of inelastic scattering takes place. In particular, the London researchers found that in many encounters with helium atoms, the Ps will split apart but that the fragmented partners continue to be highly correlated, moving through the lab with roughly the same velocities. Learning more about this fragmentation process will aid proposed schemes for using Ps beams for studying material surfaces. Furthermore, Ps is unusual in that its centers of mass and charge coincide. This allows for interactions between the electron in the Ps and electrons in target atoms to be more potent than if the electron were yoked to a much heavier proton, as in a hydrogen atom. (Armitage et al., Physical Review Letters, 21 October 2002; contact Gaetana Laricchia, 44-20-7679-3470, g.laricchia@ucl.ac.uk)

SUPERCONDUCTIVITY IN LITHIUM now has the highest demonstrated transition temperature of any element, 20 K. Great pressure, 48 GPa, was needed to achieve superconductivity. According to the physicists at the University of Tokyo and Osaka University who performed the experiment on lithium (the sample and its electrical leads are squeezed in a diamond anvil press), their result bears out an expectation that lighter elements should possess higher transition temperatures. Extrapolating this principle further, they argue, might produce room temperature superconductivity in hydrogen, but only at crushing pressures above 400 GPa. (Shimizu et al., Nature, 10 October 2002.)

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PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 609
October 15, 2002
by Phillip F. Schewe, Ben Stein, and James Riordon

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