  We report measurements of the transverse diffusion of electrons in P-10 gas
(90% Ar, 10% CH4) in a laboratory-scale time projection chamber (TPC) utilizing
a novel pixelated signal capture and digitization technique known as Q-Pix. The
Q-Pix method incorporates a precision switched integrating transimpedance
amplifier whose output is compared to a threshold voltage. Upon reaching the
threshold, a comparator sends a 'reset' signal, initiating a discharge of the
integrating capacitor. The time difference between successive resets is
inversely proportional to the average current at the pixel in that time
interval, and the number of resets is directly proportional to the total
collected charge. We developed a 16-channel Q-Pix prototype fabricated from
commercial off-the-shelf components and coupled them to 16 concentric annular
anode electrodes to measure the spatial extent of the electron swarm that
reaches the anode after drifting through the uniform field of the TPC. The
swarm is produced at a gold photocathode using pulsed UV light. The measured
transverse diffusion agrees with simulations in PyBoltz across a range of
operating pressures (200-1500 Torr). These results demonstrate that a Q-Pix
readout can successfully reconstruct the ionization topology in a TPC.
