The unstable radium nucleus is appealing for probing new physics due to its high mass, octupole deformation and energy level structure. Ion traps, with long hold times and low particle numbers, are excellent for work with radioactive species, such as radium and radium-based molecular ions, where low activity, and hence low total numbers, is desirable. We address the challenges associated with the lack of stable isotopes in a tabletop experiment with a low-activity (∼ 10 µCi) source where we laser-cool trapped radium ions. With a laser-cooled radium ion we measured the 7p 2 P o 1/2 state's branching fractions to the ground state, 7s 2 S 1/2 , and a metastable excited state, 6d 2 D 3/2 , to be p = 0.9104(7) and 0.0896 (7), respectively. With a nearby tellurium reference line we measured the 7s 2 S 1/2 → 7p 2 P o 1/2 transition frequency, 640.096 63(6) THz.Radium, the heaviest alkaline earth element, has no stable isotopes. Singly ionized radium's simple electronic structure is amenable to optical pumping and laser cooling with wavelengths far from the challenging UV of most alkaline earth type ions. Radium's heavy nucleus, atomic number Z = 88, is well suited to searches for new physics, where sensitivity to symmetry breaking forces scales ∝ Z 3 [1, 2]. Certain radium isotopes, such as radium-225, have additional nuclear structure enhancements to CP (charge-parity) violating new physics [3,4]. Setting limits to sources of CP violation will help us understand the baryon asymmetry in the observed Universe [5].Pioneering work with trapped HfF + molecular ions has made significant progress in constraining leptonic CP violation, and has rigorously studied potential systematic effects for future experiments [6]. A complementary hadronic CP violation experiment with radioactive molecular ions RaOH + , or RaCOH + 3 [7] is an intriguing possibility, where the low densities and long hold times of ion traps are well matched to working with radioactive isotopes, because low total activity is desirable. The radium-225 nucleus (I = 1/2) has octupole deformed parity doublets that enhance sensitivity to CP violating forces by a factor of 100-1000 compared to the current touchstone atomic system, 199 Hg [8-10]. A radiumbased molecular ion, such as 225 RaOH + , has an additional sensitivity advantage because of the molecule's closely spaced, opposite parity electronic states in addition to the enhancements from the closely spaced, opposite parity radium nuclear states. Trapped and lasercooled radium ions could be the starting point for generating such radium-based molecular ions, where optical pumping Ra + may provide control of chemical reactions to produce RaOH + , as seen in other alkaline earth ions Ca + and Be + [11,12].A single laser-cooled radium ion is also a candidate for atomic parity nonconservation (PNC) measurements, as the massive radium nucleus enhances PNC effects and the simple electronic structure is appealing for the requisite calculations [13,14]. There are many radium isotopes, including several that were previo...
