  We propose a novel charge sensing concept for high-pressure Time Projection
Chamber (TPC) to search for Neutrinoless Double-Beta Decay (NLDBD) with
ton-scale isotope mass and beyond. A meter-sized plane, tiled with an array of
CMOS integrated sensors called Topmetal that directly collect charge without
gas avalanche gain, is to be deployed into a high-pressure gaseous TPC with
working gases containing suitable NLDBD candidate isotopes such as Xe-136 and
Se-82. The Topmetal sensor has an electronic noise <30 e- per pixel, which
allows the detector to reach <1% FWHM energy resolution at the NLDBD Q-value
for both Xe-136 and 82SeF6 gases by measuring ionization charges alone. The
elimination of charge avalanche gain allows the direct sensing of slow-drifting
ions, which enables the use of highly electronegative gas SeF6 in which free
electrons do not exist. It supports the swapping of working gases without
hardware modification, which is a unique way to validate signals against
radioactive backgrounds. Since the sensor manufacturing and plane assembling
could leverage unaltered industrial mass-production processes, stability,
uniformity, scalability, and cost-effectiveness that are required for ton-scale
experiments could all be reached. The strengths of TPC such as 3D ionization
tracking and decay daughter tagging are retained. This development could lead
to a competitive NLDBD experiment at and above ton-scale. The conceptual
considerations, simulations, and initial prototyping are discussed.
