Neutron-Laden Nucleus Pushes Limit

Abstract

NUCLEAR PHYSICS Nuclear physicists are striving to find out how many neutrons can be packed into a nucleus. But a newly discovered nucleus suggests that the limit may be higher than theorists had thought—perhaps too high for experimenters to reach. The new nucleus, aluminum-42, contains 13 protons and 29 neutrons, so many neutrons that calculations had suggested that it could not form. Yet Thomas Baumann and colleagues at the National Superconducting Cyclotron Laboratory at Michigan State University (MSU) in East Lansing produced 23 copies of the highly unstable nucleus, as they report this week in Nature. “It's beautiful,” says Olivier Sorlin, an experimenter at the French laboratory GANIL in Caen. “I'm quite surprised that they found it. We tried and did not succeed.” Aluminum-42 could cast a long shadow in the study of rare isotopes. Physicists plot the known nuclei on a gridlike chart with the number of protons running up the chart and the number of neutrons running across it. The nuclei lie in a broad swath that is bounded above by the so-called proton drip line, which shows which combinations of protons and neutrons are too rich in protons to form a nucleus, and below by the neutron drip line, which shows which combinations are too loaded with neutrons to stick together (see [figure][1]). According to two theoretical models, aluminum-42 lies on the wrong side of the neutron drip line and should not exist, even fleetingly. ![Figure][2] A nucleus too far. Two calculations of the drip line (solid and dashed lines) suggest that aluminum-42 shouldn't exist. CREDIT: ADAPTED FROM T. BAUMANN ET AL. , NATURE (25 OCTOBER 2007) If aluminum-42 exists, then aluminum-43, −44, and −45 may also exist, says MSU's Michael Thoennessen. That's because aluminum-42 has one lone neutron in a “shell” that can hold as many as four. In fact, the team spotted one possible example of aluminum-43, as well as the nucleus magnesium-40. But the possibility that 32 neutrons could be packed into the aluminum nucleus means that the neutron drip line may lie too far away to be reached even with new facilities such as Japan's Radioactive Isotope Beam Factory at the RIKEN laboratory in Wako or Germany's Facility for Antiproton and Ion Research under construction at GSI in Darmstadt, Thoennessen says. To make aluminum-42, the MSU team blasted calcium-48 nuclei, which have 20 protons and 28 neutrons, through a tungsten target. Very rarely, the violent collision stripped off seven of a calcium-48 nucleus's protons and gave it an extra neutron to make aluminum-42. To make aluminum-45, the incoming nucleus would have to snatch up four neutrons, an event so improbable that seeing it is “really at the edge of what's possible in any foreseeable future,” Thoennessen says. Not everyone is convinced that the drip line has retreated beyond reach. “It's an open question whether this [observation] pushes the drip line out generally or if there is a little bulge in that region” around aluminum, says Richard Casten of Yale University. Witold Nazarewicz, a theorist at the University of Tennessee, Knoxville, and Oak Ridge National Laboratory, says that the position of the drip line was not known precisely to begin with. “I think that most theorists would say that the models they looked at are simply not reliable for subtle details along the drip line,” he says. Even so, the existence of aluminum-42 undermines a key concept in nuclear physics, Sorlin says. Researchers know that nuclei with 28 neutrons are generally especially stable. So the fact that aluminum-42, with 29, holds together even for an instant suggests that the “magic number” 28 disappears at the drip line, Sorlin says. No matter where the drip line lies, aluminum-42 has given physicists plenty to think about. [1]: #F1 [2]: pending:yes