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An efficient trap of {sup 221}Fr atoms

Zheng‐Tian Lu, Kristan L. Corwin, Kurt R. Vogel, Carl Wieman

Published Dec 31, 1997• 0 citations
OpenAlex

Abstract

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.

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Physical Review Letters•Aug 11, 1997•93 citations
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.
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.
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.
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.
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.
We have efficiently loaded a vapor cell magneto-optical trap from an orthotropic source of 221Fr{}^{221}\mathrm{Fr}221Fr with a trapping efficiency of 56(10)%. A novel detection scheme allowed us to measure 900 trapped atoms with a signal to noise ratio of $\ensuremath{\sim}60$ in 1 sec. We have measured the energies and the hyperfine constants of the 27P1/2^{2}7P_{1/2}27P1/2​ and 27P3/2^{2}7P_{3/2}27P3/2​ states.