| The Stony Brook group creates Fr atoms with atomic weight 210, (half-life of 3.2 minutes) by bombarding a gold target with beams of oxygen from the linear accelerator in the basement of the physics building at Stony Brook. The production required some time to develop and understand. It was critical to operate the gold target very close to its melting point and to make sure that its surface was very clean. The nuclear reaction imbeds the francium atoms deep in the gold target, and they must be removed efficiently. The atoms diffuse fast to the surface of the gold target and are released as ions. The francium ions are guided by electrostatic lenses until they land into a surface of hot yttrium and become neutral again. An easy way to remember the nuclear reaction is to think of it as inverse alchemy. We start with gold and oxygen, create francium that later decays into other elements that decay into lead. Our process of turning gold to lead is opposite to the dream of the medieval alchemists! |  Above: A picture of the
Superconducting LINACBelow: Schematic of the
accelerator used to produce the 18O beam
which produces francium. |
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The present system can deliver about one million francium atoms per second of any of the 209, 210 and 211 isotopes. The production rate of a particular isotope depends on the energy of the oxygen beam. While this seems like a lot of atoms, it is tiny compared with the 1015 atoms of sodium in a grain of salt! |
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 Diagram of the apparatus used to trap francium. An 18O beam from the Stony Brook LINAC creates 210Fr in the gold target with the nuclear reaction 197Au + 18O = 210Fr + 5n. The francium is released from the gold as an ion and is transported to an yttrium foil where it is neutralized and injected into the glass bulb. A magnetic field and retroreflected laser beams cool and confine the atoms. This is not the only way to produce francium. There are other nuclear reactions that create the neutron rich isotopes. They usually involve the bombardment of a Uranium target with a beam of high energy protons as done at ISOLDE in CERN. Another possibility is the use of a radioactive source of thorium that decays into actinium and then francium. This source produces 221Fr and the groups at Moscow and Colorado/Berkeley have used it. |
