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Measurement of $7p\ ^2P_{1/2}^o$ branching fractions with laser-cooled $^{226}\mathrm{Ra}^+$

Mingyu Fan, C. A. Holliman, A. L. Wang, Andrew M. Jayich

Published Jan 28, 2019• 0 citations
OpenAlex

Abstract

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 table-top experiment with a low-activity ($\sim 10 \mu\mathrm{Ci}$) source where we laser-cool radium ions to form radium Wigner crystals. With a laser-cooled radium ion we measured the $7p ^2P_{1/2}^o$ state's branching fractions to the ground state, $7s ^2S_{1/2}$, and a metastable excited state, $6d ^2D_{3/2}$, to be $p=0.9104(7)$ and $0.0896(7)$, respectively. With a nearby tellurium reference line we measured the $7s ^2S_{1/2} \rightarrow 7p ^2P_{1/2}^o$ transition frequency, 640.09663(6) THz.

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Measurement of 7p 2P1/2o7p\ ^2P_{1/2}^o7p 2P1/2o​ branching fractions with laser-cooled 226Ra+^{226}\mathrm{Ra}^+226Ra+
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 table-top experiment with a low-activity (∼10μCi\sim 10 \mu\mathrm{Ci}∼10μCi) source where we laser-cool radium ions to form radium Wigner crystals. With a laser-cooled radium ion we measured the 7p2P1/2o7p ^2P_{1/2}^o7p2P1/2o​ state's branching fractions to the ground state, 7s2S1/27s ^2S_{1/2}7s2S1/2​, and a metastable excited state, 6d2D3/26d ^2D_{3/2}6d2D3/2​, to be p=0.9104(7)p=0.9104(7)p=0.9104(7) and 0.0896(7)0.0896(7)0.0896(7), respectively. With a nearby tellurium reference line we measured the 7s2S1/2→7p2P1/2o7s ^2S_{1/2} \rightarrow 7p ^2P_{1/2}^o7s2S1/2​→7p2P1/2o​ transition frequency, 640.09663(6) THz.
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_{1/2}^{o} 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_{1/2}^{o} transition frequency, 640.096 63(6) THz.
We report a measurement of the radium ion's 7p2P3/27p\phantom{\rule{0.16em}{0ex}}^{2}P_{3/2}7p2P3/2​ state branching fractions and improved theoretical calculations. With a single laser-cooled 226Ra+{}^{226}{\mathrm{Ra}}^{+}226Ra+ ion we measure the P3/2{P}_{3/2}P3/2​ branching fractions to the 7s2S1/27s\phantom{\rule{0.16em}{0ex}}^{2}S_{1/2}7s2S1/2​ ground state 0.87678(20)0.876\phantom{\rule{0.16em}{0ex}}78(20)0.87678(20), the 6d2D5/26d\phantom{\rule{0.16em}{0ex}}^{2}D_{5/2}6d2D5/2​ state 0.10759(10)0.107\phantom{\rule{0.16em}{0ex}}59(10)0.10759(10), and the 6d2D3/26d\phantom{\rule{0.16em}{0ex}}^{2}D_{3/2}6d2D3/2​ state 0.01563(21)0.015\phantom{\rule{0.16em}{0ex}}63(21)0.01563(21).
We report laser cooling and trapping of 224Ra+{}^{224}{\mathrm{Ra}}^{+}224Ra+ ions. This was realized via two-step photoionization loading of radium into an ion trap. A robust source for 224Ra^{224}\mathrm{Ra}224Ra atoms, which have a 3.6-day half-life, was realized with an effusive oven containing 228Th^{228}\mathrm{Th}228Th, which has a 1.9-yr half-life, which continuously generates 224Ra^{224}\mathrm{Ra}224Ra via its $\ensuremath{\alpha}$-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 $^{224}\mathrm{Ra} 7{s}^{2}{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{S}}_{0}\ensuremath{\rightarrow}7s7p{\phantom{\rule{4pt}{0ex}}}^{1}{\mathrm{P}}_{1}$ transition frequency: $621\phantom{\rule{0.16em}{0ex}}043\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ 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 226Ra^{226}\mathrm{Ra}226Ra is $621\phantom{\rule{0.16em}{0ex}}037\phantom{\rule{0.16em}{0ex}}830\ifmmode\pm\else\textpm\fi{}60$ MHz, which disagrees with the most precise measurement, $621\phantom{\rule{0.16em}{0ex}}038\phantom{\rule{0.16em}{0ex}}489\ifmmode\pm\else\textpm\fi{}15$ MHz, which is used for the recommended value in the National Institute of Standards and Technology Atomic Spectra Database.
We have developed an apparatus integrating resonance-ionization, ion-trap, and laser-cooling techniques for an ultratrace radioactive isotope 90Sr^{90}\mathrm{Sr}90Sr analysis. Trapped 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ isotope ions were laser cooled, and their $4d\phantom{\rule{0.16em}{0ex}}^{2}D_{3/2}\ensuremath{\rightarrow}5p\phantom{\rule{0.16em}{0ex}}^{2}P_{1/2}$ transition isotope shift was experimentally measured to be $\ensuremath{-}281(17)$ MHz by comparing individual spectra of 88Sr+^{88}\mathrm{Sr}^{+}88Sr+ and 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ ions. Crystallization of 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ was carried out using the resonance frequency value confirmed in our experiment, and then 90Sr+^{90}\mathrm{Sr}^{+}90Sr+ individual ions were successfully observed.
Radioactive molecules have remained severely understudied relative to their stable counterparts, owing to difficulties associated with their radiological hazards and limited material availabilities. Despite this, their relevancy to many fields including nuclear science, chemistry, and astronomy make them attractive prospects for precision investigations. In particular, laser-coolable radioactive molecules containing octupole-deformed nuclei such as radium monohydroxide (RaOH) and radium monofluoride (RaF) promise dramatic sensitivity increases for experiments aiming to measure new physics originating from fundamental symmetry violations. Before they may be used for any of these purposes, however, new tools are needed that allow for their safe and precise study while making them accessible in a university (or similar) setting. In this thesis, we construct a tabletop cryogenic apparatus capable of studying radioactive molecules which we produce in-situ from trace quantities of starting material. We use this apparatus to perform the first spectroscopic observation of a radium-containing polyatomic molecule RaOH and its deuterated isotopologue RaOD, as well as high-resolution spectroscopy of the photon cycling transitions in RaF. To prepare for laser cooling, we develop a protocol to measure the vibrational branching ratios of these exotic species and use it to establish RaF to be among the most laser-coolable molecules known as of yet. We demonstrate the versatility of our approach by producing and detecting over twenty different atoms and molecules, ranging from transuranic radioisotopes to complex asymmetric top molecules. The species-agnostic methodology employed throughout this work allows its application to a wide class of previously ignored radioactive atoms and molecules, and makes them accessible to the many advanced experimental techniques already leveraged in stable systems.
We explore the potential of the electric quadrupole transitions 7s 2S1/27s\,^2S_{1/2}7s2S1/2​ - 6d 2D3/26d\,^2D_{3/2}6d2D3/2​, 6d 2D5/26d\,^2D_{5/2}6d2D5/2​ in radium isotopes as single-ion optical frequency standards. The frequency shifts of the clock transitions due to external fields and the corresponding uncertainties are calculated. Several competitive A^AARa+^++ candidates with A=A=A= 223 - 229 are identified. In particular, we show that the transition 7s 2S1/2 (F=2,mF=0)7s\,^2S_{1/2}\,(F=2,m_F=0)7s2S1/2​(F=2,mF​=0) - 6d 2D3/2 (F=0,mF=0)6d\,^2D_{3/2}\,(F=0,m_F=0)6d2D3/2​(F=0,mF​=0) at 828 nm in 223^{223}223Ra+^++, with no linear Zeeman and electric quadrupole shifts, stands out as a relatively simple case, which could be exploited as a compact, robust, and low-cost atomic clock operating at a fractional frequency uncertainty of 10−1710^{-17}10−17. With more experimental effort, the 223,225,226^{223,225,226}223,225,226Ra+^++ clocks could be pushed to a projected performance reaching the 10−1810^{-18}10−18 level.