Number 18, January 23, 1991 by Phillip F. Schewe and Ben Stein THE MOST PRECISE ISOTOPE SHIFT measurement ever made for a multi-electron atom has been performed by a group of scientists at Harvard (Francis M. Pipkin, 617-495-2910). Observing the same atomic transition in both helium-3 and helium-4, the scientists can study the isotope shift, whose chief components are the "mass shift," the shift due to the different motion of the two nuclei (the He-3 nucleus has two protons and one neutron, whereas He-4 has an extra neutron), and the "volume shift," the shift due to the different shape of the electric charge distribution for the two nuclei. The isotope shift was measured using laser-microwave spectroscopy techniques to a precision of one part per million, 100 times better than previous measurements, allowing checks on various theoretical calculations. (Phys. Rev. Lett., Feb. 4) TAU NEUTRINOS WERE NOT DETECTED BY THE ASTRO observatory carried on the Space Shuttle Columbia last month. The Hopkins Ultraviolet Telescope, part of Astro's package of instruments, had sought to measure the ultraviolet radiation coming from the presumed decay of tau neutrinos. Oxford theorist Dennis Sciama believes that tau neutrinos, with a nonzero mass, might account for some of the missing mass in the universe. Arthur Davidsen of Johns Hopkins (301-338-7370) said at the American Astronomical Society meeting in Philadelphia that "neutrinos could still have mass and could account for dark matter, but they do not decay at the rate predicted by this theory." (The Washington Post, 18 Jan. 1991.) GAMMA-RAY ANNIHILATION RADIATION from the center of the Milky Way, seen off and on since 1977 by balloon-borne detectors (Marvin Levanthal, Bell Labs, 201-582-3448), has now been observed by the Soviet-French gamma-ray satellite GRANAT. Actually sitting some 100 light years from dead center, the source of gammas, which arise from the annihilation of electrons and positrons and which therefore have a characteristic energy of 511 kev, may harbor a relatively small (hundreds of solar masses) black hole. This is in addition to the black hole (perhaps a million solar masses) which may lurk at the very center of the galaxy. Evidence for this will be sought by GRANAT and by the U.S. Gamma-Ray Observatory, due to be launched later this year. (Science, 11 Jan. 1991.) MODULATION-DOPED FIELD EFFECT TRANSISTORS (MODFETS) are the fastest transistors available. In modulation doping, facing layers of gallium arsenide and aluminum gallium arsenide squeeze electrons into an essentially two-dimensional electron gas, or 2DEG. The electrons confined in this way scatter less because they are separated from their donor ions and can therefore move quickly. Consumer applications of the so-called low-dimensional devices which employ the effect, such as MODFETS or high electron mobility transistors (HEMT), have been limited so far; they are used in satellite television receivers, where the low-noise behavior of modulation-doped devices and the frequency range of GaAs come into play. (New Scientist, 12 Jan. 1991.)