A prototype gas cell for the stopping, extraction and neutralization of radioactive nuclei from the SPIRAL2 Super Separator Spectrometer (S$^3$)

TL;DR

The paper presents a prototype FRIENDS^3 gas cell designed to enable fast extraction and neutralization of radioactive ions for S^3-LEB in-gas-jet laser spectroscopy. It combines ion-transport simulations (COMSOL and SIMION) and plasma-density modeling to compare approaches and optimize geometry, electrode gradients, and the neutralization channel, aiming to outperform the current S^3-LEB cell for short-lived isotopes. A key finding is that the final FRIENDS^3 design can offer shorter extraction times and higher effective efficiency for ions with half-lives below several hundred milliseconds, by separating rapid electrical extraction from slower neutralization in a fast-flow exit region, where recombination with gas-produced electrons is possible. The work also demonstrates, via electron-density simulations, that diffusion, migration, and convection govern neutralization success, with a charge-equilibrium regime emerging at sufficiently high ionization rates; beta-source neutralization could be feasible at higher pressures, motivating ongoing experimental validation and further optimization.

Abstract

We present the design and simulation of a prototype gas cell for in-gas-jet laser-ionization and spectroscopy studies using the low energy branch of the SPIRAL2-S$^3$ radioactive-ion-beam facility. The prototype aims to demonstrate the possibility to reduce the extraction time of radioactive ions from the gas cell, while implementing a controlled neutralization mechanism, necessary for laser-spectroscopy studies. Different simulation methods of ion processes in gas are comparatively discussed. Design considerations and detailed simulations of the ion extraction time and efficiency are presented. A study of the dynamics of electrons obtained in the gas cell by ionization is also performed to assess the achievable electron densities.

Figures (8)

• Figure 1: Schematic representation of the initial (left) and final (right) gas-cell designs. The initial design features a more compact geometry, a larger number of funnel electrodes, and a segmented neutralization channel, whereas the final design adopts a wider and simplified geometry with fewer electrodes, taking into account stopping-volume margins, mechanical constraints and manufacturing considerations.
• Figure 2: Examples of simulations performed for the characterization of the gas-cell performance in its initial design (see Fig. \ref{['fig:geo']}), at 100 mbar argon pressure and using the so-called low-voltage settings. The left panel shows the results of a combined COMSOL simulation of the gas-flow velocity (corresponding to the color legend in m/s) and of the electrical potential (electrode voltages and equipotential contours are shown). The middle panel shows a COMSOL simulation of ion transport using the Plasma module (the color legend presents the ion density in cm$^{-3}$). The right panel shows the corresponding simulation performed using SIMION (but with the individual ion trajectories shown, instead of the ion density). Both ion-transport simulations were performed with $^{133}$Cs$^+$ and diffusion is taken into account. The images in the middle and right panels are captured after 45 ms of ion transport. We note that the COMSOL simulation is performed in cylindrical coordinates. The properties of the initial ion density in the COMSOL simulation are matched to those of the initial ion distribution used in SIMION.
• Figure 3: Simulated extraction efficiency (top) and average extraction time (bottom) as a function of pressure, for the initial gas-cell design with the low-voltage settings, obtained with COMSOL (red) using either the CPT module, without diffusion effects (open diamonds) or the Plasma module, with diffusion effects (closed circles), compared to results obtained using the SDS model of SIMION (green) without (open diamonds) or with (closed circles) diffusion effects.
• Figure 4: SIMION simulation of the gas cell in its initial design as a function of pressure, comparing the low-voltage (blue) and high-voltage settings (red), see Table \ref{['tab_LH_V']}. (Top) Extraction efficiency, (middle) extraction time up to the entrance of the neutralization channel (dashed lines) and up to the outlet (solid lines), and (Bottom) residence time of the ions in the neutralization channel.
• Figure 5: Simulated total efficiency of the FRIENDS$^3$ gas cell (considering extraction and decay losses) at 100 mbar, 200 mbar, and 500 mbar, compared to the current gas cell at 500 mbar, for different decay half-lives of the extracted ions.