Enhanced Yield Rate of \textsuperscript{229m}Th via Cascade Decay in Storage Rings and Electron Beam Ion Traps

TL;DR

The study tackles the challenge of producing the $^{229m}$Th isomer by leveraging cascade decay from higher-lying nuclear states excited via two electron-mediated mechanisms, NEIES and NEEC, in storage rings and EBITs. By quantifying cross sections, excitation rates, and cascading branching ratios, the authors demonstrate that NEIES can boost the isomer yield by up to $10^{4}$ and NEEC can contribute additional tens-to-hundreds of times, depending on the cascade path and charge-state configuration. In SRs, cascades from the fifth and sixth excited states yield the largest enhancements, with up to ~80× over direct excitation for certain paths; in EBITs, a dual-beam approach enables resonant NEEC to dominate for specific cascades, achieving up to a ~29× increase over direct excitation. These findings offer a viable route to produce substantial $^{229m}$Th populations for nuclear clocks and provide a pathway to experimentally probe NEEC in HCIs, with practical guidance on beam energies, charge states, and trapping configurations.

Abstract

The low-energy nuclear isomeric state of \textsuperscript{229m}Th provides a unique bridge between nuclear and atomic physics, enabling applications such as nuclear clocks and precision metrology. However, efficient and controllable production of \textsuperscript{229m}Th remains a major experimental challenge. We propose an efficient scheme to produce the $^{229\mathrm{m}}$Th in storage rings (SRs) and electron beam ion traps (EBITs), using a cascade decay pathway. Highly charged ions are excited to higher nuclear states via nuclear excitation by inelastic electron scattering (NEIES) and nuclear excitation by electron capture (NEEC), followed by radiative or internal conversion cascades that populate the isomer. Our calculations demonstrate that, under typical SRs and EBITs conditions, optimized indirect excitation pathways significantly enhance \textsuperscript{229m}Th production rate. In particular, NEIES can provide an enhancement of up to four orders of magnitude through cascade de-excitation at high energies, while NEEC can contribute an additional enhancement of up to several tens of times. Such a significant increase in the \textsuperscript{229m}Th yield rate would facilitate its application in various nuclear photonics fields, especially in the development of atomic nuclear clocks.

Figures (6)

• Figure 1: Partial level scheme of 229Th showing selected nuclear excited states and their decay pathways to the isomeric state. The M1+E2 and E2 transitions in the decay paths are marked in brown and blue, respectively. For each nuclear level $\beta$, the spin--parity assignment $J^{\pi}$, half-life $T_{1/2}$, and excitation energy $E_n$ are labeled on either side of the corresponding level line. The upper-right label on each arrow indicates the branching ratio of the corresponding decay channel (in %). The energy and lifetime of the first excited state are taken from PhysRevLett.106.162501, and those of the second excited state are also taken from masuda2019x. For the branching ratios of different decay channels, the values for the second excited state are taken from masuda2019x, while the others are calculated using the formulas Eqs. (\ref{['eq_BR']}), with the relevant data obtained from NNDC. Unless otherwise specified, all other data are also taken from NNDC.
• Figure 2: NEIES cross sections $\sigma_{\mathrm{NEIES}}$ as functions of the incident electron energy $E_i$ for different reaction channels at an ionic charge state of $q_i = 90$. The vertical dashed lines mark the threshold energies corresponding to different excitation channels $0\rightarrow\beta$.
• Figure 3: NEEC resonance strength $S_{\mathrm{NEEC}}$ for the $0\!\rightarrow\!1$ excitation channel at $q_i=1$--4 when considering $n < 10$.
• Figure 4: NEEC resonance strength $S_{\mathrm{NEEC}}$ for the $0\!\rightarrow\!2$--$6$ excitation channels at a selected representative ionic charge state $q_i=90$.
• Figure 5: 229mTh NEIES production rates $R_{\mathrm{iso}}$ for different cascade decay channels as shown in Fig. \ref{['fig1-levels']} and the total yield rate versus electron energy at $q_i = 90$. Vertical dashed lines denote excitation thresholds. A SR configuration with $10^{8}$ ions, $I_{e}=200~\mathrm{mA}$, $r_{e}=1~\mathrm{cm}$, and $\epsilon_e=1~\mathrm{meV}$ electron energy spread is assumed.