Gaseous forms of $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{124}$Sn, and $^{130}$Te: new avenues to future $0\nu\beta\beta$ time projection chambers

TL;DR

This paper proposes using gaseous compounds of certain isotopes as scalable, non-xenon targets for large-scale neutrinoless double beta decay TPCs, enabling electron drift and background rejection.

Contribution

It identifies affordable, electropositive gases containing key isotopes suitable for gas-phase TPCs in future $0 uetaeta$ searches, expanding options beyond xenon.

Findings

Identified candidate gases for $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{124}$Sn, and $^{130}$Te.

Argued that 100 T and kiloton-scale gas TPCs are feasible without unprecedented infrastructure.

Proposed that these gases enable electron drift and track topology-based background rejection.

Abstract

Searches for neutrinoless double beta decay are growing larger, with tonne-scale targets in several nuclides still far from exhausting the discovery space. What's beyond ton scale? Time projection chambers (TPCs) are one option for building large (100~T or kiloton-scale) instruments, but filling them with the familiar $^{136}$Xe for a $0\nu\beta\beta$ search is a problem: xenon is a scarce element whose atmospheric-extraction supply chain is small and hard to grow. If future $0\nu\beta\beta$ searches wish to exploit TPCs' known hardware scalability, we need to fill them with non-xenon target materials. Of particular value would be a TPC that can drift electrons, rather than ions, letting us use mature readout schemes which require gas gain. In this paper, we identify a set of previously-unappreciated, affordable gases which are likely to be electropositive, allowing electron drift and gain in gas-phase TPCs sensitive to $0\nu\beta\beta$ with the help of track-topology background rejection. We identify candidate $^{76}$Ge, $^{82}$Se, $^{96}$Zr, $^{100}$Mo, $^{124}$Sn, and $^{130}$Te compounds suitable for gas-phase electron-drift TPCs; some may be suitable for liquid-phase TPCs as well. Using a figure-of-merit that emphasizes the need for track topology for background rejection, we argue that 100~T and kiloton-scale gas TPCs are realistic without unprecedented underground infrastructure.