Spectrometry of Captured Highly Charged Ions Produced Following Antiproton Annihilations

TL;DR

This work demonstrates the first capture and time-of-flight spectrometry of highly charged ions produced from antiproton annihilations in a Penning-Malmberg trap, using a multi-step nested-trap approach in AE$\overline{\textrm{g}}$IS. Helium and argon tests validate the method, enabling in-trap capture and TOF identification of ion species via $T_{TOF}$ and $m/q$ calibration. The study establishes a path toward in-trap synthesis of radioactive HCIs and high-resolution measurements of cold annihilation fragments, with potential to illuminate the neutron-rich outer regions of nuclei and constrain nuclear forces. Future improvements in cooling and mass spectrometry (e.g., MR-TOF) could achieve high resolving power and isotopic/isobaric separation, enhancing sensitivity to the nuclear periphery and related astrophysical applications.

Abstract

We report a proof-of-principle study demonstrating the first capture and time-of-flight spectrometry of highly charged ions (HCIs) produced following antiproton annihilations in a Penning-Malmberg trap. A multi-step nested-trap technique was developed using the \aegis\ experiment to identify annihilation-linked captured ions. The trapping and spectrometry of helium and argon ions demonstrates the approach. This work establishes a foundation for the in-trap synthesis of radioactive HCIs and the study of cold nuclear annihilation fragments, with the long-term goal of enabling a sensitive tool for probing the outer nuclear periphery.

Figures (7)

• Figure 1: Overview of the AE$\overline{\textrm{g}}$IS experimental setup. Antiprotons decelerated by the AD-ELENA decelerators enter the AE$\overline{\textrm{g}}$IS 5 trap after passing a degrader foil. The antiprotons are trapped in the 5 trapping region, between the HV1 and HV3 electrode biased to $V_{HV}=$-14kV. Buffer gas was injected using a leak valve at the entrance of the 5 trapping region. Annihilations occurring within the trapping region are detected via ejected mesons using external scintillators shown in blue. Positive ions are captured using a nested trap potential presented in Fig. \ref{['fig:procedure']} and ejected axially to a downstream MCP, placed at a distance of $L_{\text{TOF}}=1.05(1)$m within the 1T trapping region for Time-of-Flight (TOF) analysis of the charged species.
• Figure 2: Overview of the multi-step nested trap procedure, used for the capture and TOF spectrometry of positive ion species formed following antiproton annihilations in the 5 AE$\overline{\textrm{g}}$IS Penning-Malmberg trap. Each step is presented in chronological order from 1 to 6, showing the applied voltages on the trap electrodes together with the antiproton and ion location within the trap. The 0V potential of the trap is indicated with a blue dash-dotted line, the red dashed lines indicates relevant voltage settings.
• Figure 3: TOF measurements in Helium background, showing MCP signal observed at different antiproton storage times ($\tau_{\overline{\mathrm{p}}}$) together with measured annihilation events using external scintillators. Left and center panels: MCP signal vs TOF (left panel) together with corresponding scintillator signal vs time after antiproton capture (center panels). The MCP signal shows peaks identified as He^1+ (m/q $\approx$ 4) and (m/q $\approx$ 2) species, the center panels shows the scintillator events observed for each measurement, the grey shade shows the release of antiprotons, and the dotted line indicate the ion ejection time. The repeated spikes observed in the scintillator signal are from the AD-cycle background. Right panel: plot of the integrated fractional intensity of the m/q $\approx$ 2 peak with respect to $\tau_{\overline{\mathrm{p}}}$, fitted with the linear function shown in the inset.
• Figure 4: TOF spectrum of ejected positive ions observed during the argon/helium experiment. Data (points) are shown with the multi-peak fit (red line), further details are provided in the Supplemental Materials.
• Figure 5: TOF spectrum of ejected electron-cooled antiprotons ejected from a trap using the electrode bias $\mathrm{V_{launch}=-180}$ V and $\mathrm{V_{wall}}=-190$ V, fitted using PS-hyper-EMG function (red) and the Gaussian core (blue).