High efficiency MOT for francium

   The primary difficulty of working with francium is its availability. We are only able to to deliver ~10⁶ Fr/s to the MOT apparatus. Since we need ~10⁶ atoms for a PNC measurement and the lifetime of francium is typically 3 minutes, we must trap the francium as efficiently as possible.

   We designed and built several prototypes of apparatus for high efficiency trapping of francium. While most MOTs have a trapping efficiency on the order of 0.01%, the final design that is presented here successfully collects francium with a ~1.5% trapping efficiency. The apparatus can maintain an average MOT population of 50,000 atoms, and is capable of peak populations of over 200,000 atoms. In the future, by transfering the collected francium to a "science" MOT in a 2nd vacuum chamber with better a vacuum lifetime, we expect steady state MOT populations of ~10⁶ atoms.

Movie of a high efficiency ²¹⁰Fr MOT.

We obtain a high efficiency by integrating the following three features into our apparatus:

1. Large high power trapping beams:
   We use large 3 cm (1/e intensity diameter) laser beams with typical peak intensity of 18 mW/cm². The large high power beams guarantee a large capture velocity, since the stopping power (high intensity) and stopping distance (large beams) are both large. The large beams and intensity allow us to operate the MOT at a detuning of -4Γ from the cycling transition, which further increases the capture velocity. Significant laser intensity extends out to a diameter of 5 cm, thus guaranteeing that the atoms inside the 5 cm × 5 cm × 5 cm trapping cell will be inside the laser intersection volume most of the time.

1. Dry film coated cell:
   We use a silane-based dry film coating (SC-77, Silar Laboratories), which has the remarkable property that francium (and rubidium) atoms that land on its surface, remain stuck to it for only a few microseconds during which they rethermalize to room temperature. Untrapped atoms, with velocities above the capture threshold of the MOT, are rethermalized and have another opportunity to be trapped as they pass MOT when they escape the surface: atoms make multiple passes through the MOT with a new rethermalized velocity on each pass, thus increasing the total probability of being trapped.

2. Closed cell, pulsed MOT operation:
   We operate the MOT on cycle: we collect francium ions on a neutralizer foil for 32 s, and then load the MOT by turning on the dispenser for 1 s and releasing the atoms into the trapping cell. We use the neutralizer to plug the entrance/exit port of the trapping cell during the two part cycle:

   1. Collection phase:  the neutralizer is in the path of the ion beam, and the cell port is unblocked to maintain the ultra-high vacuum of ~10^-9 Torr in the trapping chamber.
   2. Trapping phase:  the neutralizer swings up to the trapping cell port and blocks it while releasing the francium as neutral atoms.

High efficiency francium MOT apparatus.

   By blocking the exit port of the MOT cell, the number of atom bounces will be maximized and depend only on the quality of the dry-film coating and the neutralizer.

Trapping efficiency measurement

   We measure the trapping efficiency of our MOT by capturing atoms from the neutralizer, rapidly expulsing them with a push beam (and the trap off), and then turning the MOT back on again. The ratio of the MOT populations after and before the expulsion is the trapping efficiency. We use rubidium for better signal-to-noise and measure a trapping efficiency of ~1.8% with this technique (see figure on left for a plot of the data).

   We also measure the trapping efficiency by taking the ratio of the ion beam current and and the loading rate of the MOT. With francium, we measure a trapping efficiency of ~1.2%.

Trapping efficiency measurement data.

MOT temperature measurement

We determine the temperature of the trapped francium by turning of the MOT laser beams and measuring the ballistic expansion of the cold atomic cloud by fluorescence imaging (see sample data below and in the movie on the right). With these data, we measure a temperature of 75 µK.

For more information on the high efficiency MOT, please see:

   S. Aubin, E. Gomez, L. A. Orozco, and G. D. Sprouse
   High efficiency magneto-optical trap for unstable isotopes
   Rev. Sci. Instrum. 74, 4342-4351 (2003) [PDF]