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Number 437, July 2, 1999 by Phillip F. Schewe and Ben Stein
STAR MATERIAL DISCOVERED IN SOUTH PACIFIC Interstellar matter formed in a supernova has been discovered on Earth now for the first time. Light coming to Earth from distant supernovas is recorded all the time. Likewise, a dozen or so neutrinos from nearby Supernova 1987A have been detected. But atoms from supernovas are a different matter. In a sense, all the heavy atoms on Earth have been processed through or created in supernovas long ago and far away. But now comes evidence of atoms from a supernova that may have been deposited here only a few million years ago. An interdisciplinary team of German scientists from the Technical University of Munich (Gunther Korschinek, 011-49-89-289-14257, korschin@physik.tu-muenchen.de), the Max-Planck Institute (Garching), and the University of Kiel have identified radioactive iron-60 atoms in an ocean sediment layer from a seafloor site in the South Pacific. First, several sediment layers were dated, and only then were samples scrutinized with accelerator mass spectroscopy, needed to spot the faintly-present iron. The half-life of 60Fe (only 1.5 million years), the levels detected in the sample, and the lack of terrestrial sources point to a relatively nearby and recent supernova as the origin. How recent? Several million years. How close? An estimated 90-180 light years. If the supernova had been any closer than this, it might have had an impact on Earth's climate. The researchers believe traces of the 60Fe layer (like the iridium layer that signaled the coming of a dinosaur-killing meteor 65 million years ago) should be found worldwide but have not yet been able to search for it. (K. Knie et al., Physical Review Letters, 5 July 1999.)
ELECTROPHOSPHORESCENCE GETS THE GREEN LIGHT In organic light emitting devices (OLEDs) electrical energy injected onto a host molecule is often transferred to luminescent "guest" molecules which then light up. Using this approach, OLEDs have been fabricated to emit colors ranging from violet to the near infrared and have been incorporated into displays already on the market. So far OLED researchers have concentrated on maximizing fluorescent emission of light. Fluorescent OLEDs use a process whereby the energy transfer occurs between a singlet-state (total spin of zero) host molecule and a singlet-state guest molecule. Phosphorescent OLEDs, by contrast, transfer energy from a triplet-state (total spin value of one) host to a triplet-state guest, which subsequently emits the energy as light. Phosphorescence is inherently a slower and less efficient process, but triplet states constitute the majority of electrically excited states, so putting them to work can increase the overall luminescence. This is exactly what scientists at Princeton (Stephen Forrest, forrest@ee.princeton.edu, 609-258-4532) and the University of Southern California have now done. Using both singlet and triplet states for producing green light, they have achieved quantum efficiencies (photons out divided by electrons in) of up to 8% and power efficiencies (optical power out divided by electrical power in) of up to 30 lumens/Watt. These high efficiencies are unprecedented and may have a great impact on display technology. (Baldo et al., Applied Physics Letters, 5 July 1999).
A RUDIMENTARY MUON MAP OF THE SKY has been carried out by the Soudan-2 detector, located deep in a Minnesota mine and built originally to look for proton decay. To be exact, Soudan records muons produced by incoming cosmic rays hitting the atmosphere. The muon imaging process clearly senses the shadow cast by the passing Moon, which temporarily blocks cosmic rays coming from that position in the sky. (Science, 18 June.)
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