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A pathway towards decentralized studies of radioactive post-lead elements and their applications in beyond standard model physics

This paper presents a decentralized scheme for efficiently harvesting short-lived radioactive ions and demonstrating high-efficiency gas-phase reactions to produce radioactive molecules like RaF $^+$ , thereby enabling fundamental physics research beyond the Standard Model at laboratories without local nuclear facilities.

Original authors: Moritz Pascal Reiter, Kriti Mahajan, Meetika Narang, Carsten Zuelch, Timo Dickel, Daler Amanbayev, Robert Berger, Julian Bergmann, Agnieszka Bukowicka, Mariam Fadel, Tayemar Fowler-Davies, Zhuang Ge

Published 2026-08-27

📖 6 min read🧠 Deep dive

CC BY 4.0

Original authors: Moritz Pascal Reiter, Kriti Mahajan, Meetika Narang, Carsten Zuelch, Timo Dickel, Daler Amanbayev, Robert Berger, Julian Bergmann, Agnieszka Bukowicka, Mariam Fadel, Tayemar Fowler-Davies, Zhuang Ge, Simeon Gloeckner, Gabriella Kripko-Koncz, Nasser Kalantar-Nayestanaki, Cameron Merron, David J. Morrissey, Wolfgang Plass, Christoph Scheidenberger, Makar Simonov, Nazarena Tortorelli, Jiajun Yu, Alexandra Zadvornaya, Jianwai Zhao

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep in the heart of modern physics lies a quiet but persistent mystery: the laws of nature as we know them do not fully explain the universe. We see evidence of dark matter holding galaxies together, and we know the cosmos is expanding at an accelerating pace, yet our current best theories cannot account for these phenomena. To find the missing pieces, scientists are looking for subtle violations in the fundamental symmetries of nature, specifically in how matter behaves under the mirror-like reflection of space and the reversal of time. One of the most promising ways to hunt for these violations is by studying heavy atoms and molecules. The heavier the nucleus, the more sensitive the atom becomes to these tiny, elusive effects. However, the most interesting candidates for these experiments are elements that come after lead on the periodic table. These elements are all radioactive, meaning they decay and disappear quickly, making them incredibly difficult to study. For decades, research on these fleeting atoms has been locked inside massive, expensive national laboratories, accessible only to a few.

A team of researchers has now broken that lock, demonstrating a way to study these short-lived radioactive molecules in a much smaller, more accessible setting. Their work focuses on a specific molecule made of radium and fluorine, a candidate that could revolutionize our search for new physics. The team showed that they could harvest radioactive atoms, turn them into ions, and force them to react with a gas to form the desired molecule, all within a compact setup that does not require a giant particle accelerator. This achievement proves that the study of heavy, radioactive chemistry can move out of the few large facilities and into university laboratories, opening the door for a wider range of scientists to explore the frontiers of the periodic table.

The journey begins with a simple but powerful idea: using the natural energy released when an atom decays to do the work of moving it. The researchers started with a small source of thorium, a heavy element that naturally emits alpha particles. When a thorium atom decays, it kicks out a smaller atom, radium, with enough force to bounce it off the source and into a chamber filled with helium gas. This chamber acts as a gentle brake, slowing the fast-moving radium atoms down until they are calm enough to be caught. Once captured, electric fields guide these atoms into a narrow tube where they are cooled and prepared for the next step. The beauty of this method is that it works on a timescale of milliseconds, fast enough to catch atoms that would otherwise vanish in a fraction of a second.

Once the atoms are ready, the team introduced a gas called sulfur hexafluoride into the tube. This gas is known to be very reactive with certain types of ions. The researchers wanted to see if they could use this gas to grab a fluorine atom and attach it to their heavy metal ions, creating a molecule. They tested this with three different heavy elements: radium, polonium, and lead. The results were striking. When the atoms were in a doubly charged state, meaning they had lost two electrons, they reacted with the gas almost instantly. In the case of radium, the reaction was so efficient that nearly every single radium ion turned into a radium-fluorine molecule in just a few milliseconds. This conversion happened with such speed and certainty that the team could measure the rate of the reaction and confirm that it followed the expected chemical trends for heavy elements.

However, the story was not the same for all charge states. When the team tried the reaction with singly charged ions, the results depended heavily on the element. Radium still managed to form the molecule, but lead and polonium did not react at all in this state. This difference is not just a minor detail; it is a crucial clue about the underlying physics. The researchers used advanced computer simulations to understand why. The calculations showed that for lead and polonium, forming the molecule with a single charge would require an input of energy that the system does not have, making the reaction impossible. Radium, being even heavier, has a different electronic structure that allows the reaction to happen naturally, releasing energy instead. This match between the experiment and the theory gives the team high confidence that they are observing the correct chemical behavior.

The team also observed that the newly formed molecules were stable enough to be studied, but only for a short time. For the lead molecules, they noticed a secondary reaction where the molecule grabbed a water vapor molecule from the air inside the chamber, changing its chemical makeup. This did not happen with the radium molecules, which remained pure and unchanged. This observation of how different heavy elements behave in the presence of trace impurities provides a new level of detail for understanding the chemistry of these extreme elements. Perhaps the most impressive feat was the successful creation and detection of a molecule containing polonium. Polonium has a half-life of only 145 milliseconds, meaning half of any sample disappears in that tiny fraction of a second. The fact that the team could catch the atoms, react them, and identify the resulting molecule before it decayed proves that their method is fast enough to handle the most fleeting isotopes in existence.

This work does more than just create a new molecule; it establishes a new pathway for science. By proving that these reactions can be done with small amounts of material, at high speeds, and without a massive accelerator, the researchers have shown that the study of heavy radioactive elements can be decentralized. Universities around the world could potentially set up similar systems to explore the chemistry of elements that have been out of reach for decades. This opens up possibilities not just for testing the fundamental laws of physics, but also for developing new medical treatments and understanding the limits of the periodic table. The ability to produce and study these molecules with such precision and speed transforms what was once a rare, facility-bound experiment into a practical tool for discovery, bringing the edge of the known world closer to the everyday laboratory.

Technical Summary: A Pathway Towards Decentralized Studies of Radioactive Post-Lead Elements

Problem Statement
Precision tests of physics beyond the Standard Model (BSM), particularly searches for permanent electric dipole moments (EDMs) that violate charge, conjugation, and parity (CP) symmetries, rely heavily on heavy, polar molecules. Elements beyond lead (Pb), such as radium (Ra), francium (Fr), and actinium (Ac), possess nuclear structures (large quadrupole and octupole deformations) that significantly amplify sensitivity to CP-violating effects. However, experimental progress is severely constrained by isotope availability. All elements beyond bismuth are radioactive; the most promising candidates for BSM studies are often short-lived and lack stable isotopes. Consequently, research has been largely confined to large-scale nuclear facilities with online radioactive ion beam (RIB) capabilities. Existing methods for producing these species, such as wet-chemistry separations, are too slow for short-lived isotopes and require macroscopic quantities that are difficult to obtain. This bottleneck prevents systematic surveys of post-lead molecules at university-scale laboratories.

Methodology
The authors present a proof-of-concept scheme for the decentralized production and identification of short-lived radioactive molecular ions. The method decouples molecular synthesis from slow chemical separations by utilizing a four-step process:

Alpha-Recoil Harvesting: A high-purity, cryogenic stopping cell (CSC) filled with helium gas captures recoil ions generated from the alpha decay of a parent source (specifically $^{228}\text{Th}$ ). The recoil energy (~100 keV) is sufficiently low to allow thermalization within a few millimeters of the source surface.
Ion Extraction and Transport: Extracted ions are guided via DC and RF electric fields into a versatile, modular Radio-Frequency Quadrupole (RFQ) beamline. This system allows for charge-state selection (e.g., selecting $1+$ or $2+$ ions) and continuous cooling via collisions with the buffer gas.
Gas-Phase Ion-Molecule Reactions: Inside a reaction region of the RFQ beamline, the ions are mixed with a reaction gas, specifically sulfur hexafluoride ( $\text{SF}_6$ ). The system is designed to facilitate fast ion-molecule reactions on millisecond timescales, compatible with the half-lives of short-lived isotopes like $^{216}\text{Po}$ ( $t_{1/2} = 145$ ms).
High-Resolution Identification: Reaction products and unreacted ions are transported to a Multiple-Reflection Time-of-Flight Mass Spectrometer (MR-TOF-MS). This instrument provides high mass resolving power (up to $10^5$ FWHM) and single-ion sensitivity, enabling the unambiguous identification of specific molecular species based on their mass-to-charge ratios.

The study focused on singly and doubly charged ions of Radium ( $^{224}\text{Ra}$ ), Polonium ( $^{216}\text{Po}$ ), and Lead ( $^{212}\text{Pb}$ ) reacting with $\text{SF}_6$ .

Key Results

Efficient Molecular Formation: The reaction $\text{Ra}^{2+} + \text{SF}_6 \rightarrow \text{RaF}^+ + \text{SF}_5^+$ was observed with an almost unity conversion efficiency. Within an accumulation time of 10 ms, approximately 90(10)% of the initial $\text{Ra}^{2+}$ ions were converted to $\text{RaF}^+$ .
Charge-State Selectivity: A distinct charge-state dependence was observed. Doubly charged ions ( $\text{Ra}^{2+}$ , $\text{Po}^{2+}$ , $\text{Pb}^{2+}$ ) reacted efficiently with $\text{SF}_6$ to form monofluorides ( $\text{MF}^+$ ). In contrast, singly charged $\text{Pb}^+$ and $\text{Po}^+$ ions showed no observable reaction with $\text{SF}_6$ under the experimental conditions. $\text{Ra}^+$ did react, though with lower efficiency than the doubly charged species.
Short-Lived Isotope Access: The scheme successfully produced and identified $^{216}\text{PoF}^+$ from $^{216}\text{Po}^{2+}$ , demonstrating the capability to access isotopes with half-lives as short as 145 ms. The signal buildup and subsequent decay of $^{216}\text{PoF}^+$ matched the known half-life of the parent isotope, confirming the system operates on the necessary timescales.
Secondary Reactions: In the presence of residual water vapor, $\text{PbF}^+$ was observed to undergo hydroxylation to form $\text{PbOH}^+$ . Conversely, $\text{RaF}^+$ remained inert under identical conditions, indicating higher chemical stability against hydroxylation.
Reaction Rates: Effective bimolecular reaction rate constants were determined. For $\text{Ra}^{2+}$ , the rate was $7(5) \times 10^{-10} \text{ cm}^3 \text{ molec.}^{-1} \text{ s}^{-1}$ . Upper limits were established for the non-observed reactions of $\text{Pb}^+$ and $\text{Po}^+$ .

Theoretical Validation
Quantum chemical calculations at the RECP-ROHF-UCCSD(T) level were performed to interpret the experimental trends. The calculations predicted that the formation of $\text{MF}^+$ from $\text{M}^{2+} + \text{SF}_6$ is exothermic for Ra, Pb, and Po, consistent with the observed efficient reactions. Conversely, the reaction for $\text{M}^+ + \text{SF}_6$ was predicted to be endothermic for Pb and Po (energies of 2.08 eV and 1.85 eV, respectively), explaining the lack of reactivity for singly charged Pb and Po. The calculation for $\text{Ra}^+$ yielded a slightly exothermic value (-0.92 eV), consistent with the observed reactivity of the singly charged radium ion.

Significance and Claims
The paper claims to demonstrate a broadly applicable, accelerator-independent scheme for producing and identifying short-lived heavy radioactive molecular ions at trace levels. By combining alpha-recoil harvesting with fast ion-molecule chemistry and high-resolution mass spectrometry, the method addresses key bottlenecks in the field:

Decentralization: It enables fundamental physics research (such as EDM searches) at laboratories without local nuclear reactors or accelerators, provided they have access to suitable parent sources.
Speed and Sensitivity: The millisecond-scale reaction times and single-ion sensitivity make the technique suitable for isotopes with very short half-lives and minute quantities (sub-microgram/nanogram).
Chemical Specificity: The mass-resolved detection allows for the study of specific isotopes and charge states, overcoming the limitations of bulk chemical separation.

The authors position this work as a "proof-of-concept" that establishes a practical pathway to supply $\text{RaF}^+$ and other heavy molecular ions for precision measurements. They note that while the current study focuses on Ra, the technique is generalizable to a broad set of isotopes beyond $^{208}\text{Pb}$ , potentially supporting systematic studies in heavy-element chemistry and radiochemical applications. The work does not claim to have solved all scaling issues, noting that future throughput improvements will depend on source activity and surface area, but it successfully validates the core mechanism for decentralized, short-lived isotope studies.

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