A Comparison of Micromegas with x/y Strip Charge Readouts for Directional Recoil Detection

Abstract: Detecting the topology and direction of low-energy nuclear and electronic recoils is broadly desirable in nuclear and particle physics, with applications in coherent elastic neutrino-nucleus scattering, astrophysical neutrino measurements, probing dark matter (DM) within the neutrino fog, and confirming the galactic origin of DM. Gaseous Time Projection Chambers (TPCs) offer the required gain and readout granularity, but must be large to achieve the required volume. Therefore, scalable, cost-effective readout technologies are essential. High-resolution x/y strip readouts, previously identified as the optimal balance between cost-efficiency and performance, are examined here. To guide the readout design of a 40-L detector under construction, we present a comparative analysis of 9 x/y strip configurations with Micromegas amplification. Each setup employs VMM3a front-end ASICs within the RD51 Scalable Readout System (SRS) for strip readout and a pulse height analyzer for reading out the Micromegas mesh. These techniques assess gain, gain resolution, x/y charge sharing, and spatial resolution of each setup. Configurations with a diamond-like carbon (DLC) layer exhibit improved spark resistance, allowing larger maximal gain and improved fractional gain resolution without notable impact on the spatial resolution. Although the DLC reduces the signal in the strips situated lower in the readout, this can be mitigated by narrowing the perpendicularly oriented strips above them. Our results allow us to select the optimal readout for future detectors. We also observe clear 3D tracks from alpha particles, with performance in good agreement with a simple simulation. Overall, Micromegas with x/y strip readout are promising for low-energy recoil observatories. However, dedicated amplification devices or improved electronics are needed to reach the fundamental performance limit of 3D electron counting.