Laser Trapping of ${}^{225}\mathrm{Ra}$ and ${}^{226}\mathrm{Ra}$ with Repumping by Room-Temperature Blackbody Radiation

We have demonstrated Zeeman slowing and capture of neutral ${}^{225}\mathrm{Ra}$ and ${}^{226}\mathrm{Ra}$ atoms in a magneto-optical trap. The intercombination transition ${}^{1}S_0\to {}^{3}P_1$ is the only quasicycling transition in radium and was used for laser-cooling and trapping. Repumping along the ${}^{3}D_1\to {}^{1}P_1$ transition extended the lifetime of the trap from milliseconds to seconds. Room-temperature blackbody radiation was demonstrated to provide repumping from the metastable ${}^{3}P_0$ level. We measured the isotope shift and hyperfine splittings on the ${}^{3}D_1\to {}^{1}P_1$ transition with the laser-cooled atoms, and set a limit on the lifetime of the ${}^{3}D_1$ level based on the measured blackbody repumping rate. Laser-cooled and trapped radium is an attractive system for studying fundamental symmetries.

Figure 1

Energy level diagram and decay rates for radium. Energies [8] are indicated at the left, and calculated decay rates [11] are shown (${}^{3}P_1$ lifetime comes from experiment [12]). The 300 K blackbody photon number density is shown on the same energy scale in the inset.

Figure 2

Laser-trapping of ${}^{226}\mathrm{Ra}$ atoms. (a) Spectra of the atoms in the atomic beam (multiplied by $100\times$, open circles) and laser-cooled in the MOT (solid circles). (b) The number of atoms detected in the MOT over the 1.8 s following the shuttering (unshuttering) of the slower beam at $t=0$ yields the lifetime (loading) curve which is shown by solid (open) circles. Both curves are fit by exponentials and yield the identical lifetime $\tau =1.1\mathrm{s}$ for these data.

Figure 3

Repump events (483 nm photons) observed versus time as the repump laser is shuttered and unshuttered (laser-cooling is on continuously). Experimental data (solid circles) and a double exponential fit (solid line for $t>5\mathrm{ms}$) are shown with a dotted line indicating the steady-state repumping rate. The inset shows the relevant levels and the calculated transition rates due to spontaneous emission (straight arrows) and thermal radiation (curved arrows).

Figure 4

Number of ${}^{225}\mathrm{Ra}$ and ${}^{226}\mathrm{Ra}$ atoms in the trap, as measured by fluorescence at 714 nm, vs. the wave number of the 1429 nm repump laser recorded by the wave meter. The ${}^{225}\mathrm{Ra}$ data are scaled by $100\times$ to compare to ${}^{226}\mathrm{Ra}$, which is offset on the vertical scale for clarity. The repump resonances are indicated in the inset level schematic.