A new result from a powerful atom smasher may help astronomers explain how supernovae create "heavy" chemical elements — those that follow iron on the periodic table. Physicists using the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University have produced eleven nuclei of the extremely elusive isotope nickel-78, enabling them for the first time to measure how fast it decays. NCSL's Hendrik Schatz presented the result at this week's American Physical Society's April meeting in Tampa, Florida.
Why should astronomers concern themselves with the properties of a short-lived nucleus that is 34 percent heavier than nickel's commonest isotope? Because one of astronomy's Holy Grails is explaining how all the chemical elements seen on Earth and in space came to be. And while astronomers have know for some time that the slow-burning nuclear furnaces of red giants and the cataclysmic explosions of supernovae can
create heavy elements, their calculations haven't perfectly matched the elements' observed abundances — in part because of lingering uncertainties about the rates at which certain atomic processes occur.
The eleven nickel-78 nuclei that NSCL's atom smasher produced pertain to core-collapse supernovae: massive stars whose cores abruptly implode, fueling titanic explosions. In the first few seconds after such an event, atomic nuclei in the collapsing core are bombarded by neutrons. The nuclei capture some of the neutrons, turning into heavier isotopes. But nuclei with excess neutrons are unstable. In such a nucleus, a neutron will decay, turning into a proton, thus increasing the atomic number (the number that tells you which chemical element a particular nucleus belongs to). The process can go on, with more neutrons being added and the atomic number continually increasing. Within seconds, the nucleus is ejected from the supernova, and later it decays, eventually forming a stable isotope.
At 110 milliseconds, nickel-78's half-life is less than half as long as theorists previously had calculated. In the rapid chain of events taking place within a supernova, that difference can be crucial. "Nature can create the heavy elements faster than we assumed," Schatz says. And while theoretical models of supernovae are still rough, astrophysicists can use nuclear-physics data to fine-tune their predictions of isotope abundances. These predictions then can be compared to the relative numbers of various chemical elements observed in the solar system and in second-generation stars.
The NCSL data have improved those predictions, says Friedrich-Karl Thielemann, an astrophysicist at the University of Basel in Switzerland. "It's an absolutely important step," he says. In addition, says Schatz, understanding nickel-78's properties will allow physicists to predict the behavior of other nuclei that are similar but harder to produce in a laboratory.
Another announcement in Tampa may have implications for our
understanding of the very early universe. By smashing gold nuclei together, physicists using Brookhaven National Laboratory's Relativistic Heavy Ion Collider have created a short-lived state of matter in which quarks and gluons — the elementary constituents of protons and neutrons — flow like molecules in a liquid. Similar physical conditions existed microseconds after the Big Bang. Four distinct research groups announced their results in a joint press conference on Monday.