FRANCIUM

LUIS A. OROZCO, STATE UNIVERSITY OF NEW YORK, STONY BROOK

Little did I know when I arrived at the State University of New York, Stony Brook, in 1991 and started collaborating with Gene D. Sprouse that on the night of Sept. 27, 1995, we were going to succeed in capturing some thousand atoms of francium in a magneto-optical trap. Francium is an alkali and so is one of the simplest heavy atoms. We are interested in performing precision measurements of its spectroscopic properties to find out more about the weak force--the force of nature responsible for the beginning of the solar cycle: the conversion of a proton into a neutron.

There is much less than an ounce of francium at any given time in the whole Earth. It is the most unstable element of the first 103 in the periodic table, and its longest lived isotope lasts a mere 20 minutes.

We made francium in a nuclear fusion reaction. After many trials, we ended up with a beam of oxygen ions accelerated enough to fuse with gold atoms in a target. Gold is a noble metal and does not form compounds with francium, so we could extract and transport it to the trapping region.

Only in 1978 did the team led by Sylvan Liberman at ISOLDE in CERN succeed in finding the D₂ resonant line of the spectrum of francium, opening the road for further spectroscopic studies. Meanwhile, the development of tunable lasers had enabled the cooling and trapping of alkali atoms using resonant light in combination with magnetic fields, so all the parts necessary for trapping and cooling francium were there at Stony Brook in the early 1990s.

Gerald Gwinner and John Behr joined the effort to trap radioactive rubidium on-line with the superconducting linear accelerator at Stony Brook. We succeeded in 1994, opening the way to the more challenging effort to capture francium. Jesse Simsarian and Paul Voytas took over, and after many failed attempts during the summer of 1995, we gave it another try at the end of September.

It was already past midnight, and I was preparing the class that I had to give the following morning. We were in the control room of the accelerator, far from the trapping area. We had set television monitors and computer screens to follow the signal. I was not looking into any of the monitors but was sitting reading and facing the other members of the team. I saw a funny expression on their faces, and I just thought, "Something has failed." But no, there was a signal that increased and increased as we changed the frequency of the laser, just where we expected it, but about a hundred times larger! We were all skeptical and set up to repeat the scan. A few hours later we could not stop celebrating.

	TRAPPED A fluorescence image of 200,000 francium atoms in a magneto-optical trap at SUNY Stony Brook.
	COURTESY OF LUIS A. OROZCO AND SUNY STONY BROOK

Many students and postdoctoral associates worked in developing the new francium trap. It captured a few hundred francium atoms on Dec. 10, 2002. By the end of that week, Seth Aubin and Eduardo Gomez managed to have more than 300,000 atoms. The trap was now in a different room from the target area where the nuclear reaction took place. This allowed better control over the trap.

Now, when we trap francium atoms, we continue to monitor and interrogate their fluorescence; they remain in the trap for about half a minute. Then they begin their nuclear decay. It is amazing that we somehow manage to do inverse alchemy: We begin with gold, we get francium for three minutes after the nuclear reaction, and, once francium decays, we can end up with lead. Vive le francium!

Luis A. Orozco is professor of physics at the State University of New York, Stony Brook. He moves to the University of Maryland this month. He is one of the distinguished traveling lecturers of the Division of Laser Science of the American Physical Society.

Top

Chemical & Engineering News
Copyright © 2003 American Chemical Society