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Quantum Electronics and Laser Science Conference

New apparatus for magneto-optical trapping of francium

Joshua M. Grossman, S. Aubin, E. Gómez, Luis A. Orozco, M.R. Pearson, Gene D. Sprouse, Mike R. True

Published Nov 13, 2002• 3 citations

Abstract

Summary form only given. We have constructed a new apparatus for on-line magneto-optical trapping of radioactive francium atoms at the Stony Brook Superconducting LINAC. An ion beam with an energy of /spl sim/100 MeV from the accelerator strikes a target of gold or isotopically enriched platinum mounted on the end of a tungsten rod. A heavy-ion fusion reaction [/sup 16,18/O(/sup 197/Au,xn) or /sup 19/F(/sup 198/Pt,xn)] selectively produces the isotopes /sup 208-212/Fr in the target room of the accelerator. The beam power and an auxiliary heater heat the target, allowing the francium to diffuse rapidly to the surface. The target ionizes the francium as it diffuses out. We then transport the francium as ions to a different room for subsequent neutralization and capture in a magneto-optical trap (MOT). Since the transport system is electrostatic, it is mass independent, allowing us to transport any isotope of Fr or even other alkalis. We have delivered over 3/spl times/10/sup 6/ Fr/sec to a Faraday cup in front of the trap, and tests show greater than 90% transport efficiency from the target. A small constriction in the vacuum system separates the target and transport region from the trap, permitting differential pumping of the trap.

DOI: 10.1109/qels.2001.962110

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Summary form only given. We have constructed a new apparatus for on-line magneto-optical trapping of radioactive francium atoms at the Stony Brook Superconducting LINAC. An ion beam with an energy of /spl sim/100 MeV from the accelerator strikes a target of gold or isotopically enriched platinum mounted on the end of a tungsten rod. A heavy-ion fusion reaction [/sup 16,18/O(/sup 197/Au,xn) or /sup 19/F(/sup 198/Pt,xn)] selectively produces the isotopes /sup 208-212/Fr in the target room of the accelerator. The beam power and an auxiliary heater heat the target, allowing the francium to diffuse rapidly to the surface. The target ionizes the francium as it diffuses out. We then transport the francium as ions to a different room for subsequent neutralization and capture in a magneto-optical trap (MOT). Since the transport system is electrostatic, it is mass independent, allowing us to transport any isotope of Fr or even other alkalis. We have delivered over 3/spl times/10/sup 6/ Fr/sec to a Faraday cup in front of the trap, and tests show greater than 90% transport efficiency from the target. A small constriction in the vacuum system separates the target and transport region from the trap, permitting differential pumping of the trap.

We have developed a unique neutralizer device that uses an yttrium target surrounded by a platinum wall to magneto-optically trap radioactive atoms. In general, the radioactive nucleus produced in a nuclear reaction is extracted and transported in ion form. For the magneto-optical trap, thermal neutralization must occur on the surface of a metal with a small work function. The converter can produce a neutral atomic beam with small angular divergence that, given the recycling of atoms and ions, converts ions into neutral atoms with remarkable efficiency. We demonstrated the ion neutralization process using stable rubidium and confirmed

10^6

neutralized atoms in the magneto-optical trap. Additionally, the experiment using francium demonstrated the obtaining of neutralized francium atoms.

Summary form only given. We have constructed a new apparatus for producing and on-line trapping of francium for parity non-conservation experiments. At present, we have produced and delivered 15 /spl times/ 10/sup 6/ Fr/sec to the trapping area with better than 90% efficiency. The electrostatic optics of our beamline ensure mass independent transport of all francium isotopes and even other alkalis, such as rubidium, which we use for testing most of our apparatus. After accumulation in this first MOT, we transfer the atoms to a second MOT in a separate vacuum chamber one meter below, where they are transferred to a far off-resonance optical dipole trap for PNC experiments. At present, we are constructing the apparatus for a measurement of the PNC anapole moment by probing the microwave E1 transition between the ground state hyperfine levels of francium.

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.