Number 88, July 13, 1992 by Phillip F. Schewe and Ben Stein
THE MOVEMENT OF SINGLE ELECTRONS along a chain of defects in a semiconductor has been monitored by observing the conductance of a nearby channel; the channel is formed when a two-dimensional electron gas (electrons moving in a thin layer of GaAs sandwiched between layers of AlGaAs) is pinched down to a narrow strip by voltage applied to two gate electrodes. According to scientists at the Cavendish Laboratory in Cambridge, England, the tiny "hopping" current (amounting to a fraction of an attoamp, or 10**-18 amp) gives rise to a "random telegraph signal" in the conductance of the constriction, each level of which corresponds to a different configuration of electrons in the chain. The Cavendish workers expect that this technique can be used to follow single-electron processes in real time (and not just on a time-averaged basis) in other systems. (D.H. Cobden et al., 20 July 1992 issue of Physical Review Letters.)
A NEW VALUE FOR THE HUBBLE CONSTANT has been obtained by calibrating the intrinsic brightness of type-IA supernovas, which can be observed in distant galaxies, against the intrinsic brightness of cepheid variable stars, which can be observed only in relatively nearby galaxies. Allan Sandage of the Carnegie Institution and his colleagues have used the Hubble Space Telescope to observe 27 cepheids in galaxy IC4182; the reliable relation between the cepheids' variability rate and intrinsic brightness gives a value of 16 million light years to IC4182. This allowed Sandage to calculate the intrinsic brightness of a type-IA supernova also residing in IC4182; this brightness could be compared to that of similar supernovas in more distant galaxies. This distance-finding technique results in a value of 45 for the Hubble constant and an age for the universe of at least 15 billion years. (Science News, 4 July 1992.)
A NEUTRON FOCUSING DEVICE , long an elusive invention, has been built by Russian scientists using a simple network of glass capillaries bundled together. Because of their zero charge, neutrons do not ordinarily interact very well with other matter. However, the capillary network is designed to focus a high portion of the neutron beam through multiple internal reflections. The current setup can focus neutrons to a spot less than a millimeter in diameter and increase the neutron flux by 6-7 times, with the potential, its designers claim, to reduce the beam diameter to 30 µm and increase the flux by 1000 times. A neutron lens promises improved neutron scattering studies of materials and more precise neutron therapy techniques, which would mean smaller radiation doses for patients. (Nature, 4 June 1992.) A Russian-American team has made a similar device which redirects x-rays that are usually wasted in conventional setups, resulting in a four-fold increase in intensity. (Science News, 27 June 1992.)
THE 23rd INTERNATIONAL PHYSICS OLYMPIAD, held last week in Helsinki, Finland, brought together high school students from 40 countries. The American team received two gold medals, one silver, and two honorable mentions. This was the first time all members of an American team had won an award. The two gold medalists were Eric Miller of San Francisco and Szymon Rusinkiewicz of Houston. (For more information, contact Avi Hauser at 908-615-5406 or Larry Kirkpatrick at 406-994-6182.)
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