Number 271, May 16, 1996 by Phillip F. Schewe and Ben Stein SHARPER, CHEAPER PET SCANS. At the recent APS meeting in Indianapolis, Simon Cherry of UCLA (310-825-4334) described MicroPET, a positron-emission tomography (PET) system that can resolve structures as small as 0.006 cubic centimeters (about the volume of a sesame seed), a nearly tenfold improvement in resolution over conventional scanners, at an estimated price of $300,000, less than one-fifth the cost of conventional PET systems. Used to provide images of such things as brain activity, PET detects pairs of gamma rays released when positrons from a radioactive tracer in the bloodstream annihilate electrons in the patient's body. Scheduled to be fully complete in July, MicroPET brings about increased resolution by employing smaller scintillation crystals (made of lutetium oxyorthosilicate) for detecting the gammas. The crystals are small enough that they can be read out using optical fibers, which convey light to a multichannel phototube consisting of 64 individual elements, greatly reducing the need for numerous expensive tubes. (Much of the technology was adapted from high-energy particle physics.) This system is suitable for imaging the smaller-scale anatomies of laboratory animals used in drug trials. THE ACOUSTIC ANALOG OF A LASER is being developed by Jean-Yves Prieur at the University of Paris-South (Physica B, vol. 219/220, p. 235). The active medium in this case is a piece of pure silica at a temperature of 0.5 K. An initial sound pulse "pumps" the sample by depositing acoustic energy at absorbing centers throughout the silica. A second sound pulse stimulates the absorbing centers ro reradiate phonons, which serve to amplify the second pulse. Unfortunately, the amount of amplifications is still low because the pump pulse remains in the system, where it undermines the stimulation process. The researchers believe that eventually acoustic lasers will be used as sensitive particle detectors. (New Scientist, 27 April 1996.) BINARY ASTEROIDS. Doublet craters account for 10% of all impact structures on Earth and Venus. A prevalent theory holds that such impacts come about when a single asteroid is fragmented either when it rips through the planet's atmosphere or when it is pulled apart by gravitational tidal forces just before impact. A new study by William Bottke (Caltech) and Jay Melosh (Arizona) shows that the relatively wide separation of craters in doublet events can best be explained by supposing that tidal fragmentation into parts had occurred at some earlier stage, as with Comet Shoemaker-Levy. The researchers suggest that such weakly-bound "rubble-pile" asteroids and asteroid satellites (such as the Ida-Dactyl system imaged by the Galileo spacecraft) might be more common than we thought. (Nature, 2 May 1996.) CORRECTION to Update 270: The universe is, or at least ought to be, older than its oldest stars, not younger. REMINDER ON SUBSCRIBING TO PHYSICS NEWS UPDATE: to add or delete yourself from our list automatically, send a message to <listserv@aip.org> leaving the subject line blank and, in the body of the message, merely specify either <add physnews> or <delete physnews>.