Optimal operating parameters for next-generation xenon gas time projection chambers

TL;DR

This work analyzes how operating parameters for next-generation gaseous xenon TPCs influence sensitivity to neutrinoless double beta decay. By comparing enriched $^{136}$Xe and natural Xe across detector sizes, shielding, energy resolution, pressure, and diffusion for three gas-TPC technologies (EL TPC, Topology TPC, and Ion TPC), it shows that background rates can reach fractions of a count per tonne-year in the ROI under realistic conditions. Enriched Xe consistently yields lower backgrounds, while diffusion-reduction additives and higher pressure improve topology-based discrimination, albeit with trade-offs in construction and scintillation. Overall, the study provides design guidance for tonne-scale GXeTPCs targeting sensitivities toward $10^{27}$--$10^{28}$ years, highlighting the importance of enrichment, diffusion control, and energy-resolution performance for achieving low backgrounds and robust signal efficiency.

Abstract

The next-generation of neutrinoless double beta decay ($0νββ$) searches are targeting half-life sensitivities towards 10$^{27}$--10$^{28}$ years. Gaseous xenon time projection chamber (GXeTPC) detectors are a technology that may be able to meet this challenge due to their excellent background rejection power, scalability, and energy resolution. This paper explores how the design choices of a next-generation GXeTPC time projection chamber can impact the overall performance of the experiment. We study the performance of systems using xenon enriched in the isotope $^{136}$Xe or natural xenon, focusing on scenarios that incorporate one tonne of source isotope. The detector size, copper shielding mass, energy resolution, pressure, and diffusion amount are surveyed to evaluate the overall performance dependencies on these parameters. A detector optimized for using enriched xenon is preferred, with a factor of 10 lower background rate, driven by the large intrinsic backgrounds introduced by the copper shielding used in the detector. The performance of three types of gas TPC technologies was also explored based on different gas additives used to reduce diffusion to different levels. For all TPC technologies, we find background rates of a fraction of a count per tonne year in the region of interest are achievable. These performances are contingent on suitable energy resolution and event position placement in the drift direction being achieved for the specific detector technology. When factoring in the considerations for the construction of the detector in addition to the selection performance, there is no clear optimum pressure, with advantages and disadvantages if a high or low pressure default configuration is chosen.

Figures (16)

• Figure 1: The total mass of inner shielding copper as a function of pressure for (left) enriched and (right) natural detector assuming 12 cm thickness.
• Figure 2: Example of the same signal event with different amounts of diffusion applied at 1 bar and 25 bar. These events include a 300 eV energy threshold.
• Figure 3: The fraction of events depositing energy within 2.3--2.6 MeV of energy to the number of decays of each radioisotope for an enriched and natural detector size.
• Figure 4: Top left: The distributions of background assuming 0.5% FWHM energy resolution. The blue and maroon dashed lines represent the bounds of one 0.5% FWHM window. An asymmetric cut is done on this window by increasing the lower bound (blue line). Top right, bottom left, bottom right, show the background acceptance rates assuming different energy resolutions for $^{137}$Xe, $^{214}$Bi, $^{208}$Tl, respectively. The x-axis shows the efficiency loss vs background acceptance from increasing the lower bound cut on energy.
• Figure 5: Background acceptance factors for voxelized tracks at 1 bar for different diffusion amounts for an enriched (left) and natural (right) detector configuration. Overall, the dependence on diffusion has a similar functional form.