A journey to ITACA

TL;DR

The paper proposes ITACA, a dual-tracking approach for GXeEL TPCs in $ββ0ν$ searches by imaging both electron tracks and the corresponding ion tracks. By adding trace NH$_3$ to convert Xe$^+$ to NH$_4^+$, ITACA enables slow-moving ion tracks to be imaged with sub-millimeter diffusion using molecular sensors and laser interrogation, complementing the fast EL electron image. This dual-imaging boosts topological discrimination, achieving an approximate order-of-magnitude improvement in background rejection and up to a ~20× sensitivity gain for favorable backgrounds, thereby extending the discovery potential for very long $T_{1/2}$ lifetimes. The approach relies on rapid ammonia chemistry, controlled additive concentration, and a dedicated ion-detection chain (Molecular Ion Detector, Magnetically Activated Molecular Apparatus, Transport and Shelving System, and Ion Scanning Microscope) to reconstruct ion tracks with minimal EL-induced smearing.

Abstract

A unique feature of gas xenon electroluminescent time projection chambers (GXeEL TPCs) in $0νββ$ searches is their ability to reconstruct event topology, in particular to distinguish "single-electron" from "double-electron" tracks, the latter being the signature of a $0νββ$ decay near the decay endpoint $Q_{ββ}$. Together with excellent energy resolution and the t$_0$ provided by primary scintillation, this topological information is key to suppressing backgrounds. Preserving EL, however, requires operating in pure xenon (with helium as the only benign additive), and in pure xenon the diffusion of drifting electrons is large. As a result, the fidelity of reconstructed tracks is limited both by diffusion and by the intrinsic blurring of EL amplification. We propose augmenting the detector with the ability to image not only the electron track but also the corresponding mirror ion track. Introducing trace amounts of NH$_3$ ($\sim$100 ppb) converts Xe$^+$ ions into NH$_4^+$ while leaving EL unaffected. For events in the region-of-interest, an ion sensor positioned near the cathode at the projected barycenter captures the NH$_4^+$ ions. Electrons drift rapidly to the anode, producing the standard EL image, whereas the NH$_4^+$ ions drift slowly toward the cathode. Their slow drift provides time to determine the event energy and barycenter. Laser interrogation of the sensor's molecular layer then reveals an ion-track image with sub-millimeter diffusion and no EL-induced smearing. The combined electron-ion imaging substantially strengthens topological discrimination, improving background rejection by about an order of magnitude and significantly extending the discovery potential of GXeEL TPCs for very long $0νββ$ lifetimes.

Figures (14)

• Figure 1: Principle of operation of of an asymmetric GXeEL TPC with SiPMs and PMTs.
• Figure 2: A simulated $\beta\beta0\nu$ track in HPXe.
• Figure 3: Reconstruction of a $\beta\beta0\nu$ track a GXeEL TPC with 1 m electron drift. The upper row shows the XY projection of the track in pure xenon, for the characteristic distances. Near the cathode ($\sim 100$ cm) in the middle of the drift ($\sim 50$ cm) and near the anode ($\sim 10$ cm). The bottom row shows the same projections for Xe/He.
• Figure 4: Principle of operation of ITACA.
• Figure 5: The diffusion in ion and electron tracks is anticorrelated. The upper panel shows the case in which the electrons are near the anode (10 cm), and thus ions are far away from the cathode (100 cm); the bottom panel shows the inverse case. Ions are close to the cathode (10 cm) and thus electrons are far from the anode (100 cm). The availability of the ion track allows a uniform topological reconstruction across the detector.