Sub-Penning gas mixtures: new possibilities for ton- to kiloton-scale time projection chambers

In this work, we present the concept for large low-background experiments in which an unusual gas mixture gas serves as a seamless, high-QE, near-100\%-coverage photodetector for scintillation or \cerenkov photons. We fill a large time projection chamber with a VUV scintillating gas, plus an unusually small admixture dopant gas with a low ionization threshhold (and a high ionization yield), akin to a highly-underquenched Penning mixture. Scintillation photons travel far from a primary ionization site before converting into photoionization electrons. Using standard TPC methods, we can separately count both the primary ionization electrons (which occur along a dense track) and the scintillation-ionization electrons (which will occur over a large spherical region) without the use of PMTs. The scheme is compatible with very large detectors, in both two-phase and single-phase high pressure configurations. We discuss how the drift-axis position of an event can be reconstructed, and under what constraints we can expect stable gas gain operations. We propose some detectors illustrating how this scheme---both in conventional two-phase geometries, as well as in pressurized space in solution-mined salt cavern---makes it possible to safely construct gas time projection chambers of previously-unreachable target masses, capable of studying dark matter, double beta decay, proton decay, and solar neutrinos; more speculative gas mixtures might extend the technique to reactor and geoneutrinos