87 - Francium
left, about 1,000 Fr atoms on 10 Dec 2002. right, about 200,000 Fr atoms on 13 Dec 2002.
Despite the extremely short half-life of this element, a visible sample of francium has been made! The Stony Brook Nuclear Structure lab produces francium in a nuclear fusion reaction between an oxygen beam and a heated gold target. The francium is extracted from the target as an ion (Fr+) and is transported to a second room. The Fr+ ion beam is accumulated for 20 - 300 seconds before being injected into a magneto-optical trapping (MOT) cell. The francium is dispensed by swinging the neutralizer up against the trapping cell and heating it to ~700-800 degrees Celsius. This process guarantees that all the released francium enters the cell while simultaneously plugging any escape holes out of the cell. A dry-film coating minimizes the amount of time the francium atoms remain on the cell walls before being released back into cell. The cell design and the large diameter trapping beams (~ 4 cm diameter) maximize the trapping efficiency of the apparatus. The trapping region fills up almost the entirety of the cell. The trapping measurements indicate that the trap captures at least 0.7% of the francium deposited on the neutralizer. Up to 350,000 atoms of 210Fr have been trapped.
Schematic of the francium production, transport, and trapping apparatus at the Stony Brook Nuclear Structure Lab.
This graph, which identifies francium by its radiation, is from the notebook of the discoverer, Marguerite Perey, a onetime assistant to Marie Curie.
http://fr.physics.sunysb.edu/francium_news/frconten.htm - It had been said
that francium would never be seen, but this group has done it!
http://funk.physics.sunysb.edu/lab/frnews2002/more3.htm - Fr pictures.
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