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

TL;DR

The work demonstrates a portable, accelerator-independent platform for studying short-lived radioactive molecular ions from post-lead elements by combining alpha-recoil harvesting in a cryogenic stopping cell with fast ion–molecule chemistry in an RFQ beamline and high-resolution MR-TOF-MS detection. It shows near-unity formation of RaF$^+$ via the Ra$^{2+}$ + SF$_6$ reaction on millisecond timescales, while revealing charge-state–dependent reactivity and a PbOH$^+$ byproduct from residual water, all accessible at university laboratories. Quantum-chemical calculations at the RECP-ROHF-UCCSD(T) level corroborate the observed trends, predicting exothermic MF$^+$ formation for M$^{2+}$ with SF$_6$ and endothermic M$^{+}$ channels for Pb and Po, aligning with experimental outcomes. This approach enables decentralized, rapid, and chemically selective studies of heavy, radioactive molecules, with RaF$^+$ and related species poised to contribute to precision tests of fundamental symmetries and heavy-element chemistry beyond traditional reactor- or accelerator-based facilities.

Abstract

Molecules have proven to be sensitive tools for studying physics beyond the standard model, with heavy and deformed nuclei offering decisive sensitivity to parity- and time-reversal-violating effects. However, almost all elements beyond lead, occupying the 6p~to~5f atomic orbitals, lack stable isotopes, hence molecules containing them are referred to as radioactive molecules. Among those, radium monofluoride has seen particular interest, but to date, research on radioactive molecules has mainly been limited to large-scale nuclear facilities. Here, we present a scheme that allows efficient and fast harvest of radioactive ions (including short-lived Ra), and show ion gas-phase reaction studies of singly and doubly charged Ra, Po, and Pb ions with SF$_6$ gas inside an ion trap. Our results show that the chemical reaction rate of Ra$^+$ is in line with trends of other alkaline earth elements, further support by quantum chemical computations. The reaction Ra$^{2+}$ + SF$_6$ $\rightarrow$ RaF${^+}$ + SF$_5^{+}$ achieves an almost unity conversion efficiency, making it particularly suitable for the application for studies in physics beyond the standard model. The scheme enables future decentralized research avenues with short-lived radioactive molecules for fundamental physics research at laboratories without the need for local nuclear reactors or accelerators.

Figures (7)

• Figure 1: Overview of the experimental setup at the FRS Ion Catcher, showing the cryogenic stopping cell (CSC), the versatile radio-frequency quadrupole beam line, and the high-resolution multiple-reflection time-of-flight mass-spectrometer (MR-TOF-MS). The setup has an overall length of about 4 m. Inset, section of the nuclear chart showing isotopes beyond $^{208}$Pb which can be made available for studies at university laboratories via the in-stopping cell alpha decay harvest scheme using readily available parents in comparison to long-lived (half-life $> 9.9$ days) isotopes. The elements At, Rn and Fr do not have any long-lived isotope and as such they are almost impossible to be studied via conventional wet chemistry. (Note, using online production, additional parent isotopes can be produced to further expand the harvesting scheme.)
• Figure 2: Relative intensities of $^{224}$Ra$^{2+}$, $^{224}$RaF$^{+}$ and SF$_5^+$ as a function of accumulation time, normalized to initial $^{224}$Ra$^{2+}$ intensity; demonstrating the reaction $^{224}$Ra$^{2+} +$SF$_6 \rightarrow ^{224}$RaF$^{+} +$SF$_5^+$ using the broad band mode of the MR-TOF-MS.
• Figure 3: High-resolution time-of-flight spectrum showing the observed singly charged species without SF$_6$ present in the reaction region (top) and observed reaction products from singly (mid) and doubly (bottom) charged atomic ions with SF$_6$. Note: RaF$^+$ is formed from reactions of both charge states, whereas PoF$^+$, PbF$^+$, and PbOH$^+$ are only formed from $2+$ ions. No reactions of Tl were observed.
• Figure 4: (top) Detailed reaction study showing the formation of RaF$^+$, PoF$^+$ and PbF$^+$. Fit curves with a growth model are shown to guide the eye. (bottom) Formation of PbF$^+$ and PbOH$^+$ indicating the two steps reaction path $^{212}$Pb$^{2+}+$SF$_6\rightarrow^{212}$PbF$^{+}+$SF$_5^+$ and PbF$^+ +$H$_2$O$\rightarrow^{212}$PbOH$^{+}+$HF.
• Figure 5: Schematic reaction energy diagram for the prediction of the reaction pathways for the fluorination of M$^{+}$ (dashed lines) and M$^{2+}$ (solid lines) with SF$_6$ to MF$^+$ (M $=$ Ra, Pb, Po) computed on the level of RECP-ROHF-UCCSD(T). Relative energies are given in eV with the direction of the arrows indication endothermic/exothermic reaction pathways. Only the mono-cation of Ra reacts with a negative reaction energy, while all di-cations exhibit an exothermic reaction. The individual arrows indicate the specific experimentally observed reaction pathways, matching the theoretical calculations.