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Physical Review Letters

Magneto-Optic Trapping of 210 Fr

J. E. Simsarian, A. Ghosh, G. Gwinner, Luis A. Orozco, Gene D. Sprouse, P. A. Voytas

Published May 6, 1996• 104 citations
Publisher

Abstract

The alkali element francium has no stable isotopes and has eluded extensive study because sufficient quantities have not been available for detailed investigations. More than 1000 ${}^{210}\mathrm{Fr}$ atoms have now been contained in a magneto-optic trap. This number is sufficient to allow extensive studies of Fr atomic properties that are necessary input for future tests of fundamental interactions.

DOI
10.1103/physrevlett.76.3522

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•Dec 31, 1997•0 citations
The alkali element francium has no stable isotopes and has eluded extensive study because sufficient quantities have not been available for detailed investigations. More than 1000 210Fr{}^{210}\mathrm{Fr}210Fr atoms have now been contained in a magneto-optic trap. This number is sufficient to allow extensive studies of Fr atomic properties that are necessary input for future tests of fundamental interactions.
Francium is the least studied of the alkali atoms because it has no stable isotopes. We have performed precision spectroscopy on cold Fr atoms in a magneto optical trap. We have determined the location of the first two excited states of the S series by two-photon spectroscopy. We have measured the lifetimes of the 7p levels with a precision better than 0.5%. Our measurements test the many-body perturbation theory ab initio calculations of the dipole matrix element to very high accuracy in this relativistic alkali.
A magneto-optical trap for francium radioactive atoms has been set up at the Istituto Nazionale di Fisica Nucleare, Legnaro laboratories, and its characterization and optimization are under way. The main features of the Fr+ beam line, of the target, and of the magneto-optical trap are described in detail. Measurements of the Fr-ion production rate as a function of the target temperature, the primary beam intensity, and energy have been made. Preliminary tests with other stable alkali atoms aimed at an improvement of the magneto-optical trap collection efficiency are reported. Fast and efficient trap loading of rubidium has been obtained through the light-induced atomic desorption from an organic coating. A larger number of sodium atoms, as compared with a monochromatic trapping laser, has been trapped by use of a broadband laser.
A cold cloud of rare atoms is desirable in several fields. High-resolution spectroscopy of poorly-known atomic levels and tests of the Standard Model in atomic systems are examples of studies pursued in laboratories around the world [1]. Francium is particularly suitable because it is the heaviest alkali metal and it has several isotopes with relatively long lifetimes. Also, one expects enhanced parity-violating effects and reduced theoretical uncertainties from isotope comparisons. By accumulating atoms in a trap, one partially compensates for their scarcity. Our group and the one at SUNY Stony Brook, who pioneered the field [2], are focusing on the isotopes with mass numbers in the range 208–211. A production target, a beam transport line and a magneto-optical trap (MOT) are operating at INFN’s National Laboratories in Legnaro. An overview of our work is given in Refs. [3, 4]. Here we summarize the performance of the Fr sources.
Francium, the heaviest alkali, is the ideal candidate for the next generation of experiments that study atomic parity non-conversion(PNC) effects or search for a CP-violating permanent atomic electric dipole moment(EDM). Since all francium isotopes are radioactive and have half-lives of 22 minutes or less, a highly efficient atom trap is needed to provide a sufficient sample for PNC or EDM experiments. Using an improved vapor cell magneto-optical trap, we have demonstrated an efficient trap of {sup 221}Fr(t{sub {1/2}}=5m) atoms. The 4cm cube cell has a geometry optimized for a high trap loading rate and its walls are coated with dryfilm to slow the loss of francium atoms to the walls. Produced from the decay of {sup 225}Ac(t{sub {1/2}}=10d), the francium atoms are efficiently transferred into the cell using a novel orthotropic oven. We have also measured the atomic hyperfine structure of the excited 7P{sub 3/2} and 7P{sub {1/2}} levels of {sup 221}Fr.