Laser cooling and trapping of 224Ra+

M. Fan, Roy A. Ready, H. Li, S. Kofford, R. Kwapisz, C.A. Holliman, M.S. Ladabaum, A.N. Gaiser, J.R. Griswold, A.M. Jayich
Physical Review Research 5, 043201 (2023)

We report laser cooling and trapping of 224Ra+ ions. This was realized via two-step photoionization loading of radium into an ion trap. A robust source for 224Ra atoms, which have a 3.6-day half-life, was realized with an effusive oven containing 228Th, which has a 1.9-yr half-life, which continuously generates 224Ra via its α-decay. We characterized the efficacy of this source and found that after depleting built-up radium the thorium decay provides a continuous source of radium atoms suitable for ion trapping. The vacuum system has been sealed for more than 6 months and continues to trap ions on demand. We also report a measurement of the 224Ra 7s2 1S0 → 7s7p 1P1 transition frequency: 621 043 830 ± 60 MHz, which is helpful for efficient photoionization. With this measurement and previous isotope shift measurements we find that the frequency of the same transition in 226 Ra is 621 037 830 ± 60 MHz, which disagrees with the most precise measurement, 621 038 489 ± 15 MHz, which is used for the recommended value in the National Institute of Standards and Technology Atomic Spectra Database.