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Hot iron, cold sea

点击量:   时间:2019-03-08 08:03:00

By Robert Adler RADIOACTIVE iron hauled up from the floor of the South Pacific was formed when nearby stars exploded, say researchers in Germany. This is the first time scientists have found material on Earth created by recent supernovae. Astrophysicists believe that most of the heavy atoms now present in the Universe were forged in supernovae, and that it should be possible to detect atoms propelled into space by these exploding stars. Yet scientists have so far failed to find clear traces of such material anywhere on Earth. Gunther Korschinek, a physicist at the Technical University of Munich, reasoned that a particular radioactive atom, iron-60, would be the easiest supernova product to detect. There are few sources of this form of iron close to Earth, but theorists have calculated that when massive stars explode they should spew huge amounts of iron-60 into interstellar space. The researchers examined metallic sediments laid down over the past 13 million years in samples dredged from the seabed in 1300 metres of water near Mona Pihoa in the South Pacific. Using a specially designed accelerator mass spectrometer, they were able to separate traces of iron-60 from terrestrial nickel-60. Although these atoms have identical mass, nickel-60 has two more protons in its nucleus. As a result, when the researchers ionised beams of iron-60 and nickel-60 from the sediments by passing them through nitrogen gas, the two kinds of atoms shed different numbers of electrons. They could then be separated using a magnetic field. This revealed significant quantities of iron-60 in two relatively recent sediment layers. Cosmic rays slamming into the Earth’s atmosphere also generate showers of heavy nuclei. But in the latest issue of Physical Review Letters (vol 83, p 18), Korschinek and his colleagues show that sources other than supernovae cannot account for more than a few per cent of the atoms they detected. “It’s something like a hundred-to-one ratio,” Korschinek says. Since iron-60 has a half-life of 1.5 million years, the atoms they found must have been produced quite recently, and in a relatively nearby supernova. But not too near, because the flood of high-energy particles from a supernova closer than about 100 light years would have caused an ecological catastrophe by stripping the Earth of its protective ozone layer (This Week, 4 April 1998, p 20). Korschinek’s bet is that the iron-60 came from one or possibly two supernovae that exploded within a few hundred light years of Earth over the past five million years. Brian Fields, an astrophysicist at the University of Illinois, Urbana-Champaign, describes the detection of large quantities of radioactive heavy nuclei as the “gold-plated signature” of a supernova. To confirm the findings, he suggests that Korschinek and his colleagues should examine their sediments for plutonium-244,